LMC Heavy-duty headlight harness... because I like complicating simple stuff

[The_Tomato]

Repairs and improvements to my '89 have been shelved for about 2 years, and I'm picking back up where I left off. Trying to cross off some of the simpler items from my to-do. So without further ado...

The LMC harness is simple enough and makes sense to me in theory. My problem is that the connector clip intended to attach to the old harness doesn't resemble a single connector that currently exists. I'm not sure if this is because of the year of my truck, seeing as it was produced alongside the gmt400 series trucks, but I'm stumped. If this doesn't make sense, I'll post pictures later.

 

[ZooMad75]

Repairs and improvements to my '89 have been shelved for about 2 years, and I'm picking back up where I left off. Trying to cross off some of the simpler items from my to-do. So without further ado...

The LMC harness is simple enough and makes sense to me in theory. My problem is that the connector clip intended to attach to the old harness doesn't resemble a single connector that currently exists. I'm not sure if this is because of the year of my truck, seeing as it was produced alongside the gmt400 series trucks, but I'm stumped. If this doesn't make sense, I'll post pictures later.

I'm guessing you have the 4 headlight system on your '89. If so that's why the plugs don't match.

LMC's relay harness kit for the 89-91 4 headlight system uses a plug at the headlights that only matches the upgraded clear "euro" light housings that use replaceable bulbs vs the stock sealed beams. It's in the description that it does not work with factory sealed beams. I missed it too and got annoyed when I figured it out.


So the fix is to change the plugs on the relay harness to match your bulbs or pony up another $120 for LMC's headlight assemblies.

I changed the plugs, because I wasn't going to spend that much more to make it work. Side note, I switched to the two headlight system in the process of installing mine. So the plug I got won't match.

 

[The_Tomato]

I'm guessing you have the 4 headlight system on your '89. If so that's why the plugs don't match.

LMC's relay harness kit for the 89-91 4 headlight system uses a plug at the headlights that only matches the upgraded clear "euro" light housings that use replaceable bulbs vs the stock sealed beams. It's in the description that it does not work with factory sealed beams. I missed it too and got annoyed when I figured it out.


So the fix is to change the plugs on the relay harness to match your bulbs or pony up another $120 for LMC's headlight assemblies.

I changed the plugs, because I wasn't going to spend that much more to make it work. Side note, I switched to the two headlight system in the process of installing mine. So the plug I got won't match.

Thanks for the reply! I did order the LMC headlights (which of course are backordered), but the problem I'm having is the connection from the new harness to the existing one. I think it's supposed to connect through the low-beam connection? I no longer have the terrible instructions having thrown the box away 2 years ago.

 

[ZooMad75]

Thanks for the reply! I did order the LMC headlights (which of course are backordered), but the problem I'm having is the connection from the new harness to the existing one. I think it's supposed to connect through the low-beam connection? I no longer have the terrible instructions having thrown the box away 2 years ago.

You are right. There should be two male connectors that would plug into the stock low and high beam sockets. I thought I had a pic of my setup priro to me modifying it for the two headlight setup with H4 connectors, but I don't.

Worst case scenario is just cutting the connectors off for the stock left side bulbs and wiring them directly to the relay harness.

I remember my LMC harness was wrapped up and wedged pretty tightly into the package. Took a bit to spread it out to see all the connections. I'd pull it out and spread it apart. The two connectors you need are going to have wires that go directly to the relays. It shouldn't take too much to backtrac from the relays to find the circuit you need.

 

[The_Tomato]

Here is the LMC diagram and instructions.

View attachment 372027

View attachment 372028

I'm hoping this means I'm not crazy, because here is a picture of every connection on my wiring harness and I'm not seeing a 3-prong connector.

 

[frankin5]

You have a picture of the whole kit. It appears that all is marked. i don't see any relays in what you have show. I can not read what is on the two female connectors.

 

[ZooMad75]

I'm hoping this means I'm not crazy, because here is a picture of every connection on my wiring harness and I'm not seeing a 3-prong connector.

View attachment 372034

View attachment 372035

View attachment 372036

So in the bottom pic, the blue and black connectors in the middle are the ones that should be connected to the stock headlight connectors. No, they don't match up with the stock light connectors. Also, the other black and blue connectors in the lower left (with the orange gaskets) are what would go to the headlamp assemblies. If you plan on using the stock bulbs, those will have to be swapped out for the stock connectors. You may notice that the female headlamp connectors on that harness may plug into the other male connectors on the harness that would connect to the stock light circuits (on signal). If that's true you should be able to get a pair of female 9005/6 connectors to replace the stock connectors in the factory harness to allow it to plug in correctly. You can pick them up on Amazon.

IF you want to use the stock headlight bulbs, you can change out the 9005/6 connectors for the stock connectors. Just pick up 4 fresh ones from Autozone. If you go that route you could reuse the 9005/6 connectors to install on the stock harness to allow it to plug into the relay harness.

 

[The_Tomato]

So in the bottom pic, the blue and black connectors in the middle are the ones that should be connected to the stock headlight connectors. No, they don't match up with the stock light connectors. Also, the other black and blue connectors in the lower left (with the orange gaskets) are what would go to the headlamp assemblies. If you plan on using the stock bulbs, those will have to be swapped out for the stock connectors. You may notice that the female headlamp connectors on that harness may plug into the other male connectors on the harness that would connect to the stock light circuits (on signal). If that's true you should be able to get a pair of female 9005/6 connectors to replace the stock connectors in the factory harness to allow it to plug in correctly. You can pick them up on Amazon.

IF you want to use the stock headlight bulbs, you can change out the 9005/6 connectors for the stock connectors. Just pick up 4 fresh ones from Autozone. If you go that route you could reuse the 9005/6 connectors to install on the stock harness to allow it to plug into the relay harness.

This more or less answers what I need to do. Is this harness put together wrong then? Shouldn't it have the 3-pin connector to go to the old harness? I ordered the one that LMC says fits my truck.

 

[The_Tomato]

Also, thank you so much, your answer was incredibly helpful. The truck is currently on jackstands while the axle goes for a rebuild so I now have something else I can be working on while it's undriveable.

 

[ZooMad75]

This more or less answers what I need to do. Is this harness put together wrong then? Shouldn't it have the 3-pin connector to go to the old harness? I ordered the one that LMC says fits my truck.

It's setup the way LMC wanted it. Even if they had the right connectors you would still need two so it would intercept the on signal for the high and low beam separately .