Headlights and Dash Lights go dim then bright then dim at highway speed

[GSP9]

Bought a Bronco about two months ago and finally got some items installed (roll cage, seat belts, floor shifter, etc). I have driven it around the neighborhood a few times during the day but finally got it registered and insured today and took it out before dark to get it up to highway speed to check lights and what not. Got it up to highway speed and noticed that the headlights and dash lights would go dim then brighten then dim again pretty rapidly. Not sure if the brake lights and side marker lights were doing it as well but I assume they are. I'm not sure the term flicker is correct or not as they wouldn't go completely out but pretty annoying none the less. I did some searches and ran across some posts that were happening at idle as opposed to my issue of highway speed or one component like just the headlights but not all the lights. Electrical issues are probably my weakest point but would like to try and get it figured out before it goes to get inspected the end of next week. Any suggestions on what to look for or where to start first? Thanks.

 

[Attac]

Check all of your ground points completely as well as wires for bare spots where they could have rubbed. Check your alternator belt for tension as at higher speeds it could be slipping
Chuck

 

[ugly74]

Checking grounds, or even adding extras grounds is good.
I'd be tempted to throw a voltage regulator at it

 

[DirtDonk]

Those are the three most common reasons for flickering or fluctuating.
Voltage regulator on the way out.
Alternator on the way out
Grounds deteriorating with age.

All three are common on Broncos. None of them are necessarily an automatic fail at an inspection if it's not too bad, but you're right to want to fix it no matter what.
And none of them are guaranteed to leave you stranded anytime soon either, but none of them are good for the morale and general good health of the truck.

But what year is the Bronco GSP? What general condition?
Two other things that do crop up with regard to condition of connections and wires is the '66 to '70 twin rectangular firewall connectors and the firewall mounted fuse panel.
Both take a beating and get rusty too. Cleaning all connections is a good practice with those, and in fact even later years with their glovebox mounted fuse panels can benefit from a good cleaning/scraping.

So at the very least, pull all the fuses and put them back in. This will at least tend to scrape a tiny bit of corrosion off that can cause issues. A little wire brushing goes a long way too.
Pulling the two connectors apart and simply putting them back together will also be a good practice with an older rig that you just bought.

If rust is an obvious issue everywhere, then there is one more connection that can benefit from a quick inspection. And that's the connector inline with the Black w/yellow wire adjacent to the ammeter. This is your main battery charge wire between the alternator, fuse panel and battery. If it's rusty it can cause all sorts of drama.

The pulsing indicates more towards the alternator/regulator still, but all of those little tests and cleanings will only benefit the Bronco and your knowledge of the systems it runs.

Good luck.

Paul

 

[GSP9]

Thanks for the reply's. It's a 1970 in fairly decent shape. Someone at some point has gone thru it before I got my hands on it but I'm just trying to get the last of the issues worked out.

I have some electrical cleaner so I'll spend some time tomorrow and pull some connectors and fuses and check the ground wires. Thanks

 

[GSP9]

Spent a little more time with it tonight and have some more info. I pulled the twin rectangular firewall connectors and they looked to be in good shape, cleaned them anyway and reinstalled. Quickly checked the fuse box under the dash and it appeared to be clean but will still pull them and clean them tomorrow. There is a ground cable running from the negative battery cable to the firewall and it looked good and solid. I also checked the alternator which appears to be newer but not certain of its exact age and the belt looked good with good tension.

So after checking those few things, I backed the rig outta the garage at dark and let it idle. The lights were dim as they typically are at idle but they did not pulse at all. The tach is fairly small but looked to be idling at about 600 rpm. I gave it light throttle and the lights quickly got brighter as they always do. When I hit about 900 rpm, the lights would start to pulse at a very even rate. Both the headlights and the dash lights. Once it got below 900 RPM, the lights would start to dim and the pulsing would go away. When I was driving it the other night, it didn't appear to have any rhythm to the pulsing but I'm pretty sure that was just the uneven terrain as I drove by. But after tonight, it has a very rhythmic pulsing action. The alternator gauge in the speedo cluster would also bounce back and forth between the "30's" in rhythm with all the lights. All other needles in the speedo cluster stayed nice and even.

I would assume from my limited electrical knowledge that it's not a ground issue just due to how rhythmic the pulsing was. I ordered a new voltage regulator so I think that will be my next step unless you guys can offer any insight. Any thoughts?

 

[Viperwolf1]

Could be the regulator or it could be the alternator itself. When you swap out the regulator disconnect the battery first and make sure the regulator has a good ground before hooking it back up.

 

[Mark]

Maybe my memory is faulty, but it seems I remember this being a common EB problem discussed way back even going to the email EBR-NET group emails in the 90's.

I had the problem and years ago I solved it by upgrading my headlight harness. RJM made the harness back then but there are a few aftermarket choices now.

http://www.wildhorses4x4.com/product/Illuminator_Headlight_Harness

http://shop.broncograveyard.com/Heavy-Duty-Headlight-Harness/productinfo/14831/

 

[GSP9]

Maybe my memory is faulty, but it seems I remember this being a common EB problem discussed way back even going to the email EBR-NET group emails in the 90's.

I had the problem and years ago I solved it by upgrading my headlight harness. RJM made the harness back then but there are a few aftermarket choices now.

http://www.wildhorses4x4.com/product/Illuminator_Headlight_Harness

http://shop.broncograveyard.com/Heavy-Duty-Headlight-Harness/productinfo/14831/

That's a good point.......because everything seems to be in order until the engine gets to a certain rpm which in turn means the headlights get to a certain level of brightness. I'm going to go ahead and replace the voltage regulator but that might be my next step after that. I'm not sure what bulbs are in the Bronco but if they are newer and the original harness can't handle the load, then at a certain point they start to flicker/pulse.

 

[B RON CO]

Hi, X2 on the headlight harness. The original headlight switch is overloaded, it will actually shut off the lights if it gets really overloaded and turn back on when it cools down. They were not designed for upgraded halogen headlights either. I don't think you need a regulator if you have the small electronic one, as long as you have 13.5 -14 volts at the battery (engine on). Check out the after market headlight harness and how it uses modern relays to juice up your headlights. It worked great on my 66

 

[nrramse]

Hi, X2 on the headlight harness. The original headlight switch is overloaded, it will actually shut off the lights if it gets really overloaded and turn back on when it cools down. They were not designed for upgraded halogen headlights either. I don't think you need a regulator if you have the small electronic one, as long as you have 13.5 -14 volts at the battery (engine on). Check out the after market headlight harness and how it uses modern relays to juice up your headlights. It worked great on my 66

This happened to me too, after I replaced a bunch of bulbs with LEDs. I installed a new headlight switch, to no avail. But those relays did the trick - I have excellent headlights now!

 

[GSP9]

Looks like a new voltage regulator did the trick. The Napa one that was in there looked fairly new but the one terminal was pretty corroded once I got it off. I didn't road test it tonight but did get the engine up to 2000 RPM and there was no more pulsing. So hopefully it puts that issue to bed. Thanks for the help.

 

[B RON CO]

Hi, You could probably clean the regulator and plug with a small wire brush. I use white grease to control corrosion. Save the regulator for an emergency. Think about a headlight harness upgrade. Good luck

 

[DirtDonk]

Hey, glad you got it licked on the first try! That's kind'a rare with some of these issues!%)

And I didn't mean to sound like I was trying to dissuade you from the headlight harness upgrade. It's still one of the best mods around for our old Fords. Just didn't think it would fix your particular issue.
But from reading what the others have said here, I'm re-thinking my skepticism.

Paul