Attention: Electricians

[xsteamer]

After a nice drive yesterday I was informed that I had no brake lights. I pulled the cover off the brake pedal to inspect the brake pedal switch. I shorted the two wires together and then checked the brake pedal switch contacts for continuity. Zilch!! Also noticed the centre pin of the contact loose (moving back & forth freely). Not being an electrical expert or MGB expert for that matter, should I conclude that I need a new switch? Also any precautions when changing out switch? /ubbthreads/images/graemlins/canpatriot.GIF

 

[Grantura_MKI]

You need to trace the green wire back to the current source. On most British cars, the brake lites have current all of the time...even with the ignition switched off.
Good luck!!

 

[Bret]

I’ll have to check this on a schematic but I know for a fact that my 78B’s brake lights only come on with the ignition on and won’t work at all with it off. I guess I could be wrong but I thought all MGBs where wired basicly the same way.

Back to the problem.
First things first – If you don’t already have one get a manual with a schematic or down load one from the internet for your particular vehicle. Then get yourself a basic digital multi-meter (DMM). I think Radio Shack has them for under $20. Good cheap investment anyway to have in your tool box - cause if you own an old car, you will evetually need it again.

Next check the fuses. If glass you should be able to notice if they are blown or not but if you ain’t sure use the meter set to ohms (resistance) and measure for zero ohms. Anything lower than 1ohm should be just fine. Assuming that all the fuses look good the real fun begins.

Using meter set it to dc volts, use the red (POS+) measure if any of the wires (lines) to the switch is hot (+12V) with the black (NEG-) test lead to chassis or ground. Depending if your car is a negative or positive ground you should read either +/-12 respectively. If you have twelve volts – you can use the meter’s Ohms (resistance) settings to check the brake light switch. If no voltage is detected trace back to the source.

With power off and the leads (or wires) disconnected - Measure across the switch while you engage or toggle it on & off. If you see zero (or close to zero) ohms across the switch when (on) closed then the switch is fine.

So if you have voltage getting to the switch and out the other side when it is on then you’ll need to trace back to the brake lights.

That should be enough to get you started. Try that and get back to us?

Good Luck!

 

[xsteamer]

Thanks Guys,
I will pursue your suggestions and get back to you.......

 

[Grantura_MKI]

I would suspect that you have other items that are not operational at the moment. The green wires also feed power to the guages turn signals. It is key hot and the ignition will need to be in the on position. Green wires are fed from the white wires on your fuse box. Sorry, had to get back into the MG mode and forget about what I am working on at the moment!
Good luck!

 

[Bruce74B]

That's right...the 72-on only have power when the ignition is on...just swap the switch with the key off and you should be fine. Be careful not to make it too tight and break the new one...start out slow with it loose and tighten it up a little at a time to make sure it comes on as soon as you depress the pedal.

Bruce /ubbthreads/images/graemlins/cheers.gif

 

[xsteamer]

I jumpered the two green wires and turned the ignition on and the brake lights work. I ordered a new switch. Thanks to everyone.

 

[Bret]

Good Job! /ubbthreads/images/graemlins/thumbsup.gif

See troubleshooting electronics problems can be fun. /ubbthreads/images/graemlins/grin.gif

Who needs an electrician when you have electrical engineers & technicians? /ubbthreads/images/graemlins/wink.gif

 

[Grantura_MKI]

Good to hear!!! /ubbthreads/images/graemlins/cheers.gif