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Brake lights stay lit!

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William

Darth Vader
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Hi, I'm new here, heard about this forum on Miata.net ('96 Miata owner).
We've got a weird one for you all...my Dad has a '71 MGB, on which we just had the brake master cylinder replaced (by a very trusted mechanic who used to work on my beat up '76 Midget). Now the brake lights stay lit up all the time. I thought it was the brake light switch out of adjustment, so I followed my trusty Bentley manual for adjusting the free play at the switch. Lights still stay lit up. Any ideas?
-William
PS-on the plus side, with the brake lights on all the time, people usually don't tailgate!
 
Originally posted by William:
Hi, I'm new here, heard about this forum on Miata.net ('96 Miata owner).
We've got a weird one for you all...my Dad has a '71 MGB, on which we just had the brake master cylinder replaced (by a very trusted mechanic who used to work on my beat up '76 Midget). Now the brake lights stay lit up all the time. I thought it was the brake light switch out of adjustment, so I followed my trusty Bentley manual for adjusting the free play at the switch. Lights still stay lit up. Any ideas?
-William
PS-on the plus side, with the brake lights on all the time, people usually don't tailgate!
Click to expand...


Hi William, welcome to the forum! The brake light circuit is a fairly simply circuit, so the problem should not be too hard to track down. My first guess would the the switch is simply bad (or just dirty). If you have an ohm meter (or can borrow one), set it on the 1k setting. Then, disconnect one lead from the switch (doesn't matter which one) and measure across the switch with the ohm meter. If the switch is open (as it should be with the brakes not engaged) then the meter needle should not move at all (open circuit). It should only deflect when the switch is depressed. If the needle deflects to the right (probably to near "0" ohms) when the switch is not depressed, then the switch is either bad or dirty. You can either replace with a new switch (available at many fine British Car parts stores) or you can try pulling it apart and just clean it real good, then reassemble. Hope this helps. If it's not the switch that is bad, as I suspect, then you must have a short somewhere. That's not as easy to track down, but it's not impossible either. Try checking the switch first, then get back to us. Good luck.

Basil


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basil.gif

1967 Jaguar E-Type OTS
1E13333, Silver Blue
 
Thanks for the suggestion! I'll see if I cant scrape up a meter and check the switch. You'd think I would have checked the switch already, but I never start with the obvious!
-William
 
Originally posted by William:
Thanks for the suggestion! I'll see if I cant scrape up a meter and check the switch. You'd think I would have checked the switch already, but I never start with the obvious!
-William
Click to expand...

Let us know what you find out. By the way, I know where St Charles MO is, but not sure about St Charles, IL? I used to live in Troy IL, just across the river from St Louis, MO.

Basil
 
Basil-
St. Charles, Illinois is near Chicago, technically a "far western" suburb of the Windy City.
Strangely, I know several people from the "other" St. Charles!
-William
 
Originally posted by William:
Basil-
St. Charles, Illinois is near Chicago, technically a "far western" suburb of the Windy City.
Strangely, I know several people from the "other" St. Charles!
-William
Click to expand...

I always had intended to get up that way when I was stationed at Scott AFB, IL but never made it. So, how goes the brake light problem? Any luck yet?

Basil
 
No news with the brake lights yet. Computer problems as well, so only back on line yesterday. Cant seem to find anyone with an ohm meter who is also willing to let me borrow it!
-William
 
Originally posted by William:
No news with the brake lights yet. Computer problems as well, so only back on line yesterday. Cant seem to find anyone with an ohm meter who is also willing to let me borrow it!
-William
Click to expand...

You can get a decent one for $20 at radio shack (a good meter is something that any British Car owner with Lucas electrics should invest in
grin.gif
 
I am not that familiar with the B's braking system but I do have suggestion. If you have the dual resivour master cylnder (disk brakes front and drums rear) somewhere in the brake circuit there is a pressure differentiating block (PDWA).
when the system was rebuilt it is possible that the warning switch within it tripped and is allowing the brake lights to stay on. any good repair manual will be able to tell you how to recenter the switch.
 
Some how the smoke in the circuit is not moving so the brake lights are staying ondissconnect clean and reconnect all your connections and this will hopefully prompt that smoke to move be greatful that you have not lost the smoke it can be expensive to replace if it escapes
 
William, if you have a B an ohmmeter is a neccessary thing to own. Sears had one on sale last week for $10 that is adequate. a pretty good article on their use can be found here article
 
Huh, I'd forgotten about this! It got fixed last year during the spring tuneup, along with about three thousand other things. Turns out the switch was dead. Everything works fine now!
-William
 
That is usually the way!! When the brake lites stay on the switch is toast!! I'm glad you found it!! Next time, all you have to do is pull one side off the swuitch and they will go out!!
 
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