• 2001 GMC C3500
  • 8.1L
  • V8
  • 4WD
  • 160,000 MILES
I was following a you tube video on how to cross the low and high beam so they both would work at the same time, but my project did not work now I have low beam when switched to high they go out, but my problem started several weeks before when they started flickering. At first I thought it was lose lens I replaced the bulbs, no change. Any advice would be appreciated.
Do you
have the same problem?
Thursday, May 26th, 2016 AT 8:02 PM

1 Reply

If both lights flicker at the same time, it is typically due to arced and pitted contacts inside the headlight switch or the dimmer switch. On a lot of newer vehicles the lights are switched on indirectly through relays. Check for those first in the under-hood fuse box. I'll have to dig up a wiring diagram to see what your circuit looks like. If you have low and high-beam relays, switch them, then observe if they still flicker on low beams. If they flicker on the high beams now, suspect that relay. If the symptoms haven't changed, or if you don't have relays, the best suspect is the headlight switch. They have auto-resetting circuit breakers built in that cause more trouble than they prevent.

Also look at the terminals on the switches for signs of two of them overheating. They'll be black and the body of the connector will be melted in that area. I can share how to repair that rather inexpensively.

As for combining the lights so they always work together, you can't do that because you'll be blinding oncoming drivers with your high beams. Also, you'll be doubling the amount of current going through the switches, and the contacts won't take that for very long. In fact, that circuit breaker in the head light switch will trip and reset repeatedly. If you have GM's famous daytime running lights, that adds another layer of complexity and what you're trying to do won't work. If your headlights aren't involved with the daytime running lights, like they were on older trucks, the way to do this is with a relay controlled by the high beam circuit. When the low beams are turned on, that's all that will be on. When the high beams are turned on, they will work like normal, and the relay will turn on which will turn on the low beams. The engineers could have built their daytime running lamp circuit to work the same way with a cheap ten-dollar relay, but they chose to incorporate a computer module to pulse the high beams on 80 percent of the time so they could shine them into peoples' rear-view mirrors. In case you can't tell from my sarcasm, I'm not a fan of computers or distracting head lights.

Be aware that if you do this modification, you could leave yourself open to a lawsuit in a roundabout way. Just like with the high intensity discharge, (HID), head lights, which in my opinion belong on a pole in a parking lot, you will be increasing the illumination close to the front of the truck while you're relying on the high beams to illuminate obstacles far away. People get the false sense of seeing better because things are brighter, but our eyes self-adjust for that increased brightness. A lawyer or insurance investigator could make the case that you had reduced visibility because things close to you were too bright. I don't know how that argument would hold up, but they are real good at shifting the blame to the person who was not at fault for the crash when they find any kind of modification. (This is a real big issue with altered ride height, meaning lowered cars and raised trucks).

Now, to shift gears entirely, GM has also had a real big problem with their generators since the '87 model year. Due to their design, they generate huge voltage spikes that can damage the internal diodes and voltage regulator, and interfere with computer sensor signals. The battery is the key component in damping and absorbing those spikes, but as they age and the lead flakes off the plates, they lose their ability to do that. Your generator could be the cause of the flickering lights too, especially if you see lights flickering that don't involve the head light switch. The first step is to have the charging system professionally tested, (generator always on the engine, never carried into a store). Of particular importance is full-load output current and "ripple" voltage. Without looking it up, as a guess, your generator is likely rated at least at 90 amps. With one failed diode of the six, you'll only be able to get exactly one third of the rated current. 30 amps is not enough to run the entire electrical system under all conditions. The battery will have to make up the difference until it possibly runs down over days or weeks. Ripple voltage will be high too with a failed diode.

If the generator does need to be replaced for low or no output, always replace the battery at the same time, unless it is less than about two years old. Failure to replace it will usually result in numerous repeat generator failures. This is why you'll often hear GM owners complain of going through four to six replacements in the life of the vehicle. The old battery will still crank the engine just fine, but it's not damping those pesky voltage spikes.
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Thursday, May 26th, 2016 AT 8:44 PM

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