Why do I not have 5v coming from B12 (MAF Signal) at the ECM?

Working on '89 GTA w/1227165. I cannot figure out why I do not have 5volts coming from the ECM on the B12/MAF signal terminal when disconnected. I even got some ECM pigtails from the yard and setup a bench test to prevent the possibility that something on the car is draining the power from ECM and ckt 998 (MAF 5v Signal) or even the possibility of a short. I had a couple of guys probe their MAF signal on B12 and they were getting 5.2v and 5.7 volts. The FSM states the minimum is 4volts and if it does not have 5v then the ECM is faulty or the MAF signal circuit (B12) is shorted. I have been through 5 ECMs and they all spec out the same. I simply cannot move on with this car until I absolutely for certain know why my MAF signal is so low, 3.7v I put this here because I know you guys are intimately knowledgeable of the ECMs and how they operate. Thanks for any input, been through hell with this issue.

What is your input voltage? Are your grounds good?

Remember that the voltage coming out of your ECM is coming from the voltage going in, so if you have a weak input voltage or poor ground the output ref voltages will not be correct either, in many cases.

Also make sure your battery grounds are good, along with a strong battery.

According to my diagrams (several), B12 on the '165 ECM is the MAF input, not ref voltage. The MAF gets a 12V from the MAF sensor power relay, which gets power from the MAF burn off relay. I don't have all years here, but I'm thinking there was a change in the MAF relay set-up at some point, so be sure to check a diagram for your '89 specifically.

Pin C14 is the 5V ref output for the TPS.

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Originally Posted by Six_Shooter

What is your input voltage? Are your grounds good?

Remember that the voltage coming out of your ECM is coming from the voltage going in, so if you have a weak input voltage or poor ground the output ref voltages will not be correct either, in many cases.

Also make sure your battery grounds are good, along with a strong battery.

According to my diagrams (several), B12 on the '165 ECM is the MAF input, not ref voltage. The MAF gets a 12V from the MAF sensor power relay, which gets power from the MAF burn off relay. I don't have all years here, but I'm thinking there was a change in the MAF relay set-up at some point, so be sure to check a diagram for your '89 specifically.

Pin C14 is the 5V ref output for the TPS.

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My C14 is good at 4.99v. I thought the B12 was supposed to be 5v coming from the ECM itself. The flow chart for code 33 (high maf signal) states this circuit 998/B12/MAF signal must check out between 4-6volts.

On the benchtest I know I am getting 12.48v from car battery on both 12v signals to include the A6 (IGN signal) terminal.

Grounds on the benchtest are also good as I am using alligator clips to connect to the battery.

I must not be understanding this correctly. I took the ECM and battery out of the car and put on a work bench just to isolate.

Anything w/5v is being tested on the bench, i.e. CTS, MAT, TPS and MAF Signal.

The pin you are testing (B12) is not an output, it is an input to the ECM FROM the MAF sensor. so if this signal is low, it is the MAF sensor itself, power supply/ground to the MAF sensor or the wiring between any of the components.

B12 = variable voltage MAF.

with the sensor disconnected, you should see 5 volts on that pin.... there is a 1K resistor pulling the circuit up to 5V.

seeing a voltage that low..... i imagine one of these scenarios is correct:
your meter is shot
meter draws too much current for the current limited 5V to be read correctly <-- seems likely if they all read the same
or you have a batch of bad ECMs/wiring

seeing how this is in a pretty controlled environment.... perhaps try a different meter.

Ahh yes, I forgot about pull ups on the sensor inputs. :homer:

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Originally Posted by BurgerWorld

Working on '89 GTA w/1227165. I cannot figure out why I do not have 5volts coming from the ECM on the B12/MAF signal terminal when disconnected. I even got some ECM pigtails from the yard and setup a bench test to prevent the possibility that something on the car is draining the power from ECM and ckt 998 (MAF 5v Signal) or even the possibility of a short. I had a couple of guys probe their MAF signal on B12 and they were getting 5.2v and 5.7 volts. The FSM states the minimum is 4volts and if it does not have 5v then the ECM is faulty or the MAF signal circuit (B12) is shorted. I have been through 5 ECMs and they all spec out the same. I simply cannot move on with this car until I absolutely for certain know why my MAF signal is so low, 3.7v I put this here because I know you guys are intimately knowledgeable of the ECMs and how they operate. Thanks for any input, been through hell with this issue.

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i guess ones first question should be why are you checking the MAF circuit, whats the problem or is it throwing a code?


and yes six-shooter is correct.. B12 Maf sensor in and according to my info should be 2.5v with key on engine off and with engine running it will vary with the airflow into MAF. so ya idk what wire these guys were probing or in which engine state.
seems i too at one time came across that same misleading info that your chasing durring my 870-165 conversion.

ffs once again you guys are too fast , all those answers while i was just typing mine :(

Here is my problem (give it a minute and it should be oriented correctly:

http://i1362.photobucket.com/albums/...ps25999df2.jpg

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Originally Posted by xtreamvette69

i guess ones first question should be why are you checking the MAF circuit, whats the problem or is it throwing a code?


and yes six-shooter is correct.. B12 Maf sensor in and according to my info should be 2.5v (

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Mine is 2.35v connected and KOEO. 3.72 on car and 3.10v on bench test...both KOEO disconnected. Again, I had two guys with vettes check their B12 and they had 5.7 and 5.2 disconnected KOEO.

What I am reading indicated 5v comes FROM the ECM then through the MAF which drops that voltage to it's lowest point at idle and increases that voltage to the max of 5v at WOT. PLEASE CORRECT ME IF I AM WRONG. Sorry guys all I could find was stars to bring attention to the areas indicated that 5v should be coming from B12:


http://i1362.photobucket.com/albums/...ps24c5d0b4.jpg

disconnected? isnt this supposed to be a probed test? i cant remember how i tested it finaly but i do know i ended up with only 4.05 and never had codes or problems, it could be that your meter is just a little off? i think just to double check though i would check the green wire for resistance, or just to see if anything abnormal by disconnecting from ecm and maf and ohm testing the wire.

i'm sure these guys will have better answers for you though.
also i know the relays can cause odd things, are they both good?

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Originally Posted by xtreamvette69

disconnected? isnt this supposed to be a probed test? i cant remember how i tested it finaly but i do know i ended up with only 4.05 and never had codes or problems, it could be that your meter is just a little off? i think just to double check though i would check the green wire for resistance, or just to see if anything abnormal by disconnecting from ecm and maf and ohm testing the wire.

i'm sure these guys will have better answers for you though.
also i know the relays can cause odd things, are they both good?

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I went so far as to make a jumper wire with alligator clips and completely replace the suspect circuits 998(MAF Signal) and 450 (CTS, MAT ground) with this jumper wire then probed for voltage. No issues found, so I continued backtracking and now have an ECM pigtail on the bench.

Yeah 4.00 is the minimum voltage in this terminal according to the flow chart.

Car is idling like crap, surging, terribly poor performance, GPS are jumping around erratically in WinALDL (just got TunerPro working though), very poor fuel economy and generally non-responsive to throttle. But, this issue is being traced back to the ECM, so I posted it here.

Car battery is new having 12.48v last time it was probed.

a couple things come to mind, these cars are all known to rely heavily on there main grounds not just the ones dealing with the circiut that happens to be throwing codes, if these grounds are weak or high resistance its hard to tell what issues and codes will pop up so i would make sure there all good(on frame, back of engine and above oil filter.(when checking these they should be removed and cleaned to be sure there good as i hade a frame ground before that looked great and pulling on the wire was all tight but when i went to take it off to clean i bairly put any pressure on it and it snapped..old owner musta tightened to point of half twisting it) now you may have that all covered with your bench testing idk , just thought i would mention it.

also have you tried to unplug the MAF and see if the car runs any better? also the wires at the plug are another known popular spot for those maf issues.

It does run considerably better w/MAF disconnected, but it still is quite horrible. Maybe my meter is off by .25v approximately which would put me very near 4v, which is what you had. The DVOM is not a good brand meter, but it is only a few months old. I have sure been over the grounds. Thanks for all of the suggestions guys, hopefully I can get this all taken care of and start tuning.

Ok this should be right up your alley guys. It runs the same when in cloesed loop. Sorry about WinALDL, I didn't have TP working yet. Noone is messing with the gas pedal:



http://www.youtube.com/watch?v=_l06_wvgEh4