T-Series Overheating light switch

[aroostok]

Not long ago I had replaced the stock B headlights with updated H4 halogen. This included new wiring harnesses for both lights with relays on both. What I've been experiencing, though, is that the headlight switch drastically overheats (in fact I melted one). Any thoughts as to what to do. I've even thought about going to a heavy duty toggle switch if necessary. Something certainly isn't right.

 

[Jack_Long]

If they are wired correctly, the relays should handle the heavy current for the lights and the dash switch should only turn the relays on and off, which draw very little amperage and will not overheat the switch.

Check to make sure that the headlight power is coming from the relays, not the dash switch, and that the relays are connected (through a fused wire) to a permanent hot lead (brown wire) on the fusebox.

If you are melting switches I suspect your relays are not hooked up right.

 

[Rick_Stevens]

Jack nailed it. The new switches are crappy, but with the relays wired properly the only load through the switch is the "signal" to energize the relays from the constant hot supply.

ONE NOTE: You should double- and triple-check that the leads going to the switch are wired correctly. With some configurations the lights still work as advertised, but you can end up running more current through the switch than you realize. If you don't have it, check out this PDF file at Advance Auto Wire: (https://www.advanceautowire.com/mgb.pdf).

 

[Kimberly]

Not long ago I had replaced the stock B headlights with updated H4 halogen. This included new wiring harnesses for both lights with relays on both. What I've been experiencing, though, is that the headlight switch drastically overheats (in fact I melted one). Any thoughts as to what to do. I've even thought about going to a heavy duty toggle switch if necessary. Something certainly isn't right.

What's the model year of your B? Does the marker lights get power from the blue wire?

 

[aroostok]

Got the wiring diagrams. Replaced the switch and found that, in fact, the problem was not being caused by the headlights but by the red/green (or green/red, can't remember which off hand as I'm typing here at the computer) dual wire connection. As I read the wiring diagrams I see that the connections are to the fuse block and to the rheostat for dimming the dash panel lights. Haven't checked out the rheostat yet but could it be showing too much resistance that it could cause the switch terminal to overheat. If so, could a single pole single throw switch do just as good, realizing, of course, that I wouldn't be able to dim the dash lights (like, they're bright to begin with?!).

 

[Kimberly]

Got the wiring diagrams. Replaced the switch and found that, in fact, the problem was not being caused by the headlights but by the red/green (or green/red, can't remember which off hand as I'm typing here at the computer) dual wire connection. As I read the wiring diagrams I see that the connections are to the fuse block and to the rheostat for dimming the dash panel lights. Haven't checked out the rheostat yet but could it be showing too much resistance that it could cause the switch terminal to overheat. If so, could a single pole single throw switch do just as good, realizing, of course, that I wouldn't be able to dim the dash lights (like, they're bright to begin with?!).

You can bypass the reostat altogether by connecting all the wires going to it on one side of the switch.
Look to see if the red/green wire is chaffed or pinched and is shorting against the chassis.
If possible, disconnect one of the red/green wires going to the headlight switch to determine if it is the wire going to the dash lights or the wire going to the fuse box.
Someone else had a blue wire going to a marker light in the trunk short against the body and burn up two headlight switches. Early '70s models did not have fused marker lights.