I did the TC lock mod, and of course the OD light is flashing. Now I thought of running a diode in the purple/yellow wire that goes to the trans, and hooking my ground in between the diode and the trans. But, I wanted to be sure which way the diode went, so I rigged up a test, with some 12V light bulbs. One bulb would represent if the PCM saw the lockup, and the other represented if the TC got the signal to lock. Here's how my setup is:

When I closed the switch for the TC lock mod, obviously bulb 2 comes on. And when, I ground the PCM side both bulb 1 and 2 come on. Now, here is where the strange thing happened. If I do both, ground the PCM side (both bulbs lit), and then close the switch which would ground the TC lock mod, bulb 1 (PCM) goes out and only bulb 2 (TC) remains lit. So, it's as if the PCM still loses signal when you lock the TC, even with a diode inline. I've read of someone using a Relay too, but it I believe guzzle tried it and still had an OD light flashing. Once the connection was broken between the TC and PCM using the relay, and the PCM tried to lock up the TC, it got no signal, so the OD light flashed thinking something was wrong with the transmission.

Any ideas on why this is happening. Perhaps a different diode should be used? I used a Radio Shack 276-1141 diode. I'm baffeled. I want to figure out a way to ground the TC without the PCM knowing it, but still let the PCM lock the TC on it's own by not breaking the connection between PCM and TC.

 

You are having the same results that I did alright! The only way I was able to eliminate the flashing OD light was to ditch the diode, use a relay and feed the PCM a false 12v load when locking up the T/C. By making the PCM believe that it still had control over the trasmission when locked, it went away and has never given me a problem since.

 

Can I get some more details on that relay you used to fool the trans and how you wired it up? Also, when the switch is off, does the PMC still has control over the TC?

 

Yes, when the switch is off, the relay is no longer energized so the circuit reverts back to stock configuration, only the led that monitors the lockup condition remains in the circuit. You'll need another DPDT relay though....
I have a schematic in my gallery but if it's too small to read, PM me and I can send you a larger version if you like.

 

Guzzle, got the diagram. Thanks for sending it.

Now my head's been spinning again figuring all this out. But, I think I've got it: Use the DPDT, center off switch that I already have to energize the DPDT relay that I just bought. One side will be manual, the other side still energized by the brake light, thus automatic. It will function exactly like I have now, but without that pesky OD light.

Last night I kept thinking to myself, "There has got to be a way to make this work, dagnabbit!!" I just never thought to feed the PCM a false load when the TC was locked. I'd have likely not figured this out on my own, and I've learned more about DC wiring in the past 3 days than I have in the previous 26 years. This forum is a great place.

 

The reason that happens is the route through the switch provides a lower resistance to the flow of electricity then through the diode. When power passes through a diode the voltage is dropped, sometimes up to 1 volt or more. Placing a similar diode between the switch and the ground may solve the problem, but they would have to be very close (no two diodes, or any electronic componet are exactly the same). Electricity is lazy and it always goes for the path of least resistance.

 

Yeah, that whole "path of least resistance" thing went through my head a couple of times. Although, I didn't think about putting a diode inline to the swithc too, so to try and even things out. I'm pretty sure the flashing OD light is solved with the relay, thanks to help from guzzle. But, now I know why bulb 1 would go out in my setup, and I thank you for that.