Engine/Transmission (1994 - 1998) Mystery switch problem

[SRehberg]

I decided to install the Mystery switch to use with the exhaust brake I have on my '96 auto, to lock the torque converter. A no-brainer right!? I cut the orange/black wire from the PCM and installed a three position switch. This was to provide "normal" operation, "forced lockup" and lastly, "no lockup". I provided ground to the switch directly from the battery. The wire from the PCM was mounted on one end of the switch, the wire to the trans in the middle, and the ground to the other end, thereby providing the above operation.



Patting myself on the back for a job well done, I test drove the truck. It is now in 3rd gear all the time, the OD switch doesn't operate and it won't lock up on its own or with the switch in different postions. I removed the switch, jumpered the PCM wires back together, still nothing. Tracing wiring, I found the transmission control relay wire from the PCM was not putting out a ground signal. For test purposes, I jumped a ground to the wire to see if this would bring things back to life. Nada. Nothing. All the fuses on the hot sides are good. I did the key-trouble code retrieve for giggles, and it came up with two codes: 12 & 37. The first being a battery disconnect in the last 50 key cycles, and the other says the trans temp sensor has not come up to temp. That was a new one on me. Now I'm wondering if the PCM is requiring some input that is keeping it from generating the signal it needs plus some other output, or the PCM has bit the big one. That one scares me, as I can imagine what the dealer gets for that thing. I just can't fathom what could have happened, it is so straight forward; anyone else experience this issue with this install?



Update note: I checked the temp sensor from the PCM harness connector and the resistance was 1030ohms at about 85 deg. Pretty close to what the book calls out for a hot engine, but not cold. So if it was the sensor out of spec, it should still tell the PCM it is within range. I wouldn't have thought it would have died just sitting there with the key off. I took the negative battery cables off for about 30min. seeing if the PCM needed a time out..... no help. I checked the wire at the connector that goes to the PCM to see if it might be loose in the connector... nope, just fine. The trans is basically in the "limp home" mode. Any ideas?

 

[fox]

The mystery switch wiring that you did only applies to the 47rh.

You have a 47 re. The computer senses that the wire is open so it goes to limp home mode. . You need to search for the other method. It has a pull up resistor - 1 k ohm - and some diodes.

It sounds like you also may have blown a fuse. It still should lock up manually. I think.

 

[JKouzez]

http://www.tstproducts.com/tranny_install.html



go here it will tell you how

 

[Joe G.]

The 47RE has a relay in the lockup circuit. You have to remove it and jumper two contacts in the relay socket. See the TST instructions.

 

[SRehberg]

I went to the TST site, and I had tried that method on another '96 I had; the computer would latch onto the ground someway and you could not release the lockup command. Soooo I thought I could break the wire, and force a release command if necessary. I had forgotten about the jumper, so that was my fault. I tried it with the relay in place. I have put everything back to the way it was prior to the problem, and I still have it big time. By playing with the ground on the output of the PCM I can't see where anything could be hurt. Does the PCM need reflashing or is this something that wouldn't help it? I noticed on the TST site, they stated some '96's needed reflashing for it to work. What goes awry that it needs to be reflashed?

 

[dalton4]

I read that the reflash was needed on early built 96 trucks. I put in a mystery switch and it would not work correctly. Had the reflash and it then worked. My 96 truck was an early built unit in Nov 1995.



A bad PCM or gov. pressure sensor in pan can also keep you in limp mode. This happened to me this past May. Do you know anyone with a truck in your year range that would have the same PCM. If so, maybe they will let you try their PCM.

 

[SRehberg]

dalton4, my truck was built in either Oct. or Nov. or '95, I don't remember which, so it could be the same issue you had. The PCM and the governer etc. were o. k. when I shut the truck off to install the switch, so I have to assume the governer didn't die. The PCM shouldn't have, but you never know. How did yours act after you installed the switch? Did it go into the limp home mode? I'm hoping just a reflash would do it if it involves the PCM, as I imagine they are expensive.

 

[SRehberg]

Well, I finally tossed in the towel, called the dealer and set up an appointment for diagnosis. He seemed to think it needed reflashing. Now the good part: started the truck up, and took off down the road towards the dealer... . it worked just like it should. Lockup, OD etc etc. Now I did all the troubleshooting in the driveway, in PARK with the key on/off. Did I make a major mistake having it in park or does it make any difference? If it doesn't make a difference, then I could possibly have a loose connection somewhere that will come back to haunt me. The manual isn't real clear if the trans control relay is dependent upon the gear position you're in. I thought you could engage/disengage OD while in Park. This is what scares me, an intermitent problem. Any experience along this line? I'm trying to learn because I want to take the truck to Canada in a couple of weeks and I don't need any surprises. Thanks everyone.

 

[dalton4]

SRehberg, I sent you a PM.