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CALLING ALL PCM MASTER MINDS

2K views 6 replies 2 participants last post by  QKE-F250 
#1 ·
96’ bone stock F250. Been having what I would consider typical CPS problems (quick cut outs, and died a couple times but restarted no problem). Then Tuesday ran great for a couple hours then went to leave again a few hours later and NO START. Fired up once for a split second but that was it. No tach so swaped in a spare CPS and exact same thing. One fire up out of a bunch of attempts. Paying more attention to the tach notice it works some crankings and not at all on others. Assuming wiring prob now I test CPS pigtail, ground-good, 5V-good, next wire is different every time 6v 4v 3v 5v. Checked my 99’ and found it’s supposed to be battery voltage. Googled wiring diagrams to help find the trouble spot. Found that diagrams are slightly wrong, they say the green wire in question goes through pin #39 in the 42 pin connector and ends at pin #21 at the PCM. The #39part is right but it ends at pin #49 at the PCM, not #21,21 is an empty hole. That one Ohm’d out perfect but ironically the other two wires were high at 25 Ohm’s. Jumpered a wire from the battery + to the unplugged #49 PCM connector and got perfect battery voltage at the CPS connector.

So do I assume PCM is not putting out the Battery + its supposed to at Pin #49 ??? =New PCM

Is there a way to very carefully “hotwire” the PCM (with it unplugged) on the proper pins to check/verify the voltage coming out of pin #49 ???

Would the PCM out of my 99’ work in the 96’ just to see if it will run, without blowing something up ???

Could I run a wire from the battery+ and splice it into this wire to supply the needed voltage ??? If so would I need to cut the wire going into pin #49 to keep from back feeding the PCM and causing more damage ???

ANY help is greatly appreciated. I NEED THIS TRUCK RUNNING.
 
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#2 · (Edited)
What PCM is in it (DPC-202 or DPC-203)? What diagram are you referencing? PCM pin 21 for the CPS Dark Green wire would be for a 97 California splitshot OBS that is running a DPC-311 or 312 PCM. PCM pin 49 would be the correct pin for a 96 OBS truck. I would start with the CPS and PCM disconnected and see if you get any shorts to ground, power, or open circuits by jostling the harness and 42=pin connector. The 99 PCM will not work at all (pinouts would be completely different). Cheers!
 
#4 ·
Just so you know, the Dark Green wire is the CPS signal wire and won't simply have 12V on it. The signal that comes from the CPS will be a digital frequency and not a constant 12V. Cheers!
 
#5 ·
I'd seen references to signal wire and square waves but couldn't and still don't make sense of it. Feal like giving the 411 for dummies on that subject? To possibly explain myself a little better at one time I put the black lead to battery - and the red lead to CPS connector (green wire) and voltage is JUMPING around between 4.5 and 5.5. Does that at all sound right? What's your next step advice?
 
#6 ·
See the attached. If you are using a DVOM, it'll basically try to average the voltage since it is not steady. Cheers!
 

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