dim headlights - Chevelle Tech
Electrical & Wiring Troubleshooting electrical problems.

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post #1 of 14 (permalink) Old Mar 6th, 19, 1:48 PM Thread Starter
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dim headlights

What is causing dim headlights. New battery and new alternator and nothing has changed. It is on a 1972 Chevelle
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post #2 of 14 (permalink) Old Mar 6th, 19, 2:07 PM
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Re: dim headlights

Voltage regulator on the firewall may be bad if your not running stereo amps or any other items that draw massive current.

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post #3 of 14 (permalink) Old Mar 6th, 19, 3:21 PM
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Re: dim headlights

Do you own a volt/ohm meter? If not, go buy a cheapy and we'll run you through some tests.

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An electrical test light comes in handy for other auto electrical diagnosis, but don't need one for your issue and if you know how to use a meter all the test light can do is sometimes speed up the diagnosis.https://smile.amazon.com/s?k=auto+te...b_sb_ss_i_3_15
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post #4 of 14 (permalink) Old Mar 6th, 19, 3:23 PM
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Re: dim headlights

Poor grounds will give you dim lights.
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post #5 of 14 (permalink) Old Mar 6th, 19, 3:33 PM Thread Starter
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Re: dim headlights

I have a meter. the dash lights are also dim
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post #6 of 14 (permalink) Old Mar 6th, 19, 5:00 PM
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Re: dim headlights

Check the body ground to the fender from the negative post. Many of these are omitted through the years at battery replacement time. Check voltage at the headlight connectors to confirm supply and grounds. Should read approx. 12.5 to 13.5 volts at connector when running. If ok, adding a relay will brighten them up significantly. These old systems drop a lot of voltage through the headlight switch and bad connections that develop through time.Using a relay will supply 14.5 volts directly to the headlights which will make them appear almost 30-50% brighter.
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post #7 of 14 (permalink) Old Mar 6th, 19, 9:32 PM
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Re: dim headlights

Add a temporary supplemental ground wire to a single headlight and see if the light gets brighter. If it gets brighter, you know which half of the lighting circuit needs cleaning or maintenance.


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post #8 of 14 (permalink) Old Mar 6th, 19, 9:51 PM
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Re: dim headlights

I put a 150 amp alternator, rewired the main harness with -10, -8, and -6 wires where applicable. Noticeable different in how bright my headlights got.

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post #9 of 14 (permalink) Old Mar 6th, 19, 10:03 PM
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Re: dim headlights

Not saying this is the problem but will improve your headlights and make them much brighter. There is a mod that many here do that involves adding relays to the head light circuit. Should be very easy to find a thread about that procedure.
My guess for your problem is grounds need improved.
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post #10 of 14 (permalink) Old Mar 6th, 19, 10:37 PM
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Re: dim headlights

Just to add to all the other good suggestions, I'd disconnect every + side connector and plug it back in, including the harness connectors at the firewall and headlight switch.
Good grounds as mentioned above are vital. When the car was built, all those body parts bolted together had clean electrical connection. Now they're rusty and no maybe bueno.
Relays are good too since in a stock system there's a much longer current path to run the lights.

The whole thing leads to electricity (energy) flows from the batt/alt in a circle from one side to the other. All the current that that goes to the lights in this case, goes from the alt to the lights and back. Nothing is lost. Bad connections may limit the current (making the lights dim) but end to end, current is the same. The point is, you need to make sure there are no bad wiring or connections all the way from and to the current source (batt/alt).

Yep, it's a pain in the butt...
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post #11 of 14 (permalink) Old Mar 7th, 19, 7:19 AM
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Re: dim headlights

If lights are dim, no point in doing any thinking/asking/guessing at all. Check battery voltage with everything (including engine) off, with engine running at 2,000 RPM with everything off. Then do it again with high beams on with everything else off (including engine), then with as much stuff on as you can with engine at 2,000, High Beams, brake lights, fan at high...

List those 4 voltages, should only take 2 minutes to test.
rough estimates of them should be 12.6, 14.3, 12.2, 13.9 something like that. Basically over 13.8 when running, and under 13 when off, but always over 12 (unless cranking).
Also, how old are the bulbs, they don't get brighter after 50 years...

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post #12 of 14 (permalink) Old Mar 7th, 19, 9:04 PM Thread Starter
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Re: dim headlights

Thanks for all the suggestions. I will check grounds. The headlights are at least 20 yrs old.
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post #13 of 14 (permalink) Old Mar 8th, 19, 7:08 AM
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Re: dim headlights

with dim dash lights as well, the headlight switch might be suspect, arm yourself with your meter. check battery voltage. check voltage at some fuses. should be pretty similar. then turn on headlights, and check voltage at the plug. use the ground at the plug, then try grounding at battery or alt case or something to see if you have a difference. i think you will see like a 1 volt drop, just due to old wiring. if its more, then maybe head up to the headlight switch and start testing there. I found this which might be helpful at the headlight switch.

The relay kit for the headlights is clearly worth the money. Seen a bunch of charred plugs for the headlight switch. headlights have a big load, and the relay mod will take that huge load away from the headlight switch.
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post #14 of 14 (permalink) Old Mar 13th, 19, 1:52 AM
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Bought a relay kit from EBay last year, $20 some I think it was with pre wired harness etc.

Took half an hour to install, HUGE didference in headlights, dash lights, etc 👍
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