Trouble codes. They're like the math problem the teacher puts on the chalkboard. You follow along, everything seems to make sense in a definite, orderly fashion and then, you try it yourself without anyone's assistance. Everything that made sense just minutes ago, suddenly turns into a language that you have no hope of deciphering. This is why we emphasize that the first purchase for your Corvette should be a shop manual. Whether you get one from our advertisers or find one at a swap meet, it will pay for itself when you need it most.
On a recent trip to the Corvette Clinic, Chris Petris told us about a C4 that was contributing to his balding fund by constantly throwing a code for the MAF sensor. He found the problem, and we decided to do a story for our readers, hopefully preventing the "follicle farming" Chris was subjected to.
There are three codes that pertain to the MAF sensor: Codes 33, 34, and 36. Code 33 (mass air signal voltage high) is set when the computer sees a high mass air reading at low engine speeds. In other words, there appears to be more airflow through the MAF sensor than the engine can physically breathe at a specific throttle and rpm.
Code 34 (mass air low) sets when the computer registers less than 5 grams of airflow at idle or 2.5 grams/second of air when the engine is first started. The mass air wire inside the MAF sensor should heat up after the car shuts off to clean any contaminants from the wire. When this fails to happen, the computer shows a Code 36 (mass air burn-off malfunction). We'll address and troubleshoot each code individually.
The mass air sensor is a simple design. It's comprised of a wire that heats up when the car is running. The air entering the engine through the MAF sensor cools the wire. The more air ingested by the engine, the more air must flow through the sensor. More air means a cooler wire. The computer knows how much air is entering the engine by the temperature of the wire inside the MAF sensor. The air/fuel mixture is dependent upon having accurate readings from the MAF sensor. If there were a problem, a Code 33 or 34 would be set. Once a Code 33 or 34 appears, the MAF sensor will shut down and the car will go into "basic strategy" mode, aka limp-home mode, until the key is turned off for at least 15 seconds. You'll know the MAF has stopped working when, in Chris' words, "You let off the throttle and it feels like an anchor just dropped."
A Code 33 can be set for several reasons. It appears if the dark green wire (circuit 998) has an open circuit or a bad sensor; if the black, burn-off control circuit wire (circuit 900) has power with the car running; or if the system has a bad ground. You must check for an open circuit on 998 at the ECM on connector B12. There are several places to check for a grounding problem. The cast-iron-headed '85s and '86s have the grounds on the rear of the driver-side head. You'll have to pull the wiper motor to check. The aluminum-headed '86s and '87s have a pack of grounds next to the oil-temp sensor and filter. On the '88 and '89 Corvettes, the ECM, dash, and sensor grounds are on a lower lefthand-side bellhousing stud.
A Code 34 could be set for several reasons as well. A bad ground, an opening in the ducting between the MAF and the throttle body, incorrect minimum idle-air setting of the throttle body, or a worn throttle body can all cause a Code 34. Typically, a worn throttle body or a bad ground will set a Code 34 intermittently (every third or fourth time). To see if the minimum idle air is causing the problem, open the throttle slightly as you try to start the car again. If it's easier to start, check the minimum idle air.
To check the throttle body, simply depress the cruise-control bellows and observe if the throttle-body linkage moves laterally instead of only rotating on the shaft. If there is noticeable movement, your throttle body is worn and the engine is getting unregistered air through the gaps.
'86-'89 Corvettes use a powered...
'86-'89 Corvettes use a powered fuse link and connector that supplies power to the fuel pump and MAF relays behind the battery. The '85 Corvettes do not use relays for the MAF, but instead use a module next to the ECM. This module rarely goes bad; but to check the module on '85 Corvettes, look for 12 volts at connection E on the MAF with the engine running. Then, run the engine for at least 5 minutes, turn off the key, and check for 12 volts at connection D (MAF burn-off). The powered fuse link is a common cause of intermittent MAF codes. Check the connection and wire for a possible open circuit.
The oil-pressure switch is...
The oil-pressure switch is another possible cause of intermittent MAF codes. If the engine feels as if it has an intermittent hiccup with a fuel pump and MAF code set, the oil-pressure sensor would be the place to start. If the switch is bad, the engine tries to start because power to the pump at startup is fed through the ECM's fuel-pump-relay drive (connection A1).
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Over the years, some changes...
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.
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The relays changed in late...
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. You'll notice that the new connectors are weather pack connectors to seal out moisture.
To check that the MAF burn-off...
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...
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.
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Chris has spent some time...
Chris has spent some time adding wires, meters, test lights, and jumpers, chasing down several specific reasons why Codes 34 and 33 will appear. Here is what he found: 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, Chris has seen a few cars for which the only cure was to run new wires for both circuits. This cured the intermittent codes immediately.
Checking The Minimum Idle...
Checking The Minimum Idle Air: The first step in checking the minimum idle air is to put the car into Field Service Test Mode. Do this by putting a spade connector into the A and B positions of the ALDL connector. With the key on and the engine off, the fan will turn on and the SES light will flash. Also, the Idle Air Controller (IAC) will fully close.
Disconnect the IAC to eliminate...
Disconnect the IAC to eliminate its operation. This will give you an accurate reading of the throttle body's minimum setting.
Also disconnect the timing...
Also disconnect the timing control wire to the distributor.
Remove the spade connector...
Remove the spade connector from the ALDL port and try to start the car. If it won't start, there isn't enough Minimum Idle Air. Set the Minimum Idle Air so the car will start and idle at around 450 rpm. Once the Minimum Idle Air is set, plug the timing-control connector and the IAC back in. Check the Throttle Position Sensor to be sure it's adjusted properly. It should read between .54 and .60 volt.
With that, you should now have a fun-running Corvette or at least found the problem that was throwing the code (as long as it was a Code 33, 34, or 36). Just like those math problems back in high school, you may have to go back and double-check your work; but with your trusty shop manual and professor Petris' insights, you'll do just fine. This time, rather than having a paper full of scribbles, you'll have a Corvette you can drive! Can anyone say "RECESS"?