Lights dim intermittently at higher rpm - Ford Bronco Forum
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post #1 of 7 (permalink) Old 05-03-2018, 09:19 PM Thread Starter
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Lights dim intermittently at higher rpm

Hi there guys!

79
New alt, reg and batt

Drove it tonight and lights began to dim intermittently at 2000 rpm and higher, putting the clutch in and allowing it to return to idle would cause it to stop dimming.

All lights internal and headlamps dim. Headlamps are feeding through relays directly off of the battery so switch feed is not the culprit.

Alt light came on once for just a brief moment then went back out. It illuminates when switched to on so bulb is not burnt.

Most likely culprit? It sounds like a regulator but the alt light makes me wonder.

Thanks for your time!


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Last edited by schwim; 05-03-2018 at 09:45 PM.
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post #2 of 7 (permalink) Old 05-03-2018, 10:16 PM
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Are you grounded good?

Another thing a friend had trouble with, although not the same symptoms, was his dimmer switch, even though it was fairly new. But, I don't know if that would apply since you have a HD headlight wiring system.

Just thinking out loud, hope you can figure it out!
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post #3 of 7 (permalink) Old 05-03-2018, 10:32 PM
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Is there a way to put a volt meter on the alt and with headlights on have someone rev it and see what the alt is doing? If voltage is dropping im curious if it does it with the headlights off also


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post #4 of 7 (permalink) Old 05-03-2018, 11:00 PM
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My vote is on the Alternator, I had to deal with the electrical gremlin on my '04 Mustang when it was doing something similar and only when it felt like being a problem-child. Alternator fixed her right up.
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post #5 of 7 (permalink) Old 05-04-2018, 09:35 AM
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Yo MR SCHWIM,
V Belt, then,
Regulator based on this 79 EVTM via topsig (Ziggy)

See alternator/regulator wiring tests.

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post #6 of 7 (permalink) Old 05-04-2018, 11:55 AM
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Can you replicate the problem sitting in park in the driveway with the hood up?
I'd rev it and see if the belt is slipping, they can occasionally slip with no sound, its possible you can actually see the slip
I'd hook up a meter, rev it, and watch and see if the output to battery (or straight off alternator) drops
the regulator's job is to 'shunt' and excess electricity when its rev'd
If you hook up meter straight off alt, when the lights dim you will see alt output drop or possibly not
That should divide the problem in half right there

From there on, bad connection, bad regulator, so many possibilities
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post #7 of 7 (permalink) Old 05-04-2018, 01:13 PM Thread Starter
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Hi there guys,

I woke up this morning and put my voltmeter on the battery. During rev, voltage would go from 14.2 to 12.4-13.1, etc. I rooted around in the trash and grabbed the old regulator (since it's easiest to replace) and plugged it in. Voltage stayed at or above 14.2.

I went to Carquest and they gave me a replacement(BWD R400). I think I remember having a failure problem from new with this particular brand twice before. If I'm right, that's the third BWD regulator that has been a non-operable or faulty part from new.

I haven't installed the new BWD regulator yet. The old regulator has been through three alternators now so I want to swap it but I may see if I can get a FoMoCo replacement on Amazon or eBay.

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