Hey guys, I picked up a 1992 F-150 a couple years ago for real cheap and it needed a few easy things and has been a great truck since. It is a 302 long bed regular cab with electronic 4x4 and an E4OD automatic. It did come with a 4" suspension lift (not my idea). It has 298,000 on it and is used strictly for chores. The transmission has a metal Ford Renewed tag on it from, if I had to guess, the early 2000s.

Randomly, when going down the road in a higher gear, the truck will randomly drop down into first gear. If you give it a minute, it will typically figure itself out and go back into the gear it is supposed to be in. This occurs sporadically. It seems to only happen when the truck is warm. This occurs only when the shifter is in 'D'. It occurs whether you lock out OD or not. If you drive with the shifter in '2', this issue does not occur.

Last Spring I decided to replace all the main sensors on the truck (IAC, TPS, 02, MAP, coolant temp, etc.) At the time, I found some tribal knowledge on the internet that said the TRS/MLPS (transmission range sensor) is known to fail with age and cause exactly the problem I described above. So I replaced the TRS/MLPS. The truck subsequently did not have the problem for months, after doing it frequently up until then. Granted I probably only drive the truck every other week. Now, the problem is back.

So, either the new TRS/MLPS is bad or, probably more likely, it never had anything to do with that sensor to begin with. I did read the codes with a paper clip, and it returned the following:

621 Shift Solenoid 1
622 Shift Solenoid 2
631 OD cancel light circuit
629 Torque Convertor clutch circuit
626 Transmission coast clutch circuit

Seems like a lot of codes to me. I want to believe that this is an electrical/superficial issue, and not an internal mechanical issue, because when the issue is not occuring the transmission actually operates perfectly. In fact it operated perfectly all summer and most of this fall. It's beyond my ability to diagnose but I don't want to just take it to a transmission shop and get raked over the coals for a full rebuild. Any ideas on things that would cause that combination of codes, or if any of those codes would be directly related to the issue I described?

 

A full rebuild isn't going to help this at all. Don't take this to a transmission shop, you have a problem with the wiring harness to the transmission. Take it to a shop that specializes in electrical repairs.

 

Thank you. That had not occured to me but that makes perfect sense. I may inspect the harness for obvious issues first but likely will wind up taking it to an auto electrical shop. Glad I asked.

 

Yup, if you see this, then you'll know haha:


Failed shift solenoid pack plug. This is an example of what you don't want to see! This is the main harness that the ECM uses to communicate with the transmission. If yours looks like this one,google search: E4OD solenoid wire harness repair kit 1989-94 (that will be for all idi trucks with the E40D If your truck is a '95+ you need to make sure to buy the '95+ one.)


If it looks ok, you can unplug it and inspect it for dirty and or corroded wires. Use a small mirror to look down into the pack side.
Flush it out with electrical cleaner, let it set and then soak it up with a clean lint free cloth. Apply a light application of dielectric grease and plug it back in.


You can take it to a shop like Mark suggests, if you have no luck, but be be very suspicious if they rush to recommend a full reman!

 

Hey guys, I took a quick look under the truck to see what I could find. The attached picture is of the range sensor plug, not the solenoid pack plug, but as you can see the jacketing of the wires has cracked. This picture was taken after cleaning up the plug. When I first got down there these wires were caked in mud/grease which seemed to retain moisture. This truck is a lifted 4x4 and is caked with mud underneath.

I did not remove the solenoid pack plug but visually inspected it, including with a mirror. It did not have obvious signs of wiring failue like the range sensor plug did, however it was similarly caked in mud/grease. I did try to clean to the extent possible. I did not remove the plug because I was doubtful about my ability to do so without cracking the clips off. I now plan to replace both plugs (or have an experienced auto electrician replace them). I can find the solenoid pack connector for sale readily, and I'm sure I can find the MLPS connector somewhere as well. Note that this truck despite being a 1992 has had a 1995+ style MLPS and connector as long as I have had it.

Anyways thanks for the help. I think you guys got me on the right track, after chasing this on and off for months.