TR4/4A No Brake Lights

[KVH]

I have tail lights. But no brake lights. At the switch, if I depress the brake pedal the switch plunger extends and completes the circuit. I know that much from a test light. But I don't know where now to look. Neither fuse appears to be bad. Is there also an in-line fuse somewhere?

The switch has a green and a green/purple wire to it. What is the theory of the switch and wiring? What should I be looking for?

 

[TR3driver]

The green wire should be hot whenever the key is on. When the pedal is pressed, the switch should conduct, putting power into the green/purple wire. The GP wire in turn should run to the brake lights, supplying power to them.

My next step would be to check for power on the GP wire, with the lights connected, key on and pedal pressed. Assuming they are relatively easy to get to, I'd probably start at the brake light fixtures; but of course if there is no power getting there, then the next test is at the brake light switch.

One of the things that can be very confusing; it is quite possible to measure 12v on a wire that is disconnected from the load, and still have no power when it gets connected. A bad connection, dirty switch contact or even a damaged wire may pass enough current to power a voltmeter or test light; but not enough to power the intended load. Same thing applies to using a continuity tester (aka powered test light).

As a side note, I have one of those cheap 36" F-clamps from HF, where I drilled out the rivet that held the fixed head, took it apart and reassembled the whole thing as a "spreader" instead of clamp.
https://www.harborfreight.com/36-inch-bar-clamp-96208.html
It is very handy for holding down the brake pedal when necessary. Just lay it down through the bottom of the steering wheel, with the fixed head resting on the pedal and slide the other head against the steering wheel to hold the pedal down.

 

[DNK]

On the 4 , does the power have to be on??

Well if it's green I guess it does
DOH!

 

[TR3driver]

On the 4 , does the power have to be on??

According to the diagram I have, that's true even for the TR8. As far as I know, all Triumphs were that way originally. I actually modified my TR3 so the brake lights work with the key off, as I have been known to 'switch off' while driving, especially if the engine is running hot.

 

[trglory]

And, just for giggles, make sure you have double filament bulbs (1156?) installed in the brake lights.

 

[Andrew Mace]

1156 is the single-filament (i.e., flasher ONLY) bulb. 1157 is the dual-filament (i.e., stop and tail) bulb.

 

[trglory]

Hi Andy; Wasn't sure from memory which was which, thanks for jumping in.

 

[KVH]

Now I've got brake lights. No idea what I did, but thanks for all the suggestions. I'm checking for loose wires and that switch, itself. Will report back. thx

 

[TR3driver]

Keep an eye on them! Things that fix themselves frequently also unfix themselves.

 

[DNK]

The plastic brake switch can be retro'd with a metal modern new one from other manufacturers

 

[TR3driver]

Oddly enough, though, I've had more trouble with the "metal" ones from FLAPS than with the plastic reproductions from TRF.

 

[tdskip]

Keep an eye on them! Things that fix themselves frequently also unfix themselves.

In addition to being true, also pretty darn funny.