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Brake Test Light Always On!

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Obtong

Obtong

Jedi Hopeful
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Hello again.

I hadn't driven my 74B in several weeks so I decided to take it round the block a few times but the car wouldn't start. The lights worked, pump was ticking etc., and I soon realised that the battery charge was low.

This evening when I entered my darkened garage to work on my B I noticed that the red brake test light was on even though there was no key in the ignition. It must have been this light that slowly drained the battery.

Is it likely that the switch itself is broken, or do I have a problem with the braking system. I thought the light only came on if the button was pushed and the brakes were OK.

Thanks,
Dom
 
Hi,I struggled with my test switch/indicator lamp about a year ago...your problem could be either a short, defective test switch or the shuttle valve that is in the brake hydraulic distribution manifold that could either have hung up or is actually indicating a fault your braking system. You can probably narrow it down with a mulitmeter but be careful to disconnet the battery of course if you are using the ohmmter...

Jim
 
Hi Dom et al,

There are some blocking diodes which might be the problem, or so I've been told by my local expert. These might also be the possible problem with engines not turing off (not running-on) and the radio loosing power when you flash the passing lights.

Might be something to look at. My experet says they can be replaced by Lucas parts or plain dioeds from Radio Shack.

Dave
 
[ QUOTE ]

Is it likely that the switch itself is broken, or do I have a problem with the braking system. I thought the light only came on if the button was pushed and the brakes were OK.
Thanks,
Dom

[/ QUOTE ]
Your car has a shuttle valve that moves off center when the two brake hydraulic systems get "out of balance". There are two separate hydraulic systems. i.e. when one of the two systems loses a little fluid the valve moves off center toward that side & operates the warning light switch to let you know that something may be leaking. Over time, even a slight leak can cause the shuttle to move off center. I would first check for brake fluid leaks & then re-center the valve to turn the warning light off or on as the case may be.

You can determine which way the valve is off center, crack the opposite brake pipe fitting connection slightly & lightly push the brake pedal to cause brake fluid to move the shuttle back to center & turn the light off. Might take a little experimenting. Be sure that both master cylinder chambers are full first, & top up afterward.

I don't know about pushing a button, I'm not that familiar with MGB's, but the principal is the same, the valve needs to be centered. "Normal" cars would have the light only come on when the hydraulic system has experienced a fluid loss. & they wouldn't have a button. Someone more experienced than I can tell you if the light should be off or on. Probably a shop manual gives the whole procedure. The shuttle & switch are parts are #29 & #30 here.
https://www.mossmotors.com/Shop/ViewProducts.aspx?PlateIndexID=29029
Good luck,
D
 
In thinking about this even more, I'd suggest the simplest thing to do would be to just pull the connector off of the sensor that goes into the shuttle valve. If the light on the test switch goes out, then you know that the shuttle valve isn't centered and that you might have a problem in the brake's hydraulic system. If the light stays on, then there is a short between the connector and the switch. Good luck.
 
Thanks for your suggestions. I will certainly check them out. I have little experience with auto wiring/troubleshooting, so this will be a learing experince for me. (As has been everything else with my B!) What I thought was interesting was that as soon as the battery was fully charged the brake check light went off! (I used a charger that charges the battery in the car.)

I'll be sure to follow up to this thread as soon as I have pin-pointed the problem.

Dom
 
/ubbthreads/images/graemlins/shocked.gifWell Dom, I sure am glad you asked that question as I have been expierencing the same thing with my 74 "B". I had a right rear brake cyclinder leaking so I replaced the shoes and both of the cylinders. But to no avail, the light would not go off. SO, I pulled out the fuse, which is also the interior light and horn fuse. Figured I would get around to figureing out the light problem later. So thanks again for asking that question. /ubbthreads/images/graemlins/grin.gif
Dave
 
I pulled out and inspected the brake test switch. It was a little corroded, but it appeared to test OK with multimeter. The light stopped coming on, but if the car wasn't driven for a couple of weeks the battery would lose enough charge to prevent the MG from starting. I have since installed a "trickle charger" ($26.99 from AutoZone) behind the passenger seat. The car starts every time now, even if not driven for a while.

Dom
 
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