CUCV Headlight Circuit Explained

[Warthog]

The headlight circuit causes many problems for most CUCV users.

The GM design could use some improvements as the Headlight Fuse actually provides power for multiple circuits. We will cover those shortly.

Let's look at the headlights them self first.

Whenever troubleshooting any electrical issue, make sure that your batteries are fully charged and load tested. This is clearly stated on the first page of the Troubleshooting section of the Technical Manuals.

Also take time to read the first few pages of the Troubleshooting section of TM 9-2320-289-20 or the TM 9-2320-289-34 manual. It explains how to read the diagrams and what all the symbols mean and how the wires are labeled for tracing the circuits.

I have taken the wiring diagrams from the Appendix for the above manuals and saved then as a jpeg file. When I am working on a problem I open the picture and color the wire for the circuit I am working on.

Let's get started.

Here is the circuit path for the headlight circuit.

Pic1 - 12v is supplied from negative terminal of the rear battery via wire (8 RED-2A) to the ENG WRG HARNESS BLOCK. This is the diamond shaped 12v Terminal Block on the firewall near the Brake Master Cylinder. The circuit is protected by the Blue fusible link (3 BLU-2B).

Pic2 - The circuit travels thru the Red fusible link (1 RED-2F) and wire (3 RED-2J) to the Firewall Wiring Harness Connector. It travels thru the firewall and out the other side via wire (3 RED-2G)

Pic2a - By reading the instructions at the first of the Troubleshooting section it shows where to find the wires on the connector. This is the engine compartment side of the firewall connector

Pic2b - This is the under dash side of the firewall connector

Pic3 - Wire (3 RED 2G) travels to the fuse box to the 30amp Headlight Fuse. It enters the left side of the fuse holder.

***IMPORTANT NOTE ***

If you took the fuse box apart you would not find a wire on the other side of the fuse. Instead you will find a metal jumper that feed the Stoplight Fuse. This is not shown in any of the diagrams and confuses many people.

Pic4 - From the right side of the Stoplight fuse, but still protected by the headlight fuse ( are you confused yet???), the circuit travels thru the Orange wire (3 ORG-914) to the Military Headlight Service Switch on the left side of the dash. The Service switch was added to insure that the "enemy" couldn't see you if you accidently turned on the lights in the battlefield. Without the Service switch turned on many items in the electrical system are disabled. This is all covered in the TM 9-2320-289-10 Operations Manual.

Pic5 - When the Service Switch is engaged the circuit travels the Orange and Black wire (3 ORN/BLK 912A)

Pic6 - Continuing on to the Headlight switch. It attaches to Pin #1 of the switch. The headlight switch supplies power to headlights, tail lights, dash lights and the "dome" light under the dash.

*** IMPORTANT NOTE ***

You will also notice that there is a second orange and black wire connected to Pin #1. We will discuss this shortly

Continued.

For a bigger picture, Right Click on the thumbnail and chose "Open Link in New Window"

 

[Warthog]

Pic7 - When you pull out the headlight knob the internal connections engage and supplies power to Pin #6. Power travels thru the Yellow wire (1 YEL-10) over to the dimmer switch on the top of the steering column.

Pic8 - Depending on how the dimmer switch is positioned, power will flow to the low beams (.8 TAN-12) or high beams (1 LT GRN-11A)

Pic9 - Depending on the dimmer setting, power travels thru the Firewall connection out to the headlights. When the HIGH BEAMS are engaged, power is also provided to the High Beam Indictor on the instrument cluster.

Pic9a - dash side of firewall connector

Pic9b - Engine compartment side of the firewall connector.

Now lets talk about the additional loads that are on the Headlight fuse.

From Pic3 we see that the Headlight fuse is jumpered to the stoplight fuse

Pic10 - Power travels out the left side of the fuse via wire (.8 ORN-140)It come back to the Service Switch via wire (.8 WHT/BLK-913) and then to the turn signal switch via wire (.8 WHT-17A)

Pic11 - Shows the brake light switch and the turn signal switch.

Pic12 - From the NOTE on Pic6 the Orange/Black wire also supplies power to the wire (3 ORN/BLK-912B)

Pic13 - Following the wire (3 ORN/BLK-912B) we see that the Flasher Fuse and the Courtesy Light Fuse are supplied power. Believe it or not the Courtesy Light fuse also supplies power to the horn fuse.

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Thanks to Doghead for catching this. I had posted the horn connection, but didn't expound on it.

If the horn is having issues, It will put an additional load on the headlight fuse. If your headlight fuse is melting, check your horn.
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Wow I'm worn out!!!!!

So now you can see why the 30a Headlight fuse and socket will overheat and melt. It is important that all the fuse connections are clean and tight. A dirty or loose connection will increase the resistance and create additional heat.

That's all for today's lesson.

 

[Warthog]

This thread was not started to be a question and answer session.

People where posting totally unrelated information and questions.

If you have something that you think needs to be added or corrected, please send me a PM and I will add the info.

Thanks,
Warthog

Thread CLOSED

 

[Warthog]

reserved3

 

[twomorestrokes]

This OS a GREAT thread, quickly explaining the quirky lighting system. I lost all lights and horn out in the middle of nowhere last night with 50 miles to go. Logged onto the forum to look for emergency help and here it was!!! Many thanks!

 

[culverjoe]

Thanks for all the Info! And you're right, it is easy to see why the circuit gets overloaded. The second best way to overcome this (aside from periodically inspecting/cleaning connections) is to add relays to power both Low and High beams. By bypassing the switch and extra several feet of already undersized wire, this is also the singe best way to increase light output! Far superior to Silvania Silverstars, or other "30%" or "50%" brighter H6054s.

 

[Warthog]

As a followup:

If you want to upgrade your headlight wiring, you can build your own wiring harness with all the connectors and relays or you can go the easy route.

The easiest way to upgrade the headlight wiring is to buy the $30 wiring harness upgrade from LMCtruck.com (or other suppliers)

The LMC part number is 36-3580

http://www.lmctruck.com/icatalog/csb/full.aspx?Page=80

What it does is it draws power directly from the battery and uses the existing wiring and fuse to energize a relay only.

Easy to install. (simplified version)

Unplug the headlight plugs
Connect the plugs to the new harness
Connect new harness to headlights
Connect grounds
Mount relays and secure wiring
Connect fused power leads to front battery
Your done

The high amp load is now removed from the 25+ year old wiring, headlight switch and fusebox. It is moved to new wiring.

When I get time I'll try to draw up a wiring diagram for this upgrade.

 

[Warthog]

Here is my rough draft of the LMC harness. The picture is a LMC harness that has different ends than the CUCV, but gives you the idea of what you will get.

I was thinking about building some for sale, but my cost just for quality materials alone would be over $50. Not cost effective to build them one at a time and make them for sale. Now if you want to make one yourself, you can build one with heavier parts than the LMC version.

 

[Warthog]

For detailed instructions of the LMC harness installation TOP_PROP created this how-to:

http://www.steelsoldiers.com/showthread.php?124720-LMC-Truck-Head-light-circuit-mod