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Old 03-10-2011, 04:06 PM   #1
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2003 GMC Headlight Flicker

Hey - I'm just back from my shakedown run in a newly acquired 2003 GMC Sportsmobile, and I'm tracking down a few things that have cropped up. The first of which is that the headlights will flicker through a few different levels of brightness as you drive along at night. You get a new brightness about every 3-5 seconds - sometimes longer intervals.

I'm new to owning an Isolator but having read the post from Dave Boyer, I'm wondering if what I'm seeing is the isolator switching back and forth between the two batteries.

The charging system appears to be working - it starts and it's charging the house battery. The van has 73k, and the lifeline it came with was shot. I have a smaller 32 aH AGM in there now.

I'm open to any thoughts on what else it might be.

Thanks -


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Old 03-11-2011, 07:38 AM   #2
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Re: 2003 GMC Headlight Flicker

Isolator and Separator are two different things. The an isolator looks like a brick, separator looks like a small can and they function differently. Isolator might be in your engine compartment, separator is probably inside.

Either one can be disconnected to eliminate it as a problem. The separator also clicks, but I'm pretty sure a 2003 would have an isolator.

Also check your voltage on both sides, while running and not. The dash power ports should be on the van battery, and the ones in back on the house- one of those cheap plugin volt readers should tell you if the voltage in the system is fluctuating with the lights.

and then
everything changed
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Old 03-15-2011, 05:29 PM   #3
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Re: 2003 GMC Headlight Flicker

x2 what Jage posted. I did have a similar problem with my headlights on a pickup and it ended up being the headlight switch itself, but that was a 93 and I don't know how the newer versions work. It finally gave up and I lost my lights one night. If the lights dim out I would guess a bad connection or switch. If its easy to get to, check the starting battery connections...that's part of normal maintenance anyway. There is a post about problems with the isolators and a specific breaker. Don't know if thats the problem but if I find it I will post it.
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Old 07-20-2012, 07:37 AM   #4
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Re: 2003 GMC Headlight Flicker

Hey all;,

Wanted to thank you for the input. This one took me a LONG time to track down but in the end I got it.

This turned out to be a compound of a few issues. But the real source of the problem was the massive amount of corrosion the van has from it's lifetime in Florida.

The headlight flickering from a high and low intensity state went on to become spikes in voltage that you could see clearly on the dash battery gauge. That one reared it's head coming up a steep pass when the voltages spiked enough to reset the dash cluster - which also triggers the start-up chimes. I tell you what - being in the passing lane cookin' up a big hill and suddenly hearing alarms and a taking a quick look down to see your gauges dead is a bit of a shocker. Fortunately the van was still running and the steering and the brakes were still powered.

I didn't get it figured out until after I had to replaced the gauge cluster - the spikes fried it. I replaced the Isolator with the correct one from SMB and that helped some but it didn't stop the spikes altogether. BTW an OBD II reader works great as a guage cluster if you ever need one. We keep ours on board.

In doing a little research on the latest gen GMC alternator I learned that this unit senses the current load and actually varies the field inside the alternator accordingly. Pardon my junk explanation there... but it turns out the system is very sensitive to grounding. So I went back through and recleaned all of the grounds - including a relatively small one that bolts to the inner fender next to the battery. Also gave them all a good coating of dielectric grease.

In the end that got it. The headlights now run steady, the voltages sit at a fixed spot on the gauge and the cluster does not reset.

Oh and I also installed a plastic shield underneath the seam where the hood and fender meet. On the GMC the isolator is installed just below that opening and rain collects and pools between the terminals. I also filed a little notch into the top plate of the isolator so water can drain off it. SMB has since stopped installing them here. Now it stays dry when parked and somehow the area doesn't get wet when driving in the rain.

I suppose once I get ahead of all the other honey-do's I'll relocate the thing but for now it's all working great.
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