On my 96' Cummins I have a Fisher Minute Mount plow (all hydraulic). Actually, it's more of a Frankenstein with an older speedcast A-frame cut and welded to fit the Minute Mount frame and a wiring harness that looks like it was fabricated by the owner at the time.

Ever since I've owned this truck there's been a problem with the headlights. When they are connected to the plow harness, the passenger headlamp is very dim. When I disconnect the plow harness and plug the truck harness directly in on that side, the headlight functions normally. I tried replacing the bulbs twice with halogens to no-avail and eventually just left the harness disconnected.

With plow season approaching and me wanting to have the plow harness connected, I decided to try relamping with LEDs (thinking with the lower power draw they might just function normally without tweaking a thing). Now I have another issue I'm trying to diagnose:

• In low beam when the truck first starts/lights are first turned on, both lights display low beam as expected.
• In high beam both lights display high beam, as expected.
• When returning to low beam, the driver's side remains in high beam and will not return to low beam. The passenger side behaves as expected.
• When a halogen bulb is swapped back in on the driver's side, it behaves as normal.

Any thoughts on why I can't get my driver's side to return to low beam?

 

How is it wired?

I would take a long, hard look at all the grounds for the headlight system. I would also make sure the headlight switch is ONLY operating relays, and not drawing all the power for the headlights.

 

I didn't see any relay where I was expecting one in the relay box next to the battery. My understanding is that in a 1996, there are no headlight relays. My first thought was that the high beam relay was failing, but it doesn't seem there is one.

When you say headlight switch, you mean the pull/twist headlight and dimmer control in the dashboard? Perhaps the issue could be there?

I think I've decided to try this to fix it:

1. Swap LED Bulbs from one side to another to see if it follows the bulb.
2. Swap Ballasts from one side to another to see if it follows the ballast.
3. Try connecting to the truck harness bypassing the plow harness.

 

Something is being grounded to your truck frame when you connect up the plow. You must have positive that is nicked or otherwise shorted yo the plow ground.

 

Re: the led's...
When they are acting up that way, I bet your plow is hooked to the truck. Disconnect it from the truck but leave the harness connected, I bet it works like it should with the led's (just like normal bulbs).

 

Yeah, switch in the dash. The are a rather notorious weak point in the system stock. Add the running lights from the heavier duty trucks, and then plow lights...... I can see how that would exacerbate the problem.

There aren't any relays stock, unless you live somewhere that DRLs were a requirement in 96...... which I suspect you don't. Check out LMC truck for a headlight relay harness. It takes the load off the switch, and may just solve your problem.

 

All of the above symptoms occur while the plow is disconnected from the truck. The plow headlights work just fine (and always have). The dim light only on the truck headlight occurs when connected to the plow harness and not when it is connected directly to the truck harness. My guess is a failed solder joint or ground with the plow harness, but a royal pain to find that.

I've actually isolated the issue to one of the LED ballasts with the new headlights. It's a real shame because the new headlight bulbs are very well constructed and very high quality. Hopefully the seller will replace the ballast.

 

Sounds like a grounding issue.