I have a 93 3500 6.5TD and every once in a while I will loose overdrive and will not shift back in till I shut off the ignition and restart. Sometimes this will only work for a mile or so then I will loose overdrive again. It is most annoying at hiway speeds doing 50mph/80kph at 2500 rpm. If anyone can help it would be greatly appreciated, Doug

Doug,
Check your TPS...Throttle Position Sensor..
Had the same problem with the same truck. Changed it out and the problem went away(until the next ones, LOL). I do not recall if it gave me a code or not(did not light the check engine light), but caused the same symtoms and worried the heck out of me.
The part is approx $130.00 USD or maybe less. Check around and I will almost guarantee that is your problem.
Hope this helps,
Mark

Thanks for the help Mark, I will check it out tomorrow.

Hope it helps.

Sounds like DTC 68, component slip, it sets when input speeds are 400 rpm higher than output for a few seconds. You might try cleaning the input and output sensors of metal fragments.

Thanks Quantum, What I noticed on the weekend was my alternator would read low when my heater fan was on leading me to believe it cannot sustain itself with loads on, I put the fan on high and the winshield wipers on high. This instantly kicked out the overdrive. I had to shut off the engine to get it to work again. I can get it to kick out every time when I do this. I am going to start by sending out the alternator for repairs and hope this will fix it. It may also be a weak overdrive solenoid cause it's the only solenoid affected by the low alt. output. Does this make any sense to anyone? Thanks, Doug

Yes, the tranny is electronically controlled and problems in the harness or power supply could give you trouble. Make sure all grounds are good, especially on the battery terminals.

I am going to join this club too. I noticed that if I have the lights on and use the power windows, my speedo jumps about 20 mph. Tonight I lost overdrive while I had the lights on. Restarted the truck and it seems fine. Also getting radio static.

Time for a new alternator? Can you replace the guts in these like the old GM ones?

- Shawn

If you have no codes... replace the 1-2 and 2-3 shift solenoids in the rear of the valve body. About $20 each or less. The solenoids available are updated fom the originals and address hydraulic leak of the solenoids causing loss of fourth. Fairly common problem. Otherwise follow diag for any codes stored.
:)