Hello,

Been reading the forums for a few years now but never been a member until now. Previous posts have been quite helpful with other issues i've encountered (tps replacement, coolant bypass, throttle body cleaning) but i've recently come to an issue where there seems to be limited info out on the forums. I'm hoping someone can provide me some guidance on my latest problem.

My 1985 C4 is has the first year bosch fuel injection setup along with a maf sensor setup powered by a module buried in the dash. There are no relays in this car to replace. This is a one off year design for 85 and was changed in 86 forward. I've been struggling with a code 34 maf signal voltage too low for about half a year now and want to get some opinions before I go digging apart the brittle dash to test the maf module located under the breadbox.

Following the service manual for a code 34, when the maf is disconnected, a code 33 is thrown which leads me to chart c-13 to make sure the burnoff module is working correctly. Following Chart c-13, the air metering connection from the maf is disconnected and the voltage between connector terminal E and ground is >10 volts for about 5 seconds. Folowing the next step, voltage measured between terminal D of metering harness and ground is less than 1 volt. Up until this point in the service manual everything seems to check out ok.

The next step instructs me to ground the ALCL terminal E, ignition on for 10 seconds, ignition off and then measure the voltage between terminal D and ground of the metering harness. Manual says that after 4 seconds the voltage should read greater than 2.5 volts for one second. From my measuring I am only getting 0.81-0.84 volts for 1 second here.

The service manual instructs me to check continuity of circuit 994 (maf harness) and if connections are ok then the burnoff module is faulty.

Am I missing anything here that I could check before I start taking apart the dash and breadbox to get to the wiring for circuit 994 and the maf burnoff module?

Are there any other things that could set a code 34 here?

Sincerely appreciate any input or opinions,

Bart.

 

How are you making the one second measurement you mentioned?

 

For each step on chart c-13 I am using a volt meter on the air meter connection going to the maf. First I have my leads on the air meter connection under the hood terminal E and terminal A (ground) and after turning the key to on position the voltage there stays above 10 volts. Then I move the leads to terminal D and terminal A and the voltage is lower than 1 volt. Finally I keep my leads connected to terminal D and terminal A in the air meter connection and proceed to ground the ALCL diagnostic port inside the car by connecting a wire between terminal E and terminal A(ground). That does not yeild a result on the volt meter that is greater than 2.5 volts but instead 0.8 volts or so.

 

There a schematic to go with the flow chart ?


analog or digital meter?

 

Can you hear the MAF module click after shutdown, if you can I'd dismiss the module for now. If you're only getting 0.8V on a 2.5V circuit that would make me suspect bad connections, it may be contact oxidation on the relay inside the module, but i have my doubts on that. If you pull the pass side hush panel you should see the burn off modules connector closer to the top, verify resistance from there to term D on the MAF connector. If it's good, unfortunately you'll be pulling the dash, if its some wierd resistance that isn't logical you've got a break somewhere, I'd also verify that there is no continuity to ground with both sides disconnected.

Edit: Added schematic

 

Thought I would post an update to this even though it's been quite awhile!

I eventually got around to fixing this problem and it turned out to be a combination of a bad MAF and a faulty connection to the burnoff module.

Before I tore apart the dash I sprung for a quality replacement Delco MAF, this did not solve this issue..... initially. I ended up taking the dash apart, testing all wire continuity going from the under-dash burnoff module to the MAF, and then cleaning up all connections/reassembling. This was the ticket and the car has never run as good as it does now. I will note that I did try to use the original MAF after the wiring fix, but it immediately threw a light and caused the car to run poorly.

Thank you everyone who helped and responded. Going back to basics and ThatOneKid's suggestion of bad connections/wiring really got me thinking in the right direction.

To anyone else facing a similar issue with 1985 corvette, it really seems the connections, wiring, and sensors on this car are the main culprit of many issues.

Really happy I didn't let this issue or the multiple others I was able to solve using these forums beat me. The car has been a joy to drive this summer.

 

I had similar MAF issues in my 85. I had a bad Burn off module. They do not make new Burnoff Modules (1985 only part), so I bought a used one at a local junkyard and got lucky it was good... Beautiful 85 BTW.