My vette is a 1988 Auto

I have had the code 36, failure of the MAf burnoff, for a long time.

Previously I had checked relay and maf and isolated the problem down to the black wire from maf burnoff relay to ECM.

I could light the MAF wire by grounding this black wire.

Yesterday I unplugged the ECM and I could make the maf wire glow hot by earthing D12 onthe ECM connector plug.

As far as I can see this means everything is working and it must be the ECM.

I have a second ECM - so I swapped them.

I'm still getting the code.

Either both ECM's have the same fault (and otherwise work perfectly) or I've missed something.

The ECM must be getting a good ground or it would be noticable.

I have checked all wires to the relay, all working and relay works when you ground the black wire and the MAF lights up.

I have checked continuity of the black wire between MAF & Relay.

I can make the MAF wire glow red - but the ECM can't.

Is the ECM not getting a signal to burnoff the maf?


Does anyone have any ideas?

 

What year Vette??

 

woops sorry, have edited post - it's an 88 auto

to recap-

The relay is working

All wires check OK

MAF wire glows red hot when D12 on ECM plug is shorted to earth which confirms the entire circuit is OK

Two ECM give the same result, they don't burnoff the MAF on switch off and they both throw code 36 - checked with several battery disconnections

Both ECMs show no other signs of being faulty

 

I wonder if your problem has something to due with the MAF signal line info. I don't know how the ECM uses that info, but it seems like it could be a cause.
Have you looked into the MAF upon shut down. Maybe it is burning but the ECM doesn't know ??

 

While dealing with my recent MAF problems (code 33), if you put a Volt Meter on ECM terminal B12 (green wire) you should see the voltage raise from .5volts to around 4.5 to 5 volts for 2-3 seconds upon shutdown, then it will go to 0 volts. If it does this then the MAF is getting its burnoff signal.

I would imagine that this is how the ECM monitors burnoff - BUT I do not know for sure.

 

there are two relays.. behind battery.. one is the burn off relay and the other is the power relay, which powers the Maf, should this relay fail, the ecm will set code 33, The MAF MUST be powered for ecm to read..if the ecm
cant , will not send command to burn off relay,,hence first code 33,,burn off fail,

To check that the MAF burn-off is getting power, look for 12 volts at the MAF connector position E

If you get a Code 36, MAF burn-off malfunction, it should immediately reveal itself at startup and it won't affect performance. '85 Corvettes will never show a Code 36, only '86-'89 Corvettes. You can check to see if the ECM is calling for an MAF burn-off by running the car for at least 5 minutes, and checking the black wire at connection F (circuit 900) on the gray connector. It should be grounded for approximately 30 seconds. This is the ECM grounding the wire to actuate the relay. If there is no ground, the ECM is at fault. If you have ground, check the power to the relay. If the relay is sending power to the MAF, check that the dark blue wire (position D) on the MAF sensor connector has power.

change both relays,,the parts number hopefully still readable on old ones..i cant recall if both relays are the same (Over the years, some changes were made to the relays for the MAF. The '86 and early '87 relay for MAF power was PN 100067925; the burn-off relay was PN 10094701.__)

The relays changed in late '87. Relay No. 14089936 operated the MAF power, burn-off, and fuel pump. Although not always accurate, the connectors for the 9936 relay are the gray connector for the burn-off circuit and the black connector for the MAF power circuit

chasing down several specific reasons why Codes 34 and 33 will appear. The dark green wire from the ECM to the MAF (circuit 998) can short to ground or short together with circuit 450 (black or black/white) and cause a Code 34. To get a Code 33, circuits 998 or 450 could have an open circuit. Recently, the only cure was to run new wires for both circuits. This cured the intermittent codes immediately.

 

Thanks Agent86 and korvette4u - you're always around to help, much appreciated. Thanks also cgantner5150- eventually we will find the answer to this seemingly UNSOLVABLE puzzle.

sorry to say I have already done all tests mentioned and several others.

I only have code 36 - no 33 or 34 or any other showing and I have been testing off and on for 18 months.

The MAF wire glows red hot when I ground pin D12 at the ECM connector, but the ECM does not ground it.ie the ECM does not make the wire glow hot. That eliminates the idea that the ECM is doing it's job then not knowing it has.

The FSM says if all the tests are positive, that it must be the ECM- I have two, I swapped them over- same result- so both my ECM's are faulty? yet in every other respect they works fine

I brushed the contacts and then sprayed with contact cleaner. The D12 socket appears just same as all the others, ie it's not pushed back or somehow not making contact.

So we have a complete MAf burnoff circuit working, right up to the ECM plug, yet the ECM doesn't burn the MAF - it HAS to be the ECM?? how does it know when to burn the MAF? maybe that signal is not being sent?

 

CHANGE THE RELAYS, This sound to me that the power relay to MAF is not working,,IF the MAF is not energized then the ECM will not fire up the burn off,, This is the problem... change the relays first!! ecm fails are as common as winning the 6/49..

 

5 seconds after ign off, the burn off cycle should run for 1 second.

300 ms into the burn-off cycle, the MAF signal A/D will be monitored.

The burn-off test will fail if the MAF A/D is < 20 counts (0.40 volts) or > 95 counts (1.9 volts) for 6 samples.

Also there is a condition to skip the burn off if the voltage has been greater than 17 volts during the ignition cycle. Are you experiencing any issues with high voltage?

Is the MAF still powered 5 seconds after ign off? I suspect it is not.

It sounds like there could be an issue with the MAF power relay that is preventing the MAF from being powered during burn off.

Are you getting 12 volts on Pin E when you manually burn off the MAF by shorting D12?

This voltage should be provided by the energized burn-off relay, and then pass through the (de-energized) power relay to reach pin E.

If the power relay isn't cleanly switching "off", then the MAF won't be powered during the burn off test.

See Agent 86's diagram to see the voltage path:

http://members.shaw.ca/corvette86/En...agram%2086.pdf

 

Exactly! The FSM tells you this as the last resort in the Code 36 diagnostic. On the 87' we have, everything checked out and even the relays worked when powered. Replaced the relays as instructed (both of them!) and never had that problem again.

 

I have replaced the relays of course, it was the first thing I did, 18+ months ago. In fact for the lasy 6.5 years this code has appeared regularly every 2 years or so and was always fixed by replacing the relays until last time.

Tequilaboy, thanks for the explanation, I will replace both relays first then if the problem is still there, test as per your info

thanks all
l