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MG Midget and Sprite Technical - Brake light switch

The brake lights on my 73 midget arn't working. They were OK prior to a major repair after an acccident (Off side and rear) but now having got all the rear lights working the brake lights arn't.
Done all the normal chacks and ended up at the switch (operated by pressure in the brake lines). 2 queries:
1. The wiring diagram in the Haynes manual shows a green/purple wire coming up from the brake lights and a single green to the fuse box - I have got 2 greens! See picture. What is the 2nd green for?
2. Checking the operation of the switch with a continuity meter I would assume that when the brake system is pressurised the switch would 'make' and pass current from the fuse box supply to the brake light - mine dosen't switch so this is probably the fault. Can anybody confirm this before I change the switch?

Thanks in anticipation of the usual great help

Regards

John Turner
John Turner (Midget & MGB)

Picture


John Turner (Midget & MGB)

Isn't that your brake pressure warning switch? Your brake light switch should be operated by the pedal on the front of the pedal box.
CJCharvet

John,

Yes - the switches are notoriously rubbish.
I'm on my third in five years. Change it and you will almost certainly be sorted. Some folks fit a relay in order to reduce the load across this switch - they'll be along shortly to tell you how it's done.

CJ - Home models never had a brake pressure warning switch - we have the foot pedal version: when it goes to the floor it's a warning that brakes need attention.
Steve Clark

The brake light switch up in the pedal box can be easily cleaned and lubricated without removing. A can of contact cleaner with a lubricant can be inserted at the back of the switch at the switch cover seam and shot in. The long red tubes that come with the cans direct the spray so that most if not just some of the cleaner will penetrate the switch and fix the problem. Much better solution than an aftermarket switch which is of poor quality and will last for about a year.
Good luck
Bob
Bob 76 Midget

John
simple test for switch and circuit, connect the wires and take the switch out of circuit. If the brake lights light, therein lies the problem.

The extra green may be going off to power the reversing lights, or the windscreen washers, or the windscreen wipers. Or any of the circuits normally powered off ignition live. My money if I had any would be off to the reversing lights, based on the "take-off's" proximity to the little feed link down to the gearbox.

Haynes's wiring diagrams show that there are several places that the green wires can come off and all seem to vary model to model.

HTH

If you have to fit a new switch, two things.
First, if you are quick you will "probably" not have a serious "Bleed the brakes problem" I have had to fit two or three switches without a re-bleeding needed.
Second, I had to fit new switches because we (in here) think the replacement switches are no longer capable of working well under the load imposed. We mostly advise fitting the new switch and using it to switch a relay instead of the pair of 21 watt 12 volt bulbs. This saves the internal contacts of the switches from burning out.

The wiring is quite simple as shown in the diagram




Bill

Bob
Only 1500 Midgets have "mechanical" switch on the pedal box - previous models had the hydraulically actuated one we're discussing here.
I've often thought about fitting the 1500 switch next time my 'drolic one goes wobbly...
Steve Clark

The brake pedal and pedal box are both slightly different to accommodate the pedal activated switch.

The hydraulic switches can be encouraged with a tap by a hammer. They are apparently very discouraged by silicone brake fluid.
Daniel Thirteen-Twelve

Thanks to all for the advice -sounds good. I will do the test Bill suggest tomorrow and report back.

I also like Bill's mod so will think about that for the new switch if needed.

Also the explaination of the extra green makes sense - I can test that too tomorrow.

John

P.S. Yes the UK 1275 has the pressure switch in the brake hydraulic circuit and not the micro switch in the pedal box.

John Turner (Midget & MGB)

Steve, the microswitch must be a USA thing. My '72 has the green an purple and green wires from the harness going to the box and no pressure switch. If it makes you feel better John my microswitch has been a PITA as well.
CJCharvet

My US 1500 Midget needed a new brake switch when I bought it.

The PO (bless his poor stupid soul) had wired up a momentary push button to a set of wires that went from the cockpit out to where the wires for the brake lights were previously hooked up to the switch.
When he stopped he would find the button and push the switch (it really was just dangling there by the wires).

I bought a new switch from Pats MG parts on Ebay, green LUCAS box. It lasted about 6 months. I sent a note to Pat and he mailed me a new switch - no charge.
Six months later it died again. It would read open / closed on the meter but would not pass enough current to light the stop lamps.

I decided this time to connect a relay, I had a spare anyway. No more trouble since.

I thought you would enjoy the story about the PO.
Rick Bastedo

For information on how to make up a relay/arc suppression circuit to protect the notoriously crappy replacement brake light switches (whether hydraulic on the brake line or the mechanical switch operated on the brake pedal) see the article, Brake Light Relay in the Other Tech Articles on my web site at: http://homepages.donobi.net/sufuelpumps/
The other alternative for a hydraulic switch is to order one of the heavy duty switches (P/N SW 32) from Ron Francis Wiring at: http://www.ronfrancis.com/showpage.php?page=main.htm
Cheers - Dave
David DuBois

Connected the lights direct to the supply and they worked. Reconnected to the switch and gave it a clout with a heavy object - no luck so looks like a new switch.
As the car is only used for sprints and hillclimbs I will see how long the replacement lasts before wiring in a relay. Thanks for the guidance on this.

Rick, I liked the story about the PO - how many accidents did he have??!!

Thanks again for all the advice

Regards

John
John Turner (Midget & MGB)

I rebuilt my present Midget about 25 years ago. Decided to simply use the existing switch which is probably original. Still working today without the need for tricky relay circuits. Must be that Lucas electrics were far superior to todays efforts. Prince of darkness? my 4rse!
Bob (Robert) Midget Turbo

I have to agree with Daniel, i changed to Silicone & not long after the brake switch went, followed by the master cylinder seals, seems a bit of a coincidence.
I'm back on the proper paint stripper stuff now.
Brad (Sprite IV 1380)

" I will see how long the replacement lasts before wiring in a relay." If you are lucky, 6 months to a year. If luck is not with you, two weeks. This is from experience.

"Decided to simply use the existing switch which is probably original. Still working today without the need for tricky relay circuits." The original switches were pretty well bullet proof. They will finally wear out and quit working from old age (kind of like us), but they give a very reasonable life time of service - not so the replacement crap of today (see above).

"They are apparently very discouraged by silicone brake fluid." Silicone fluid is not the culprit, although the manufactures of the crappy replacement switches would really like everyone to believe that. The original switch in our TD quit working some 25 years after I had installed silicone fluid and the replacement would die anywhere from 1 to months after installation, but once I put the relay/arc suppression circuit in the brake light circuit, I have not had any failure in 7 or 8 years. The worst luck I had with crappy replacement switches was with mechanical switches working off the brake pedal in our MGB (failed in just 2 weeks) and it was nowhere near the silicone fluid used in that car also (so much for the claims from manufactures of crappy replacement switches that silicone fluid is the reason for their switches failing. Cheers - Dave

David DuBois

New switch - brake lights working - but for how long I wonder!
I have kept the diagrams for the relay mod so if this one fails in the near future I shall go down that route.
Thanks again for all the advice

Regards

John 'brake lights working' Turner
John Turner (Midget & MGB)

The new brake light switches seem to last a short time before failure (the will still often work with extreme pedal pressure). I managed to buy a small stock of Lucas NOS switches recently, so, fingers crossed, I should be OK for a while....

Mark.
Mark T. Boldry

Mark

Any chance of a Lucas NOS switch / beer token exchange situation?
Steve Clark

Mark & Steve - " I managed to buy a small stock of Lucas NOS switches recently, so, fingers crossed, I should be OK for a while...."

If those NOS switches are red box Lucas switches, they should last a good long period of time (I still installed the relay/arc suppression circuit to insure that the switched outlast me). I have had problems with the green box Lucas switches (sometimes it is hard to beat old technology). Cheers - Dave
David DuBois

I have heard that a switch for a Harley Davidson motorcycle will do as a replacement for the Spridget switch.
Alan Anstead

' I have heard that a switch for a Harley Davidson motorcycle will do as a replacement for the Spridget switch'

There must be plenty of replacement reliable switches out there - any suggestions for easily opbtainable reliable replacements in the UK?

Regards

John Turner
John Turner (Midget & MGB)

When I had the sprite for 2 years I had to change the switch every year. Now I kick the pedal very hard (I am rowing the masters world championship) at the beginning of the season and yes! A nonworking switch is doing its job again!

Flip



Flip Brühl 948 frog 59

Flip, I have had this in the past, occasionally, not always, a good hit or two on the pedal will bring the switch back to life.

Mark.
Mark T. Boldry

The good old "stamp on the pedal" fix

Problem is that although it pushes the contacts closed through the built up burnt out metal, it will be no time at all before the reconnected bits re-burn out. Crap metal is always going to do that.

(Experience talking here, the stamp treatment gave me a couple of weeks before it failed again.)

Alan aren't the Harley switches pedal based mechanical ones? If they are in-line fluid based it might be worth looking them up....
Bill

This thread was discussed between 15/11/2008 and 02/12/2008

MG Midget and Sprite Technical index

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