In my experience the safest way is to isolate the light from the TC circuit by using a DPDT switch or relay or if necessary a 3PDT. I was concerned earlier about forgetting the TC is locked, the light is a good idea.

 

Youcould also use a regular switch and aseperateled and a 1k ohm resistor. This would totally isolate the 12v from the tc circuit. If you put the 12v on the switch and, while the switch is off, something shorts across the 12v terminal and the terminal going to the tc control wire you may fry your pcm.

Also if this does not work, I would leave it alone. I would not take my truck into the dealership and have them update the powertrain software. You could potentially leave there with a completely different animal.

 

Thanks for you replies. The other sites that I looked at also recommended the DPDT switch. I just haven't found a good wiring diagram or pictures to show how to wire the switch and the light. I want to get that all figured out before I start on the project. Even with the resistor and the LED light, I'm not sure how it all gets connected together. I'm not proficient in electronics, so I need to study up before getting too involved.

My truck does have the "PCM Update" sticker on it. I'm not sure what all was done with the upgrade, that happened before I bought the truck.

 

I don't remember where I saw it and I can't find the link,but another cool tranny mod I was thinking about at one time is to get the truck to start up with od off. Basically the od switch sends a ground pulse to the pcm that toggles od on and off. So you get a 12vrelay and:

Connect aground to one side of the relaycoil and the common of the normally open contact
Slice into the starter relay wire in the column harness (yellow according to the 2001 FSM) and connect it to the other side of the relay coil.
Splice into the od wire in the column harness (orange w/ white tracer-2001 FSM) and connect it to the other side of the normally open contact.

Test the starter relay wire. Thefsm has two wires listed. One yellow for start and one dark blue for start/run. I'm pretty sure it's the yellow wire.

This will send a ground pulse to the pcm when you start the truck, thereby turning od off.

 

I think your Hypertech PPIII overwrites the PCM update. Maybe someone else can confirm that.
It sounds like you only have a SPDT switch. If that is true then the DPDT or 3PDT switch you buyshould have a diagram showing how the light in the switch gets wired. If no diagram give me the brand and part# of the switch and I may be able to tell you which terminals to use.

 

Here is a good link for an explanation of switches: http://www.kpsec.freeuk.com/components/switch.htm

My assumption. If I want the ability to use operate a light, then I should go with a double pole. Two grounds into the switch and two wires out that would ground the light and the wire to the TC. Now I'm assuming the DP switches are internally insulated. I guess a short could occur internally in the switch, causing problems like Mopowar described.

Some of the instructions that I read talked using a douple throw switch (ON - Off - On). To do that, you need to cut the wire from the PCM to the tranny, and connect two wires that will run back to the switch. This gives you the option of having the TC locked, the tranny operating without the TC ever locking, or having the tranny operating normally.

I don't see the need in my case to ever run without the TC locking up. In that case I can install a single throw switch and just splice into the PCM to tranny wire, like the instructions on this page described: http://www.4x4wire.com/dodge/tech/tc_lockup/ So if I use the DPST switch, I just have to figure out how to wire the light to isolate the circuit.

 

You could use a relay to ground the tc control, and power the relay with the switch. This would allow you to hook the switch up as designed.It just occured to me that if the light in the switch is an led, then it will not work by hooking 12v into the ground terminal.
Also,I remember reading something when I was researching this about a guy thatused a relay, and somehow either wired it through his brake switch oran auxillary brake switch that he installed so that when he applied the brake it would disengage the tc.You may already have wiring in place to accomodate trailer brakes that could be used. I will see if I can find it. Some of the links that I had saved no longer work.

 

Yea, I figured that out also. I stopped by NAPA but they didn't have any DPST switches available. I like your idea of using the switch to power a relay. I'll have to study up on that. If I was providing current through the switch rather than enabling a ground, this would be a piece of cake. Well, it's a learning experience for me. I've definately increased my knowledge of electronics by researching this topic.

Another question though. I've learned a diode is basically a one-way valve for electricity. So an LED is simply a diode, or valve, that also gives off light? So if I wired an LED into the sytem, that would prevent a short from affecting the PCM? I'm still not quite sure how the phyiscal connections are made, but I'll get it figured out eventually.

Also, here is the link you were looking for on the OD switch: http://dodgeram.org/tech/mods/transm..._inverter.html

 

UPDATE: I wired a regular switch in today. I decided I wasn't going to worry about the light for the moment. Went for a test run, through the switch and immediately got a check engine light. I assume its the code described above. I'll check it tomorrow with the Hypertech.

The TC stayed locked in 3rd gear like it was supposed to. The only problem is the TC would not lock in 2nd gear. I dropped the shift lever down to hold it in 2nd and threw the switch, no change in the RPM's. So maybe this trick doesn't work with the 46RE tranny. I know the diesel guys do it, but they have a different transmission. So I'll check around some more. Hopefully someone can chime in. I wonder if it could also have to do with the year of my truck.

 

How did you wire the switch to the TC circuit?