flatfish

There are lots of threads here on MAF problems, so I'll keep this short.

Car shows code 34 (low voltage) when MAF connected.

With MAF disconnected there are no codes. FSM says it should show 33 (high voltage). Voltmeter shows 5 volts at air flow signal (pin C).

I suspect the ECM is bad. Both MAF and ECM are remanufactured units and were replaced to successfully resolve entirely different symptoms several months ago. I am having trouble believing the replacement parts are bad, but maybe they are.

Burn off module checks out OK. (This is an '85). Provides power to MAF at right times and lights the wire after run.

Can anyone suggest any other diagnostics or should I return the parts?

Thanks

Cliff Harris

My expertise is with the 1227165 ECM ('86 to '89), but I suspect the innards are very similar in this case.

There is a pull-up resistor on the MAF input. That causes it to go to 5 volts when the input is open, as your FSM states. Why it doesn't show an error code 33 is a mystery.

On the error code 34 problem, can you measure the MAF output voltage when it is connected? I did it by sticking a hat pin into the back of the MAF connector. The voltage should be pretty low when the engine is off, ignition on. I saw something less than about 0.5 volts. I measured this a while ago but I'm not at home right now so I don't have access to that info. The point is that the voltage was not zero, which would seem to be what the ECM would report as an error code 34.

flatfish

I put a pin into the wire and measured 0.5 volts with ignition on, engine not running.

With engine running at idle I measured about 0.8V. When revving the engine, I measured around 2.5V.

I am going with the idea that the ECM is bad and will return it under warranty.

Cliff Harris

You might have a bad connection (broken wire or corroded connector pin) between the MAF and the ECM. You need to measure the voltage at the ECM to see if it's getting there.

flatfish

There was +5 at the MAF connector when the MAF was disconnected. This would have been coming from the ECM.

Cliff Harris

That is correct (in most cases). If there is a high resistance connection somewhere then it will show 5 volts with no load but drop when a load is applied.

The only way to truly know what is happening is to use a scanner and then you'll know exactly what the ECM is seeing.

flatfish

Replacement ECM fixes the problem completely.

The one that failed was an AC Delco remanufactured unit purchased from Rock Auto in May this year. Rock Auto has replaced the unit at no charge under 1 yr warrantee. The original ECM for the car was damaged when a remanufactured alternator ran the voltage past 20.