I cannot find any other posts close to what I have going on so I figured i'd start a new thread. 1996 F-350 PSD Crew cab long bed 4x4 XLT, had the trans rebuilt about 60k ago (triple disc converter, shift kit, clutches etc). I have only gotten the truck to do this 3 times in the last month, so here the issue: I will be driving along at about 45-55 mph and my torque converter will unlock. Tranny still shifts fine but it will not go back into lockup. However as fast as you can shutdown and restart the truck, put it back in drive and everything goes back to status quo. no O/D light blinking, havent scanned for codes yet but plan to, brake lights all appear to be functioning, fluid level is good and appears clean and does not smell burnt. On another note it seems that it will act up when going over a bump. All plugs appear to be ok on a quick inspection, but i have not really investigated yet. I am going to change the VSS and the Brake Pressure Switch today as cheap insurance. The real difficulty of this issue is I cannot get the truck to act up all the time. I want to believe its an electrical issue


Has anyone experienced anything like this?

 

It's not the VSS and not the brake pressure switch. It could be the brake light switch. It could also be the third brake light. If that doesn't work it will cause the converter to unlock.

 

Well I never got to changing the sensors yesterday anyway. I did however change all three brake light bulbs. The 3rd brake light is much brighter now than it was with the old bulb (same # bulb). Drove it to work today and all seems fine so far

 

I was going around a bend this morning shortly before slowing down to turn and it did it again. However I did take note that when it shifted back down (all the way to 1st) and i made my turn everything was dandy again. does anyone have any ideas? Mark ruled out those sensors, but other than the solenoid pack I'm at a loss

 

I had an idea, but you didn't seem to like it. It still could be the problem.

 

Mark- Im not saying your wrong and this is my fault because for whatever reason I overlooked that. I have a vegetable farm here at home and ive been pulling my hair out with other nonsense going on this week and the pickup gets kind of put on back burner even though its toting my rear end everywhere in the meantime .... Im going to order a new switch today however, and again im not doubting you, how can it be the switch if it seems to be operable? And thank you sincerely for your help

 

If the switch momentarily turns on the brake lights it will turn off the lockup.

 

I understand that portion of it but in theory shouldn’t the converter immediately re-lock?

 

There is a several second delay before it relocks. It could get another false signal in that time and not relock.

It could also be a problem with the converter, seals, solenoid, or valve. If one of them malfunctions and allows the converter to slip it will set a code and not allow the converter to lock again until the PCM powers down and starts again.

 

That makes sense. The new switch is ordered and I should have it this afternoon. Given the fact it relocked on its own without restarting makes me think more on the electrical end then. I will post back with more info as to what happens

 

I changed the brake light switch last night, the old one had a bent terminal and aside from that, you could just about breathe on it and have the brake lights come on.... Drove it around this morning and all seems fine so far. knock on wood

Hopefully I'm not saying this too pre-maturely..... Mark thank you again

 

I suppose I spoke too soon, it did it again this morning slowing down briefly for a car to turn (without foot on the brake and i only had let my foot off for about 2-3 seconds) and wouldn't re lock until I was almost at work about a minute down the road. It decided to relock as soon as i put my signal on to turn into work, but I assume thats just coincidence. I guess its time to journey into the transmission next... Im going to plug a scanner into it later today just to check there are no codes.

 

My truck did that once. Found that the trailer wiring back by the plug in had some wires lose
Took me forever to find it.

Good luck

 

It just drops out of lockup or goes all the way to first gear?

If only u locking. Have you had the tail light bulbs out lately? Alex had a deal on his way to rre where the filaments broke off and fell down into the bulb. It would short out periodically. If I wasn't with him I would not believe it.
If it's going to first gear have you done an MLP lately? We used to have LOTS of weird things as a result of MLP and water intrusion.

 

It dropped down to first?!