Hey guys so i have a 1995 f150 xlt 5.8l e4od 4x4. Back in august the truck neturaled out meaning we had nothing not even park. So i got it towed when the tow truck came park caught. Drove it off the tow truck in resverse. Hopped online said it might be the gear selector switch so i first adjusted it the sensor, then the truck delayed in shifting and only shifted when you lifted off the throttle and it was a harsh shift. Next i asked some mechanics they said replace the trans so i did replaced the transmission kept the old T-Case and Solenoid Pack still same issue.Next i replaced the PCM with a reman one from automodulesourceusa.com So i brought it to my Vo Tech school were ive been working on it for the past 3 months. We started with the rear end good passed nothing wrong went to VSS found that it was lifted up replaced it seated well still have the issue. Went to the PSOM check connections grounds etc all good. Check transmission pressure pass, check t-case pass, next we moved to the solenoid pack in the transmission we dropped the pan removed the solenoid pack and actually we replaced the seperator plate gaskets in the transmission everything went back together perfect and solenoids worked. We then wired up with a remote starter SS1 Solenoid SS2 Solenoid and EPC solenoid and manually controlled the transmission the truck shifted gorgeous like nothing ever happened. Plug the solenoid pack into the harness truck goes back to its normal issue. Next using BOB or break out box we did a full pin out voltage drop test everything passed. We checked our major load sensors TPS and MAP they passed as well. Next we thought maybe we got a defective reman pcm. So we sent it back got a new one today and still same issue. We grounded the TPS wire still same issue Unplugged MAP still same issue Unplugged TPS now heres when it gets confusing the truck shifted without us letting off the throttle when TPS was unplugged it still was very much delayed and harsh but it shifted on its own when usually if TpS was plugged in You could probably rev the truck up to 6000rpm or get to 20mph in first gear and it would never shift unless you let off the throttle. Me and my teacher have almost run out of ideas he basically said its like you have a calculator and you enter 1+1=7 and everything looks right. I really need help im getting to the point were i wanna chop the roof off and make it a planter -said my teacher. Any ideas appreciated like i said i really really really need the help!

 

Start with the basics...

That transmission is controlled entirely by the EEC (PCM) EXCEPT for Park position, which is 100% mechanical. To do this job it needs to know vehicle speed, throttle position, load, RPM, and what gear it's in. So the truck has the VSS/PSOM, TPS, MAF, PIP, and TRS to get this data to the EEC.

EEC makes some calculations and uses the shift, EPC, and TCC solenoids to operate the transmission/torque converter.

Now you said you checked the transmission pressures and found it within spec. That means the EPC solenoid is doing it's job as commanded by the EEC.

Shift solenoids, you operated them manually and everything was good there too. So this, plus the pressure test, nearly rules out mechanical problems in the transmission.

Replaced the computer why? Ever hear of Garbage In, Garbage Out? It means when you feed a computer garbage data, you get garbage output in return. Bad sensor data = bad decisions by EEC = trans won't work right. I didn't see any mention of scanning for fault codes, but for most problems you'll have a code, and for transmission faults you'll have a flashing Overdrive OFF light, even if OD still works. Any codes? How did you prove the computer was bad?

The truck shifted on it's own with no TPS because the TPS is probably fine and without data from it, it's using default values to operate the transmission. The TPS takes the place of the TV cable on older transmissions, and lets the computer know how far open the throttle is so it knows how early or late to shift.

It sounds like you have a problem with your speed signal somewhere between the VSS and the EEC. Not a hard diagnosis, just something to look at again. Nothing wrong with eyeballing the connections, but you can't possibly determine the signal quality just by that alone. You'll need to measure the signal in/out of the PSOM, which can be done with an analog multimeter if you know what you're doing, but a scope is better if your school has one, of course.

VSS input goes to the PSOM, then it gets conditioned and this output goes to RABS module, EEC, and speed control amplifier. Look these up in the schematics, isolate them, retry test with EEC controlling the transmission.

As for the old trans not having park, well that could have been anything from out of adjustment to busted parts. No computer control there.

 

Incorrect...the VSS signal goes to the RABS module first:


The RABS module is in parallel with the PSOM, therefore the signal is snagged by the RABS module on it's way to the PSOM. The PSOM buffers the VSS signal for use with the speedometer, PCM and cruise control.

 

We didnt prove the PCM was bad. Back in august i was like well ive done everything lets replace its brain plus i kept getring codes 121 and 538 even after tps replacement when i replaced it 538 went away but 121 stayed. Which didnt work then my teacher said send it back last week seeing it was under warranty why not didnt do anything. Codes i dont have a single tranamission code i did once in july for no or little signal from vss i replaced the sensor went away and this was before my problem. I still have code 121 it hasnt come on since the we replaced the pcm again though. Thanks for the diagram though you wouldnt happen to have what the signal for the psom would look like same with the rabs module.

 

I do not have a diagram of the VSS signal but I do know the output is a square wave with a frequency the varies based on the speed of the vehicle. The PSOM functions as a divide by X to provide a standard 8000 pulses per mile to the speedometer, PCM and cruise control.

This URL shows the photo I posted earlier and some information to troubleshoot the circuit: 1983 Ford Bronco PSOM picture | SuperMotors.net

Courtesy of Steve83.

 

The thing is.. in these trucks one PCM controls both engine and trans and it's programmed to "protect" the trans by raising line pressure and restricting shifts if it loses contact with any of the major powertrain sensors(TPS, VSS, MLPS), so it's actually more common to see engine sensors causing issues with transmission operation than the transmission itself.

 

I know this sounds weird, but check the tail light wiring. LED lights DO NOT WORK with these trucks. But it doesn't have to be led's. Bad connections in the tail lights or trailer plug-ins will do strange things. Voice of experience here.

 

Thanks for the correction rla2005, I was working from memory.