i'll be swapping out my PCM today to get rid of the start-up ROMP after my ZF swap, and want to check some sensor connections and things while its unplugged. I also have an idea about maybe fooling the PCM into thinking the range selector is present by jumping the range selector wires with a resistor - but i'll have to take some measurements on the range selector first - or maybe just put the range selector back in place.

anyway - my question is - what is the story with the "sig rtn" or Signal Return pin from the pcm - pin 91. it looks like it goes to (or comes from) a bunch of sensors, like the MAP, BARO, ICP, but also seems to power the trans temp sensor and the trans range sensor. the shop manual says it should read 5V (+/-0.5), just like the Vref Signal (Pin 90), but whats the point of having 2 5V signals to sensors like the MAP. is it just an error checking thing?

 

Maybe it's a typo in the book. Pin 91 is signal ground for the sensors you mentioned including TRS. If you are getting a PCM for a standard transmission then you don't need to connect up or do anything with the TRS.

 

yeah, that's what I would have thought as well, but following the engine wiring diagram a bit more, that same circuit (pcm pin 91/sig rtn) also appears to supply power to the Intake Air Sensor and the Engine Oil Temp sensor, and those sensors only have one input and one output, so this common input would have to be power, right? also, it connects directly to pin 2 on the IDM. does the IDM not make its own 5V reference signal, maybe and pick it up from the PCM on this circuit. if that's the case, it would seem that this circuit is pretty darn important!

and yes, that's right - I confused two separate plans I was toying with. I was thinking of trying fool the auto PCM into not romping by mimicking the TRS resistance in Neutral, but i'm now thinking i'll just go ahead and put the manual PCM in and be done with it...

I think i'll test out all (or at least a few) locations of this 'Sig Rtn' circuit before and after the swap though, just out of curiosity.

 

Still common ground as PCM supplies power to the other inputs, it's a voltage divider type of circuit. IDM pin 2 and PCM pin 91 tied to ground buss together to prevent bad connections if they were tied to chassis ground separately.

 

well - you're right. I tested out all the (easy to get to) locations of Vref and Sig Rtn before taking out my auto PCM last night, then checked resistances to the PCM connector on these two and most of the engine sensor signals. everything came up OK and the Sig Rtn lines were all 0.04 V to the battery ground terminal - definitely a ground i'd say and all my connectors checked out. thanks for the input Jim.

 

That wire goes to terminal 2 on the range selector, which then goes through one or more resistors and eventually back to pin 64 on the PCM. Which resistors it goes through is determined by the shifter position. It's not separate switches/contacts for each position. So to "fool" the auto PCM that it's in N or D, you'd have to determine what the resistance is between terminals 2 and 3 on the TRS while it's in those positions, and put your own matching resistor between pins 91 and 64 on the PCM. 'Course, you may find that you've outsmarted yourself, and if the auto PCM thinks you're in P or N all the time, it won't, for example, engage the cruise. There may also be other such side effects.

 

yeah, I decided against that approach and just swapped in the manual PCM I had. I was wanting to keep the auto PCM in there a while longer (without the romp) in hopes of pinpointing a lingering power-loss problem I've had. in pulling the auto pcm, though, it seems that may have been the culprit as it showed signs of pretty high internal heat. I plan to crack it open and have a look - I just hope it was purely an internal failure and not caused by a short or bad connection somewhere else in the truck - which would then fry my manual pcm as well - hence all the connection checking last night.

the manual pcm seems to be working great, so far, though.

...finger's crossed.

 

Where did y'all get the pinouts for the PCM?? I'd like to have a set of pinouts for our PCMs and SD PCMs for a potential project I've been thinking about for awhile.

 

here you go!

https://www.ford-trucks.com/forums/9...ml#post8976470

I have a copy on my computer, but a google search for it, too me back to this forum. ...of course.