Electrical Question for Dim Headlights - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Johns 66 · 09-13-2021, 08:05 PM

Hello Peeps,

Got a question and not sure where to turn.

My headlights are dim, when I rev the motor they get brighter, at idle they go back dim.

I thought it may be a grounding issue so I changed the battery ground and added a better ground to the headlights.

so Im clueless at this point....

Im sure its an easy fix except Im not that knowledgeable about it.

I appreciate it,
John

lutronjim · 09-13-2021, 09:10 PM

Check battery voltage at idle - sounds like alternator is not putting out at idle.

Johns 66 · 09-13-2021, 09:34 PM

Quote:

Originally Posted by lutronjim

Check battery voltage at idle - sounds like alternator is not putting out at idle.

Will.defintely try that...Thanks so much

**bhap** · 09-13-2021, 10:03 PM

You may want to install a couple of relays up near the voltage regulator / horn relay. Taking power directly from the alternator/generator to the relay will eliminate the long run to the headlight switch and the high beam switch. Do a search for "headlight relay" on this forum - you should find several wiring diagrams for the relays. It made a big difference with my headlights.

**cwcarpenter98** · 09-14-2021, 06:57 AM

I remember having that same problem on my truck. After I put the headlight relay harness on, I think that issue went away, but I'm not remembering for certain if that's the case. Since I'm not remembering correctly, maybe that means the relay harness fixed that for me.

If your truck doesn't have a relay harness, definitely add one. They aren't too expensive, easy to install, and well worth the money. The stock headlights in my truck got way brighter with the relays. One of them burned out a couple weeks later, but then I was able to swap to some halogen bulbs which were even better. At the moment, my c10 throws better light than my 95 Dakota. Apparently, Dodge didn't start using headlight relays until some time after 1995, so that'll have to get fixed at some point too

Bigdav160 · 09-14-2021, 09:01 AM

Too many places to lose voltage on the power side of the circuit. Relays should solve that problem

**The Rocknrod** · 09-14-2021, 10:38 AM

For your year truck here is a great thread with great ideas on how to do it along with many links on the subject. http://67-72chevytrucks.com/vboard/s...d.php?t=397695

Johns 66 · 09-14-2021, 10:04 PM

Quote:

Originally Posted by The Rocknrod

For your year truck here is a great thread with great ideas on how to do it along with many links on the subject. http://67-72chevytrucks.com/vboard/s...d.php?t=397695

Thanks for the info guys it is greatly appreciated....I will be re-wiring the entire truck in the coming weeks and the relay seems to be the key to headlight function...I think Andy used these on his truck as well so I will definitely be adding these to mine.....brighter headlights are definitely a plus

Dead Parrot · 09-15-2021, 12:02 AM

My 75 had that problem. Switching from the 10SI Alternator to a 12SI fixed it. The 12SI puts out about 7 amps more at idle then the 10SI.

If you are still running an externally regulated alternator, swapping to an internal regulated when you do the re-wire might be a good idea. BTW, the internally regulated 10SI and 12SI both use the same connectors.

**VetteVet** · 09-15-2021, 11:05 AM

check my FAQS in the threads at the top of the electrical forum page.

**Wrenchbender Ret** · 09-15-2021, 11:16 AM

You have some real good replies here. Also check the battery. A bat. getting weak can cause that also.
George

09-13-2021, 08:05 PM	#1
Johns 66 Registered User Join Date: Apr 2017 Location: Talladega Alabama Posts: 860	Electrical Question for Dim Headlights Hello Peeps, Got a question and not sure where to turn. My headlights are dim, when I rev the motor they get brighter, at idle they go back dim. I thought it may be a grounding issue so I changed the battery ground and added a better ground to the headlights. so Im clueless at this point.... Im sure its an easy fix except Im not that knowledgeable about it. I appreciate it, John __________________ Former Decorated Combat Marine...Semper Fi!! Instagram jorourke322 Build Thread http://67-72chevytrucks.com/vboard/s...d.php?t=734699 2019 GMC AT4

09-13-2021, 09:10 PM	#2
lutronjim Registered User Join Date: Dec 2013 Location: Dunwoody,Ga Posts: 443	Re: Electrical Question for Dim Headlights Check battery voltage at idle - sounds like alternator is not putting out at idle.

09-13-2021, 10:03 PM	#4
bhap Senior Member Join Date: Mar 2006 Location: Pittsburgh, PA Posts: 240	Re: Electrical Question for Dim Headlights You may want to install a couple of relays up near the voltage regulator / horn relay. Taking power directly from the alternator/generator to the relay will eliminate the long run to the headlight switch and the high beam switch. Do a search for "headlight relay" on this forum - you should find several wiring diagrams for the relays. It made a big difference with my headlights.

09-14-2021, 06:57 AM	#5
cwcarpenter98 Senior Member Join Date: Jan 2018 Location: Hickory Flat, GA Posts: 4,421	Re: Electrical Question for Dim Headlights I remember having that same problem on my truck. After I put the headlight relay harness on, I think that issue went away, but I'm not remembering for certain if that's the case. Since I'm not remembering correctly, maybe that means the relay harness fixed that for me. If your truck doesn't have a relay harness, definitely add one. They aren't too expensive, easy to install, and well worth the money. The stock headlights in my truck got way brighter with the relays. One of them burned out a couple weeks later, but then I was able to swap to some halogen bulbs which were even better. At the moment, my c10 throws better light than my 95 Dakota. Apparently, Dodge didn't start using headlight relays until some time after 1995, so that'll have to get fixed at some point too __________________ Christian Carpenter 1963 C10 - Frankentruck 283, 3 speed with overdrive Overdrive wiring here 1963-ish truck bed trailer - Half-Wit 1981 C10 - Penny 305, th350 1995 Dodge Dakota Sport "I'll put it simple: if you're going hard enough left, you'll find yourself turning right." - Doc Hudson

09-14-2021, 09:01 AM	#6
Bigdav160 Registered User Join Date: Apr 2011 Location: Klein Texas Posts: 3,852	Re: Electrical Question for Dim Headlights Too many places to lose voltage on the power side of the circuit. Relays should solve that problem __________________ My Classics: '72 K20 Suburban + '65 Dodge Town Wagon '72 Corvette Roadster +'67 Corvette Roadster '73 Z-28 Camaro '63 Ford SWB Uni Pickup '50 Ford Coupe

09-14-2021, 10:38 AM	#7
The Rocknrod Moderator Join Date: Aug 2011 Location: China Spring, TX Posts: 7,280	Re: Electrical Question for Dim Headlights For your year truck here is a great thread with great ideas on how to do it along with many links on the subject. http://67-72chevytrucks.com/vboard/s...d.php?t=397695 __________________ Master Chief, US Navy, Retired 1964 C10 Step-side Short Bed - In Progress 2009 2500HD Duramax LTZ 1948 GMC Panel 1956 Big Window Stepside 1953 5 Window Chevy Stepside 1932 Coupe New Seat Foam for Bench Seats?

09-15-2021, 12:02 AM	#9
Dead Parrot Registered User Join Date: Aug 2012 Location: Oklahoma City, OK Posts: 2,472	Re: Electrical Question for Dim Headlights My 75 had that problem. Switching from the 10SI Alternator to a 12SI fixed it. The 12SI puts out about 7 amps more at idle then the 10SI. If you are still running an externally regulated alternator, swapping to an internal regulated when you do the re-wire might be a good idea. BTW, the internally regulated 10SI and 12SI both use the same connectors.

09-15-2021, 11:05 AM	#10
VetteVet Msgt USAF Ret Join Date: Jan 2005 Location: Kalamazoo, Michigan Posts: 8,703	Re: Electrical Question for Dim Headlights check my FAQS in the threads at the top of the electrical forum page. __________________ VetteVet metallic green 67 stepside 74 corvette convertible 1965 Harley sportster 1995 Harley wide glide Growing old is hell, but it beats the alternative.

09-15-2021, 11:16 AM	#11
Wrenchbender Ret Registered User Join Date: Oct 2005 Location: Overland Park, Ks. Posts: 5,190	Re: Electrical Question for Dim Headlights You have some real good replies here. Also check the battery. A bat. getting weak can cause that also. George



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