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no dwell signal at O2 test point

posted by  someone claiming to be Simon on  Sun Jul 15 04:38 CST 2001 [RELATED]

I have been chasing a lean mixture problem on my '83 245 for a few weeks now. I have tested every sensor, at the sensor, and at the ECU. According to the Bentley 240 manual, the wiring and sensors are all good. I have checked very carefully and do not think that there is any false air.

I do not get a dwell signal at the O2 test point. It should vary between 20 and 70 as the duty cycle is adjusted to get the optimum mixture.

I tried swapping the AMM, in case the amplifier was malfunctioning. On both units I get 2.7 volts at idle and about 3.5 at high speed. I also swapped the ECU with no change in the mixture.

I have not checked the fuel pressure yet, I need to plumb up some sort of an adapter, but the mixture does become richer if I remove the vaccuum source to the pressure regulator, so it seems that the fuel pump is operating properly and can produce the needed pressure. Is this the problem? Could the pickup sock be clogged up? Or if the transfer pump has failed does it create enough resistance to lower the system pressure?

I also have not bench checked the injectors. They all look clean and have soft pliable o-rings. They all sound similar when the engine is running. Disconnecting them one at a time results in a 75+- rpm drop and a rough idle.





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