The Stovebolt Site Forums The Shop Area The Electrical Bay Short in lighting circuit

while driving my 54 3100, with 235 engine in the dark. Lights went off. a few seconds they came back on. this happened until i got home. reading the manual it says there is an interupter in the head light switch that would cause this if there is a short which caused a current draw over 42 amps. I replaced the switch while working on the truck. This was the first time i have driven it at night. Does anyone have any suggestions for finding the short. i drove the truck in the daylight with my bright lights on and saw the interruption in the bright light indicator light on the dash. Alny help would be appreciated. it started after driving for a few miles with the lights on.
Thanks

Could be a bad ground on the headlights, or bad connections on the hot wires to the head lights causing high resistance.

Check da connection at da dimmer switch. Know issue area.

So the switch was replaced during rehab, not failure?
Is the rig 6 volt?

Thanks to 55 Shaker and to Glenn's towing. Do you think the interrupter could be reating the intermittent outage?

A light switch has a bimetal strip that acts as a self-resetting circuit breaker so you don't lose the lights entirely like would happen with a blown fuse. The dimmer switch is a good place to start looking for a problem. It lives in a very bad environment- - - -lots of road splash, salt, etc. It can develop a short that will cause the breaker to cycle. Try disconnecting the wires at the headlights one at a time to see if the problem goes away. If not, keep backing up along the wiring, from headlights, to the dimmer switch, to the headlight switch, disconnecting wires until the circuit breaker stops cycling the circuit on and off.

Did you happen to install brighter headlights recently, like halogens or LED's? Some of those types of lights draw enough current to need relays in the circuit to prevent overloading the breaker in the headlight switch.
Jerry

Another suggestion is that it could be a dirty connection causing resistance at the bimetallic strip that Hotrod Lincoln references. This happened to me and there was no short, just needed to clean the points at the connection. What happens is that the dirty connection gets hot with a normal current draw, as you would have with the headlights on, and the heat causes the bimetallic strip to open the circuit. After 5 minutes, things cool down, lights come back on, only to repeat every few minutes.
Kent

First thing I would do is a thorough inspection of the wiring. Follow the wires from the headlights back to the switch. Look for places where the wires could be grounded by chafing against metal. Look for areas where wires could have had the insulation burnt off by hot parts such as the exhaust. Look for wires spliced together and wrapped up in tape (possibly highlight pigtails); a good place for short circuits.

Reread Hotrod Lincoln's post about disconnecting the wires at the headlights and working back to the switch.

Does anyone know if the new headlight switches have the thermalcircuit breaker in them? The supplier who i bought it from acted like he never heard of it. I had seen it discussed in the 1954 Chevrolet Shop manual, page 12-37. my problem sounded like exactly what would happen. I'm not sure how long it takes for the breaker to cool so light would come back on. mine were only a few seconds to a minute. Scarry driving when it was a minute.
Thanks for all the suggestions.

This one does (attached pic). It's from Classic Parts.

99% out of a 100% of the time, the problem is in the dimmer switch.

If you truly have a short, your are risking a fire or frying your wiring.
You need to nail this problem ASAP.

1. Remove both head lamp sealed beam bulbs
2. Remove dome light bulb
3. Remove fuses (firewall box) for tail and stop lights.
4. Connect voltmeter across battery
5. Engine not running or cranking

You are looking for a significant voltage drop when you pull the knob on the headlight switch.
Watch your dash ammeter gauge for a significant deflection.
If your current or voltage has a significant change then you may have a problem somewhere else.
Report what you see.

I just quickly scanned your previous post about converting to alternator.
Are you using a 6 volt light switch in a truck converted to 12 volt?

Same thing I was getting at, question wasn't answered.

The 6 volt light switch circuit breaker carries more current than the 12 volt item- - - -it's a non-issue.
Jerry

I am with Gord. It can be a loose or half broken wire. Making and breaking due to vibration/shaking while driving. It can be a connector with corroded contacts and the vibration, again, causes intermittency, You would want to see if it does it sitting still with motor off. Then start motor and check. Then shake truck and it's wires.

If you think it's the dimmer switch, there are three terminals. Battery (B) terminal and the other two terminals are Lo beam and Hi beam. Does not make any difference which is which for those two. Just take off either the lo or hi wire off it's terminal and put it on the battery terminal. (Two on one terminal). Turn on lights and take it for a ride. Then put the wire back on it's location and try the other onto the battery terminal. Drive it again. This will bypass the switch and tell you good info. Logic will then solve it. Report results.

It's OK can to consider a short, but don't focus on that alone. May not be a short. I would also check those terminal blocks on the inner fender. I always recommend getting rid of those and splicing wires. (Solder wrap and heat shrink sleeving)

Do not dwell on the "interrupter" or the headlight switch.

I believe each headlight has it's own separate ground wire attached to the immediate chassis. So if both headlights are going out, it's not that particular ground. One will not affect the other. The only other grounds involved are the large frame to battery, body to battery, motor to battery ground combination you have.

I am 12 volt and have a 12 volt head light switch. Thanks for the suggestions. i have been getting power for the turnsignals from the down side of the brake fuse because it was always hot. Turn signals and flasher were itermittent with the head lights. Today was my birthday so the family thought they should keep me busy. i did take the turnsignal power wire off the brake fuse and drove it. did not see the problem.
Took the poitive cable loose from the battery and put a volt meter between the cable and the post. got 12+ volts with the everything off. Sure wish each of the circuits was fused individually. it would be easier to trouble shoot. Dimmer switch is new and wiring is clean and tight. thanks again for your help.

**** Happy Birthday *****

Agree with Jerry on the 6V vs 12V light switch. Unfortunately it does not eliminate a possible over-current causing the bi-metallic 12V switch to open.

A shot in the dark here. Clrh2ofrank, when you converted your truck to 12V, did you happen to add relays to switch on your head lights?

Your statement:
Took the poitive cable loose from the battery and put a volt meter between the cable and the post. got 12+ volts with the everything off.

If everything is turned off, you pulled off the positive terminal cable and measured 12V between the cable and positive battery post then my poor brain says you should have read 0 volts. If you read any voltage at all then there has to be current flow through your meter. This means something is still connected or using power. You didn't mention anything about problems with your battery draining and having to recharge it.

Using a voltmeter for that test is a waste of time. it's going to show battery terminal voltage regardless of whether current is flowing or not. Try using the "milliamp" scale on a digital multimeter, or an ice pick style test light. If the light glows, or the parasitic drain is over about 75 milliamps or so, you've got a leak to ground somewhere. Neither one will have anything whatsoever to do with the circuit breaker on the light switch cycling on and off.
Jerry

Did not add Relays to switch on head lights.
i was expecting to get zero also. since i go 12 volts led me to believe i had a short.
i returned on alternator thinking i had a diode failed. have not had the problem since. It would take a week or two to run battery down.
thanks for your help.

i just drove it again. Using the bright light inicator as my guide. drove 23 miles with bright lights on before the lights started cycling. Returned home about 10 miles and lights cycled all the way home. pulled in shop and the lights were still cycling. removed the wire from brake fuse to turnsignals no change. replaced wire to turnsignals. turned headlights off for about five minutes. when i turned them back on waited with engine running all this time the cycling did not start again.

thanks for the heads up on the test light will go try a test light.

Used an ice pick test light. No light. I did check the light across the battery to make sure the bulb was not burned out. I'm speculating that means i do not have a continous short and it is probably an open?

Listen carefully- - - - -you do not have a short, nor an open circuit. You have enough current draw to make the circuit breaker built into the light switch cycle on and off. The fact that it takes some time to begin cycling means there is not a "short" in the classic sense of the term. A short would make the breaker begin cycling immediately, as soon as the lights were turned on. An "open" would keep the lights from operating at all. Do the lights flash when they're on low beam? If not, you need to start investigating the current draw ONLY in the high beam circuit. Forget about looking for parasitic drains or other extraneous Bovine Scatology. It's simply distracting you from finding the true cause of the problem.
Jerry

it seems to me that if it was an open it would need to be in the mainpower for the headlights, tail lights, brake lights and turnsignal to go off. am i missing something.

Hot Rod. thanks > Generally to solve a problem you have to identify the real problem. I will have to make sure if cycling happens when low beam are operating. i have been using the high beam indicator as the indication of lights being on or off. i will switch horses and try to identify what the current is in the various circuits. Thanks for eveyone who has offered suggestions. By the way i sent the last post before i saw your last post.

That ice pick tester can be used as an on/off indicator for the low beam circuit. Just run a temporary wire from the low beam connector of the dimmer switch into the cab and connect it to the probe of the tester. Ground the pigtail and the tester will illuminate when the low beams are on.
Jerry

Ok I will try using that for a test tomorrow.

I measured some current in head lights ( i have no idea what they shoud be) here is what i measured:
Headlights

Head lights left bright 4.25 amps
Left dim 2.48
Rt bright 4.07
Rt dim 2.47
Are these high, low or about right?
Thanks
Frank

That looks about right. Less than 10 amps total current draw on high beam should not cause the circuit breaker to cycle. Try this- - - -disconnect the wiring harness at the dimmer switch, and jumper the wires together so the high beam lights are on, then take a test drive. Is there a plug at the dimmer, or do the wires connect individually with ring terminals and screws? Either way, the idea is to bypass the dimmer to see if there's an internal current leak that's overloading the circuit breaker. If the lights don't flash or flicker with the dimmer switch bypassed, replace it. If they continue to flicker, replace the light switch. Getting an unwanted current leak to ground in a dimmer is a pretty common problem due to corrosion inside the switch. As I believe I said before, dimmer switches live in a pretty nasty environment.
Jerry

I know that the headlight switch is new, BUT, the contacts on the built in breaker could be the culprit. I had the same issue, but with the original headlight switch. My lights operated flawlessly on low beams but when I changed to the high beams, after a 1/2 minute or so the lights would come on the go off. The high beams draw more power then the low beams. On my truck, which is twelve volts, the high beams draw 60 watts and the low draw 50 watts. That extra 10 watts caused the breaker to heat up and open. The contact points were burned. That troubleshooting drove me crazy until I finally gave up and replaced the switch. Issue solved.

I’m not saying that is your problem, but I am saying it’s a possibility. The repop overseas parts are not the quality that the old “Made in USA” were.

The whole issue can be resolved with a couple of Bosch-type relays, one for the low beam circuit and the other for high. They're about $5.00 apiece or less on Ebay. Then the current draw through the light switch and the dimmer switch will be down in the 2 amp or less range, just enough to energize the relays. The heavy current to actually run the lights will come through a 10 or 12 gauge wire and a fuse or circuit breaker wired directly to the battery or some other source of full battery voltage. As an added advantage, the headlights will also burn a little brighter.
Jerry

I also followed Jerry’s advice and installed a set of Bosch style relays when I installed my new switch. After spending over fifty dollars on a new switch, I found (4) Bosch style 60 amp relays on eBay for 13 dollars! A bit of rewiring and all is good.

I'm glad you guys solved the problem.

Hot rod,
thanks. Ordered relays this morning will install as soon as i get them Thanks to everyone for your help. i'll let you know how that goes.

Frank

I think I've got a circuit diagram stashed away somewhere in the computer memory for those relays. I use the Bosch 60 amp relays instead of 30, to take advantage of the larger diameter wires in the mating plugs, not necessarily because headlights need a relay with that much capacity. Basically, you need to use the high/low beam signal from the dimmer switch to trigger one relay for each beam. It's OK to parallel both headlights through one relay, one for high beam and the other for low. Tap the dash indicator light wire into the downstream connector for the high beam relay. I've also gotten into the habit of running the headlight ground wires back to the frame or engine block, rather than relying on rusty body sheet metal to provide a ground.
Jerry

Do you use separate wires from the battery to the relay or can you put the two relays in parrallel. I plan to mount the relays above the dimmer switch. i was planning on running #8 wire from the battery (or Starter) to the relays. I have already ordered 30 amp relays. They are cheap enough that if you think it is necessary i can order a couple of 60 amp also. Thanks again for you help

While waiting for the relays, ti took The truck out and ran with low beams for 25 miles. No cycling. thanks for the help and suggestions.
Frank

One #8 wire will handle both relays since only one at a time will be working, or even a #10 would be plenty. I always mount the relays on the core support close to the lights and run the big power wire out there. That shortens the circuit from the relay to the light. You can use the original headlight wiring from the dimmer switch to trigger the relays. The only reason I use 60 amp relays is the size of the wires in the socket, but I'm always into "overkill"- - - -that's from my dirt track racing days- - - -"Build it three times as strong as you think it needs to be, and try not to be surprised when it breaks!"
Jerry

The high and low beams of the very bright LED headlight I just put on my Harley are wired through relays, and the "passing lights" on both sides of it have a relay circuit powering them. It's a lot easier to swap a failed relay, if that ever happens, than to replace a beam selector switch with wiring running through the handlebars!
Jerry

Thanks.
Having spent a career in the Aircraft industry, i'm use to seeing the battles between the structure engineers and the weight engineers. Sometimes it is tough for the project engineer to make the final decision. Thanks for your help.
Frank

Isn't that called a "Mexican Firing Squad"- - - - -everybody standing in a circle and shooting at each other?

Jerry

Here’s how I wired mine. I used two 60 amp Bosch style relays and located them on the firewall above the hi/low beam switch. I used a 40 amp fused single 10 gauge wire to supply both relays. It works great.

I picked up the 12v supply from the starter lug but you can use the alternator. Either place works, whatever is easier.