twavor

Hey all,

New here, but in a jam also...got a '98 Ram 3/4t 4x4 with the 24valve and an automatic OD in the shop. It came in for no charging, well, the alternator was bad(bad stator), and I find out that the PCM is also not giving signals to the alternator OR that I had bad wiring from the PCM to the alternator....well, I searched on here aplenty and figure that converting over to the old style regulator is quicker, easier, cheaper, and more reliable....so I convert it over....works great...charging 14.8volts tops. BUT now the transmission is starting out in a gear higher that 1st and won't go into OD. I have the codes P1594, P0753, and P1765. I've been through every ground I can find...just seems funny that the problem didn't show up until I disconnected the factory wiring from the alt and wired up the external voltage regulator.
What did I do wrong?
I'm still searching for answers on here, and I appreciate how much this site helps everyone out...I'm sorry for the quick first post, but I'll be reporting back. Thanks so much for any help!

Travis

edcasey

If you look at the wiring diagram below, you will see that the wire from the PCM to the alternator also closes the transmission relay. This is how the PCM controls the transmission and puts it into limp mode if a problem is detected. Without power to this wire, you will get the codes you have and the transmission will be "stuck" in third gear. If there is no output from the PCM on this wire, you will need to replace the PCM to close the transmission relay. You could try rewiring the relay to another ignition source. This would close the relay and the transmission will work normally (I've done this as a temporary repair when my truck went into limp mode) but you might still get a 1765 and 1594.

This wiring diagram is for a 2001 but I know on my 2000 the 85 terminal and 86 terminal on the relay are backwards from the wiring diagram. I'm not sure if the diagram is wrong or if they switched the wiring in 2001.

twavor

edcasey,

Thank You so much for the reply! I busted my butt today tracking this down....just finished it up and got it out the door a 6:20pm....kinda' kicked my butt, but got it going like you said. Between the customers coming in and tires needing fixed, and taking care of everyone else, it finally all came together. I finally tracked the wires down, and lit up the trans relay from a switched voltage source and got things going.
The biggest reason I didn't just put a new PCM in it is because the the truck has over 255,000 miles on it...It still looks and drives great, but the guy didn't want to put a lot of cash in it.
I am still getting the P1594 for high voltage, but it doesn't light up the CEL, and the charging system is working great with the old style regulator. I can't see any other problems that it would be causing. I probably should get a service manual for this gen of Dodges, but there are so few around my area that I just haven't yet.
Anyways....like I said, things are working great...and thanks for the reply!

Travis

P.S. forgot to add...he never said a thing about it losing OD when he lost charging...maybe I need to look at the diagram again, but I think that if the PCM would not light up the charging circuits that it would not light up the trans relay either....maybe I'm wrong.???

twavor

Alright....came back later and looked at the diagram....the alternator(generator) is what grounds the Transmission control relay.....bad alternator=no transmission control relay....but, bad PCM also equals no way to operate the alternator or Transmission. Looks like a good place to put a couple of diodes? I shoulda' caught that in the first place. I guess fighting a faulty alternator and PCM kinda put my head up my ****. Thanks for the schematic!

Travis

edcasey

Glad to see you got it fixed! I wish I had seen your post yesterday. I spent quite a bit of time earlier this year tracking down a P0753 with my transmission stuck in limp mode. Basically the same circuit you were working on.

It's nice information to know that providing a different switched voltage source other than the PCM to the transmission relay wont throw a P1765. When I temporarily jumped mine I still had the P1765 but it was a stored code and at the time I had no way to clear it. If my PCM voltage regulator ever fails, I will definitely add an external regulator.