Hey guys, need a little help on this one. 1989 auto. with L98. I've had car for 10 years. I use a Actron scanner, older model but seems to work fine on all my cars. Car runs very well with no problems. Only thing is when I scan it the MAT always reads 392 degrees. Same reading cold or hot. Today I finally decided to start trying to find out why. Replaced MAT sensor in plenum, no change. Disconnected harness from sensor, no change. Discinnected extension (FSM calls this a jumper) from main harness, no change. Checked voltage on tan wire CKT 472 for the 5 volts as per FSM and OK there. If I read the diagram correctly this sensor CKT 452 black wire at ECM shares the ground with CTS and TPS. No problem with these sensors, so I assume the ground is good at ECM. Also no codes at all, I would think a 25 should have been set(high temp). Car is bone stock. Only other thing is that car does have a replacement ECM. Label says :Remn. by GM Service Part# 16198445. Prom says APKC 1304. All this should be the correct ECM and PROM unless I'm wrong. Any thoughts?

Its not shorted to ground anywhere is it?

That is what I thought could be the problem, but if shorted why would there still be the 5 volts on the power wire? Thanks .

Joel

Im not too sure of that mate, id have to see it to know.

How much for a round trip ticket?

Joel

Well another afternoon working on this MAT problem and no luck. Does anybody know which bulkhead the MAT wires run to? I see one behind the battery and one behind the right side valve cover. Thought I would disconnect bulkhead and try to isolate the tan wire as my next step.

Joel

Quote:
Bit over a couple grand id imagine!!

I'll get back with on that! What is your procedure to verify a shorted/open circuit. I know I need a new DVM before I probe on the ECM. Thanks


Joel

Well if its saying the max temp it can show and wont change, it has to be shorted to ground at some point i reckon, but without seeing the wiring diagram, i really cant comment yet.

Have you followed every flow chart in the FSM relating to this component? Look up DTC 25, with the reading youre seeing, you should be seeing this DTC. Without the Genuine Service manual infront of me for your model, i dont know if it will or wont set the MIL.

Yes I have followed the FSM on both High temp and even Low temp Problem trees. Do agree with you, it seems there should be a "short/ground" issue. I put it back together yesterday afternoon, really just got fed up with for now. I'm going to drive it soon today with scanner hooked up and monitor some functions. Really more concerned with proper EGR function. Will post results later if you're interested. Thanks for your reply. I guess my posts are just too long and dull.

Joel

OK, I drove around some today with my scanner hooked up. The EGR valve looks good as I could see the duty cycle changing when it should. It almost appears this MAT sensor reading is redunant, since the coolant temp is feeding same info to ECM? Just a thought. I'll get back on this issue later. Thanks for all replies.

Joel

MAT sensor measures maninfold air temperature. CTS measures coolant temperature. They are 2 totally different things.

Just a quick update on this issue. I purchased a newer scanner for OBDII several weeks ago,along with a OBDI adaptor cable. Well, I hooked this one up today and guess what I saw! MAT sensor reading 54*. I ran the engine a few minutes and the reading did start to climb just as it should. No morale to this story, just wanted to close this problem. Seems like nothing can be trusted 100% when dealing with electrical. That's what I get for bragging about my "old" scanner!

Joel