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lights dim when idling #683243 Tue Oct 05 2010 01:15 AM
Joined: Jul 2010
Posts: 8
Apache_Jake Offline OP
New Guy
Hey all,
I have a 61 C20 that sometime in its history was converted to 12V. I notice that the lights get brighter when I rev the engine, and dim when I let off. What's the fix? Also, I have a headlight issue. The lowbeams work, but when I step on the bright button, no brights...and the low beams turn off. I replaced the switch as it was pretty corroded, but that didn't correct the problem. Any ideas?

Re: lights dim when idling [Re: Apache_Jake] #683254 Tue Oct 05 2010 01:40 AM
Joined: Sep 2001
Posts: 29,131
tclederman Offline

Generator or alternator?

A 1961 truck would have originally been 12v.

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Re: lights dim when idling [Re: Apache_Jake] #683368 Tue Oct 05 2010 01:40 PM
Joined: Feb 2004
Posts: 19,483
Hotrod Lincoln Offline
The most common cause of bright or dim lights failing completely is a bad dimmer switch. It's also possible that the same filament on both headlights is burned out, so test that first with an ohmmeter across the bulbs or by putting voltage directly to the lights with jumper leads.

If you're running a generator, it's not uncommon for the voltage to rise noticeably as the engine is revved up. There's really no fix for that, as DC generators don't turn fast enough at idle to develop max voltage. Be sure all the connections are clean and tight, especially the battery post terminals. Try checking the voltage at the battery posts at idle, then again with the engine running 1500 RPM or more. I wouldn't be surprised to see 1/2 volt or more of difference. If an alternator is doing the same thing, there's a problem with the alternator or the regulator.

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Re: lights dim when idling [Re: Hotrod Lincoln] #683384 Tue Oct 05 2010 02:46 PM
Joined: Jun 2004
Posts: 8,153
Wrenchbender Ret. Offline
Master Gabster
A certain amount is normal but can be an indication of a bad battery or too high of a voltage regulator setting.
The cause of the no bright lights is in the dimmer switch or connector. They are in a horrible invironment & corrosion is common.

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Re: lights dim when idling [Re: Wrenchbender Ret.] #683413 Tue Oct 05 2010 04:00 PM
Joined: Oct 2009
Posts: 141
Old Dollar Offline
Wrench Fetcher
sounds like dimmer switch to me.

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Re: lights dim when idling [Re: Old Dollar] #683679 Wed Oct 06 2010 03:48 PM
Joined: Nov 2005
Posts: 5,553
3B Online
Master Gabster
Hy Apache_Jake, firstly your truck was 12 volt from the factory, as were all since the 55B series as far as I know. A certain amount of light intensity change from idle to driving rpm is completely normal as has been stated. A check with a voltmeter will verify your charging voltage, as has been stated. If you have already replaced the dimmer switch, then I would check the headlights to make sure the highbeam filaments are not burned out.

Re: lights dim when idling [Re: 3B] #683725 Wed Oct 06 2010 06:57 PM
Joined: Jan 2008
Posts: 4,658
crenwelge Offline
Shop Shark
A 61 would have been born with 12 volts. If it still has the original generator, and you have other stuff running like a heater, it is normal for the lights to get dimmer at an idle because generators don't charge at an idle like alternators do. I seriously doubt that the problem is in your dimmer switch. However, to eliminate this, reach under there and touch the dimmer switch. If its hot, it is defective. A dimmer switch works by switching between two circuits and does not work with resistance like a heater switch.

Re: lights dim when idling [Re: crenwelge] #683747 Wed Oct 06 2010 08:30 PM
Joined: Sep 2010
Posts: 722
johntsmith Offline
Shop Shark
Don't forget to check the terminals in the plug that attaches to the dimmer switch. If they are corraded or you have a break in the wire and not the insulation, that will also cause this problem.

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Re: lights dim when idling [Re: johntsmith] #684017 Thu Oct 07 2010 05:21 PM
Joined: Jul 2010
Posts: 8
Apache_Jake Offline OP
New Guy
Thanks for the help fella's!

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