Tools & Fabrication - How to use a Multi-function meter?
12-28-2005, 02:16 PM
I have a question regarding checking resistance. Not sure if this is the best place for it, but its mainly directed at using a multimeter. If there is a better forum for it, please advise...
Background info. I'm blowing a 20A "engine sensor" fuse as soon as the keys are turned. I blow the fuse with all sensors on the circuit (O2, MAF, Skipshift, reverse) plugged and unplugged. After some research, it seems that this indicates a wire from one of the sensors is grounding out somewhere. Time to track down the sensor. I'm trying to figure out how to check the resistance in each wire from the sensor to the "engine sensor" fuse.
When checking the sensors, do you put one lead (from the meter) to the sensor side of the fuse (not the 12v side) and the other to the sensor itself? Or, do you put one lead to the sensor itself and the other lead to a ground?
I'm confused on how to check. I have a bad wire somewhere.
Also, what is an acceptable resistance level? <10 Ohms?
FWIW, I tried putting one lead to the fuse and the other lead to each sensor. I found that the MAF 12v wire saw infinite resistance. I cut the connector off of the MAF pigtail and rechecked the resistance. This time the Ohm value was ~4-5 Ohms. I thought I found the problem but I'm still blowing fuses.
12-28-2005, 02:28 PM
ignore your MAF test. The MAF has a mini heating element, it takes some juice to fire that puppy up.
if you want to check for a grounded wire, you need to find wires that aren't supposed to be grounded, and then put one lead of the tester on ground, such as your negative battery post, and then the other lead on both ends of the wire. if you got <10 ohms or so, you found your problem.
its kind of hard to explain.
find the sensor wires that are supposed to be hot, they are "positive". they take juice from the fuse box/voltage source, pump it through some sensor or something, and then send it to the PCM or a guage or whatever. if that "hot" wire from the fuse box gets grounded, it will blow a fuse. you should not get 0-very low resistance on those wires. some wires are grounded by design though, so make sure you know which ones are supposed to be hot.
hope that make sense.
if you have some way of scanning your PCM for DTC's, theres a good chance the relevant sensor will be throwing a code as well.
12-28-2005, 02:37 PM
Thanks for the reply. If I follow correctly, you are saying that I should see high resistance values in those wires when testing each end?
Thinking out loud:
The test theory is that we are looking for a closed circuit. A shorted wire is grounding out somewhere. Putting one lead on the ground - for example, battery (-) - and the other on the sensor wire with not complete the circuit (high Ohm value). If the Ohm value is low, then I will know that the circuit is being ground to the frame (therefore completing the test circuit).
Am I understanding what you are saying?
I hope my terminology is correct.
12-29-2005, 08:23 AM
I am oversimplifing the automotive wiring, hopfully this example will help.
Say you have this type of circuits, each with 6 test points......
Testing points between 5 & 6 should short to ground = low resistance/audible tone on continuity meter
Testing points between 4 & 6 should yeild high resistance, else the device has an internal short and needs repair/replacing;
the internal hot in the device could be grounding to the case/enclosure which contacts usually the frame body and therefore would cause this to shunt, unintentionally.
i suspect this is not so since you disconnected it and the fuse still blew.
if not the case, move on up the circuit to the next point.
Testing points between 2/3 should yeild high resistance as well, else this is where your short/shunt is and would definately keep popping the fuse, regardless of whether the device is plugged/unplugged.
i suspect this is where your problem is.
Two troubles are knowing the exact wiring schematic so you can test, debug, & diagnose accurately & effectively AND tracing the wires that are all taped, tie-wrapped, molded, throughout the damn car. I know I get confused a bit when trying to test the fuse box and you can ge ta manual to help out on the first one, but the second, boy would that be tough. I only hope that I never have to troubleshoot factory wiring. Hope this helps.
12-29-2005, 12:25 PM
How it is explained by "steel chicken and blaster" is correct but you really need the diagram if you do not have it already. It will make it so much easier. You need to break the system down (only testing small segments of the circuit at a time). With it blowing the fuse immediatly it is a BAD short, which is good, makes it easier to find. Find were the circuit breaks off and goes to your ( O2, MAF, Skipshift, and reverse) and ohm that single wire with it disconnected from the sensor or component, (*if you do not disc from the component you can get false readings, depending if that component is de-energized closed or open) now you are just reading the wire that goes directly to that component,(one lead on a ground (black lead) red one on the wire and do not touch the metal part of the lead, it will read through your body) that wire should be completly open (checking the diagram to make sure it goes through nothing else.) If you get anything on your digital meter other than O.L (fluke multimeter), its shorted out. If you get a reading of 0.5 to 0.0 its a complete short to ground, so you want to see O.L.(or whatever sign your multimeter has for "open circuit") If you find it and its a long wire find the middle of the wire, and cut it to break the system in half (unless you can actually find where its shorted to ground) that way you dont have to replace a long a$$ wire through half your car. And always check your meter leads, they all go bad, especially if you have had them waded up. Set your meter to ohm reading and contact your leads together, you should not read above 0.7 or you need new leads. You can get completly wrong reading just from your leads not being able to "zero" out. sorry its so long!
12-29-2005, 05:28 PM
I'd look for a burnt O2 heater wire lying up against headers
/ manifold, first.
Most DMMs have a relatively low current max and a 10A
inline fuse. It's worth looking at the downstream end of
the fuse using low resistance range, and a good connection
to whatever you believe the return point to be (check the
diagrams, probably a PCM ground rather than chassis although
you would porefer that these be electrically identical, by some
robust straps). You need the resistance to be above 0.6 ohms
to not pop a 20A fuse. However realize that some loads are
nonlinear and the low voltage most modern DMMs apply, in
the name of not-damaging electronic components, may not
swing the load to full current or minimum resistance.
You could replace the fuse with a low (like 0.1 ohm) wire-
wound high power capable resistor and measure across it
with the voltage scale, then current is the voltage reading
times 10 (if this is indicating more than a volt drop then
step on down to a 0.01 ohm one, and current = voltage
times 100 instead; might be handy to go ahead and solder
a set of spade lugs on them and keep 'em in the electrical
tool box). Also, Vresistor/Vbatt = Rresistor/(Rresistor+Rload)
if you wanted to calculate up the actual short resistance.
But if it's a real, insulation-fault short, it may also be an
intermittent one (needing vibe or G-load to make/break)
and you may not see it sitting there in the garage.
12-30-2005, 12:09 AM
Thanks guys. I went out and picked up a little Fluke MMeter ($99 from Sears). I took off my O2 extensions and checked the resistance. All was well so I pulled up the O2 connections and put a new fuse in the box. Turned the key and no blown fuse.
I'm going to slowly and methodically make a single small change and check the fuse. Obviously its the O2, but I can't see an exposed wire in the extension, nor the harness. I'm glad that its not blowing the fuse, but with no visible culprit, its not as promising as I'd hoped.
Thanks guys. GREAT info. I surely appreciate it. This is great info for troubleshooting in the future.