My PCM doesn't like my transmission anymore?

[PToombs]

Hi all! I have a 97 with a fresh rebuilt 47RE in it. The converter clutch went bad with about 50k on it and plugged the trans filter and she was junk! I bought a rebuilt trans from a reputable local builder and put it in, and my trans was in limp mode. So, after digging around yesterday I discovered that the trans relay in the under hood fuse box wasn't coming on. Using the schematics in my FSM I found that power to activate the relay comes from the alternator and the ground is by the PCM. When I turn on the key it comes on for a second and then goes off. When the engine is started it comes on for about 5 seconds and then goes back off, so I have no voltage to the trans solenoids, etc.
The crank sensor is working, I have tach and AC, the alternator is charging 14 volts. I grounded the relay direct and the trans works just fine.

Does anyone have any idea why the PCM isn't keeping the relay on? I saw nothing in the schematic to tell me why it works like this. I want to make sure grounding the relay isn't going to burn something up.
Thanks!

 

[fest3er]

The PCM activates the trans relay by activating its own relay to ground; directly grounding the control side to activate the relay won't hurt it. If the PCM's grounds are OK, the engine-to-body grounds are OK and the connection between the trans relay and the PCM is solid (prolly less than 1 Ohm), your PCM may be getting weak. (Mine's been 'weak' for at least 10 years now; cruise control doesn't work and PCM control of A/C clutch was bypassed years ago.) Check the FSM to see *when* the PCM activates the trans relay; you might be able to fudge something.

 

[PToombs]

I hunted all thru the FSM and it didn't say anything about when it activates it. There was a decent explanation in the trans section what the relay does but it didn't say why or when it does it like that. Weird thing is it worked fine until the converter came apart, then the truck sat for 5-6 weeks while I worked on it, and then it acted up.
I'll check the grounds.

 

[Timd32]

The TCR will place the transmission into "3rd gear only" when a system fault is detected.

There are a bunch of codes associated with the transmission, some more important then others.

Could be a connector issue when your did your swap, or it could be another issue related to your failure you had.

I would pull the codes and go thru the list amd see whats up.

Its $40 for the ATSG manual, will save you alot of time and is super valuable to have on hand, but it does not go over all the codes and what actually disables the TCR. You already have your FSM to cross reference the codes listed in the ATSG book.

 

[PToombs]

Timd32, no codes at all . Only time I had a code pop up was when I removed the relay with the key on. I cleared it and haven't had any since.
The original failure was the lock up clutch in the converter disintegrated and plugged the filter, so it starved for oil.

 

[Ozymandias]

Just an idea, could there be a to high current draw on that circuit so the PCM shuts it down for its safety?
Possible??

 

[Timd32]

Timd32, no codes at all . Only time I had a code pop up was when I removed the relay with the key on. I cleared it and haven't had any since.
The original failure was the lock up clutch in the converter disintegrated and plugged the filter, so it starved for oil.

The ATSG book has some additional diagrams of the controls and relay pin power descriptions. I dont know if their PDF version is downloadable, or just a CD.. But it could just be a weak PCM and bypassing it would be a quick fix. Amazon also has the book for a few bucks cheaper.

 

[PToombs]

Update! After the initial issue it worked ok with the PCM bypassed, then it suddenly wouldn't go into overdrive. It was intermittent too, I would drive the 4 miles to the highway and it would be fine, I slow down for the onramp and then OD was gone for 6 mile highway portion of my trip to work. I dealt with that for a couple days and started digging, everything was fine, wires, solenoids, grounds for the PCM. So I added a switch in for the OD, just in case. I drove it, when OD didn't work, the switch didn't either, so I was thinking when it got hot the OD solenoid went bad. Wrong. I think it was just bad connections in the darn plug. I rechecked the plug going into the trans, and since then the darn thing has worked great.
Thanks for the help!