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MG MGB Technical - Brake Lights staying on

My brake lights have started to stay on permanently on my 1972 mgbgt. All other lights seem to work OK.

I beleive they are activated via a sensor on the brake pipe union in the engine bay.

Is it likely that the sensor has gone and needs replacing? In addition, if it is the sensor does it require the system to be drained to replace?



C Broadhurst

If the sensor is in fact the problem, you will need to bleed the system once you replace it. Plus, the switch needs to be "balanced" once it's reinstalled.

But - have you checked the brake switch itself? It could be going bad, or could just be out of adjustment due to a missing or loose lock nut.

R.
Rick Stevens

I doubt that it is the sensor in the brake lines that is causing the tail brakelights to stay on. That sensor only lights up a light in the dash telling you that there has been a loss of fluid in one half of the system. Like Rick, the first place I would look is the brake light switch located on the master cylinder housing. It adjusts in and out and is touched by the brake pedal lever at the top end to break the circuit for the brake lights. The switch likely needs to be turned in a couple of turns to get the switch to contact the pedal lever. You might want to check into why this is not working at its current setting. Is the locknut loose on the switch itself? Is there enough wear in the pivot bolt running through the pedal lever to create slack (potentially to the point of it breaking during a hard stop)? These bolts do wear noticeably over time, and should be replaced from time to time.

Erick
Erick Vesterback

A faulty brake light switch could cause your problem. You need to confirm which type of brake light switch you have fitted in your car:-

1. mechanical switch fitted in the pedal box cover (could be faulty or just need adjustment)
or
2. pressure activated switch fitted in the brake pipe junction (no adjustment possible and will require replacement if faulty, and brakes bled after fitting).

You can use a multimeter or test bulb/battery to confirm if the switch is working (should be open circuit when not pushing brake pedal/ closed circuit when pedal pushed.

Regards

SL
Steve

If the brake light switch is the hydraulic type, fitted to a union on the right side of the engine compartment, the problem could be that thereis some residual pressure in the hydaulic system. First, disconnect one of the wires from the switch (either one) and see if the brake lights go off - this will eliminate a short from somewhere else. The next step will be to unscrew the switch from the union (disconnect both wires from the switch to unscrew it), then connect the wires back tot he switch and see if the lights come on. Note: before unscrewing the switch, place a piece of plastic wrap over the M/C filler and screw the cap back on over it. This will slow the rate at which brake fluid will dribble out of the union while the switch is out. You should also position paper towels or rags under the union to capture any fluid that dribbles out. If the brake lights still come on with the switch uncrewed and the wires connected, you have a bad switch and will have to get a new one. If, on the other hand, the brake lights stay out with the switch unscrewed, you have a hydraulic problem that is not releasing all the pressure when you release the brake pedal. It is not necessary to blead the brakes after doing this procedure or after replacing the switch as no air is introduced into the system. The only thing that will happen is that you will loose a small amount of brake fluid while doing the checks and/or replacing the switch. Oh yes, when finished fussing around with the switch, be sure to remove the plastic wrap put over the M/C filler.

If your brake switch is the type that is on the pedal box cover, it may only need to be screwed in a slight amount to get the brake lights to go out. If this is the case, be sure to tightn the jam nut after adjusting the switch. Cheers - Dave
David DuBois

Thanks to all those who have replied. To confirm it is the hydraulic brake light switch fitted to the union on the right side of the engine bay.

Dave I will follow the procedure you have listed to try and diagnose.

Cheers

Craig
C Broadhurst

UK is much simpler than US, a 72 simply has a hydraulic brake switch - no failure sensors or any other gubbins, nor the mechanical switch mounted on the pedal box.

First remove one of the wires from the switch and check they go out, it could be the pair if wires to the switch has rubbed and shorted out. If they go out with the wire off and come back on with it reconnected (ignition on of course) then it is the switch. However these usually fail open-circuit i.e. fail to light the lamps.

The other problem is that current supply from many stockists is of very poor quality and failure of the new unit will occur in a very short time. The only way to get round that is to use the switch to operate a relay and the relay to operate the lights, but the relay will still need a quenching diode across its winding as even that inductive load can cause the switch to fail.
Paul Hunt 2

The other problem that I have found is that of a fault in the brake servo which casuses hydraulic pressure to be trapped in the system. If this happens then the lights and for that matter the brakes to a lesser extent will also remain on.
Iain MacKintosh

"The other problem is that current supply from many stockists is of very poor quality and failure of the new unit will occur in a very short time. The only way to get round that is to use the switch to operate a relay and the relay to operate the lights, but the relay will still need a quenching diode across its winding as even that inductive load can cause the switch to fail." See my article for building and installing a brake light relay/arc suppression circuit at: http://www.omgtr.ca/technical/brakelightrelay/brakelightrelay.htm
Cheers - Dave
David DuBois

This thread was discussed between 08/04/2007 and 10/04/2007

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