dot 5 brake switch

Well I got the tr3 out of the garage and drove it another 75 miles and the oil looks good and turning black with carbon, what a nice site for a water leaking system. I drove it a little harder and ran it up 3K 4K and pulled off the gas to help the rings pull out and seat also. Anyways the neighbor comes over to tell me I have no brake lights. The switch is new and the system has had dot 5 since the restoration or compilement of this car. I bought the switch at the local shop. And it has maybe 2 hundred miles on it. Should I buy something special for the dot 5 systems or what?
steve

I use DOT5 and had no problem with the after market switch offered by Moss.

Have you put an ohmmeter on the switch to be sure it has failed?

Are the connections clean and tight (mine are the old screw connectors and in that case clean & tight is essential).

I do use a relay on the brake light system in the hope of extending switch life (the old switch gave out after only 50 years).

Maybe it's just luck of the draw or whatever. I had similar problems on my 3A with DOT 5 & the stock hydraulic switch, which at the time I attributed to uprated brake lights. Two replacements from Moss, plus one from FLAPS (off a Ford pickup as I recall) all failed within a year or so.

My solution was to convert to a mechanical switch. On the 3A, I mounted it on the pedal box (where the stop screw was located on earlier cars). Ugly but effective.


On the 3, I decided on a more stealthy approach:


I also added some relays so the corner turn signals double as brake lights

Well I went and unplugged the wires and connect them up, and it is the switch. I am going to get another switch and see what happens?
Steve

Since DOT-5 use became more common there has been a fair amount of discussion about hydraulic brake light switches. In particular I have seen this a lot with the BMC cars. You can try another hydraulic switch but there is no guarantee the exact same failure won't happen in short order. The long-term fix is to install a mechanical switch like Randall did. I have done this on the Mini and don't regret it.

I'm a bit confused...

Are you guys saying the DOT5 is breaking down the switches?

John

That's what they seem to be saying. I'll be in the test group on this one as I have DOT5 and the switch sold by Moss -- Intermotor brand p/n 51620, made in England.

Switch installed 9/9/2009, 87,853 miles. Now at 92,997. So far, so good.

Yes, that is what I now believe. Apparently what happens is that the switch diaphragm leaks just a microscopic amount of brake fluid (perhaps only under very vigorous braking), which then coats and eventually fouls the contact mounted on the nominally dry side of the diaphragm. Why that isn't a problem with conventional fluid, I can't say.

Of course, I could be wrong, it's just a SWAG. And many people (like Geo) seem to have no problem, so I feel one could reasonably blame it on poor quality switches. But the first switch I had fail had been working fine for many years with DOT 3/4, and failed within a year (IIRC) of switching to DOT 5. There was still no noticeable weeping from the switch when I removed it, but like the mechanical switch, the inside of the cavity had that "wet" look that I associate with a microscopic layer of silicone. (Like the "wax" they put in the water at the car wash, which is mostly silicone.)

FWIW, I actually had the same thing happen with my pedal-mounted switch. One of the fittings on the MC was weeping a bit, and over time fluid apparently migrated down the pedal and into the switch. When I took the switch apart to investigate, the cavity appeared damp and there was a hard substance on the contacts which I now believe was silicon deposited when the silicone DOT 5 got heated by a poor contact (caused by the DOT 5 in the first place). Fixed the weep, replaced the switch, and it's been working fine for several years now.

Interesting...thanks for the explanation. I have noticed that the silicone seems to thicken, almost like some of it evaporates over time.

I still have the original switch in mine. It'll truly be sad if the DOT 5 kills it after all these years...

John

It has nothing to do with the DOT 5 brake fluid. I had my hydraulic switch fail on the way home from TRA in Williamsburg back in 1996. So I ordered two new ones from TRF, one to install and one as a spare. It worked fine for 5 years but then it failed. I put in the spare one I had been carryng with me in the TR3A for 5 years. It didn't work - right from the start. I turned both these last two switches on a lathe to open them up and found that for both, the disk that the hydraulic pressure activates to close the circuit had turned black. YES ! The one with the DOT 5 fluid and the one that had never seen any fluid.

When I bought both of these new switches in 1996, I connected them to my air compressor hose to see that they worked and the continuity was fine for both. They were in a blue box marked "Made in England" and INTERMODAL.

Well I went to my favorite parts shop and purchased the best they had of that style, spade fittings 20.00. Made in China, but still they claimed it was the best they had to offer at, plus money back guarantee. Anyway time will tell

Don, in your case the failure may have nothing to do with DOT-5 fluid. However, there is a lot of anecdotal evidence out there among DOT-5 users that implies that somehow DOT-5 can (but doesn't always) result in hydraulic switch failure. I have also seen threads that discuss where people have found similar aftermarket switches that were labeled/marketed as being compatible with DOT-5 fluid. Such a marketing ploy should not be needed if there isn't some failure mode unique to DOT-5.

The problem is not unique to British cars. If you Google using keywords:
+"DOT-5" +"brake switch"
You will find threads for Studebakers, Porsches,... just about every collector car that is available.

Aloha Steve,

Like Don, I've openned up a couple of failed switches and in both cases, the contacts where badly pitted and coated with a black substance. I assume it is carbon. I didn't detect any fluid contamination like others, but may be wrong.

Another member of the forum, David DuBois has written an article describing his take on the problem with the switches. I have tried his solution and have had good results. here is the guts of his article:

Brake Light Relay

By David DuBois

The replacement brake light switches sold today are extremely light duty and don't hold up to the 3 amps drawn by the brake lights for very long before the contacts burn again (I have had them burn in just two weeks). Adding a relay to the brake light circuit will remove the heavy current from the brake light switch.

The relay to use is a general purpose 30 Amp automotive relay, sometimes known as a Bosch relay. They are available from Radio Shack and most auto parts stores. The relay can be mounted anywhere that is convenient. I mounted mine close to the original brake light switch, but it can just as well be mounted in the trunk (for the MGAs or MGBs), close to one of the tail light/brake light assemblies (if you do this, you will need to bring a source of 12 volts, other than the 12 volts that is switched by the brake light switch, back to where the relay is mounted). In the accompanying diagrams, I show a 0.47 microfarad capacitor across the contacts of the brake light switch and a diode across the coil of the relay. These are optional parts and can be left out if you wish. I added them as additional protection for the brake light switch. The capacitor is just soldered across the switch terminals. The diode can be soldered across terminals 85 and 86 of the relay. The advantage of the capacitor is that the capacitor acts as an arc suppressor and the diode collapses the field of the relay coil, eliminating any inductive surge across the brake light switch when it opens.

FWIW, on the TR3, my switch only carries current for one bulb. Looked like perhaps the original switch, worked fine at first, failed within a few months using DOT 5.

I question the wisdom of adding a capacitor across the switch contacts. When the switch is open, the capacitor will charge to full battery voltage, then dump it's entire charge through the switch when the switch closes, with the current limited only by the internal resistance of the capacitor. This is why you never put a capacitor on a generator field, for example.

Aloha Randall,

I'm not an electrical engineer, haven't played one on television or stayed at a Holiday Inn last night so I can't with any authority address your concern.

It was explained to me that the capacitor acts as a snubber to reduce arching as the switch contacts open when pressure is released from the hydraulics. This is similar to how the condenser (old name for a capacitor) reduces arching and piting of the points in the distributor.