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MG MGB Technical - Brake light switch/ headlights
| 1972 B. My brake lights don't work and i can't find the brake light switch to check that. Do i have to take of the pedal box cover in the engine bay to find it ?|
That box is a real pain to remove and i don't want to fiddle with it unless i have to.
Also, one of my sealed beam headlights no longer works on high beam. Can i get replacements from my local Auto store or do i have to go through Moss ?
Thanks in advance
|On a 1972 B, the brake switch should be prominently displayed right on the front side of the black steel stamping that covers the brake and clutch master cylinders.|
It works in the reverse of what you'd expect. When you depress the brake pedal, the arm pivots and the metal blade above the pivot naturally moves back towards you. There's a spring loaded plunger in the switch that comes out and completes the circuit, turning on the lights. The switch default is always "on" and the brake pedal in its normal location works to interrupt the circuit.
Usual issues are:
1) poor ground at the rear
2) disconnects or corrosion in the pin connections
3) worn switch
4) improperly adjusted switch
I've had a long and frustrating relationship with my switch. things to do:
- Check the grounds and connections (live wire IIRC is green)
- have a supply of spare sleeves, I've had sleeves disintegrate
- Switch itself can be repaired if you'd rather not wait for a replacement. This past year mine has had its posts re-soldered, e.g.
- Adjusting the switch is tricky.
How's that? If adjusted "in" too much, you can inadvertantly engage the brake. I once cranked it in so much there was a slight drag on the brakes, the hydraulic fluid expanded and locked the brakes solid after a short drive. That took a little time to figure out. This year, my brake lights worked when I picked the car up from storage and switch was soldered. The lights failed on my first longish drive.
It turned out that the switch had been turned in a bit too much and turning it out about 60 degrees restored function.
The tricky thing about the adjustment is that the top of the pedal arm is a blade about 1/8" thick. The plunger from the switch has a little play in it, the blade itself gets worn by the plunger over time, and the pedal arm will rock a bit on the pivot. All of which means that the adjustment is critical. In my case, its initial position (after my mechanic's repair) permitted intermittant operation. A slight outward adjustment sorted it.
Hope this helps.
PS - old style lights should be available at your local discount auto store. If you haven't already, this should be an opportunity to change to halogens. Taking off the chrome trim right can be tricky, if memory serves, a hooked wire works best.
|Just bridge the green and green/brown wires on the switch that John Z describes with the ignition on. If the lights work now then the switch is faulty, if not then there is a break in the wiring somewhere. Check with a test-lamp or meter for 12v on both wires with the contacts bridged and the ignition on. No 12v, check and connections back to the fusebox, 2nd fuse up. If you have 12v on both sides of the switch check the 4-way bullet connector by the right-hand rear light cluster. Don't forget you could have two blown bulbs, or one blown bulb and a bad connection, or various other combinations of problems.|
If it *is* the switch, then be aware that some replacements are poor quality, can't handle the load, and fail soon after fitting. If that is the only type you can get you may have to fit a relay with diode protection.
With the headlight again check the connections before shelling-out for a new sealed-beam unit. There are bullet connectors by each headlight, and the one by the right-hand (facing forwards) can affect the left-hand bulb as well as the right hand-bulb. These connectors are the most likely to have problems as they are exposed to the elements. In my case it was neither headlamp nor connectors, but the insulation in the wiring between the two had cracked, and the copper conductors had corroded through. Could also be the headlamp connector.
|Gerry - Follow John's and Paul's advice for troubleshooting the brake lights. If it turns out that the switch is bad, you will need to install a relay/arc suppression circuit to make the replacement switch last for more that a couple of months (I had one that only lasted two weeks). For information on making and installing a relay/arc suppression circuit, or purchasing one, see my article at: http://homepages.donobi.net/sufuelpumps/Other_Subjects/Electrical/General/Brake_Light_Relay.pdf|
Cheers - Dave
|David, would your capacitor/diode setup be better than the capacitor arc suppressor used in older SU fuel pumps?|
|Robert - The capacitor on the older fuel pumps was used in conjunction with a swamping resistor of about 100 ohms that was would around the outside of the coil. If you go back even further, the only arc suppression for the points in the fuel pump was the swamping resistor alone. In spite of this rather rudimentary arc suppression circuit, the points in the pumps lasted for a surprisingly long time because of the heavy duty construction. The replacement brake light switches of today however are so light weight that I have had one switch last only two weeks. The diode that I use is placed across the relay coil to shunt any reverse voltage spikes to ground, rather than feeding back to the switch contacts. The capacitor across the switch contacts is to further reduce and spike that might be left over. It is a bit of a belt and suspenders approach and probably more than a bit of overkill. When I designed the circuit, I had gone through so many brake light switches between our TD and MGB that I was getting a bit desperate for a cure. since coming up with the circuit, I have not had any further failures of the brake light switches in either car. Cheers - Dave|
|Thanks David. I always learn something from you. I have an 65 with the hydraulic switch. Aftermarket ones failed in a matter of days to weeks as you say.|
I did find an OEM Lucas switch from LBCarco that has worked perfectly for many years. But, I imagine they're all gone now. When I need it I've got your plans to fall back on.
By the way, Burlen says their new pumps are multi-polarity. Does that mean they've gone back to capacitors instead of diodes to save money?
|Robert - "By the way, Burlen says their new pumps are multi-polarity. Does that mean they've gone back to capacitors instead of diodes to save money?"|
No, they have gone to a bidirectional Transient Voltage Suppressor. It looks like a diode but works in both directions and doesn't slow the operation of the fuel pump the way straight diode will. Cheers - Dave
This thread was discussed between 01/05/2008 and 06/05/2008
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