[Gmecm] HELP: MAF Burn-Off problem

Rick McLeod dunvegan
Mon Oct 2 14:02:05 UTC 2006 

This is great information, but it still doesn't quite answer my question:
 
Now, my question is: What does the ECM/code look for at shutdown to determine that the MAF burn-off cycle completed.


----- Original Message ----
From: Roger Danzy <rdanzy at cox-internet.com>
To: Rick McLeod <dunvegan at sbcglobal.net>; gmecm at diy-efi.org
Sent: Friday, September 29, 2006 10:11:43 PM
Subject: Re: [Gmecm] HELP: MAF Burn-Off problem


Rick,

For CODE 365, per my 1986 Camaro/Firebird manual for the maf burnoff the ecm 
grounds circuit CKT 900 (the ground to the burnoff relay, terminal B) which 
cause the relay to close and supply battery voltage to MAF terminal D and 
also terminal E.  The suggested test sequences check for:
1) 12 volt supply to the maf burnoff relay
2) grounding CKT 900 closing the relay and checking for voltage at MAF 
terminals D and E.

If you don't get voltage at the maf terminal D, check the circuit from 
terminal A of the relay to terminal D of the maf.
If you don't get voltage at the maf terminal E, check the circuit from 
terminal A of the relay to terminal A of the maf power relay.  If there is 
no voltage at terminal A of the maf power relay, check the voltage at 
terminal D of the maf power relay.  If there is voltage at terminal D the 
maf power relay is bad.  Terminals A and D of the maf power relay are 
normally closed.  Terminals A and E are normally open and are close when the 
relay is grounded at terminal B.

The ecm is connect to terminals A, B and C of the maf.  I would assume that 
the ecm is monitoring the effect of the voltage of terminal E from the 
burnoff relay from terminal C of the maf which is the maf signal to the ecm. 
Terminal A is the system ground and B is the analog ground.

If you need more information, I can supply you the details of the Code 36 
test charts.


----- Original Message ----- 
From: "Rick McLeod" <dunvegan at sbcglobal.net>
To: <gmecm at diy-efi.org>
Sent: Friday, September 29, 2006 1:29 PM
Subject: Re: [Gmecm] HELP: MAF Burn-Off problem


> Been there, done that, even swapped the MAF for known good unit, I've got 
> a Z/28 of same vintage and all and makes a decent comparo, except I'm 
> trying not to have to disturb the wiring on it also, it's isn't throwing 
> codes, which makes me suspect corroded grounds or connectors, the vette is 
> parked longer periods between it's exercise runs.
>
>
> ----- Original Message ----
> From: Mark Tonazzi <rmtonazzi at charter.net>
> To: Rick McLeod <dunvegan at sbcglobal.net>; gmecm at diy-efi.org
> Sent: Friday, September 29, 2006 8:27:52 AM
> Subject: RE: [Gmecm] HELP: MAF Burn-Off problem
>
>
> I got MAF error codes when my ECM was crapping out.  It would run fine 
> until
> warmed up then intermittently it would throw a MAF  voltage high or 
> voltage
> low code and go into limp home mode.  If you can get your hands on another
> known good ECM may want to try that.
>
> -----Original Message-----
> From: gmecm-bounces at diy-efi.org [mailto:gmecm-bounces at diy-efi.org] On 
> Behalf
> Of Rick McLeod
> Sent: Friday, September 29, 2006 2:57 AM
> To: gmecm at diy-efi.org
> Subject: [Gmecm] HELP: MAF Burn-Off problem
>
> 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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