Hi all, Just picked up another Cummins diesel wrapped in a '94 Dodge 2500 2wd truck with auto trans. Trans shifts fine but the torque convertor does not stay locked up - except when you are going up hill.....get to the top of the hill and it starts locking/un locking. This is not the overdrive, but the convertor. It would seem like a sensor or positioner or something is out of spec or adjustment. I've only put 60 miles on it. On a flat road it locks then unlocks, back and forth, whether you do 55 or 75. Any suggestions? Thanks
While Gary's right that a scanner is the best way, if you don't have access to one, you can do something to determine whether the unlocking is being commanded by the PCM, or if it's a problem with the torque converter or valve body. Install a temporary switch in the cab. Connect one side to a ground somewhere. Run the other side of the switch to the PCM, and tap the orange/black wire, coming from pin #54. Don't cut the wire - use some type of temporary tap (you'll remove this and seal the tap with a little liquid electrical tape or RTV when you're finished).
Now, with that done, make SURE the switch is in the off position, and start the truck. Get it up to highway speed, where it's locking and unlocking. Throw the switch. If the converter locks and stays locked, you have an electrical problem. If it still unlocks, you have a transmission problem. Remove the switch, the temporary wiring, and seal up the hole(s) in the wire insulation as mentioned above.
If it's electrical, two common problems are a bad Throttle Position Sensor, or a bad brake light switch. To check the TPS, back-probe the middle connector, measuring 0-5VDC. An analog is actually preferable here, becuase (with the ignition on) you want to slowly move the throttle pedal through it's range, while observing the meter. You should see a nice, steady sweep in sync with the pedal. If it's jittering, or if you find a spot where it goes either open (0V) or dead short (5V), that's your TC problem, and you need a new TPS.
On the brake light switch (mounted in front of the brake pedal arm - give it a twist to remove it from the bracket). There are six pins, with #1 being furthest from the plunger. Pull the plunger all the way out. Using an ohmmeter, check for continuity on pins #1 and #2. At this point, you should read no continuity. Push the plunger in, slowly, and at some point, you should get continuity. Continue depressing the plunger all the way. Once you first read continuity, you should never lose it. If this behavior isn't observed, your brake light switch is probably shot.
To re-install either this switch, or the new one, pull the plunger all the way out, then push it in 4 clicks. Push the brake pedal down, insert the switch into the bracket (at an angle), and rotate it into position. Then release the brake pedal, and pull up on it gently, to set the final position of the plunger.
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