Last night my 850T wouldn't start after refueling. I had lights, plenty of juice, and the car turned over strong, yet no spark and no warning lights -- with key in II pos. After jiggling all the connections, and checking the fuses, I found out that when I remove the ECU fuse, the warning lights returned and the fuel pump kicked in. But obviously, no spark. After sitting at the gas station for a while, I decided to check the TCU. So, after I removed it, I checked for warning lights and fuel pump. They engaged. So I tried to start it -- it started. I thought, the tranny won't engage, but I'll try anyway (desparate times, desparate measures) -- the tranny engaged. So, I drove home all while the TCU sat on the front passenger seat. It was in "limp home" mode, but I made it home. Question: DOES THIS MEAN THE PROBLEM IS THE TCU -- all along? Any potential for damage driving home with no TCU? Sorry for the long post, but it needed a thorough description...

 

I wouldn't feel great about running without the TCU. After all the TCU takes input from about 9 or so sources to decide when and how to shift. I wouldn't do any driving unless it's to get the problem resolved.

It sounds like the TCU is the issue, but without the wiring diagrams in front of me I really can't be 100% on it.

 

But did it start after you reinstalled the TCU?
If it did I would check all the wiring around the control modules.

If you plug the TCU in and it won't start again then most likely the TCU is bad.

 

ok -- here goes. Tech, I did what you said, only a day later. I put the TCU in, after sitting awhile, and the car started. So, I thought -- as you said, there could be some wires to check. But first a test drive. I drove it for 10 minutes -- enough to get the TCU warm, like the previous night when it died. It drove the same -- limp home mode. When I got home, shut it off and tried to start it again, and it was dead. Removed the TCU, and it started. Put it back in, wouldn't start. Removed it one more time, and it started. I installed and removed it 4 times -- each time the TCU was in, it wouldn't start. TCU?

 

Yes now I might say TCU.

You might want to try to find a wiring diagram first and look it over.The TCU's are pretty pricey.
It would be best to confirm 100% first.If you get a wiring diagram look between the ECM and the TCU and see what they share.It might only be that if the signal is missing then when in limp home mode it bypasses some sensors allowing it to start.

 

Tech, thanks for the tips, I'll try to locate a wiring diagram and look over it this weekend. As you said, the TCU's are pricey. I've got a quote from autocraft direct, and they'll rebuild mine for $350 -- doesn't seem too bad, considering. But I would sure like to find it's something less!

 

Keep us posted.

 

Any special procedure for replacing the TCU, once I get one?
Anything to reset?

Thanks

 

No not really just install and go.

If you want to you can disconnect the battery before you install it.

 

Ok, Thanks -- didn't know if I had to do something special, except prayer of course, which I'll do anyway!