OK i am having a weird issue with my '90 Y-body, with the MAT reading high on a bone cold engine. usually reads 185 degF or even 203, on cold engine at 60 deg. Now i run car and do a datalog, and the temp SLOWLY cools down, to maybe like 150 by the time i get to work. and the engine temp is up to around normal, like 200. So today i pulled the plenum, and looked at the MAT sensor, reads normal about 3k ohm at room temp. Took a heat gun to it and the resistance went down as it is supposed to. I even checked a few other sensors from my Camaro, and they read the same. I even bought a new MAT from dealer, thinking this one is bad. Gonna take that back if i can.
Now i know there a few tables on the MAT, and it is a VERY important part of fuel calc in a speed density system. I even put in my stock memcal/chip, and checked the cold engine temp, and it was still 185!
WTF? i thought i did something wrong in my one PROM.
Need help!
o this is 90 L98 car, with 727 ecm, and $8D def file.

 

Here is some more info from today:
Ok, this is what i just found.
Tried 2 different used sensors, both read about 160 deg, with cold engine.
Connected several resistance values across the MAT connector.

Short = -40 degF
150ohm = 45 F
681ohm = 104 F
1200ohm = 130 F
10kohm = 253 F
47.5kohm = 361 F
open = 392 F

Seems backwards compared to what i should have, in the table.
So do i have the wrong sensor? or maybe it is the wrong ECM? It has been changed, i think it is 1227727 model.

Also, checked for 5 volts on one side of the MAT connector, open ckt, and it was good. And with the 681 ohm resistor, i had about 2.02 volts across it.
My FSM says these MAT sensors are the same as the CTS.

 

So nobody has any ideas? Is there another MAT sensor out there that would work? Seems like it is working backwards. Or this 727 ECM, could be the wrong one? I know they used it on several GM autos.....

 

Seems to me that the scan tool is reporting the wrong temperature.

RBob.

 

Well i think that was it. Been using my old standbye scan tool by Ease Diagnostics, had it long time, know it well and easy to use. So last night i got the correct files downloaded for TunerPro RT, and had a chance to try it late this pm. I didnt fire the engine, but with key on and connected, found the MAT to be within 2 degC of the coolant temp. Now the data stream was kind of skipping in/out, and the screen was flickering, but i see enough data to convince me the MAT was reading correct.
I havent used Tpro very much, and i dont think it is setup perfect for my app, but i will be using it more very soon!
The EASE tool i have had since 99, it is very basic but does all GM OBD1 from 82-95. But not much support for that anymore, if any.
I just love troubleshooting a non-issue!
Now the car will go back togethor, and the new MAT sensor i bought will go back to the dealer. Time to datalog!

 

Have you considered the plastic cage style IAT for mounting in pre throttle air intake? Much more accurate than the stock style mat's that look like the coolant temp sensors

 

Absolutely. I put one of those MAT to IAT relocation kits in my IROC back in June of 99. Got it from TPIS, fit perfect. But that was before i knew that the MAT/IAT did not do much with a MASS AIR controlled engine. It mostly just affects EGR operation. Now that MAT is stuck on -40F for some reason....i just dont look at it much in my datalogs.

Now on my '90 Vette with MAP and speed density.... much different story. This relocation is on my todo list, just not a high priority right now. I want to get the tune dialed in, and a few other things. Only had the car a few months.