Attention: TDR Forum Junkies 
It took about six months to solve this problem! 1994 Cummins Automatic Trans.
THE PROBLEM: Transmission would slip in and out of lockup. At first this was intermittent, but eventually it got to the point where it was happening all the time. In addition it would slip in and out of OD as well. Transmission shifting is controlled by at least 6 devices: TPS, Transmission Temperature Sensor, Brake Switch, Vehicle Speed Sensor, Engine Speed Sensor, and PCM, so isolating the problem was very difficult.
Normally, the inherently poor design of the TPS makes it the best candidate for such a problem, but when I replaced it, no change whatsoever. One day the trans. temp light came on. Ah ha! A check of the trans by hand revealed normal temperature. Also, the dip stick oil was hardly warm. Faulty sensor, right? Nope. No change at all when I replaced it.
Reluctantly I took the truck to the local dealer in Lenoir, NC. He said that they "adjusted" the TPS (which cannot be done except to alter the length of the throttle linkage as far I know). This did appear to "fix" the problem, but the price was I was limited to 60% throttle..... HHmmm, not a good solution. I then took the truck to a local garage where I know the owner is competent. Problem is he is so overwhelmed with work that the truck was there for 9 weeks!!! A Guinness World Record I would wager.
CLUES: The biggest clue that I saw and kept telling the mechanics was that the truck ran well... . until it reached NOT (normal operating temperature). A second big clue was that several (5 at least) diagnostic codes showed up, but there appeared to be no problems with the systems identified. A third big clue for me was that when the truck was cold voltage was a steady 13. 9. But when it started acting up, voltage fluctuated wildly from 12. 2 to 15. 5!
THE SOLUTION. So you probably have guessed the PCM. Right... . mostly. Replacing the PCM solved the problem... . except for one single instance on a 200 mile trip. In and out of lockup for a short period of time. My heart sank! Fortunately, another powerful clue appeared at the same time. She dropped out of cruise control! Ah ha. The brake switch. I hit the brake pedal a few times with my hand and cruise control came back and no more trans problems. Since I replaced the brake switch, no further problems. When you have 6 variables, it can be (and was) a nightmare. I guessed early on that the PCM "board" had "cracked," or something in the circuitry had cracked, so that it worked OK when cold, but when heated the "crack" affected the entire electronics. Something like that must have happened.
Thanks to Joe George and Howard Durand for giving their time to help point me in the right direction.
THE PROBLEM: Transmission would slip in and out of lockup. At first this was intermittent, but eventually it got to the point where it was happening all the time. In addition it would slip in and out of OD as well. Transmission shifting is controlled by at least 6 devices: TPS, Transmission Temperature Sensor, Brake Switch, Vehicle Speed Sensor, Engine Speed Sensor, and PCM, so isolating the problem was very difficult.
Normally, the inherently poor design of the TPS makes it the best candidate for such a problem, but when I replaced it, no change whatsoever. One day the trans. temp light came on. Ah ha! A check of the trans by hand revealed normal temperature. Also, the dip stick oil was hardly warm. Faulty sensor, right? Nope. No change at all when I replaced it.
Reluctantly I took the truck to the local dealer in Lenoir, NC. He said that they "adjusted" the TPS (which cannot be done except to alter the length of the throttle linkage as far I know). This did appear to "fix" the problem, but the price was I was limited to 60% throttle..... HHmmm, not a good solution. I then took the truck to a local garage where I know the owner is competent. Problem is he is so overwhelmed with work that the truck was there for 9 weeks!!! A Guinness World Record I would wager.
CLUES: The biggest clue that I saw and kept telling the mechanics was that the truck ran well... . until it reached NOT (normal operating temperature). A second big clue was that several (5 at least) diagnostic codes showed up, but there appeared to be no problems with the systems identified. A third big clue for me was that when the truck was cold voltage was a steady 13. 9. But when it started acting up, voltage fluctuated wildly from 12. 2 to 15. 5!
THE SOLUTION. So you probably have guessed the PCM. Right... . mostly. Replacing the PCM solved the problem... . except for one single instance on a 200 mile trip. In and out of lockup for a short period of time. My heart sank! Fortunately, another powerful clue appeared at the same time. She dropped out of cruise control! Ah ha. The brake switch. I hit the brake pedal a few times with my hand and cruise control came back and no more trans problems. Since I replaced the brake switch, no further problems. When you have 6 variables, it can be (and was) a nightmare. I guessed early on that the PCM "board" had "cracked," or something in the circuitry had cracked, so that it worked OK when cold, but when heated the "crack" affected the entire electronics. Something like that must have happened.
Thanks to Joe George and Howard Durand for giving their time to help point me in the right direction.
