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

Hi, I am working to resurrect my 93 C1500 Suburban 5.7L K L05 with 16168625 pcm and BDJZ code.
The truck is my "go to home depot" truck and sits out side. Dang pack racks have been in it and it's slowly degraded in drivability and is sometimes erratic. Still, however, surprisingly reliable! I'd like to get it running better.
I'm sure there are many sensors, wires fraying, etc. - so I bought the Moates cable and have TunerRT Pro running. ( I found .adx, .xdf, & .bin files needed on this site - great resource! thanks!. ) The idea is I can find specific problems vs just replacing every sensor and wire.

The only code ever surfaced is a EGR DTC - and it's intermittent. The other two overt problems are: 1) sometimes the speedo will move up to about 20mph in park if I rev the engine. 2) The charge voltage seems too high - sometimes around 16-18 V.

Looking at output from TunerRT Pro - first thing I see is a Injector 2 Fault Detected (constant), Quad Module Driver 2 Fault Detected (constant), and Quad Module Driver 1 Fault Detected (intermittent).
I read on a thread here somewhere that (summarizing) - unless the Tech 1 surfaced the error you can't be sure the error is real as GM didn't necessarily make sure the code for that error was correct. I was wondering if anyone has experience on this PCM and these errors?

I have the Helm shop manuals for the vehicle. It does not list a QDM DTC code (I see that some cars in this timeframe had a DTC 26 for qdm faults, apparently mine does not) nor does it list the quad modules as something the Tech1 would check or monitor.

Regardless, I want to check the wires/relays which the quad modules drive - seems likely problem areas. I can find info on other PCMs showing what pins and relays the qdm's drive. I can't find anything for my specific PCM. If it exists that would be very helpful! Short of that, I've found the pinouts in my PCM for the same components in cars around same time and can assume. Or, I guess, open up the box but I'd prefer not to mess with that unless needed.

With respect to the Injector 2 fault. I pulled air cleaner and just did simple look/see. Both injectors appear to have nice spray fan when I rev the engine. I can't see anything from either @ idle. So it doesn't seem there's an obvious problem. Suggestions on what to do here?

With respect to speedo weirdness - looking at circuit the speedo output comes directly from the VSS. I can't see how a pulse train from the engine (is RPM from the crankposition sensor or distributor?) could get to the VSS. I'm thinking either it's a bad ground or the high charge voltage is messing up a reference in the VSS. or the VSS is bad.

I'd appreciate any insight, experience, good or bad suggestions from group.
Thanks
Ed

I generally don't troubleshoot the 16168625 down to the circuit board component level.

The PCM triggers ground signals (measured in milliseconds) to the injectors. The longer the PCM "hold" ground on the injectors the longer (measured in milliseconds) the injector will "spray" fuel. I'm wondering if GM uses a watchdog timer circuit to monitor the injector ground signal "hold" time. If the watchdog timer circuit was not reset by ground not being removed from the injector driver, then an injector driver error would be set. I have absolutely no idea if GM uses a watchdog timer circuit on the injector driver or not. Likely an Oscilloscope will be needed to measure the injector "hold" time to determine if the "Injector 2 Fault Detected (constant), Quad Module Driver 2 Fault Detected (constant), and Quad Module Driver 1 Fault Detected (intermittent)" are valid errors. The injector driver ground signal seen on the Oscilloscope should be a clean square wave, any saw tooth wave form the injector driver would be suspect.

The VSSB / DRAC is used to convert the 40 pulses per driveshaft revolutions from the transmission Vehicle Speed Sensor (VSS) into 2,000 pulses per mile to the PCM. The VSSB / DRAC share the same ground wiring as the PCM.

I suspect the ground wiring to the VSSB / DRAC / PCM is needing repair. One option would be to purchase new PCM pins ( https://www.mouser.com/ProductDetail/829-12146448 ) and rewire with new wire / new pins all the ground wiring to the VSSB / DRAC / PCM.

dave w

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.

Quote:

---

Originally Posted by 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.

---

.
.
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.

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.