Brake help needed
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Freshman User
Joined: Aug 2010
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Brake help needed
B700 with hydraboost brakes, the backup brake motor(Electric) keeps going on and off while driving down the road. Light and buzzer going on and off. Eventually the relay sticks and it stays on constant, even with the key off. I change out the relay and all is well for usually several hundred miles, then it starts all over. I have changed the belt on the fluid pump, and changed the pressure switch in the boost unit. Dont know if I need to replace the belt driven boost pump, or the electric back up system next. Any Ideas? Thanks Dennis
Senior User
Joined: Feb 2010
Posts: 290
Likes: 2
From: Seattle Washington
So the problem is that the relay is failing?
The contacts are welding together? And not allowing it to turn off once the engine is running?
Or is the problem that the back up motor is having to run constantly because the pump isn't creating the pressure required to switch the safety switch?
The contacts are welding together? And not allowing it to turn off once the engine is running?
Or is the problem that the back up motor is having to run constantly because the pump isn't creating the pressure required to switch the safety switch?
Laughing Gas
Joined: Nov 2003
Posts: 898
Likes: 1
From: southeren Oh
So the problem is that the relay is failing?
The contacts are welding together? And not allowing it to turn off once the engine is running?
Or is the problem that the back up motor is having to run constantly because the pump isn't creating the pressure required to switch the safety switch?
The contacts are welding together? And not allowing it to turn off once the engine is running?
Or is the problem that the back up motor is having to run constantly because the pump isn't creating the pressure required to switch the safety switch?
As allways when checking electics Check ALL grounds and If there are extra wires the vehicle didn't come with unhook them.
(I hate butchered wireing harnesses)
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Freshman User
Joined: Aug 2010
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The buzzer/light/ pump kick off and on while driving most of the time for 1 to 2 seconds, then eventually sticks the relay and runs constant. Wiring has not been altered, bus has about 60,000 miles on it
Senior User
Joined: Dec 2010
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From: Lajord, Sk,Can.
I have a 1985 F-700 which i believe has the same system. There should be an oil pressure switch by the rear axle mine is on the passenger side. My wire had a break on it, so i put a new end on it. Another time it did the same thing, this time it burn't out the electric pump motor. There is a black box called a sensor module on left lower dash 2 screws hold it on. I took it off openned it up to find the circuit board was burn't. Replaced it 2years ago. Mine has also gone off if fluid level is low. Hope this helps Pat. I have a wiring diagram somewhere if you need more info , if i can't find i can get it from one of my mechanics.
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