Code 56 on 1995.

[WarWagon]

Any other causes for a code 56 than a missing resister? Yes, PMD still on pump.

Just set the timing to find that it was spot on at -1.5. Next startup the SES light was on. No codes prior to that and it sat w/o power for 4 months and has a new PROM chip in it. Ya think it would have had an issue sooner!

 

[Crankme69]

That is what I ran into on my Tahoe, from what I gather that code can be generated from running TDC time set/learn function when the resistor value is out of the min max spec range.

I could not clear mine until I swapped out the resister...and being as though mine was also on the pump like yrs I ended up with new ext cable cooler plate resistor & PMD from Leroy.

I still don't know if the PMD on my pump has a resistor or not. Might find out when I swap the intake manifolds this spring to remove the EGR BS restriction...

 

[bk95td]

There was a thread recently about a fried resister. Maybe a possibility?

 

[buddy]

On OBD1 trucks the resistor value is learned every 50 starts or when running TDCO learn, but the value is stored in the PCM, not on the PROM. So it is possible that there was a different PMD attached to this PCM last time it learned it. There was a burnt resistor seen recently. Maybe the resistor got damanged since last time it learned, and would have no affect until the next time it learns. Has there been any jump starting or any issues starting this engine up since you installed that may have spike the voltage?

You can try running the TDCO learn again to see if it reads it this time.