Long time listener, first time caller. I'm hoping some of the experts here can help me troubleshoot my problem. Excuse my longwinded-ness. I've put a lot of time into diagnosing this to no avail and I want you experts to have the details.

The Truck: 1995 F350 Crew Cab, 7.3 Powerstroke, 4x4, E4OD Automatic. Basically Stock and Unmolested. 300k miles

Issue Summary: Trans intermittently will not perform 2-1 downshift or the 3-4 upshift. I have checked all the codes and there are no codes stored in the system. Verified by my Auto Enginuity Scanner and Snap-on or the like scanners at two different shops.

Background: Bought the truck off of FB marketplace over a year ago to tinker with. Previous owner said it wasn't shifting correctly. It was in limp mode with the speedo bouncing. I had some experience with VSS issues before so hubris got the best of me and I thought it would be an easy fix at a good price. I chased the limp mode issue for a while replacing VSS sensor and harness replaced and having the PSOM rebuilt, at this point speedo quit bouncing but still in limp mode. Then I replaced the TPS and MLPS all to no avail, then I found one random thread that mentioned the barometric pressure sensor under the steering column- Replacing that did the trick no more limp mode. I say all this to tell you what I have replaced because we have not even gotten to the issue yet.

Symptoms: On startup the truck will usually shift up to 2nd or 3rd then back down to first normally, but after a few minutes of driving it comes down to 2nd and will not go to 1st even on a complete stop. You can manually shift to 1st and it will perform the shift. Anytime you shift to Park and/or turn the truck off it starts back off in 1st just like it should. I do not drive it far but I have noticed that if you drive for a while it will begin to perform the 2-1 shift randomly but usually will revert back to only downshifting to 2nd. As for the 3-4 issue- this is somewhat more intermittent than the 2-1 issue but regularly the truck will perform 1-2-3 shifts then will not shift into 4th. This can be overcome by manually shifting down into 2nd then back into D and the truck will shift 2-3-4 at which point it will generally up and downshift between 3-4 up and down hills but occasionally it will stick and in 3rd and you have to manually shift down to 2 and back to D and it will then go up into 4.

Theory: The 2-1 downshift and 3-4 upshift share the disengagement of the No. 2 shift solenoid. My Auto Enginuity scanner shows that the computer is commanding the shifts but obviously they are not being performed. I think that circuit is either shorting to ground somewhere, or for some reason the valve that solenoid controls is getting stuck. I'm not a transmission guy so I investigated the latter theory by taking it to a trans shop. Those guys said that the solenoid was good and and that the valve was in spec. Previous owner had the trans rebuilt and valve body replaced (probably chasing this issue) and my transmission shop seemed to think that this is electrical, not mechanical after examining the solenoid and the valves and doing general diagnostic.

I have pulled out the harness for the transmission and continuity checked the No. 2 shift solenoid circuit and it has continuity. I have examined the harness and other wiring and see no obvious breaks. I'm thinking about buying a used harness on ebay and throwing at it, but knowing there is great input here I wanted to get input and see if anyone had any thoughts on what is going on. I greatly appreciate anything you tell me.

 

Can Autoengenuity read data? Can you monitor vehicle speed? If not, get Forscan at www.forscan,org. That will do what you need.

When it won't shift to first, is the vehicle speed reading in the PCM staying above 0 MPH?

With the harness installed in the truck, but disconnected on both ends, you need to check the #2 shift solenoid circuit for short to power and short to ground. Just checking continuity only checks one third of the potential problem.

 

Thanks for the quick response Mark. I was hoping you'd chime in. I've read quite a few of your posts thoroughout this diagnostic process.

I believe my AE will give me speed data. If not, I have a Forscan somewhere. I'll need to throw it all back together to drive it. I knew I should have waited before pulling the harness. I don't want to jump the gun, but what could cause it to 0 out? Should it be above 0 for it to shift?

As soon as I get the harness back in I will check for power and short to ground. I know that the power circuit that goes the transmission solenoids has power because I checked that before I pulled it, but I did not check for short to ground.

Again thanks for you help. I'll report back once I've tried this.

 

I think a bad PSOM could affect the reading, and If i rememeber right , a bad RABS module could theoretically interfere with the cleanliness and reliability of the speed sensor that the transmission will need to know. But I could be wrong. @rla2005 were you the one to tell me this about RABS?

 

Stopping would cause it to read 0. We are talking about vehicle speed. If the truck is moving, there should be a number higher than 0, corresponding to how fast the truck is moving. For example, if the truck is moving at 25 MPH, this number should read 25. Once you stop it should read 0. If it is reading a number higher than 0 when the truck isn't moving the trans won't shift to 1.

 

Finally got it back together and was able to test some things:

Vehicle Speed on the Scanner: PCM vehicle speed mirrors actual speed. It returns to 0 when I stop.

Harness Tests In Truck, Both Ends Disconnected: SS2 Circuit continuity is good as previously reported. No short to ground. No Short to Power. This should indicate that SS2 circuit in this harness is good.

Ground Test Trans from Trans End of Harness with Fender Connector End of Harness Connected. SS2 Circuit has a path to ground. I then disconnected the fender well connectors and back-probed the SS2 wire from the fender connector back to the PCM. It shorts to ground (as I would expect since we've ruled out a bad harness between the fender connector and the Trans).

This leads me to one of two conclusions: Either there is a short in the harness that runs from the fender well to the PCM, or there is a short in the PCM. Thoughts? I'm out of shop time tonight but hopefully tomorrow night I can test the harness right before the PCM.

Interesting thing I noticed: Solenoid commands seem normal while driving except that if I manually shift to 2nd, both solenoids are open 0s. This should be a 4th gear solenoid command, but the trans shifts to 2nd and stays in 2nd despite the solenoid command. I do not think this is normal. I do think this is somehow related to my ability to force a 4th gear shift by shifting to 2 then quickly shifting back into D. Could be unrelated to my issue but I found it odd so felt the need to share.

I appreciate your input.

 

I think I have found the culprit. I removed the harness from the PCM and back-probed the SS2 circuit on the 101 pin with the fender well end of the PCM harness disconnected and I have a short to ground. I'll need to take the fuse box off and figure out how to get that harness where I can check the wires but that SS2 wire must be chaffed or broken somewhere in that bundle of wires. I might just have to run a bypass wire but I want to do some further investigation.

Broader question- If SS2 is always shorted to ground, why does it sometimes perform the 1st and 4th gear shifts? Seems like the trans would never go to 1st or 4th if SS2 was always grounded.

 

transmission circuits for 7.3 power stroke

power distribution diagrams

power distribution diagrams

power distribution diagrams

 

Thanks AuroraGirl. Those are much easier to read than the ones I've found.