Voltage on MAP ground circuit
Voltage on MAP ground circuit 93 Firehawk, LT1
This is interesting.
I found that I have a 70mV signal on my MAP ground wire.
Which is identical to the signal I find on the MAP/IAT/AC Pressure low-voltage signal wire.
I tested resistance and found that there is not likely a short (50kOhm)
The ECM ground wires go to the block, and I found a 0mV drop between that junction and the battery terminal (which is what I'd expect)
Any ideas?
Diagram for reference.
Note that IAT, MAP and AC Press share a common LO reference circuit (blk).
What I am seeing, is an identical signal on the MAP's Ground (lt green)
This is interesting.
I found that I have a 70mV signal on my MAP ground wire.
Which is identical to the signal I find on the MAP/IAT/AC Pressure low-voltage signal wire.
I tested resistance and found that there is not likely a short (50kOhm)
The ECM ground wires go to the block, and I found a 0mV drop between that junction and the battery terminal (which is what I'd expect)
Any ideas?
Diagram for reference.
Note that IAT, MAP and AC Press share a common LO reference circuit (blk).
What I am seeing, is an identical signal on the MAP's Ground (lt green)
Last edited by James Montigny; Dec 4, 2008 at 12:32 AM.
So you're saying I should have voltage on both the lt grn and blk wires?
It seems like the reference should be at least a little different from the signal
unless it happens to be 101.32kPa
For example, the other day both wires read just over 40mV
Today, with atmospheric pressure a little higher, both read 70mV
I don't see why both are the same.
It seems like the reference should be at least a little different from the signal
unless it happens to be 101.32kPa
For example, the other day both wires read just over 40mV
Today, with atmospheric pressure a little higher, both read 70mV
I don't see why both are the same.
Well, 70mV is maybe a 3% error / noise adder at
half scale. Maybe enough to be a minor nuisance
to fueling. But realize this is probably very averaged
and question whether a more substantial peak might
be caught randomly or repeatedly to worse effect.
Look at where the ground is coming apart. PCM to
chassis, PCM to block, block to chassis. Should be
very tight electrically, enforced by various ground
braids.
At idle the MAP signal should be somewhere in the
middle, 2V or so with a +5V and 0V for the high and
low side references.
half scale. Maybe enough to be a minor nuisance
to fueling. But realize this is probably very averaged
and question whether a more substantial peak might
be caught randomly or repeatedly to worse effect.
Look at where the ground is coming apart. PCM to
chassis, PCM to block, block to chassis. Should be
very tight electrically, enforced by various ground
braids.
At idle the MAP signal should be somewhere in the
middle, 2V or so with a +5V and 0V for the high and
low side references.
Yeah, 3% is about what I am seeing VS the weather station.
Then someone pointed out that they correct their numbers for sea level (I'm 1100 above).
So the real underlying issue here is that the tune is dumping way too much fuel
into the motor at low vacuum and fouling plugs. I can't even drive it the way it is.
I just want to make sure I take care of every other issue before I waste the tuner's time
trying to fix something that isn't really his issue.
Then someone pointed out that they correct their numbers for sea level (I'm 1100 above).
So the real underlying issue here is that the tune is dumping way too much fuel
into the motor at low vacuum and fouling plugs. I can't even drive it the way it is.
I just want to make sure I take care of every other issue before I waste the tuner's time
trying to fix something that isn't really his issue.
I see it at both ends of the wire (ECM and sensor) Sensor plugged, or not.
It seems to be that the only place this could be coming from is a short inside the ECM.
All of my ground sites measure 0V drop from the battery terminal (IE: they're good)
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If you're looking to check the ground for volt drop, the bestest way is to check it while cranking. You have to pt a good load on it. Checking it while cranking pulls on it, puts a good load on it.
If it's not over .100 volts, don't worry about it.
If it's not over .100 volts, don't worry about it.
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