Electrical Troubleshooting 101
11-14-2008, 03:28 AM
#1
Electrical Troubleshooting 101 Introduction:
Well, I've seen a lot of posts on here lately about people asking for help in troubleshooting electrical issues with sensors or driveability and notice that a lot of the posters have done absolutely no troubleshooting of their own using a DVOM (digital volt-ohm meter).
The way some see troubleshooting as a "part swap until I just happen to find the problem" can be very costly.
As an Instrumentation and Electrical Technician that has to troubleshoot problems with multi-million dollar machinery where each hour of downtime costs $6000, I cannot fathom why someone would not take time to learn some basics of electrical troubleshooting on their own project vehicle instead of paying a fortune in "parts swapping".
You pay thousands to build a fast car, why not a hundred bucks on a halfway decent piece of test equipment?
I decided to start this thread so that guys who REALLY want to do it themselves can have some reference.
Anyone can add ideas or comments at anytime and I truly welcome it.
Well, I've seen a lot of posts on here lately about people asking for help in troubleshooting electrical issues with sensors or driveability and notice that a lot of the posters have done absolutely no troubleshooting of their own using a DVOM (digital volt-ohm meter).
The way some see troubleshooting as a "part swap until I just happen to find the problem" can be very costly.
As an Instrumentation and Electrical Technician that has to troubleshoot problems with multi-million dollar machinery where each hour of downtime costs $6000, I cannot fathom why someone would not take time to learn some basics of electrical troubleshooting on their own project vehicle instead of paying a fortune in "parts swapping".
You pay thousands to build a fast car, why not a hundred bucks on a halfway decent piece of test equipment?
I decided to start this thread so that guys who REALLY want to do it themselves can have some reference.
Anyone can add ideas or comments at anytime and I truly welcome it.
Last edited by Dan Stewart; 11-14-2008 at 03:55 AM.
11-14-2008, 03:33 AM
#2
Equipment:
DVOM - Digital VoltOhmMeter. This is one of those digital meters I'm sure you've seen in the hardware store. Avoid the really cheap ones, as they can be extremely inaccurate. For about $100, you can find a nice one at the local hardware store. Want even more accuracy? Expect to spend several hundred dollars. Pick one that reads voltage in DC and resistance.
Test Light - Avoid it like the plague. This was fine in the days before cars had sophisticated electronics and all you needed to know was whether or not there was 12 volts present, but not today. Because they have such a low resistance, they can damage the circuitry in the vehicle.
Click on the thumbnail below for a picture of a DVOM. (This is the one I use for work and is extremely expensive. We are just using it as an example for visual reference)
DVOM - Digital VoltOhmMeter. This is one of those digital meters I'm sure you've seen in the hardware store. Avoid the really cheap ones, as they can be extremely inaccurate. For about $100, you can find a nice one at the local hardware store. Want even more accuracy? Expect to spend several hundred dollars. Pick one that reads voltage in DC and resistance.
Test Light - Avoid it like the plague. This was fine in the days before cars had sophisticated electronics and all you needed to know was whether or not there was 12 volts present, but not today. Because they have such a low resistance, they can damage the circuitry in the vehicle.
Click on the thumbnail below for a picture of a DVOM. (This is the one I use for work and is extremely expensive. We are just using it as an example for visual reference)
Last edited by Dan Stewart; 11-14-2008 at 07:25 PM.
11-14-2008, 03:52 AM
#3
Direct Current:
Much is truly misunderstood when it comes to how electrical circuits function in an automobile.
Cars typically utilize a 12 volt direct current (DC) system that gets it power from two sources. The first is the battery, which is used for providing power for starting the engine, running accessories when the engine isn't running, and electricity during high consumption periods in which the alternator cannot simply keep up with the load.
Most OEM batteries are of the 12 volt, 6 cell, lead-acid variety. This means that when you test the potential difference between the positive and negative terminal post on the battery, you will read about 12 volts. Typically, with a fully charged battery, you will actually measure slightly more than the rated voltage. On a 12 volt, this is around 12.6 volts and is perfectly normal.
There are two theories of how electrons flow in a DC circuit. The positive electron flow or conventional flow theory uses the principle that most of us are familiar with in that the "positive" terminal of the battery is the "hot" side. The negative electron flow or just electron flow theory uses the principle that the "negative" terminal of the battery is the "hot" side and is technically correct. But, for simplicity, we'll use the positive electron flow principle for all of our discussion here.
DC or direct current is simply a reference to how the electrons flow in the electrical circuit. In DC, the current of electricity is always moving in one direction, where in AC or alternating current, the electrical current changes direction. The only concern we have with AC is that the alternator produces an AC current, but has circuitry built into it to convert the AC current into DC. This involves something more than what I would consider "basic", so let's just keep in mind that all we want out of the alternator is DC.
In my next installment, I'll show you how to use the basic functions of a DVOM with pics and maybe a video or two (anyone want to host files, let me know)
(contribution thanks: Frost, edcmat-l1)
Much is truly misunderstood when it comes to how electrical circuits function in an automobile.
Cars typically utilize a 12 volt direct current (DC) system that gets it power from two sources. The first is the battery, which is used for providing power for starting the engine, running accessories when the engine isn't running, and electricity during high consumption periods in which the alternator cannot simply keep up with the load.
Most OEM batteries are of the 12 volt, 6 cell, lead-acid variety. This means that when you test the potential difference between the positive and negative terminal post on the battery, you will read about 12 volts. Typically, with a fully charged battery, you will actually measure slightly more than the rated voltage. On a 12 volt, this is around 12.6 volts and is perfectly normal.
There are two theories of how electrons flow in a DC circuit. The positive electron flow or conventional flow theory uses the principle that most of us are familiar with in that the "positive" terminal of the battery is the "hot" side. The negative electron flow or just electron flow theory uses the principle that the "negative" terminal of the battery is the "hot" side and is technically correct. But, for simplicity, we'll use the positive electron flow principle for all of our discussion here.
DC or direct current is simply a reference to how the electrons flow in the electrical circuit. In DC, the current of electricity is always moving in one direction, where in AC or alternating current, the electrical current changes direction. The only concern we have with AC is that the alternator produces an AC current, but has circuitry built into it to convert the AC current into DC. This involves something more than what I would consider "basic", so let's just keep in mind that all we want out of the alternator is DC.
In my next installment, I'll show you how to use the basic functions of a DVOM with pics and maybe a video or two (anyone want to host files, let me know)
(contribution thanks: Frost, edcmat-l1)
Last edited by Dan Stewart; 11-15-2008 at 12:55 AM.
11-14-2008, 08:42 AM
#4
Moderator
iTrader: (11)
Join Date: Mar 2002
Location: East Central Florida
Posts: 12,604
Likes: 0
Received 6 Likes
on
6 Posts
A DMM good enough for basic use can be as cheap
as $4 (Harbor Freight; have 8 of these myself) but
one which reads frequency can be very handy
(such as checking the MAF output, RPM, etc.) Got
one on sale at Radio Shack for $39 that had this
feature plus came with a thermocouple & adaptor
so reads temperature too. Don't be scared off by
the "$100 and up" notion, just like you don't -have-
to buy Snap-on for your first set of wrenches.
as $4 (Harbor Freight; have 8 of these myself) but
one which reads frequency can be very handy
(such as checking the MAF output, RPM, etc.) Got
one on sale at Radio Shack for $39 that had this
feature plus came with a thermocouple & adaptor
so reads temperature too. Don't be scared off by
the "$100 and up" notion, just like you don't -have-
to buy Snap-on for your first set of wrenches.
11-14-2008, 09:26 AM
#5
Agreed:
The DVOM and test light will get you by for the ~12 volt and ~5 volt common stuff, but the frequency and CAN diagnostics may be beyond your intended scope?
This is where the easier but decidedly "not cheap" solution gets into the Tech ll. The really extensive GM diagnostic procedures employ the Tech ll.
The DVOM and test light will get you by for the ~12 volt and ~5 volt common stuff, but the frequency and CAN diagnostics may be beyond your intended scope?
This is where the easier but decidedly "not cheap" solution gets into the Tech ll. The really extensive GM diagnostic procedures employ the Tech ll.
11-14-2008, 11:34 AM
#6
Direct Current:
Most OEM batteries are of the 12 volt, 6 cell, lead-acid variety. This means that when you test the potential difference between the positive and negative terminal post on the battery, you will read about 12 volts. Typically, with a fully charged battery, you will actually measure slightly more than the rated voltage. On a 12 volt, this is around 12.2-12.3 volts and is perfectly normal.
Most OEM batteries are of the 12 volt, 6 cell, lead-acid variety. This means that when you test the potential difference between the positive and negative terminal post on the battery, you will read about 12 volts. Typically, with a fully charged battery, you will actually measure slightly more than the rated voltage. On a 12 volt, this is around 12.2-12.3 volts and is perfectly normal.
11-14-2008, 11:35 AM
#7
Agreed:
The DVOM and test light will get you by for the ~12 volt and ~5 volt common stuff, but the frequency and CAN diagnostics may be beyond your intended scope?
This is where the easier but decidedly "not cheap" solution gets into the Tech ll. The really extensive GM diagnostic procedures employ the Tech ll.
The DVOM and test light will get you by for the ~12 volt and ~5 volt common stuff, but the frequency and CAN diagnostics may be beyond your intended scope?
This is where the easier but decidedly "not cheap" solution gets into the Tech ll. The really extensive GM diagnostic procedures employ the Tech ll.
Trending Topics
11-14-2008, 11:51 AM
#8
....
There are two theories of how electrons flow in a DC circuit. The positive electron flow theory uses the principle that most of us are familiar with in that the "positive" terminal of the battery is the "hot" side. The negative electron flow theory uses the principle that the "negative" terminal of the battery is the "hot" side and is technically correct. But, for simplicity, we'll use the positive electron flow principle for all of our discussion here.
DC or direct current is simply a reference to how the electrons flow in the electrical circuit. In DC, the current of electricity is always moving in one direction, where in AC or alternating current, the electrical current changes direction. The only concern we have with AC is that the alternator produces an AC current, but has circuitry built into it to convert the AC current into DC. This involves something more than what I would consider "basic", so let's just keep in mind that all we want out of the alternator is DC.
.....
There are two theories of how electrons flow in a DC circuit. The positive electron flow theory uses the principle that most of us are familiar with in that the "positive" terminal of the battery is the "hot" side. The negative electron flow theory uses the principle that the "negative" terminal of the battery is the "hot" side and is technically correct. But, for simplicity, we'll use the positive electron flow principle for all of our discussion here.
DC or direct current is simply a reference to how the electrons flow in the electrical circuit. In DC, the current of electricity is always moving in one direction, where in AC or alternating current, the electrical current changes direction. The only concern we have with AC is that the alternator produces an AC current, but has circuitry built into it to convert the AC current into DC. This involves something more than what I would consider "basic", so let's just keep in mind that all we want out of the alternator is DC.
.....
My schooling and subsequent work taught me that these theories are called:
1. Conventional current [flow] - current travels from positive to negative; used often by high school teachers, hobbyists, and I am sure some others
2. Electron flow - current travels from negative to positive; used by all of the physicists engineers that I have worked with. The basis of my own college schooling as well.
11-14-2008, 12:18 PM
#9
Moderator
iTrader: (11)
Join Date: Mar 2002
Location: East Central Florida
Posts: 12,604
Likes: 0
Received 6 Likes
on
6 Posts
Troubleshooting tips
People tend to think about voltages, but there are
some things to consider. First, voltage is "potential"
but current is generally what gets something done.
Voltage is a span and current must travel a loop
(the "circuit"). In automotive the voltage is referred
to ground. A ground. Which ground? And current is
also generally returned from the load to the source
via a ground path. People think about shorts when
they see lack of voltage, but it's opens that kill
current loops. Think about the loop that the signal
or power has to take, in its entirety. "Be the ball".
For ground to be solid and clean, the ground path
needs to be continuous (zero resistance ideal, low
resistance really). Ground quality is enforced by
a set of braided straps connecting engine block
to chassis sheet metal, battery to block, PCM to
chassis and so on. With high current loads on the
motor (coils, injectors, A/C) and body (fans, lights,
boom-booms) grounds, any resistance produces
an offset voltage which can induce error on sensor
outputs (if block-referred), cut available headroom
for other loads (fuel pump, battery charging, etc.)
and generally flake things out. Even things like a
taillight that works abnormally, often has to do
with a ground fault rather than the high side source.
Ground braids might look like they aren't important
and can easily be left to flap in all the excitement
of trying to get to fire-up after a head swap or
removing the AIR plumbing, etc. But you want 'em.
If you are looking for a voltage and read none on
the meter, there are really two ways for this to
happen. One is that the source is really off (open).
The other is, your reference ground might be
instead. So when you get a bad reading, measure
not only from ground to wherever, but from a
+12V solid feed to the same point. Something
that is dead open will read zero for both; if you
have full voltage in one case or the other, it says
the open-circuit fault is between the two points
in question.
Sensors such as ECT are block-returned and can
be jacked by losing the block-chassis ground straps
at the back of the cylinder head and/or near the
alternator. Others ('most any with a multi-wire
harness that includes a black wire) are returned
to PCM ground in a continuous loop.
The high-current ground braids mount to body
studs and block bolts. Corrosion will eventually
raise the resistance of these connections and
on any older vehicle if you're chasing electrical
oddities, a regimen of find, remove, wire-brush
and tighten before trying to be smart, may save
you a lot of head-scratching.
Some sensors require +12V power and/or +5V
reference / power to work. The MAF for example
needs 12V for the hot wires and has a PCM-internal
pullup to +5V for the output signal. The ENG SEN
fused power runs a lot of sensors and widespread
sensor faults indicate this common point.
People tend to think about voltages, but there are
some things to consider. First, voltage is "potential"
but current is generally what gets something done.
Voltage is a span and current must travel a loop
(the "circuit"). In automotive the voltage is referred
to ground. A ground. Which ground? And current is
also generally returned from the load to the source
via a ground path. People think about shorts when
they see lack of voltage, but it's opens that kill
current loops. Think about the loop that the signal
or power has to take, in its entirety. "Be the ball".
For ground to be solid and clean, the ground path
needs to be continuous (zero resistance ideal, low
resistance really). Ground quality is enforced by
a set of braided straps connecting engine block
to chassis sheet metal, battery to block, PCM to
chassis and so on. With high current loads on the
motor (coils, injectors, A/C) and body (fans, lights,
boom-booms) grounds, any resistance produces
an offset voltage which can induce error on sensor
outputs (if block-referred), cut available headroom
for other loads (fuel pump, battery charging, etc.)
and generally flake things out. Even things like a
taillight that works abnormally, often has to do
with a ground fault rather than the high side source.
Ground braids might look like they aren't important
and can easily be left to flap in all the excitement
of trying to get to fire-up after a head swap or
removing the AIR plumbing, etc. But you want 'em.
If you are looking for a voltage and read none on
the meter, there are really two ways for this to
happen. One is that the source is really off (open).
The other is, your reference ground might be
instead. So when you get a bad reading, measure
not only from ground to wherever, but from a
+12V solid feed to the same point. Something
that is dead open will read zero for both; if you
have full voltage in one case or the other, it says
the open-circuit fault is between the two points
in question.
Sensors such as ECT are block-returned and can
be jacked by losing the block-chassis ground straps
at the back of the cylinder head and/or near the
alternator. Others ('most any with a multi-wire
harness that includes a black wire) are returned
to PCM ground in a continuous loop.
The high-current ground braids mount to body
studs and block bolts. Corrosion will eventually
raise the resistance of these connections and
on any older vehicle if you're chasing electrical
oddities, a regimen of find, remove, wire-brush
and tighten before trying to be smart, may save
you a lot of head-scratching.
Some sensors require +12V power and/or +5V
reference / power to work. The MAF for example
needs 12V for the hot wires and has a PCM-internal
pullup to +5V for the output signal. The ENG SEN
fused power runs a lot of sensors and widespread
sensor faults indicate this common point.
11-14-2008, 10:21 PM
#10
Using the DVOM:
In our first picture below (click on the first thumbnail to enlarge), we see how to set up our DVOM to check voltage in DC. Before beginning using the meter, we need to verify that the meter leads are inserted into the correct plug holes. The black or negative lead will always be inserted into the plug hole that is labelled "COM" or "-", depending on how your particular meter is built. The red or the positive lead will always be inserted in the plug hole that is labelled "V" or "+", again depending on the meter you have.
Now that we have our leads correctly inserted into the corresponding plug holes, we need to do a simple test to make sure that the leads are good and the meter is functioning correctly. Skipping this step can have you scratching your head for hours because you can't figure out why you aren't reading anything on the devices that you are testing.
Using the selection **** on the meter, turn it until the "continuity" or "resistance" function is selected. On most meters, this will be the symbol that looks like an upside-down horseshoe. This symbol is a greek letter called Omega. It is the universal symbol that is used to describe the unit of electrical measurement of resistance called the Ohm. We'll cover more on resistance and Ohms later, but for now we just want to get to know our test equipment better. (click on the second thumbnail to see the setting for this)
Now that we have the leads inserted (note that the red lead uses the same plug hole in the meter for voltage as it does for continuity, your's may or may not use the same hole. if not, skip this), we can test to make sure our meter is working properly.
Take the metal prong of the red lead and simply touch it to the metal prong of the black lead and read the display. It should read close to 0.000 ohms. Remember this, everything we test in this setting will be displayed as the measurement "ohms". The 0.000 (or 0.001, 0.002, etc. is fine) tells us that there is little to no resistance between the leads and the meter is working properly (see the 3rd thumbnail below). Now, with the leads not touching, you will see the display read "0.L" or "0L". This is an abbreviation for "Open loop", meaning that the meter sees that the circuit it is trying to measure is not complete and that there is an "open" in the circuit. If you read "0.L" or "0L" when touching the leads together, recheck to make sure that the leads are in the correct plug holes. If the are in the correct location, you have a problem with either a bad lead or a bad meter (see the 2nd thumbnail below for how the "0.L" or "0L" looks like).
In our first picture below (click on the first thumbnail to enlarge), we see how to set up our DVOM to check voltage in DC. Before beginning using the meter, we need to verify that the meter leads are inserted into the correct plug holes. The black or negative lead will always be inserted into the plug hole that is labelled "COM" or "-", depending on how your particular meter is built. The red or the positive lead will always be inserted in the plug hole that is labelled "V" or "+", again depending on the meter you have.
Now that we have our leads correctly inserted into the corresponding plug holes, we need to do a simple test to make sure that the leads are good and the meter is functioning correctly. Skipping this step can have you scratching your head for hours because you can't figure out why you aren't reading anything on the devices that you are testing.
Using the selection **** on the meter, turn it until the "continuity" or "resistance" function is selected. On most meters, this will be the symbol that looks like an upside-down horseshoe. This symbol is a greek letter called Omega. It is the universal symbol that is used to describe the unit of electrical measurement of resistance called the Ohm. We'll cover more on resistance and Ohms later, but for now we just want to get to know our test equipment better. (click on the second thumbnail to see the setting for this)
Now that we have the leads inserted (note that the red lead uses the same plug hole in the meter for voltage as it does for continuity, your's may or may not use the same hole. if not, skip this), we can test to make sure our meter is working properly.
Take the metal prong of the red lead and simply touch it to the metal prong of the black lead and read the display. It should read close to 0.000 ohms. Remember this, everything we test in this setting will be displayed as the measurement "ohms". The 0.000 (or 0.001, 0.002, etc. is fine) tells us that there is little to no resistance between the leads and the meter is working properly (see the 3rd thumbnail below). Now, with the leads not touching, you will see the display read "0.L" or "0L". This is an abbreviation for "Open loop", meaning that the meter sees that the circuit it is trying to measure is not complete and that there is an "open" in the circuit. If you read "0.L" or "0L" when touching the leads together, recheck to make sure that the leads are in the correct plug holes. If the are in the correct location, you have a problem with either a bad lead or a bad meter (see the 2nd thumbnail below for how the "0.L" or "0L" looks like).
Last edited by Dan Stewart; 11-15-2008 at 12:58 AM.
11-15-2008, 12:23 AM
#11
A DMM good enough for basic use can be as cheap
as $4 (Harbor Freight; have 8 of these myself) but
one which reads frequency can be very handy
(such as checking the MAF output, RPM, etc.) Got
one on sale at Radio Shack for $39 that had this
feature plus came with a thermocouple & adaptor
so reads temperature too. Don't be scared off by
the "$100 and up" notion, just like you don't -have-
to buy Snap-on for your first set of wrenches.
as $4 (Harbor Freight; have 8 of these myself) but
one which reads frequency can be very handy
(such as checking the MAF output, RPM, etc.) Got
one on sale at Radio Shack for $39 that had this
feature plus came with a thermocouple & adaptor
so reads temperature too. Don't be scared off by
the "$100 and up" notion, just like you don't -have-
to buy Snap-on for your first set of wrenches.
I know I wouldn't want to try and adjust a throttle position sensor with a meter that was off .1 of a volt when I need resolution down to .01 of a volt (although I prefer .001 volt accuracy and resolution). You get what you pay for in accuracy. Even Fluke. Although the Fluke 87 III will read milliamps with the positive lead inserted in the "A" or amps plug hole, it is not accurate as it is with the lead inserted in the "mA" plug hole.
But, I agree with jimmyblue. You do not need very highly accurate DVOM's to troubleshoot simple problems with the vehicle and something that is roughly accurate is just fine.
Last edited by Dan Stewart; 11-15-2008 at 12:29 AM.
11-15-2008, 12:26 AM
#12
11-15-2008, 12:51 AM
#13
Agreed:
The DVOM and test light will get you by for the ~12 volt and ~5 volt common stuff, but the frequency and CAN diagnostics may be beyond your intended scope?
This is where the easier but decidedly "not cheap" solution gets into the Tech ll. The really extensive GM diagnostic procedures employ the Tech ll.
The DVOM and test light will get you by for the ~12 volt and ~5 volt common stuff, but the frequency and CAN diagnostics may be beyond your intended scope?
This is where the easier but decidedly "not cheap" solution gets into the Tech ll. The really extensive GM diagnostic procedures employ the Tech ll.
As for the frequency testing, this would require a DVOM with frequency capabilility and doesn't necessitate a Tech II. Needless to say, that and the CAN troubleshooting are beyond "basic" troubleshooting and will not be a part of this discussion. Besides, if you can use a Tech II, you should already know how to use a DVOM and basic electrical troubleshooting. Kinda moot, huh?
11-15-2008, 01:00 AM
#14
My schooling and subsequent work taught me that these theories are called:
1. Conventional current [flow] - current travels from positive to negative; used often by high school teachers, hobbyists, and I am sure some others
2. Electron flow - current travels from negative to positive; used by all of the physicists engineers that I have worked with. The basis of my own college schooling as well.
1. Conventional current [flow] - current travels from positive to negative; used often by high school teachers, hobbyists, and I am sure some others
2. Electron flow - current travels from negative to positive; used by all of the physicists engineers that I have worked with. The basis of my own college schooling as well.
11-15-2008, 01:04 AM
#15
Troubleshooting tips
People tend to think about voltages, but there are
some things to consider. First, voltage is "potential"
but current is generally what gets something done.
Voltage is a span and current must travel a loop
(the "circuit"). In automotive the voltage is referred
to ground. A ground. Which ground? And current is
also generally returned from the load to the source
via a ground path. People think about shorts when
they see lack of voltage, but it's opens that kill
current loops. Think about the loop that the signal
or power has to take, in its entirety. "Be the ball".
For ground to be solid and clean, the ground path
needs to be continuous (zero resistance ideal, low
resistance really). Ground quality is enforced by
a set of braided straps connecting engine block
to chassis sheet metal, battery to block, PCM to
chassis and so on. With high current loads on the
motor (coils, injectors, A/C) and body (fans, lights,
boom-booms) grounds, any resistance produces
an offset voltage which can induce error on sensor
outputs (if block-referred), cut available headroom
for other loads (fuel pump, battery charging, etc.)
and generally flake things out. Even things like a
taillight that works abnormally, often has to do
with a ground fault rather than the high side source.
Ground braids might look like they aren't important
and can easily be left to flap in all the excitement
of trying to get to fire-up after a head swap or
removing the AIR plumbing, etc. But you want 'em.
If you are looking for a voltage and read none on
the meter, there are really two ways for this to
happen. One is that the source is really off (open).
The other is, your reference ground might be
instead. So when you get a bad reading, measure
not only from ground to wherever, but from a
+12V solid feed to the same point. Something
that is dead open will read zero for both; if you
have full voltage in one case or the other, it says
the open-circuit fault is between the two points
in question.
Sensors such as ECT are block-returned and can
be jacked by losing the block-chassis ground straps
at the back of the cylinder head and/or near the
alternator. Others ('most any with a multi-wire
harness that includes a black wire) are returned
to PCM ground in a continuous loop.
The high-current ground braids mount to body
studs and block bolts. Corrosion will eventually
raise the resistance of these connections and
on any older vehicle if you're chasing electrical
oddities, a regimen of find, remove, wire-brush
and tighten before trying to be smart, may save
you a lot of head-scratching.
Some sensors require +12V power and/or +5V
reference / power to work. The MAF for example
needs 12V for the hot wires and has a PCM-internal
pullup to +5V for the output signal. The ENG SEN
fused power runs a lot of sensors and widespread
sensor faults indicate this common point.
People tend to think about voltages, but there are
some things to consider. First, voltage is "potential"
but current is generally what gets something done.
Voltage is a span and current must travel a loop
(the "circuit"). In automotive the voltage is referred
to ground. A ground. Which ground? And current is
also generally returned from the load to the source
via a ground path. People think about shorts when
they see lack of voltage, but it's opens that kill
current loops. Think about the loop that the signal
or power has to take, in its entirety. "Be the ball".
For ground to be solid and clean, the ground path
needs to be continuous (zero resistance ideal, low
resistance really). Ground quality is enforced by
a set of braided straps connecting engine block
to chassis sheet metal, battery to block, PCM to
chassis and so on. With high current loads on the
motor (coils, injectors, A/C) and body (fans, lights,
boom-booms) grounds, any resistance produces
an offset voltage which can induce error on sensor
outputs (if block-referred), cut available headroom
for other loads (fuel pump, battery charging, etc.)
and generally flake things out. Even things like a
taillight that works abnormally, often has to do
with a ground fault rather than the high side source.
Ground braids might look like they aren't important
and can easily be left to flap in all the excitement
of trying to get to fire-up after a head swap or
removing the AIR plumbing, etc. But you want 'em.
If you are looking for a voltage and read none on
the meter, there are really two ways for this to
happen. One is that the source is really off (open).
The other is, your reference ground might be
instead. So when you get a bad reading, measure
not only from ground to wherever, but from a
+12V solid feed to the same point. Something
that is dead open will read zero for both; if you
have full voltage in one case or the other, it says
the open-circuit fault is between the two points
in question.
Sensors such as ECT are block-returned and can
be jacked by losing the block-chassis ground straps
at the back of the cylinder head and/or near the
alternator. Others ('most any with a multi-wire
harness that includes a black wire) are returned
to PCM ground in a continuous loop.
The high-current ground braids mount to body
studs and block bolts. Corrosion will eventually
raise the resistance of these connections and
on any older vehicle if you're chasing electrical
oddities, a regimen of find, remove, wire-brush
and tighten before trying to be smart, may save
you a lot of head-scratching.
Some sensors require +12V power and/or +5V
reference / power to work. The MAF for example
needs 12V for the hot wires and has a PCM-internal
pullup to +5V for the output signal. The ENG SEN
fused power runs a lot of sensors and widespread
sensor faults indicate this common point.
11-17-2008, 06:25 PM
#17
11-29-2008, 02:23 PM
#18
Teching In
iTrader: (1)
Join Date: Oct 2008
Location: Norfolk, VA
Posts: 26
Likes: 0
Received 0 Likes
on
0 Posts
im an electricain in the navy...i have a volt meter. But I have no clue where to start on an electrical system for a car. I just bouth a camaro SS 99. Seems there is a short...somewhere. Maybe there isnt. But my elect system will just restart while im driving. All the gauges jump all the way right and then back to zero then back to the spot they are supposed to be in. Radio restets everything...Any ideas? Besides their being a shirt somewhere...?
