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MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - Bad brake light switch.

My brake lights are on as soon as I turn the ignition switch on (54 TF). I haven't had a switch apart but does this mean that the contacts are always made and therefore it is toast? Does the switch work using hydrolic pressure to make the contact? Is there a comerically available switch and when I unserew the switch part of the brake line connector will the brake fluid leak out? Thanks for any advice.

John
John Progess

The brake light switch is hydraulic...it is closed by the pressure of the brake fluid... Moss sells a switch however, i have had two of them go within the last 6 months...
Check the archives for "brake relay" and with that in combination you should have one that lasts ... (some have had no problem with the Moss switches).
Very little brake fluid leaks out when changing them....good idea to bleed afterwards, however depends on how fast you change it.
Before pulling the switch is there anything else that could be causing the lights to stay on.... ? (When my last one went i had to really push down hard to get the lights to come on...was told that is usually the symptom of a bad switch).
gordon lawson - TD 27667

John, here's copy from an archive posting:
For you folks here in the Colonies the solution is at your neighborhood NAPA store. Their Echlin line of stoplight switches have a Model SL144 for grub screw connectors or SL 147 for push-on connectors.

Not in umbrance, but I beg to differ with Gordon. Unless you move the brake pedal, there should be no loss of fluid when you r/r the switch.
Bud Krueger

John - Check the wires to and from the brake light switch and make sure there is no external short between them. With the wires off of the switch terminals, check for continuity between the two terminals with a multimeter set to read resistance - it should read infinite until the brake pedal is pressed. If it reads 0 ohms, then it will need replacement. If the switch is reading ok, the next place to go is your turn signal switch to make sure that it is wired correctly, sinced the brake light circuit goes from the switch through the relay, to the lights.

If you find that the switch needs replacement, get the switch from NAPA as Bud suggests, they are the same switch as Moss sells only a lot less expensive. If they don't have the grub screw model (someone reported that they are no longer made), consider converting to the spade lug (push-on) model. You will find that from NAPA, the spade lug model is about 1/3 the price of Moss Motor's grub screw model.

If you find that the switch does need to be replaced, read my article on brake light switches and how to make a relay and arc suppression circuit at http://www.omgtr.ca/technical/brakelightrelay/brakelightrelay.htm
and install that at the same time you install the new switch. Doing so will save the new switch from premature failure.

I have to side with Gordon regarding loss of brake fluid while replacing the switch. Since the system is vented throught he master cylinder cap, fluid will leak out of the open fitting when the switch is removed. The leakage is very slow, so the loss will be on the order of one or two drops, even if you drop the switch while replacing it. Likewise, because of the leakage of fluid resulting from the vented master cylinder, no air will be introduced into the system while replacing the switch, so ther is no reason to bleed the syatem after doing so. Good luck - Dave
David DuBois

John,
David covered it all ...except one other small thing...."fill" the new switch w/ fluid before you install it.
I used the NAPA switch and have had no problems with it ....but I was pretty sure it was actually not only cheaper, but also built from metal instead of "baklite"...am I wrong here?
Cheers,
David 55 TF1500 #7427
David Sheward

David - All of the switches are the same, metal body and thread, bakalite contact and terminal section, plated copper or brass fixed contact, spring phospher bronze fixed contact, neoprene diaphragm and a plated mild steel pressure disk all crimped together. The newer, after market switches don't have as much material used in the contacts and as a result they usually don't hole up very long under actual operating conditions (although some do - something that I have not determined why as yet).

As for filling the switch hole with fluid before installing, the very small air column in the threaded section of the switch is such a small volume that it doesn't make any difference in the operation. I have done it both with and without filling the switch with fluid and couldn't tell any difference. If it makes you feel better, fill the hole, it certaainly doesn't hurt.
Cheers - Dave
David DuBois

David,
Where did you get the 0.47 microfarad capacitor. My local radio shack and the 18 year olds inside who know less about electronics than me (which is not saying a lot), don't list/sell this capacitor. There are .33 4.7 47 4700 and a lot of multiples of 1 microfarad. Where do you get this specific capacitor?
Also what NAPA part number for the 30 amp relays are you guys using. Our local NAPA has a book with about 3 pages of relays and they all look the same to me.
Thanks, Rob (electronicallly challanged!!).
Rob Silverman

To John in Utah, Are you by any chance running DOT 5 (silicon) brake fliud. The usual brake light switches I have been able to get all fail very soon when exposed to silicon brake fluid. The gasket inside the switch apparently swells in the presence of silicon fluid. Dave Ahrendt put me on to a very satifactory ( though expensive)solution. Ron Francis Wire Works P/N SW-32 ($19.00). I have had one in each of my T series for over 2 years now with no failures.
Good luck with your problem, Bob
Bob Jeffers

For Rob in Syracuse, You can put two Radio Shack P/N 272-1070 in parallel which will get you .44 mfd. Close enough for govermenbt work. Alternatively you could order from Mouser Electronics. They have a 1 3/4 in. thick catalog crammed full of electronic goodies. They don't have a minimum order either. Try <www.mouser.com> Phone is (800) 346-6873 But you will want to know their P/N for the capacitor before you call.
Good Hunting,
Bob
Bob Jeffersn

Rob - The .33 microfarad from Radio Shack will work just fine. I came up with .47 microfarad through a very extensive use of electronics theory and reams of calculations and the fact that the .47 microfarad was the one I had in my junk box. I have had a problem getting them also and finally ordered a batch from G C electronics.

Bob - I have had silicone fluid in our TD for nearly 20 years and the brake light switch finally failed. When I cut it open to conduct some failure analysis on it, because of all the noise about silicone fluid causing the switches to fail, what I found was that the original Lucas switch simply had failed of old age. The failure had nothing what so ever to do with the use of silicone fluid. This business of silicone fluid causing the switches to fail is the result of someone on one of the BBS reporting that they called Echlin (who makes the switches sold by NAPA) and Echlin told them that silicone fluid would cause this type of failure. After doing some experimentation with switch contacts purposley contaminated with silicone fluid, I can state catagorically that this is nothing more than a red herring being thrown out by manufactures of present day replacement switches to take the heat off of their inferior switches. The real clincher for me was when I changed our MGB from the hydraulic switch to a mechanical switch that works off of the brake pedal and had it fail in less than two weeks and it was never anywhere near the fluid. The business of the silicone fluid causing the diaphragms to swell and cause a problem is also something that manufactures are using to take the attention off of their inferior products - all brake fluid that carries a DOT number, be it DOT 3, 4, 5 (silicone) or 5.1 (synthetic) is required to be 100% compatible with ALL materials found in vehicle brake systems, including natural rubber I seriously doubt that there are any brake systems on any working vehicle that has any natural rubber components in them any more, but if they do, all DOT approved brake fluids can be used safely with them. You can read this for yourself in the Federal Motor Vehicle Safety Standards, Standard No. 116; Motor vehicle brake fluids at: http://frwebgate.access.gpo.gov/cgi-bin/get-cfr.cgi?TITLE=49&PART=571&SECTION=116&TYPE=TEXT

Since adding the relay and arc suppression circuit to the brake light switches in our TD and MGB, I have had no further failures in either of them in over 5 years. Both cars still have silicone fluid in the brake system, which is further testamony that the problem is not being caused by silicone fluid. While the MGB has a NOS mechanical switch installed, the TD is using the hydraulic switch made by Echlin, the same as the two switches that had failed earlier before I added the relay and arc suppression circuit. Cheers - Dave
David DuBois

Answer to Dave in Bremerton, ----- I would give Echlin credit for knowing something about what causes their switches to fail, after all in this age of lawsuits they better know what causes failures.

Now if what you say is true, howcome brake cylinders fail with original UK rebuild kits and don't fail with neoprene replacements from the local auto parts emporium? I have thought that the UK kits were made with the natural rubber that they used back in the 50's and that the piston rubbers that I got from my local auto parts store were neoprene, hence no failures.

I used to work at Fasco Ind. which made brake light switches in Rochesater N.Y. for the big three auto manufacturers back in the 50's (17 cents each in bulk pack). There was, in their version of the switches, a gasket to seal the phospher-bronze diaphram and prevent the brake fluid from getting into the electrical part of the switch. What that gasket was made of is now beyond my recall (senior moment).

Anyway your relay modification reduces the current that the contacts have to carry and is probably a worthwhile upgrade. But the Ron Francis Wire Works switch is simpler and so far works very well.

Should we be starting another thread to continue this discussion? Remember I'm a newbie.
Best regards,
Bob
Bob Jeffers

Ooooohhh!!! - Controversy in the TD-TF forum - how quaint, and pretty unusual in the subject, too!

Do companies lie about failure analysis? You bet! Whether it's due to monetary concerns, apathy and indifference, or sheer ignorance, it happens more often than you may care to admit. I've seen several instances of it, and the general discussions when the perpetrators were caught was, "How can we limit our exposure?"

Anyway, that's NOT my reason for posting. What I want to know is, "Does the thread pattern of these aftermarket switches match the original, in terms of size, tpi, and taper, or do you just force them to fit?"

Thanks for any info - - Alec
Alec Darnall

Alec,

The NAPA switch is a direct replacement for the original. works great.

Good luck,
Evan
Evan Ford - TD 27621

Bob - I am not familiar with the Ron Francis Wire Works switch, can you direct me to a web site or something that shows what it is? Or are we just talking about a specific switch manufacture? I am always open to something that is easier or better.

I am afraid the I have to agree with Alec, I don't have much faith in large corporations being truthful with the consumer. I do believe that sometimes it is a matter of the person answering the question not knowing what they are talking about. After reading in the shop manual for a Audi that we had a number of years ago that silicone fluid was not to be used in the brake system, I went to the local shop and asked the service manager why. The answer I got was that they used natural rubber... I cut him off, saying that I had heard that song and dance about British cars and it wasn't true about them either, at which point he admitted that he didn't know. I later found out that silicone fluid should not be used in any system that uses proportioning valves, ABS or other such systems that have very small orifices that the fluid passes through because silicone fluid will cavitate when going through such small openings and become aireated. This I have seen for myself and was the answer I should have been given by the service manager at Audi.

I have also used both UK kits supposedly made of natural rubber and the American made neoprene seals with equal success with both DOT 4 and silicone fluid. I have been told that DOT 3 fluid is not compatible with the natural rubber seals, but have no experience with that combination. Cheers - Dave

David DuBois

To Dave ---- I put "ron francis wire works" into Google and the first 2 entries were for their web site. I went to the site and clicked on "view catalog" and put in "brake light switch". Several things came up and then SW-32. Which is the switch I was refering to. The catalog states that it is specifically rated for silicon fluid and has heavy duty contacts to last 200-300% longer. They also showed a relay for use in brake systems, might be interesting. Hope this info helps, Bob
Bob Jeffers

Just installed a new switch from Moss and a relay from Dave... certainly can't beat the price on Dave's (if he still has more left!)

ddubois at sinclair dot net


gordon lawson - TD 27667

Bob - Thank you for the info. I will look into the switches. I hope that the 200 to 300% longer life is not compared to the Cutler Hammer mechanical switch I used in our MGB, the one that only lasted 2 weeks ;). Cheers - Dave
David DuBois

I looked at the website for Frances Wireworks and wondered about the following:
--will the switch fit in our stock fitting ?
--what kind of terminals are on the switch and how would you connect to the terminal ends on that switch?

Dave
There is a guy on ebay that is selling 30/40 amp relays and a wiring harness and a diode for $1.95 each!! Would these work or is the relay rated at too high an amperage ?
Cheers,
Rob
Rob Silverman

I thought the brake light switch on my TD failed so I bought a new switch.
Before I had a chance to install it MY BRAKES LOCKED UP!
Problem was a new master cylinder was holding pressure!
After one press on the pedal light would stay on.
Afer ten stomps on the pedal pressure built sufficiently to lock the brakes.

Did this happen to you?

Mark Sherman
mrkshrmn@hotmail.com
Mark Sherman

The forward relief hole in the MC was/is plugged... had it happen to mine... took it apart 3 times before i discovered how small that hole is. I used a needle to clean it out as a pin was too large. When you look down into the MC, there are two holes... the forward one looks to be about 3/16ths or so...however, it only goes about 1/4" deep... In the middle of that hole is the tiny relief one....you have to find it by feel and work your pin/needle/cleaning pick into it.
At work i saw our welding torch cleaners and one of them would have been perfect, like a little needle sized file.
Am sure there is a dental instrument as well, but don't want to go there....
gordon lawson - TD 27667

Rob - The 30/40 amp relay should work just fine, the guy is probably getting them from MCM electronics for $0.99 each (or less in large quantities). If he gets the diodes and the socket/harness from the same place in quantity, it would be easy to put the three together for $1.95. I don't care for the wires on the sockets that come from MCM (I'm sure that it is adequate, but it looks a bit light to me and is the wrong colors), so I crimp lugs onto the proper color and size wire and insert them into the sockets, crimping the capacitor in with the wire. The diode, I solder onto the coil of the relay inside the case.

I'll let Jeff answer your question on the switch from the Ron Francis Wire Works, since it appears that he is using one of them. Cheers - Dave
David DuBois

For Rob Silverman ----- The thread on the Ron Francis switch is 1/8 in. pipe thread and fits the 4-way brass fitting without trouble. The contacts on the switch are, I believe, what we called Douglas connectors 50/60 years ago. Today I believe they are called spade connectors and the mating fittings for the harness I got at the local auto parts emporium. I crimped them onto the harness and then plugged the ends of the harness onto the switch,everything worked out real good. I think you will be real happy with that switch. Cheers, Bob
R. K. Jeffers

This is for Rob Silverman ----- I relied on my memory and it has done me in (again). I was under the TD today and took a peek at the brake light switch. It does not have the spade terminals I told you it did, it has a connector that Connects to both terminals at once. And it is right on the harness so I must have put it on the harness when I installed the switch.I don't recall having to buy the connector so it must have come with the switch.

The spade terminals were probably on the switch I took out to put the Ron Francis switch in.

I hope my bad info does not mess you up.
Regards,
Bob

R. K. Jeffers

Thanks everyone for all your help. I will try the relay and capacitor trick first and if no luck I will go with the expensive switch.
Rob
Rob Silverman

Rob - Be sure to install the relay and arc suppression circuit at the same time you install the new switch. I have found that the switches start to burn as soon as you start using them and can develope enough resistance that they will not operate the relay even though the resistance is low enough to operate the lights. Cheers - Dave
David DuBois

Still a bit confused here - how can a switch using a 1/8" American Standard Taper NPT (.405" OD and 27 tpi) correctly fit a hydraulic brake union using the British Standard Pipe Taper (.375 OD and 28 tpi) without damaging one, or both, threads?

Or is the brake union actually plumbed with American sizes?

Very curious - - Alec
Alec Darnall

This thread was discussed between 30/04/2005 and 09/05/2005

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.