Just wanted to mention the reason my friend is leaning towards computer is because he noticed the check engine light doesnt come on after it stalls..

 

Well, that could also be wiring ... if the CPU thinks you've turned it off, of COURSE it won't light the CEL *grins*

But if all wiring is 100%, then a new to you ECU is possibly the cure. I just am resistant to saying that's it ... in my job as a tech, I find that easily 50% or more of the "Dead Computer" problems are wiring and interconnects.

RwP

 

Pulling the bed to do the fuel pump is the way to go, 6-8 bolts two clamps for the filler tube and unplug the tail lights, its light enough two guys can Lift it, I always thought fuel pumps worked or they didn't but mine proved me wrong, I'd check the positive wires going to the fuel injectors too I had a hell of a time tracking that splice that shorted down, and it acted just like your describing, the only way I found mine was (dumb scary luck) the fuel that I depressed out of the fuel rail caught fire from the short.

 

My check engine light never comes on, but when you cycle the key it will show you the stored codes.

 

I don't think the check engine light is supposed to come on for all codes. I think it is limited to major codes that need immediate addressing.

 

It's OBD1 so the codes are minimal yeah.

The plan is to check the voltages @ stall via wires 'Ignition on/off' wire to PCM, 'Battery(+)fused' to PCM and two ground wires to PCM. Saw the PCM pinout diagram in the faq on here.

edit; one more thing, when you go to buy a PCM on oreilly or napa's website (or even the pcm re-manufacturers website) they always have this link saying "Engine computers rarely fail by themselves" then it gives you some reasons. One of which they say is the Transmission Control Solenoid? I mean I guess it would be a matter of testing the component... which involves driving the truck jacked up in the case of the TCS I believe.

Just something I've kept in mind over the looong time figuring this out off and on.

 

I have the 42rh behind a 3.9. I don't see how the Solenoid could make the things stall or affect the PCM but when I was trying to figure out my problems (shifting too soon & failure to get RPMS up) a tranny guy told me to disconnect the solenoid and drive it to see if it behaved differently. Apparently that would tell me something that has left accessible memory for the moment. It will drive like a 3 speed since it can't get into OD but everything else will be normal. If the Solenoid is the problem then disconnection should let it run.

 

OD or Lockup relays have no feedback to the PCM....the only way it might cause something is if it's shorted and temporarily kills power to the PCM when activated...but I'd be surprised there wasn't a fuse to protect against that.

 

Thanks for your input guys.

I am now trying to track down PCM pins 11 & 12 ground circuit Z12 termination point.

From the PCM they are black wires with brown stripe, that splice together about 1 foot from the PCM (S121), and also continue into the main harness on the firewall to two wires of the same color, but bigger gauge.

My friend was supposed to come over and take a look and re run through how i've checked the voltage on the pin 9 'Ignition switch on/off' circuit to the PCM. Which seemed normal to me; 11.6v key on only, 13.7v truck running, and ~12.7v after stall slowly fading to 12.
NO CEL
Eventually I got a CEL (after like 5 keys on/off & checking voltage under hood), and the truck started & then ran fine, drove it for 10 minutes fine.

But ya I was trying to be sure of where the black w/ tan grounds, pins 11 12 terminate. using the 1994 manual on here while my truck is a 1992.. the 94 manual on here says

"Circuit Z12 connects to cavities 11 and 12 of the PCM. The Z12 circuit provides ground for PCM internal drivers that operate high current devices like the injectors and ignition coil.
Internal to the PCM, the power (device) ground circuit connects to the PCM sensor return circuit (from circuit K4)"

"The grounding point for circuit Z12 is the rear of the right cylinder head."

Now I have the obvious wide silver ground strap from firewall to right rear cylinder head (passenger side right?).. but thats all I could see terminating there so that seemed strange.

Guess if all else fails i'll be opening up more of the harness and following these black/tan ground wires to be 100% sure of their continuity. Odds seem slim something is wrong there though.

 

I believe this is my PCM ground pin 11 & 12 circuit z12 termination point; G108 (though this is from a 94/95 manual and my truck is 92).

Its stopped raining so investigation should resume.