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MG MGB Technical - Brake lights (lack of them)

Haven't had much chance to use the '65 mgb roadster this year , so when a friends wife's car failed the mot about 6 weeks ago lent her the car. They popped down in it today and the brake lights aren't working. Shes bringing it back on Thursday for me to have a look at, going to check the bulbs then the switch but dose any body have any idea if there is a in line fuse and if there is were is it and what one is it ?
So far she loves the car and has done around 40 miles a day in it 4 days a week with the roof down when ever possible, another convert in the making....

marc

M C Lovell

Marc - The brake lights get their power from the same fuse that provides power to the turn indicators, heater fan and instruments. From the fuse block the power goes through the switch, to the lights. If the brake lights on both sides are out then the problem is either a bad ground (under one of the license plate bracket bolts through the back of the boot ore the switch. My bet woudl be the switch. Try pushing very hard onthe brake pedal and see if the lights come on. If they do, then you need a new switch. Since the replacement switches available today are so wimpy, they wont hold up for more than about 6 months, youshould make up a relay/arc suppression circuit to go between the switch and the lights. For instructions on making the circuit, see my article at http://www.omgtr.ca/technical/brakelightrelay/brakelightrelay.htm

And before you ask, the brake system doesn't need to be blead after replacing the switch. Have the switch ready to thread in as soon as the old switch is removed and you won't even loose more than a drop or two of brake fluid.
Good luck - Dave
David DuBois

Hi Marc.

Sound advice from David, if it does prove to be the switch I would consider seeking out a good but old second-hand one if originality is important to you.

If originality is not important you could consider making up a switch to operate directly from the brake pedal. I would consider using a normally closed microswitch with a long operating arm that operates when the pedal is NOT pressed, has anyone tried this ?.

If the rear earth connection was at fault it would also affect the tail lights and rear indicators.

Don
Don

Don - "...a normally closed microswitch with a long operating arm that operates when the pedal is NOT pressed, has anyone tried this ?" This is exactly how the later MGB brake light switch works. the only problem is that the replacement switch is terrible, The one that I used lasted exactly 2 weeks. Regarding the hydraulic switch, a very heavy duty version of the original is available from the Ron Francis Wire Company http://www.ronfrancis.com/showpage.php?page=main.htm
The only problem is that they are quite expensive, even before shipping charges to the UK. The relay/arc suppression circuit can be built and installed for less than $5.00 (I'll let you do the math to put that in ) on top of the cost of the switch. Cheers - Dave
David DuBois

Hi Dave.

I wonder if the UK / US language barrier is causing confusion ?.

I think of a microswitch as a small rectangular switch that has a very definite spring assisted toggle action, so that it makes and breaks cleanly.
As I am sure you know this toggle action results in significantly less arcing when the switch operates, and such switches usually have a very long operating life provided they are not given inappropriate loads.

I suspect that both the early hydraulic brake light switches and the later mechanically operated switches have no toggle action, though I have never investigated this. I further suspect that their contacts don't have a wiping action, which action does help to maintain reliable connection.

Don
Don

Not sure on the 65 but on later cars the third fuse down controls brake lights and also fuel gauge and indicators or wipers if I remember correctly. In other words, if it''s the fuse, lots of things won't work.
Steve Postins

Sorry Dave, you said that already. One other point then - I had the problem that the lights came on very late rather than not at all. It turned out to be a sign that the brakes need bleeding and the rears needed resetting as the pedal wasn't creating enough pressure to activate the switch until the pedal was relatively low. Of course I'd replaced the switch before I woke up to that.
Steve Postins

One thing about the rear light clusters, they do not have a wired ground (from the factory at any rate) but rely on the physical fixings to the wing, same as chrome bumper front parking/indicatir units. Only rubber bumper front indicator units have a wired ground. As such any ground problem will only affect one side at a time, but of course both sides could have the same problem, but both will need fixing independantly! Even then in practice as long as you only use one of the light circuits at a time it will appear to work correctly, only when you brake with the running lights on will neither work, and if you use the turn signals you will get various lights on both sides of the car flashing on and off.

All years of car powered the brake lights from the green circuit, but this is common to lots of other circuits in the car. However there are usually two green spade connectors on the fusebox so just one of these could be faulty, but normally some of the other green circuits would also be non-operational even when others were still working. A common failure point affecting both sides could be the switch, its connections, bullet connectors near the fusebox where applicable, and the 4-way bullet connector near the right-hand rear light cluster. Of course you could have one or more faults all contributing to non-operation and affecting one or other sides.

A 65 has a normally open switch which is operated by fluid pressure. Currently available replacement switches are said to be very poor quality and unable to handle the current, fitting a relay is quite common to reduce the load on the switch, and even then a quenching diode must be used. The later normally-closed switches are quite fiddly to set up, you must screw them in so that they are mid-way between the contacts just opening with the pedal fully released, and the pedal pushing the button in to its physical stop. Too far one way can result in the the lights intermittently not going off at all (endured hundreds of miles of people flashing me in a brand-new hire car before I worked out that one) and too far the other can cause the brakes to lock on when the fluid warms up.

Like Don to me a micro-switch is a snap-action switch, which neither the early hydraulic or later mechanically operated switches are. Wiping action or not, the original switches in both cases usually last for many many years and tens if not hundreds of thousands of miles.
Paul Hunt 2

Thank you all for your help. Going to check the Bulbs first, then the ground and then the switch. Also going to give the car a full service and hope to get the car back soon. Ive started to miss it now the sun is shining.

marc
M C Lovell

Don --Sorry, I didn't catch the "micro" part of the switch in you earlier post. Cheers - Dave
David DuBois

On my 70GT I had a switch that went high resistance rather than fail outright. When connected up correctly the brake lights wouldn't come on at all since not enough current would flow. Unplug the lights and measure the voltage on the wires though and there was 12 volts everytime the pedal was pressed. Took me ages to figure out what was going on. As Dave says replacing the old switch is easy. Almost no fluid came out when I unscrewed the switch.
Simon Jansen

I've been intending to solder the offending wire directly to the fuse block, but haven't. Whenever I take the car out, I turn on the wipers. If they work, I know the brake lights are okay. If not, I pop the hood and jiggle the connector until the wipers work.
Fred Doyen

Fred - Instead of soldering the wire to the fuse block, why not replace the spade lug conector on the wire so it make proper contact with the lug on the fuse block. Cheers - Dave
David DuBois

Thanks for all your help. A wire had come of the brake light switch so a easy repair (for once). Gave the car a quick oil and filter change, cleaned and gaped the plugs and went round all the grease nipples with the grease gun. Car is ready for this summers motoring, if I ever manage to prise it out of my mates wife's hands.....
marc
M C Lovell

Simon - that's why I always recomend taking voltage measurements on a suspect circuit with everything connected, rather than looking for a voltage on the end of a disconnected wire, or taking resistance measurements.
Paul Hunt 2

Hi Paul, yes, I should have known that. I guess I didn't because I couldn't get to the bare terminals on the wires when they are connected and I was too lazy to pull the connector socket off and use a jump wire between the bare terminals.
Simon Jansen

This thread was discussed between 16/05/2006 and 18/05/2006

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