TR2/3/3A Brake Lamp

[Got_All_4]

Just noticed that I have to push the brake pedal fairly hard to get the lights to come on. Is there a adjustment. I turned the big 7/8 nut in a bit and had no effect.

 

[charleyf]

There is no adjustment on this original switch. Sounds like you need a new switch.

 

[TR3driver]

I agree, first symptom of a failing switch.
Just curious, do you use DOT5?

 

[Dr_Mike]

A related question: Is it possible to check the switch before installing it? ( Have a switch that came with the car, and am reluctant to fill the hydraulic system and then have to bleed it again if the switch is a dud.)

 

[charleyf]

If you are willing to find some way to put pressure into the intake of the switch ( air compressor) and see if you can get continuity across the terminals with an ohm meter.
I doubt that you would affect the pressure in the line much at all by removing a switch and immediately replacing it.

 

[Geo Hahn]

I agree, I have always been able to swap the switches w/o having to bleed the system.

Does anyone know... are the threads on these the same as on a VW pressure switch? I ask as there is a German-made version I am using on my VW that seems very durable. Costs a bit more but I think this is a place where a few more bucks are worth it.

 

[Dr_Mike]

Thanks: that is very comforting. I will go ahead and see what happens. I 'll try th air compressor first.

 

[TR3driver]

Usually I get fluid running out rather than air going in, but it might depend on whether you have the residual pressure valve or not. To reduce the amount of fluid lost, I use a bar to hold the brake pedal slightly depressed.

IIRC the threads are at least compatible with NPT. I once used a switch from an old Ford pickup and it fit the threads.

Later I got frustrated with frequent failures, and mounted a mechanical switch on the pedal box.

Then the most recent time around, I built a simple bracket to mount the switch inside above the brake pedal.

 

[Got_All_4]

Yes I am using DOT5. Why do u ask? I remember Randal installing the electrical switch. Sound like a quick fix. Maybe permanent too.

 

[Geo Hahn]

Yes I am using DOT5. Why do u ask?...

There has been discussion linking DOT 5 with premature switch failure.

https://www.britishcarforum.com/bcf...ot-5-brake-switch&highlight=brake+switch+moss

Half way down that thread I mention:

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.

I has now been 9 years and 15,000 miles and the switch is still okay. I do have a relay on that circuit which may help preserve the switch.

 

[3798j]

Ron Francis Wiring has a low pressure, Dot5 compatible switch. I use one on a '55 Ford F100 converted to front disc brakes and using Dot5. Pricey compared to regular switches.
https://www.ronfrancis.com/prodinfo.asp?number=SW-32

 

[Got_All_4]

I wonder what makes the original switch not compatible with DOT5.

 

[TR3driver]

I wonder what makes the original switch not compatible with DOT5.

On the one I cut apart, it appeared that the diaphragm was a metal on metal seal with no O-ring or similar, and enough Dot 5 had crept through the joint to foul the contacts.

I don't know if DOT 3/4 doesn't creep as well (although I suspect that us the case), or if it doesn't foul contacts like DOT5 does.

Supposedly, there is also a Harley-Davidson switch that will fit and stand up to DOT5. But I haven't looked into that myself.

 

[CJD]

I wonder what makes the original switch not compatible with DOT5.

I think it is. However, much like when you change your engine oil to synthetic...if you have a bad seal the DOT5 will compound it.

My 63 year old TR2 switch is still ticking along on DOT 5 for 1-1/2 years and counting. My TR3 lost it’s switch immediately on the change to DOT5, but went fine for 6 years afterward.

 

[Got_All_4]

It's probably been 6 years for mine. The problem could have been there longer and just didn't notice. I was backing into the garage at night with a big white box behind me and that's when I noticed there was no reflection. So the molecules are smaller in DOT 5?

 

[TR3driver]

Seems to be a lot more complicated than that.
Eg, https://www.sciencedirect.com/science/article/abs/pii/0043164876902374

 

[Got_All_4]

OK thanks Randal! I fully understood that. I get now.

 

[Dr_Mike]

I did not read the original article, but, if I am wildly optimistic, having a switch that has never seen DOT 3/4 might be a solution , easier than distilling something to get rid of impurities. I guess this shows there is a scientific background to our practical difficulties, but no real solution except to try it! I shall try, and frequently back up to a white object to make sure the lights work!. I shall also use LEDs

On to the next "problem"

thank you

 

[TR3driver]

FWIW, I have seen rapid failures with new switches that were never exposed to DOT 3/4 (unless the manufacturer did it).

Silicone oil does have a tendency to find it's way through gaps that other things won't go through; and spreads to cover the surface even when it has to oppose the force of gravity to do so. It will actually climb uphill! There has been some debate as to just how it does that; as shown by the article summary I referenced above.

 

[Got_All_4]

The next time I went out for a ride the brake light did not work no matter how hard I pushed the pedal. Couple days later I puled the car into the working garage and it work as it is suppose to. Go figure. Anyway not trusting it so went to Summit Racing and bought a universal brake switch. Brain stormed it for a while and it turned out a lot like Randles. Works perfect.