I'm getting a constant -40*F reading off the Manifold Air Temp seonsor on a stock TBI 305 in a 92 Camaro. The sensor, near as I can figure, is actually located in the base of the air cleaner. New sensor there didn't help. So I tried both unplugging it as well as running jumper between the terminals of the plug that attaches to it, hoping that I would peg the reading at a full 392*F (max reading the ECM will register). No dice- constant -40*F reading no matter what I do. I can only thing of 3 possibilites:

1. I'm playing with the wrong sensor.
2. The wiring or ECM is screwed up to that sensor (everything else seems to read "sane.")
3. Diacom for whatever reason can correctly display the true MAT sensor reading for a 92 TBI engine and just defaults to -40*F.

Engine seems to run just fine in all respects.

Thanks in advance for any help.

 

That is the correct sensor (MAT is in the air cleaner on TBI's). Were you getting a SES light at all? If not then the problem is with Diacom. The -40*F is basically an open sensor/wire and will be detected by the ECM for a solid SES light.

If the SES was off, disconnect the MAT sensor and start the car. Should have a solid SES within 10 seconds. If so then it is Diacom.

Also check the trouble codes via Diacom. Although you may see MAT codes due to the testing you did. Clear ECM and check again.

RBob.

 

Bob- I'm not getting any trouble codes or a a lit "check engine" light. The SES light works (various Diacom modes light it, as does jumpering the ALDL terminal, etc- the usual stuff. In short, it works).

I unplugged the IAT and it runs indefinitely without a check engine light. You sure the ECM monitors the IAT cndition that closely? Even my 94 Formula doesn't know if you unplug the IAT until it turns on the cooling fans (don't ask me why it checks the IAT circuit when it turns on the cooling fan- maybe becuase it couldn't possibly be -40*F outside and need the cooling fan??).

 

Bob, my aplologies. Turns out you were about 99.99% right. Diacom isn't reading things correctly- there's no actual problem with the sensor or anything else.

Although it didn't appear that my observations were lining up with your analysis I decided to run some "if/then" kinda tests to try to triangluate in on the truth. I respect and listen to ALL advice if it comes from someone who sounds like they know what they're doing (like you). So I proceeded from your basic concusion: Diacom ain't seeing things right.

I figured if your understanding of how it works is true then unplugging the IAT and driving around should shortly light the check engine light. From a cold start I drove around for about 4 minutes. About the time I started thinking you were wrong- BINGO! Check engine light comes on. I scurry home and plug in Diacom without ever turning the engine off. It sees the trouble code (and it's still showing -40*F from the IAT in Diacom) and it's a "IAT sensor reading low" code. OK, so we got a code. A code that never popped up before if the sensor was connected.

I plug the sensor back in without turning the engine off. About 20 seconds later the check engine light on the dash goes off. Diacom still says there's a trouble code but the ECM in the car is not showing any continuing problem. Diacom STILL shows -40*F reading from the IAT.

I shut down the car and reboot Diacom. Restart and link up Diacom. Trouble code still showing in Diacom but the ECM is happy- no SES light (I guess it stores codes for a while even after the check engine light goes out). With the sensor plugged in I go back out and take the same drive. No check engine light. Diacom still reading a constant -40*F from the IAT the whole time.

In short, Diacom ain't seeing things right, just like you said. It took a while longer to light the check engine light in my first test probably becuase I went from a cold start. Subsequent tests on a fully warmed up engine, an unplugged IAT would light the check engine light much sooner (but never quite as quick as 10 seconds). I also beleive there is a component to the ECM's logic that requires you to actually drive around. The SES light is much more reluctant to come on if you just idle motionless in the driveway, which might be another contributing factor to my earlier flawed tests.

Close enough to win you the gold star in my book for the night! Many thanks for your help. Note to self: Diacom don't work so great on TBI Camaros!

 

Well, that is good news, and gold stars are good too.

The trouble code will be erased after (typically) 50 starts as long as no new codes are set. Or just disconnect the battery for a minute or so.

RBob.