4wd indicator light wiring

Finally installing a lot of the wiring into my rebuild. When looking at the sub harness for the 4wd indicator light the white wire that runs from the tcase switch is cut. I'm not sure where it should go. I've dug through the wiring diagrams and it seems it should go to a 4wd indicator relay. Only problem is I have no idea what that looks like or if I even have it. My harness was pretty modified by the P.O.

Anyone have any idea what that relay looks like and where it is located?

my 4wd light ran down to a switch mounted on/in the transfer case.
i had to replace the switch, then the light worked.

good luck

Adding some info and bumping this post.

Here is the harness in question. The plug at the right end is what plugs into the transfer case switch. The light around the center of the harness is the illumination light on the shifter mounted in the cab. The left side is what has me confused.



Here is a close up of it. I know the green connector goes to the fuse block and the black connector goes to the ground block on the parking brake. The question is where does the white wire go? The wiring diagram in my previous post talks about a 4wd relay, but I cannot find any other references to it anywhere. Can this wire just go right to the gauge cluster connector to light the 4wd indicator?

I’m thinking so. Do you have continuity from either contact on the tc switch plug to the white wire in question?

Yeah the white wire is continuous from the switch connector. I'm curious how this 4wd relay is used though. It is also tied into the tcc lockup so that when in 4wd, there is no lockup.

Others may correct me Darien, but I've not heard of TCC being inhibited when vehicle is in 4wd, not seeing that in the schematic

The light green, circuit 384, is the connection between the 4wd relay and the tcc lockup.

I read about it here as well

Do you have a 700R4 trans? The relay looks like it has something to do with controlling converter lockup while in 4WD.

*Well, I see questions get answered before I even write them.

Actually the relay seems to connect to a modulator (ASM?), so perhaps it modifies shift points in 4WD.

I do have a 700R4. From everything I have read about it, everything that runs through that 4 slot connector to the trans is for convertor lockup only. At most, the connector will have 3 connections (depends on vehicle options how many are filled out). 2 of them are to "allow" lockup to occur if they have a voltage signal. The internal plumbing of the trans still dictates if lockup occurs though. So the brake switch and other connections just interrupt those voltage signals. This 4wd relay would also interrupt these signals if I'm reading the diagram right. I'm mostly looking for what this relay looks like and where it is located as I'm sure I don't have one. Just want to investigate it fully as I haven't seen any real documentation on it besides what I've brought up here. I've been digging into this and tcc lockup a lot recently haha.

On a side note, the third connection on the trans connector is actually a way for the transmission to send a signal to an EGR solenoid. Kind of a load sensing thing for emissions I think. If you look below, you can see an alternate version of the above schematic that has an additional wiring connection for this EGR solenoid on the left side.

Wanted to come back and wrap up this thread for any future searches.

The sub-harness for the 4wd system has 3 wires:

Grey is the power for the illumination light on the floor mounted shifter. It has a green connector and plugs into the bottom left slot of the fuse block. It is keyed and will only go in one slot. It draws power from the headlight switch and is only powered when the headlights are on. It only runs about half the total length of the harness.
Black is the ground wire for this harness. It has a black connector on one end that goes to the ground strap on/around the parking brake. This wire is actually two wires. It runs in one wire from the ground strap to the floor shifter light bulb, and then a second wire runs from that light bulb to the connector that plugs into the tcase.
White is in a way also a ground wire. The way this system works is that the switch in the tcase opens/closes the ground connection. When in 4wd, the switch is closed and passes the ground connection from the black wire to the white wire. This wire runs from the tcase connector back to the behind the dash area. I am unsure of what it's original length is.

The white wire being a switched ground ends up being exactly what is need to illuminate the indicator light in the gauge cluster. If you look at the circuit panel/board on the back of the gauge cluster and do some contact tracing you will discover that the 4wd indicator light always has keyed +12 volts to it. It just so happens that the wiring harness connector that plugs into the gauge cluster also has one empty spot in it and that spot correlates with the ground connection for the 4wd indicator light. It's like it was meant to be.

So in short, using a spare wiring harness I removed a pin from the gauge cluster connector and attached it to this white wire. I then added that pin to my gauge connector in the truck. And it works perfectly. Hope this helps someone out there.




In case you were wondering, this wiring joint is done mostly proper with one of those cool marine heat shrink butt connectors that has solder in it. I just taped it shoddily for more protection.

EvilGenius, I’m in need of your evil genius for my 86 K5.

I really appreciate your work on this thread. It helped me get my 4WD shift indicator light working properly with an American Autowire wiring harness. Ended up tapping into parking light wire on light switch.

Now that I’ve rewired about 90% of truck and have a different fuse block, I’m hoping you or anyone else can guide me through getting instrument cluster 4WD light working again.

Here’s some pics to show what I got:


New Fuse Block


Current 4WD shift harness


4WD relay plug/wires


4WD Indicator socket w white/brown wires


86 wiring schematic…I think…

Switch on transfer case appears to be connected. I tried a jumper wire on switch connector…nothing.

Any guidance would be much appreciated!

Looks like you have all the parts I was missing on my harness. I never did determine how the 4wd relay worked as I never ended up seeing one. From my investigation, it isn't required to get a functional dash light. Looking at your pictures you also have the factory 4wd indicator plug. I rigged mine up to work with a gauge cluster that had the spot filled with something else originally (ecu light maybe?). I just swapped the films around.

So some questions for you to help troubleshoot. Does the shifter light come on on the floor as it should with the headlight switch? If not I would investigate your ground connection. The dash light and the floor light are powered off different circuits but share the same ground. If one works and the other doesn't, look at the positive voltage side. If they are both out I would look at your ground first.

Assuming your gauge cluster is the same layout mine was, you can check for 12 volts on the plug that goes into the gauge pod as it should be hot all the time with key on I believe.


If you are ok ditching the 4wd relay wiring the light circuit itself is actually really simple. Worst case scenario you can rig it up to work pretty easily.

Have you used a meter to determine if the switch in the TC is working?

Thank you both for your replies!

So, I got up under truck again and decided to give jumper wire one more go on switch connector. It worked. Grounded out harness and triggered 4WD indicator lamp. Woohoo!

As for the floor shifter lamp, since I wired it to light switch (parking light wire), it works fine when I turn on lights.

How exactly do I test switch? You’re talking to an amateur knows-very-little about electricity. Am I testing continuity?

Also, based on this original thread and others I've seen, it appears a picture of the 4WD relay and location has been elusive. Below are two pictures of mine and where it's located. Hope this helps those still searching.

Yeah you can test the switch in the transfer case via continuity. It is just a ball switch that gets depressed by a big steel ring in the transfer case. If you put a lead on each terminal of the switch while it is in the transfer case and put it in 4 high/low you should see continuity. In 2 high and neutral there should be no continuity.