1990 Chevy heavy medium truck (28k gvwr), gas engine and a 5 speed, with hydraulic brakes, with the brake booster powered by the power steering pump. This truck has an electric motor that should give power to the brakes when the ignition is on but the power steering pump isn't putting out enough pressure, (stall, belt broke, etc.). When you first switch the truck on, without starting the engine, it works. After you start the engine, it doesn't. If the engine dies, you have no brakes. The "Aux brake" light is always on. If you check the voltage at the motor, it is only about 5 volts. Does anybody here know how this system is wired, or have access to tech info on this thing? Thanks guys, Mitch
Small metal relay above brake booster to the left. Check voltage coming in... The wiring is pretty simple you should be able to trace Em. So what exactly is wrong? Just the light on?
Hey Mitch, I'm not a Chevy guy, Ford uses the Bendix HydroMax booster, does Chevy use the same?
Willie, this is a similar unit to the one pictured. Pro94, The older ones I have worked on have been very simple, but this one adds an electronic control module to the old sensor-relay-motor circuit, according to the overview in the operators manual. When you first start the engine, the motor comes on, but if the engine chokes off, it doesn't work. I started trying to draw the wiring on this thing with Paint, but now I'm going to try using words: There is a sensor in the hydraulic outlet of the booster with a blue and a black wire coming from it. The small black wire goes from the sensor across the firewall and into the right side of the cab. The small blue wire leaves the sensor and tees off. One end goes to the coil on the relay, the other end goes into the large bulkhead connector on the left side of the firewall. The other wire on the relay coil goes into the cab in the same place as the small black wire from the sensor. The large black wire from the motor goes down the right frame rail in a large wiring harness. The large red wire goes from the maxi-fuse block thru the relay to the motor. When you short across the relay connector on the large red wire, the motor works right. If you short across the sensor nothing happens. When measured to ground: the red wire shows 5 volts all the time, the blue wire on the sensor shows 3.5 volts, and the black sensor wire shows 3.1 volts. I assume the electronic control module is under the dash, I haven't looked under the dash yet, I am giving the wasp spray a few days to work.
I had one that the pressure switch connector was not making a good connection I had to zip tie it...