[Gmecm] HELP: MAF Burn-Off problem

[Dan Grostick]

Rick, the MAF burnoff is initiated by the ignition key being turned off. 
The following is comments from the burnoff code. Looks like the MAF 
sensor output voltage must fall within a range during the test. The test 
is a fixed time initiated after a key-off delay. As far as I can see 
with a quick inspection of the code,  MAF burnoff failure  (code 36) 
being set  doesn't affect subsequent ECM program usage of the MAF. You 
should hear the burnoff relay click when burnoff starts and again when 
it completes.

         ; A/D Ch A, (Mass Air Sensor Ch)
         ; Fail B/O test if HLM A/D > 1.90 VDC
         ; Fail B/O test if A/D LT 400 mvdc


        ; 1 sec, MAF BURN OFF TIME.
        ; 5 sec, MAF BURNOFF DELAY TIME from IGN OFF
      
Dan

> IIRC: running $32b on '165 in '86 L98 vette, it's setting the 'obligatory' code 36 MAF sensor burn-off voltage missing. I've been down the road of all the classic 'fixes' such as replacing the relay, cleaning the contacts, etc., and now tearing into wiring to try to find the problem.
>  
> Part of the problem is that this signal only occurs after a warm engine shutdown, and IIRC the engine has to have been running for some period of time prior to trigger this diagnostic/service circuit into action at engine shutdown. Makes for difficult diagnostics at best, but I've got to find the headache.
>  
> Plan of action is to 'tap' the wires to the MAF for monitoring signals. Once that is done, I'll be able to monitor the activity since the 'tap' is going to be a 4-wire trailer light socket so I can plug a 4-wire 'extender' into it and temporarily monitor it in the cabin until I get the problem fixed. Then I'll weather seal the stub connector and tuck tidily away into the loom in case I need it again (suspect I may, this is a chronic problem w/ this version of MAF).
>  
> Now, my question is: What does the ECM/code look for at shutdown to determine that the MAF burn-off cycle completed. IIRC the ECM has a line that enables the burn-off relay a few seconds after shutdown (which is what I'll be monitoring at the MAF diag dongle above) but since there is NO air flow, does it look for some delta in the signal from the MAF (and, isn't the BOSCH MAF a frequency based signal vs. the voltage based on the DELCO MAF variant?). Once I confirm that the MAF is receiveing burn-off voltage correctly at shutdown, then my next step is obviously to diag the signal at the ECM, but what am I looking for, is it just the presense of signal due to the MAF being energized post shutdown, or is it a change in the signal from the ECM due to the burn-off causing a very hot heated wire, then cooling, hence that should cause a delta in the MAF signal seen by the computer.
>  
> My hunch is that I'm actually fighting a poor current path, like maybe a high resistance fusible link or a corroded ground connection to the MAF which would cause it to work correctly but the high current demand by the burn-off cycle causes excessive drop in the supply/return path and not enough current passes to actually burn-off the element correctly.
>  
> At any rate, knowledge is key, I can get lots of data, but what is it going to tell me? What is normal opperating condition, so I know what to 'fix' on this pesky critter.
>  
> BTW, it works wonderfully from a drivability standpoint if I reset the codes on the ECM and then drive, it warms up fine, and performance is normal but after that the next restart will have a check engine light and the code will be set for this, and performance sucks big time, since it is not using MAF data I presume is using static or even worse I suspect LIMP mode?
>  
> Sorry for wordy post, but info is key to getting quality responses and this group has that quality.
>  
> Cheers, thanks in advance
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