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Old 09-26-2012, 08:50 PM   #1
Maker Scream
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Unhappy High beam switch?

I have an electrical problem that i'm not able to figure out. Any help would be greatly appreciated. I just got through replacing the headlight switch. Replacing that beast cured alot of problems i'd been having but one problem remains and i am STUMPED. My driver side headlight is very dim, while my passenger side is very bright, possible even in high beam mode. I know what you're thinking "so what happens when you step on the high beam switch"? ... Both headlights go out! Since the replacing headlight switch cured so many problems, my first thought is that maybe the high beam switch is bad. I ran out of daylight today so i'm going back at it again tomorrow & i need a few good ideas on where my troubleshooting should begin. Anybody able to point me in the right direction? ... and by the way, i'm very new at this so talk to me like i'm an idiot, it'll probaby help
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Old 09-26-2012, 09:12 PM   #2
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Re: High beam switch?

... i forgot to add the model ... 1970 stepside
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Old 09-26-2012, 10:04 PM   #3
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Re: High beam switch?

I'll assume you check the headlight connectors for corrosion already and swapped headlights one for the other.

The feed for the headlights coming out of the high/low switch is a single wire, which then gets spliced off in the engine bay to feed the left and right headlights. Check your wiring paying attention to the wiring from the left headlight to where it splices & branches out of the main wiring harness. Might have something pinched in some sheet metal, bad splice, crushed wire, previous owner fix hidden behind tape.

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Old 09-27-2012, 07:08 AM   #4
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Re: High beam switch?

thx Alex. I replaced both headlights with new ones. The terminals look good but there are previous owner splices all over the place. When you say left side, you are talking about the passenger side, correct?
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Old 09-27-2012, 10:13 AM   #5
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Re: High beam switch?

The headlight circuit begins at the switch where a single blue wire runs to the dimmer switch on the drivers side floor ( left is the drivers side). From there two wires run to the fire wall connection block and out to the headlights. One is a tan wire and one is light green.

When I trouble shoot I always start with the voltage source which is the blue wire from the headlight switch. Pull the dimmer switch connector off the switch Check the blue wire for 12 volts with the headlight switch on.. Then jump between the blue wire and either of the other two wires and watch for the headlights to come on. A cotter key works good for this. Corrosion on the connector or the dimmer switch is very often the cause of your problem.

If they both do then the problem is in the dimmer switch. If they don't then the problem is in the wiring to the headlights or the headlights. To check the headlights run a ground wire from the negative terminal on the battery to the ground wire off the headlight and see if it comes on. A poor connection on this wire to the radiator mounting support is another common cause of dim headlights.

Here's the headlight switch showing the blue wire to the dimmer switch. It's on the bottom center.

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Here's the dimmer switch showing the three wires to the headlights. Tan and green go to the headlights and the blue wire is power from the headlight switch. It's on the bottom left.

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Here's the engine bay and the headlight wiring at the left edge of the diagram. The tan and green wires are for the headlights. The left headlight connector will have two greens and two tans to power both lights

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Old 09-27-2012, 03:56 PM   #6
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Re: High beam switch?

replace dimmer switch and clean the ground wire connection to the dim side and you should b good to go
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Old 09-28-2012, 09:47 PM   #7
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Re: High beam switch?

thx guys. i cleaned the grounds, replaced the dimmer switch & soldered a few bad connections. what do ya know, everything worked perfect ... but, of course my state of zen would be short lived. the lights were both functioning perfect. i was smiling from ear to ear and getting ready for the final cruise in of the season. went and picked up an air filter and noticed a weak crank when i started the truck leaving auto zone so when i got home, i turned it off, gave it a few seconds and tried starting her back up. it was dead. now what? i tested the alternator and found 11.9v going in but only 9v coming out, even at 2000 rpm... a whole new problem presented itself. i've recently (over the last 2 weeks) replaced the alternator, it tested bad. immediately after, i replaced the battery, it tested bad also. i assumed the bad alternator asked too much out of the battery & killed a cell in the battery. so i'm working w/ new stuff. only thing left is voltage regulator, right? well, when i replaced the alternator, i found out that the stock alternator w/ external voltage regulator had been replaced w/ an aftermarket alternator w/ internal voltage regulator. but the old external voltage regulator appears to still be installed and wired. My new alternator now appears to not be working so i carried it back to auto zone and it tested as good. i believe the guy didnt test it correctly so i raised enough hell to have him replace it, as it was only a week or 2 old and i may as well be paying rent at auto zone for as much as i've been there over the last month... now i'm asking myself & yall, are these 2 voltage regulators fighting against each other? since my alternator has an internal vr, should i unhook the external vr?
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Old 09-29-2012, 12:21 AM   #8
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Re: High beam switch?

If you are running the external VR and the internal regulated alternator then you will get too much resistance on the alternator exciter wire and the alternator may not turn on or will not charge sufficiently. After all the reason for installing the internal regulated alternator is to get rid of the external VR.

Here is the correct wiring conversion for the change over.

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We can't see what the PO has done to the truck so you will have to sort it out.
Does the alternator plug go in the back or in the side of your alternator. If it goes in the back it is externally regulated and if it goes in the side it is internally regulated.
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Old 09-29-2012, 02:57 AM   #9
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Re: High beam switch?

the alternator plug goes into the side/top. but i dont use a plug bcuz the original wire harness was scrapped, i assume to support the aftermarket alternator. the external vr was never physically removed although, i've found so many deadend wires that it could have been removed from operation just by cutting wires at some point in the wiring harness. i'm going to have alot of questions that pertain to functionality and tests that i can perform to show whats going on in my electrical bcuz the PO poorly rewired most of the system & hes prolly the only person that has a clue whats going on w/ it. i felt like the external vr may be operating just bcuz theres so many electrical issues popping up & logic tells me that i shouldnt find 2 VRs. thx for your help. im learning ... slowly but surely
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Old 09-30-2012, 03:11 PM   #10
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Re: High beam switch?

You need to open up the original regulator and see if the previous owner internally jumpered it for use with an internally regulated alternator. I havn't got the time to look for it right now, but the faq over at 442.com covers what you need to do to convert an externally regulated vehicle to an internally regulated one.

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Old 09-30-2012, 10:19 PM   #11
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Re: High beam switch?

here's our FAQ plus the diagram I posted above.

http://67-72chevytrucks.com/vboard/s...d.php?t=119379
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Old 10-01-2012, 12:40 PM   #12
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Re: High beam switch?

Alex & VetteVet I think y'all are onto something. The old external vr hasn't been touched, inside or jumpered on the outside. I unplugged it and can't get a high enough voltage leaving the alternator to charge the battery and before i unplugged it, i wasn't getting good voltage either. I followed the link you posted & as soon as it dries up in Tn again, I'm going to jump that external vr and purchase one of the stronger alternators mentioned in the above link. The parts guy did mention that the aftermarket alternator that i replaced put out less volts or amperage (can't remember which) than the stock alternator that it was intended to replace. Supposed to rain for a cpl days so i will post my results to the forum as soon as I'm able to get outside and complete the task. Thx again for yalls patience & input as I'm working tirelessly to resolve this electrical issue.
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Old 10-02-2012, 09:30 PM   #13
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Re: High beam switch?

in that wiring drawing, i believe i see 2 fuses and 1 fusible link. i broke the one between the alternator red wire and the engine harness. the 18b/w. is that also a fusible link or a fuse as marked? if it's just an inline fuse, what size inline fuse do you suggest i replace it with? i pretty much destroyed it trying to open it up and check the fuse. i'll probably break the one between the junction block and the engine harness too, so what size is that also? these things are old and brittle. might as well replace them while i'm there
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Old 10-02-2012, 10:39 PM   #14
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Re: High beam switch?

Those are the fuses for the ammeter and they are 4 amp regular glass fuses. If you want the ammeter to work you need those fuses, if you don't care if it works or you want to use a voltmeter then you don't need them. They are not fusible links.
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Old 10-04-2012, 08:32 PM   #15
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Re: High beam switch?

thx guys. i tackled the problem, with ya'lls help. previous owner had tried jumping the external vr and failed. he was feeding the alternator with the white wire & had jumped the voltage regulator on the hidden underside. i'm now getting 14-14.4V off the alternator (13 something under a load). i went with the 85 silverado alternator which puts out 78 amps. in the engine compartment, i'm golden. going under the dash this weekend, hopefully... this forum is a powerful resource. thx again
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