93 Suburban - injector fault, quad module driver, and other sins

[sabino56]

Thanks for suggestions! I see you have a saguaro in profile pic - not too many places in US with them. I'm in the Sonoran desert - Tucson. (which brings the pack rats into cars outside problem..)

I pulled the connectors from PCM and measured resistance of both injectors to ground - both were identical @ 15.3ohm. Then I swapped the plugs at injectors to see if fault would follow cable or stay with injector. Fault went away! Don't know why. I plan to swap back and see if it stays away or comes back. Either way it seems like something along lines of what you are suggesting. Some criteria for rise time or dwell by PCM code was failing - maybe connector after 25 years was oxidized a bit or something. I'll check resistance again before swapping back and see if it's changed.

With respect to the VSS. I checked ground at VSS connector and it looked good with voltmeter. I checked with Tunerpro while in park and the PCM sees the same as speedo while rev'ing in park. It also seems while driving the speedo is doesn't fluctuate with RPM. Searching online I found that this is not uncommon with GM cars of this vintage. One person said it was due to noise when speed sensor signal degrades. I'm not sure what kind of signal the speed sensor would send when car was not moving. I assumed either constant high or low depending on position of tooth on output shaft - so not sure it makes sense. But it's an easy thing to try and I remember changing one once & it was covered with a pile of fines from wear in transmission. So I'll try that first.
Thanks again for suggestions.

[sabino56]

I pulled the connectors from PCM and measured resistance of both injectors to ground - both were identical @ 15.3ohm. Then I swapped the plugs at injectors to see if fault would follow cable or stay with injector. Fault went away! Don't know why. I plan to swap back and see if it stays away or comes back. Either way it seems like something along lines of what you are suggesting. Some criteria for rise time or dwell by PCM code was failing - maybe connector after 25 years was oxidized a bit or something. I'll check resistance again before swapping back and see if it's changed.

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I pulled PCM connector and checked resistance. It was higher (40 ohm) which confused me, not sure why it would be higher. I swapped connectors and checked again and resistance through injectors for both were 10ohm. I pulled connectors and checked I was looking in correct place, replugged several times and always measured 10 ohm for both. Put connectors back on original injectors and fault is gone now. Either I screwed up first measurement or something weird - but regardless, I measured 10 ohm many times which is less than original and the fault is gone. So I'm going to write this one off as some connector to injector resistance until/if it comes back.

[sabino56]

I replaced speed sensor and it resolved the speedo moving in park when rev'ing engine. I guess the VSS is sensitive to noise and old sensor had lower signal levels.

I also opened up the ECU and sorted out what was being driven by the quad driver modules. It had same modules as referenced in this thread http://www.gearhead-efi.com/Fuel-Inj...ighlight=34992. Interesting thing is only the 3 transmission solenoids (A,B, and 2-3) & TCC solenoid are on the two modules. Each module has two of the four outputs tied together & one output not used. I didn't attempt to trace what or whether there was an input for the unused outputs - all I had was ohm meter and wasn't sure about logic upstream. I'm not sure but it seems from data sheet that having a floating output on the qdm would set a fault - so I'm believing that the qdm fault in datastream is not monitored in code/not used. This vehicle doesn't have a DTC for a qdm fault - I only found it by monitoring the datastream. I also checked resistances of cables/solenoids and all is OK, the transmission shifts fine. So I'm going to write off the qdm errors as an artifact.