I assume you’re concerned about external noise or scope and/or setup problems. I tried a few setup changes and checked for noise from my fluorescent trouble light, but couldn’t find any problems. The back emf of 2v on the EPC solenoid seemed pretty big, but ?? I had determined earlier that the heavy impulse switching spike peaks increased further I got away from the battery post, but there was no deterioration in the DC level, so I wasn’t suspecting power or ground problems.
The good news is that I was able to get the ECM out and discovered 3 leaking and/or bad electrolytic capacitors in residence (after only 21 years). One was a tank/filter for the TPS and EGR pressure feedback sensor reference voltages. I think it’s reasonable to assume that all three are power supply related (but I couldn’t find a schematic anywhere) so I’ll stay with my instability/oscillation theory. Anyway, after replacing the caps the problem is solved, engine is performing well and the tranny is shifting like an old lady car again.
EPC solenoid and alternator output voltages. Still looks like 2.3 volts of back emf unless this is an overall level shift, but I don’t know how. The little jitter on the power line may be a probe connection issue.
EPC solenoid and power at MAF sensor (far down line from battery post)
EPC solenoid and MAF sensor signal
EPC solenoid and MAF sensor, wot test. Still some random spikes but I’ll have to wait for them to become more permanent.
I opened up the EVR valve to have a look. The unit does have a calibrated vacuum leak and mine had a failed shorted snubber diode. After replacing the diode the valve is working and all the dtc’s are gone.
Thanks again for the help. I’ll summarize this episode and put it in the case history file so maybe I can win the prize 8 channel, 20 GHz Pico scope kit…..