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T-Series MGB Headlight Switch Gets Very Hot

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MGTF1250Dave

MGTF1250Dave

Jedi Knight
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Aloha All,

I'm working with a friend with a '74 MGB that has a recurring problem. The headlight switch becomes very hot, to the point that over time the plastic melts around the power supply terminal (brown wire). The headlamps are stock wattage and nothing has been added to the circuit. I would think that the wiring and switches were designed to carry the electric load required. Electricity is an subject I have enough understanding to be dangerous. I would appreciate any suggestions on the cause of the problem and how to fix it.
 
I would suggest that you team up with a knowledgeable person and isolate the head light circuit from the switch using a suitable relay. However, there is something, Possibly a High resistance short in that light circuit which is causing an abnormal amount of current to flow. A very close examination of the entire wiring harness in the Head Light circuit may reveal the problem.---Fwiw--Keoke
 
Definitely sounds like a partial short somewhere. Likely culprit is the switch itself. The site of the short is a hotspot. However, it should blow the fuse. Did someone put a larger fuse in to stop it from blowing?
 
Aloha, Dave! We've been worried about ya - hope you escaped harm from the earthquake.

/ubbthreads/images/graemlins/cheers.gif Mickey
 
Aloha Mickey,

Thanks for your concern, I was in Maryland during the earth quake but I didn't have any damage. The power was out 8 or 9 hours at my house, but that was it. Others were no so lucky.
 
Thanks for the suggestion. We had considered adding a relay as a solution, but it would seemed we would be jumping around the problem. A partial ground that adds load to the circuit is a logical fault so we will check that. Would poor or corroded connections be another cause of increased load?

The headlights are not a fused circuit, so a short can become a big problem like a fire if not fixed.
 
Ohh Yea, bad ground is normaly the problem but poor and corroded connections are the other primary cause in my experence, this sounds more like a badly corroded switch? Maybe.
 
Poor grounds increase resistance. Increased resistance means less current and less power. It needs to be a cut in insulation or contacts too close to ground.

I forgot about the lack of fuse. Is there a fusable link or anything like that in these cars? Or just let the fire disconnect the battery?
 
[ QUOTE ]
Poor grounds increase resistance. Increased resistance means less current and less power. It needs to be a cut in insulation or contacts too close to ground.

I forgot about the lack of fuse. Is there a fusable link or anything like that in these cars? Or just let the fire disconnect the battery?

[/ QUOTE ]

OH! lets see David some of these LBC's come with a 50 Amp fuse in them to protect a 10 Amp circuit.---Keoke- /ubbthreads/images/graemlins/laugh.gif--Its true- /ubbthreads/images/graemlins/yesnod.gif /ubbthreads/images/graemlins/crazyeyes.gif
--- /ubbthreads/images/graemlins/savewave.gif
DRe----I spruced it up so "U" no what I be going on about.--Keoke- /ubbthreads/images/graemlins/grin.gif
 
[ QUOTE ]
Thanks for the suggestion. A partial ground that adds load to the circuit is a logical fault so we will check that.

Would poor or corroded connections be another cause of increased load?



[/ QUOTE ]

Yes it is possible; 1] that a loose connector inside the switch itself would produce a "Hot Spot" capable of melting plastic. However, it would not initially result in an increased load. As a secondary condition it could and therefore contribute to the eventual demise of the circuit.---Fwiw---Keoke
 
MGB's lighting circuits are not fused. There's a bad ground or a corroded Lucar connector someplace in the circuit. Don't neglect to check parking light run as well, since ALL lights are on with the "second" position on the switch. Maybe isolate the two (parking and headlamps), pull all bulbs out and actually ohm out the grounds and the "power" wires at each light fixture? I know it'd be a chore, but it'd likely point out the faulty one on the first go.

Just my opinion.
 
Well DRe, I would not suggest the use of an Ohm Meter if I were looking for a High resistance short in a wiring harness. I would think a "Megger" would be a more practical instrument to use. OTOH, removing the light bulbs would be required to isolate the harness wiring from it's normal ground path. Should it be a Lucar connector in that circuit that bad connector should produce the "hot spot" and relieve the switch. Regarding the Lamp bulbs in their sockets a badly corroded one could produce a high resistance short at that location and simply removing them as you suggested might clear up the problem.--Fwiw--Keoke
 
[ QUOTE ]
Aloha All,

I'm working with a friend with a '74 MGB that has a recurring problem. The headlight switch becomes very hot, to the point that over time the plastic melts around the power supply terminal (brown wire). The headlamps are stock wattage and nothing has been added to the circuit. I would think that the wiring and switches were designed to carry the electric load required. Electricity is an subject I have enough understanding to be dangerous. I would appreciate any suggestions on the cause of the problem and how to fix it.

[/ QUOTE ]
This is Dangerous... have Fire-E in Car at all time if you continue to Operate this Vehicle is this Condition.

Most likely, your Switch is Bad and is techically trying to become a Light Bulb ;-)

Replace or Update to Relays--- U can do it yourself or buy from vendors such as us... it's your Baby ;-)

Cheers!
M
 
[ QUOTE ]
Aloha All,

I'm working with a friend with a '74 MGB that has a recurring problem. The headlight switch becomes very hot, to the point that over time the plastic melts around the power supply terminal (brown wire). The headlamps are stock wattage and nothing has been added to the circuit. I would think that the wiring and switches were designed to carry the electric load required. Electricity is an subject I have enough understanding to be dangerous. I would appreciate any suggestions on the cause of the problem and how to fix it.

[/ QUOTE ]
This is Dangerous... have Fire-E in Car at all time if you continue to Operate this Vehicle is this Condition.

Most likely, your Switch is Bad and is technically trying to become a Light Bulb ;-)

Replace or Update to Relays--- U can do it yourself or buy from vendors such as us... it's your Baby ;-)

Cheers!
M
 
Hot switches would make me think dirty connectors on them, or dirty contacts inside them. I was really surprised to find that the lighting wasn't fused on my Midget, either. Could explain why the PO had to have the wiring harness replaced after a fender bender shorted out the left taillight. I'd say try cleaning the contacts on the switch, and/or upgrade to relays.

I had my dimmer switch overheat to the point of losing lights one night (fortunately, the cop didn't seem to be concerned that my lights were blinking on and off as I was trying to resurrect them by holding the lever at the right position /ubbthreads/images/graemlins/eek.gif). I upgraded to relays while adding fuses to the headlight circuit, and now the headlights are brighter, the rest of the electrical system isn't getting big voltage droops when the lights are on, and no more switches getting warm.
 
Thanks to all for the suggestions. I'll post an update on what we find out over the weekend.
 
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