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Danspeed1 MAT IAT SENSOR Reading -40 all the time
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Hello everyone,

Still working on the 1990 Corvette we purchased years ago. I am slowing figuring out all the issues one by one. I recently replaced the distributor which took care of all the idle dodging. Now I have a low rpm misfire. My scan tool indicates that the MAT or IAT Sensor is reading -40*C all the time, no matter what I do. At the wires, one reads 0 volts (as I'm guessing it should, and the other reads whatever the battery voltage is. I replaced the ECM 3 times with no improvement. I checked the harness it looks good. I am totally lost. Found a flow chart on third gen, but it was no help... states I should replace the ECM lol! Need help,

Dan
Posted on: 2016/8/18 18:36
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1996 Ford Explorer
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Danspeed1 Re: MAT IAT SENSOR Reading -40 all the time
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Also, My tan wire is acting as a ground. When I apply my DVOM to 12 volts and the black wire lead to the tan wire I see 12 volts. When I do the same to the pink/black wire I get .07 volts. If I apply my red lead to the Pink/Black Wire and ground the black, my DVOM shows the pink/black wire giving off 12 volts... this is so strange.

DG
Posted on: 2016/8/18 19:02
Edited by Danspeed1 on 2016/8/18 19:40:32
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Schrade Re: MAT IAT SENSOR Reading -40 all the time
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3rd Gen? Is this L-98?

40* sounds like a default reading.

I got '90 FSM to scan + post for IAT Sensor, unless you already have it...
Posted on: 2016/8/18 19:57
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Matatk Re: MAT IAT SENSOR Reading -40 all the time
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-40 means it's not plugged in or there is no signal.

Your other wire issues might be something chafed through and crossing/grounding. Im not looking at a fsm to follow the chart.
Posted on: 2016/8/18 20:22
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Danspeed1 Re: MAT IAT SENSOR Reading -40 all the time
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Just found out diagram I have been working off of is wrong. I did locate the correct wires on the ecm. At the ecm im getting the correct reading and when I wire in the sensor I am finally getting a temperature, but I have no idea whats wrong with the harness. I might take the easy way out here and splice the wires directly from the ecm but I have to be honest I have a list of other issues here that need to be resolved including no horn and heater controls inoperable which likely means there are still bad grounds/wires hooked up incorrectly. This is proving to be quite the project. We have resided ourselves to the fact that this is just the way it is and it might take years to really resolve. I have been making progress, just very slowly.

Also, when I rev the engine SYS flashes on the speedometer read out. What the heck does that mean?

DG
Posted on: 2016/8/18 21:22
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Matatk Re: MAT IAT SENSOR Reading -40 all the time
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SYS usually means alternator/battery/charging problem.
Posted on: 2016/8/18 21:56
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Matatk Re: MAT IAT SENSOR Reading -40 all the time
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Is this the 1990 that had the fire?
Posted on: 2016/8/18 21:57
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Danspeed1 Re: MAT IAT SENSOR Reading -40 all the time
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Quote:

Matatk wrote:
Is this the 1990 that had the fire?


Yes! Lol, it is.

DG
Posted on: 2016/8/19 2:45
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Matatk Re: MAT IAT SENSOR Reading -40 all the time
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Back to the original problem, is the car throwing a code for MAT (manifold air temperature)?

The sensor actually gives a resistance that you can compare to a chart for temp. What resistance is it showing compared to actual ambient temp?

I believe there is a connector in the harness about 20" back from the sensor. That would be easier to get to than the sensor for testing purposes.

Can't rule out the sensor is bad, yet.
Posted on: 2016/8/19 22:37
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1989 Corvette...RIP
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Schrade Re: MAT IAT SENSOR Reading -40 all the time
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Do you have OEM Circuitry?

I can scan n post schematics for '90 from FSM here...
Posted on: 2016/8/20 0:33
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Danspeed1 Re: MAT IAT SENSOR Reading -40 all the time
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Ill just be honest, I don't know what I did to fix it but its fixed. Had to cut the wires exactly at the ECM and send out two new ones. Makes no sense. Only thing is that I've had to work a couple of bugs out of the this harness already. Its for a 91, I have a 90, and the guy before me made some "adjustments" as well which needed to be corrected. This car is quite the labor of love. Now I am exhibiting some tuning related issues but I have PCM's of NC helping me out with that. They have been great so far.

Thanks,

DG
Posted on: 2016/8/31 16:04
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1972 Chevelle SS 454
1995 Corvette Convertible
1996 Ford Explorer
2009 Cadillac CTS-v
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Matatk Re: MAT IAT SENSOR Reading -40 all the time
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It makes sense if you replaced the bad wire
Posted on: 2016/9/2 1:21
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1989 Corvette...RIP
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Danspeed1 Re: MAT IAT SENSOR Reading -40 all the time
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Quote:

Matatk wrote:
It makes sense if you replaced the bad wire


Yea, but whats interesting was that I did not receive the correct reading until I cut the wires. I can only assume something was back feeding through those wires causing the false readings and then failed tests.

The good news, even though I only get a couple of hours a week to work on it we are making really good progress. The car looks amazing now its just a matter of getting it to run amazing...

Which brings me to my next thread

Stay Tuned!

Posted on: 2016/9/7 0:45
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1972 Chevelle SS 454
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