About 6 weeks ago, I replaced the wiring harness with a new one from Lectric Limited. At the same time I replaced the headlight switch because the reostat did not work in the old one. About 3 weeks later, probably no more than 3 hours of night driving, the dash lights failed. I was perplexed but installed another new headlight switch. Last night, the dash lights failed again, after probably only a total of 3 or four hours of night driving. Each time they failed, it was within 5 or 6 minutes of turning on the lights. Has anyone had a similar problem or does anyone have any suggestions as to the cause of the problem??? Thanks
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Re: '57 Light switch and dash lights
Yes, 'anyone' has a suggestion/input... Although you spelled rheostat wrong, the concept came across loud and clear AND you're 100% correct. Dash lamp diming is achieved by a three port variable resistance called generically a potentiometer.
It works by dropping voltage internally to supply 'downstream' components with a lower supply rail. The H/L switch uses a wire-wound resistor with ceramic body to do the voltage drop function. You twist the H/L switch and dash lights dim or brighten in response.
When they dim, where is the balance of the supply energy going? Answer, it's being dissipated in the form of heat in the wire wound resistor integral to the H/L switch (you're throwing away energy in the form of heat to create the lower dash lamp supply voltage and lower illumination output).
Somewhere/somehow the heat generated in the H/L switch dimmer rheostat has to be dissipated. If it isn't dissipated outside the dimmer rheostat, the wire windings will overheat, burn up and create an open circuit (all the dash lights go out and the H/L switch has to be replaced)....
The native heatsink for the H/L switch IS the dash panel itself. So, if you haven't torqued the switch down crisply, there's an intermittent heat conduction path. The little sucka will try it's darn best to radiate the heat, but it's a race condition that's typically won by the switch burning up.
This scenario can be 'complicated' by wiring integrity fault(s). If there's H/L switch IS properly torqued into thermal contact with the dash, but it's asked to deliver too much branch current the same 'funny' can occur. You can pull to much branch current by:
(1) Running incorrect dash lamps that call for too much current.
(2) Having one or more wiring faults (skinned insulation on one or more lamp wiring leads making partial short to ground situations).
You can check hypothesis #1 by reading the Owner's Manual and checking the bulb generic numbers against specified replacement numbers. The only way to really trap/kill #2 is to feel the temperature of individual lamp wires to find one that's getting hotter than the others.
Syndrome #2 'can' result from various radio supression capacitors installed behind the dash. If a given cap is leaking badly internally, it'll look like a pseudo short to ground (UNTIL it failes totally--then it'll pop the fuse). Without a good capacitor check instrument, you can continue to tear your hair out! But, you can locate the individual radio RF 'hush up' components (should be called out in the radio option section of your AIM) and selectively disconnect them to see if the problem goes away....
Tracking down who's at fault can be a lengthy process without the correct diagnostic tools and knowledge.... -
Re: '57 Light switch and dash lights
Yes, 'anyone' has a suggestion/input... Although you spelled rheostat wrong, the concept came across loud and clear AND you're 100% correct. Dash lamp diming is achieved by a three port variable resistance called generically a potentiometer.
It works by dropping voltage internally to supply 'downstream' components with a lower supply rail. The H/L switch uses a wire-wound resistor with ceramic body to do the voltage drop function. You twist the H/L switch and dash lights dim or brighten in response.
When they dim, where is the balance of the supply energy going? Answer, it's being dissipated in the form of heat in the wire wound resistor integral to the H/L switch (you're throwing away energy in the form of heat to create the lower dash lamp supply voltage and lower illumination output).
Somewhere/somehow the heat generated in the H/L switch dimmer rheostat has to be dissipated. If it isn't dissipated outside the dimmer rheostat, the wire windings will overheat, burn up and create an open circuit (all the dash lights go out and the H/L switch has to be replaced)....
The native heatsink for the H/L switch IS the dash panel itself. So, if you haven't torqued the switch down crisply, there's an intermittent heat conduction path. The little sucka will try it's darn best to radiate the heat, but it's a race condition that's typically won by the switch burning up.
This scenario can be 'complicated' by wiring integrity fault(s). If there's H/L switch IS properly torqued into thermal contact with the dash, but it's asked to deliver too much branch current the same 'funny' can occur. You can pull to much branch current by:
(1) Running incorrect dash lamps that call for too much current.
(2) Having one or more wiring faults (skinned insulation on one or more lamp wiring leads making partial short to ground situations).
You can check hypothesis #1 by reading the Owner's Manual and checking the bulb generic numbers against specified replacement numbers. The only way to really trap/kill #2 is to feel the temperature of individual lamp wires to find one that's getting hotter than the others.
Syndrome #2 'can' result from various radio supression capacitors installed behind the dash. If a given cap is leaking badly internally, it'll look like a pseudo short to ground (UNTIL it failes totally--then it'll pop the fuse). Without a good capacitor check instrument, you can continue to tear your hair out! But, you can locate the individual radio RF 'hush up' components (should be called out in the radio option section of your AIM) and selectively disconnect them to see if the problem goes away....
Tracking down who's at fault can be a lengthy process without the correct diagnostic tools and knowledge....Comment
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Re: '57 Light switch and dash lights
Rex -
Have you checked the tiny glass AGA 3-amp fuse in the holder at the end of the headlight switch? It's easy to overlook (since it's the only fuse in the whole car, except for options), but it controls the circuit for the dash lights. Replacement switches may or may not have the correct fuse installed, as this fuse is pretty ancient, very small, and hasn't been used for many years.Comment
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Re: '57 Light switch and dash lights
Rex -
Have you checked the tiny glass AGA 3-amp fuse in the holder at the end of the headlight switch? It's easy to overlook (since it's the only fuse in the whole car, except for options), but it controls the circuit for the dash lights. Replacement switches may or may not have the correct fuse installed, as this fuse is pretty ancient, very small, and hasn't been used for many years.Comment
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