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spankyjr1, Journeyman
Category: Medium and Heavy Trucks
Satisfied Customers: 2601
Experience:  I worked on all types of equipment semis,forklifts,farm,ect.
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I have a Nissan us bus cmf87f. The air pressure light and

Customer Question

Hi, I have a Nissan us bus cmf87f. The air pressure light and buzzer are going off all the time, even when the air pressure builds enough to release the brakes. I can't find a great deal of info on it I.e schematics of the brake circuit and electric circuit.
Submitted: 10 months ago.
Category: Medium and Heavy Trucks
Expert:  spankyjr1 replied 10 months ago.

Morning Mat let me see if I can assist here

The air pressure reading on gage is only from one air tank and each one of your tanks are separated by a one way check valve, the check valve can be located on outside of tank or inside of tank, If a check valve fails your gage reading can be good but air pressure to buzzer signal may be low causing it to stay on.

Also note each air tank is separated internally so each tank is actually two tanks.

start by installing Schraeder valves in both sides of each tank (click here) ,, tee them in so you can leave in place for later use or to use for filling tanks,, find the tank that is lower on pressure than the others, then if you have external check valves between tanks replace the bad check valve, if you don't have external check valves and a tank has one side reading higher than the other the tank needs to be replaced.

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Customer: replied 10 months ago.
My gauge has 2 needles reflecting air pressure of both tanks.
Its only when the park brake is released that the pressure is sensed by swithches. My problem is electrical as all functions are working as they should i.e brakes releasing, sevice brake working , no air leaks. I need to know what is down stream from the pressure switches making the buzzer go off and the brake light to stay on to fix the problem.

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