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Headlight switch

Started by R Sotardi #11719, September 01, 2012, 01:46:32 PM

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R Sotardi #11719

Is it possible to replace the circuit breaker in the headlight switch of the 50-51s. I was curious. If  a CB fails the lights will begin to flash off and on then off again, no? Most cars of that vintage have a replaceable CB, but it appears the GM switch does not and the whole assembly must be replaced. I ran a relay a few years ago, and  presently don't have an issue. I also have a NOS switch assembly. Looking at it, it appears that the CB is not replaceable. Is that true? Ron

Doug Houston

You are not taking a couple of things into account with the action of your headlight switch.

First, just why is there a circuit breaker on the feed side of the light switch? It's to protect the various high load circuits that are fed through the headlight switch; particularly the headlights themselves.  Now, you've never mentioned just what the headlights are on that car, and I'm gonna guess (though I'm often wrong). I'll bet that you have four halogen headlight bulbs in your headlights. If so, they draw a lot more current than the original sealed beam lights that the car came with. If I don't miss my guess, the additional current would constitute enough overload on the breaker, and the breaker will simply do its job, and open.

Secondly, the wiring harness in the chassis has wire gauges that are calculated for the lighting load in amperes, that the original sealed beams drew. The quartz-Iodine (Halogen) lamps hadn't gotten into our cars when your car was built. Now, it is possible to get a circuit breaker with a higher current rating, and it is possible to replace the breaker with one of greater capacity. That way, you can have the bright lights and be happy.......until the overloaded headlight wiring has run warmer (hot, maybe) softened or melted the polyvinyl insulation in the harness. With heat in a harness, the copper conductors ultimately can come in contact with each other, and it's anybody's guess what happens then.

Our cars (and everybody else's) are designed for the electrical equipment that came with the car. You probably never heard it, but voltage drop is designed into the headlight wiring harness, so that the headlight lamps are run at nearly their rated voltage, which is around 12 volts. The car's electrical system runs steadily at around 14-14.5 volts with the engine running. If you were  to apply the full 14 volts to the headlights, you could be replacing them every week, in some instances.

So, as the wise Sergeants used to say in Basic training; That's all there are to it.
38-6019S
38-9039
39-9057B
41-6227D
41-6019SF
41-6229D
41-6267D
56-6267
70-DeV Conv
41-Chev 41-1167
41 Olds 41-3929

R Sotardi #11719

#2
Thanks for your reponse, but I don't have a problem, or halogen lamps. I was interested in the "repairability"of the switches. There is no part # in the Master Parts book. So I presume that in a GM car of this era (1950 = 6volt), when a circuit breaker( bi-metal contacts) wears out the whole switch needs replacement, except the rheostat which is listed.

Doug Houston

That's a hard question to deal with. Firstlish, I think we all understand that the breaker is an integral part of the lighting switch.

While it could be that the bimetal in the switch could fail, the part of the breaker that is most likely to fail is the contacts. If the car has been runniing at high electrical loads for some time, the contacs would be heating from carrying high currents. The bimmetal strrip is carrying the high current, and if enough heat builds up for enough time, the breaker could fail just from long periods of load at near its trip current.

You might look into using a flat "jeweler's?) file to burnish the breaker contacts onnthe switch. Just cleaning the point that way will not alter the breaker's ratings, so it's safe.

I've had circuit breakers in my home fail because they were carrying currents that were close to their trip ratings. It takes a good bit of time, but they do fail....and I have VERY good breakers in the panels (Square-D "QO" type).
38-6019S
38-9039
39-9057B
41-6227D
41-6019SF
41-6229D
41-6267D
56-6267
70-DeV Conv
41-Chev 41-1167
41 Olds 41-3929

R Sotardi #11719

I happen to be studying at an old switch on my shelf ,that was in pieces vs a NOS one. The only way that I can see that  the CB can be replaced is by bending the small retainer tangs holding it to the base removing it, then replacing it with a different CB from a cannibalized part. In my Hudson, when the CB (20amp) starts to fail  the assembly can be removed, and the CB can be replaced with a new NAPA part for $10 & 30 minutes labor. However, the dash light rheostat is separate, like new cars. I was curious why GM devised such an elaborate switch, that was so difficult to remove, and undoubtedly costly to produce. What is the advantage?

Dave T

Under normal conditions how hot to the touch should the metal part of the circut breaker feel , Also how many amps should the stock headlights draw on high beam?
                             thanks Dave T CLC22554
Dave Treuhaft
CLC # 22554

R Sotardi #11719

They get hot, especially on 6 volt systems.Touch it & it will feel like a just extinguished match. Because of the heat, the bimetal "tangs" stretch when on  and contract when turned off. Eventually,  the the tension is lost & the contacts won't stay closed. Off the lights go until the switch cools enough to close contact, and the circle continues.   If a relay is used, the current is passed from battery to lights  and the dash switch acts as an on/off switch for the relay. Way less curreent ... less heat. I never put a meter on my 50 Cadillac,  but my Hudson draws about 14 amps( 6v).

Dave T

I am still having problems with my 1946 Cadillac headlights (all stock). The circut breaker metal tab  gets very hot, up to 190 deg.I have changed out dimer switch and headlight switch with other used parts and have the same problem I have run a wire to ground test on each wire with an ohms meter and no shorts show up. I know that I can install a headlight relay to reduce the heat on the headlight switch, but that just seems like a bandaid. Any other suggestions would be very helpful.
                               thanks Dave T ID 22554
Dave Treuhaft
CLC # 22554

Doug Houston

After all of the checking that you've done, it would be beneficial to get the numbers of your problem. Up to now, we've assumed that the circuit breaker in the light switch is working properly  within its ratings. Circuit breakers of other styles do go bad in one way or another.

Get a DC ammeter with a range of around 50 amperes. It's probably OK to connect it in series with a battery terminal and the cable. With nothing else turned on beside the headlights, read the current drain. The headlights alone should draw around 20-25 amperes. That represents the full current drain that's coming off of the battery. There would be no other load from anything else. While I can't recall ever seeing the actual characteristics of those breakers, my guess is that they would trip at 35 amperes, or so. If your breaker trips at a 25 ampere headlight load, something is wrong with it.  If the headlight current reads higher than 35 ampres, there is trouble elsewhere in your system......even in the switch, itself.

One thought here. Separate the contacts on the breaker, and burnish them with a flat jeweler's file. If the contacts would be somewhat oxidized from age and service, they would heat up, and might cause enough heat to heat up the bimetal strip. Otherwise, it may be that the breaker has deteriorated beyond repair.
38-6019S
38-9039
39-9057B
41-6227D
41-6019SF
41-6229D
41-6267D
56-6267
70-DeV Conv
41-Chev 41-1167
41 Olds 41-3929