J71 AUTOMATIC APPLY PARKING BRAKE (aapb), Toubleshooting and Repair Information

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1 J71 AUTOMATIC APPLY PARKING BRAKE (aapb), Toubleshooting and Repair Information By sb1 at irv2.com, Aug 14, I have a 2003 Damon DayBreak 3285 with 25,000 miles. Though the coach is a 2003 model, I believe the Workhorse P32 chassis is a 2002 model. The auto parking brake failed recently, the light switch leaked oil and blew a fuse, left me stranded. My breakdown away from home was preventable, I should have stopped the motorhome 2 weeks earlier when the "auto park" light came on intermittently. Everybody with an aapb should study the irv2 postings about warnings that you may or may not get from the system when it is about to have a potentially catastrophic failure. This document summarizes what I put together after about 40 hours of studying hundreds of posts on this subject. Maybe it can save others from having to spend the same amount of time, and it might lessen some repair and towing bills. I am submitting it in "Word" form instead of pdf so others can update it from time to time with any added info or corrections, feel free to use and change this in any manner desired. If we are lucky, oldusedbear may adopt it! Most of the credit for the information shown here should go to oldusedbear and irv4fun, whom I have never met but who have been extremely generous with their time for all of us regarding this subject. Sorry about the plagiarism or lack of specific credit to others. Also, NO WARRANTIES ARE BEING MADE HERE. ALSO, WORK SAFELY, AND CHOCK THE WHEELS BEFORE GOING UNDER ANY VEHICLE. Contents: Rest of page 1: brief overview of how aapb works. Page 2: electrical schematic, annotated. Page 3 to end: discussion of components as annotated in schematic, in a way to make troubleshooting and parts-ordering easier. For more information I suggest contacting oldusedbear, or see his website at the AutoPark Library at This site has a lot of part numbers, history, and other information not included below OVERVIEW: The J71 automatically applied parking brake system consists mainly of (a) a spring operated device called an actuator, located between the transmission and drive shaft, which, when the motorhome is parked, causes brake shoes located beside the actuator to grip the drive shaft so the motorhome rear wheels cannot rotate, and (b) a hydraulic pump system, controlled by the items on the following pages, which overcomes the spring in the actuator to release the brake shoes from the driveshaft so you can drive the motorhome. The hydraulic system has to function properly all the while the vehicle is moving, otherwise the spring in the actuator can bring you to a very sudden stop. Every owner of this system should study it well before having problems, and know what to look for, and what preventative maintenance can do to prevent problems. If everything is working fine, to release the auto park brake, move the shift lever on the steering column from the park position, and push in the park brake switch push-button if it had been pulled out. Note both conditions must occur for the brake to release. Putting the shift lever back to park, turning off the ignition, or pulling the push/pull switch will de-energizes the hydraulic system. As the hydraulic pressure then decreases, the spring tension in the actuator moves a piston that pulls a cable to apply the parking brake. 1

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3 The letters below, such as A, B, C, etc refer to the red letters on the schematic on Page 2. A. The FUSE. Usually this is 10 amps, and was supplied by coach manufacturer. In my 2003 Damon it is in the fusebox by left foot when you are in drivers seat. If the fuse blows, something else is usually wrong that caused it to blow, probably a short in one of the items mentioned in the rest of this section. As mentioned in various sections below, current flows through this fuse for most the main components of the entire aapb system. When the vehicle is in park with the ignition turned on, current from this fuse runs from A to C to E to ground. And when the vehicle is in any other gear, as long as the yellow button D is pushed in, current from this fuse runs through D and then through F, and then splits with some of it going through I then J then ground, and some of it going through H then ground. So there are many components which could short out and blow this fuse. B. The PARK BRAKE INDICATOR SWITCH on the dashboard. This switch indicates an adjustment is needed in the cable that the actuator spring pulls on to set the autopark brake shoes. It is usually not part of the problems encountered with the rest of the aapb system. This light staying on may mean the cable has stretched between the actuator and brake shoes, or the brake shoes are worn. Check with your manual or a WH dealer on how to adjust the cable and/or brake shoes that grab the drive shaft for the aapb system. Also, in some motorhomes this light comes on with or without a buzzer sound, or a buzzer may sound without this light coming on, indicating the cable or brake shoes on the driveshaft need adjusting. C. The PARK/NEUTRAL POSITION SWITCH is located on the left side of the transmission near the middle. When the ignition is on, and the column shifter is in the park position, this switch is closed to supply current from the fuse, A, to the park/neutral relay coil, E, which opens the relay contacts, F, which cuts off any power to the park brake motor switch, I, and the solenoid valve, H. Cutting off the power to I and H ensures that the hydraulic pump does not turn on, and so the vehicle will stay "parked" due to the spring pressure in the actuator. When you are ready to drive away, so the ignition is on, and you move the shifting lever out of park, this switch opens, so power from the fuse, A, no longer reaches the relay coil, E. That causes the contacts F to go into their normally closed position. Then, if the yellow button D has also been pushed in, current can travel from fuse, A, though D and then through the relay contacts, F, and then continue on to energize both the park brake motor switch, I and the pump relay coil J, which closes the relay contact K, turning on the pump, L. And, in a parallel path to I and J, the solenoid valve, H, is also energized; this closes the plunger within the valve to prevent pumped oil from returning to the pump reservoir. So with H, I, J, and K and L all energized, the pump builds up the pressure to 1600 psi, more than enough to retract the spring in the actuator, which pulls on a cable to take the pressure off of the brake shoes that were grabbing the driveshaft. The switch I turns the pump off at 1600 psi, but if the pressure even goes down while you are driving, the I will turn the pump back on briefly to build the pressure back up. 3

4 D. The PARK BRAKE PULL SWITCH is the large YELLOW BUTTON switch mounted on the instrument panel. This is a manual activation switch for the park brake. This switch is normally closed, pushed in. When pulled out, this opens the circuit between the fuse A and the relay contacts F, so no power will ever get to the pump, ensuring that the spring in the actuator will keep the park brake shoes engaged. The body manufacturer determines final location of this switch. E. and F. The PARK/NEUTRAL POSITION SWITCH RELAY is a normally closed relay. In the schematic, E is the relay coil that, when energized, magnetically pulls the contacts F open. Its function is described more fully in A and C above. It is located above the engine, mounted to the right hand side of the driver s island on 1999 and 2000 chassis; refer to a service manual for location on 2001 and newer chassis. (I believe it is often to right of radiator, above the tank and pump). G. The Brake Warning System. The AUTO PARK WARNING LAMP on dash turns on when the hydraulic system pressure is less than 450 psi or when the electric/hydraulic pump is running due to the park brake motor switch being closed. The lamp will flash at partial release pressures. NOT SHOWN ON SCHEMATIC: Down by the transmission, on the hydraulic line that goes into the actuator (see page 1), is a pressure switch often called the PARKING BRAKE LIGHT SWITCH. It is tee'd into the hydraulic line which supplies the actuator assembly with oil to keep the actuator from applying the brake while you are driving. The switch controls the groundside of the AUTO PARK lamp. The lamp switch closes, and the dash light lights up, when system pressure is below 450 PSI if the ignition is on. When you first turn on the ignition, with shift lever in park, this pressure switch turns on the auto park light. When you shift to a gear such as "drive", the hydraulic pump L is turned on by other circuits, and the pressure builds up enough to make this switch open up, which turns off the auto park lite. After that, when you are driving, there is circuitry, not shown on page 2, which will make the auto park lite come on whenever the hydraulic pump runs. This pressure switch is notorious for leaking oil, leading to a system failure. Sometimes the oil causes a short that blows the fuse. A spare should be stocked, and this switch should probably be changed out every few years. This switch can be gray or brown. The GM part number is , supposedly it is the same as Napa number PS305 which is available at Amazon. If you see a brief flicker of the auto park warning light while driving, it may just be that the pump kicked in for a moment to keep the hydraulic pressure up so the spring would not apply the brake to the driveshaft. But if you see the auto park light stay on for any length of time at all while driving, it usually indicates a serious problem. The auto park light may be staying on because the pump is running continuously, possibly because of a leak causing low oil in the reservoir which starves the pump, or else a weak pump. But the auto park light may also be staying on because the pressure switch down by the actuator is saying you have less than 450psi, and the brakes are about to be applied by the spring at any moment, no matter what speed the motor home is moving. So if the auto park light stays on while driving, get off the road and stop and find the problem asap. You may have just a few miles before the whole motorhome becomes immobilized by the aapb system. H. The PARKING BRAKE SOLENOID VALVE consists of a solenoid (coil with plunger) and a hydraulic valve which is tee'd into the hydraulic line between the pump and the actuator. 4

5 It is located in the component box in the underside of the vehicle on 1999 and 2000 model year chassis. On 2001 through 2004 models it is on the pump assembly on the driver's side of the radiator. When there is no power to the solenoid, this valve is normally hydraulically open and will bleed hydraulic oil back into the pump reservoir, thus allowing the spring in the actuator to set the parking brake. Power applied to the coil closes the valve so that hydraulic pressure can be built up in the actuator to overcome the spring in the actuator and thus release the parking brake. There are reports of the solenoid coil shorting out from time to time and causing fuse A to blow. In that case the plastic case on the coil may show signs of melting. I. The PARK BRAKE PUMP MOTOR SWITCH is often called the pressure maintenance switch, or just pressure switch. It mounts to the parking brake pump housing. It closes and turns the pump on whenever system pressure is below 1200 psi, and it opens to turn the pump off when system pressure reaches approximately 1600 psi. When this switch closes it allows current to flow through I, and then J and then to ground, as mentioned in A. and C. above. This is the notorious switch often called the "RGS", or rotten green switch. In later models it is brown. This switch can fail internally, and show no signs of leaks, yet cause the pump to come on and stay on while you are driving. At that point the pump will go to 1800 psi, its relief valve setting, and that high of pressure can the blow the seals out of the other pressure switch mentioned in G above. A spare switch should be stocked, and this switch should probably be replaced every few years. The GM part number is for the green one, and the GM part number is for the brown one. They appear to be interchangeable, you can use either. Amazon usually has them. When replacing this switch, be careful to remove the switch only, not the adapter fitting in the pump housing. If you remove the adaptor you may lose a small spring and/or poppet that keeps the switch from cycling on and off too fast. J and K. The PARK BRAKE PUMP MOTOR RELAY is a normally open relay. In the schematic, J is the relay coil that, when energized by I, magnetically pulls the contacts K closed to turn on the hydraulic pump L. It's located in the component box underneath the vehicle on the inside of the passenger frame rail in 1999 and 2000 model year chassis. On 2001 and 2002 models the pump assembly, including this relay, was moved to the front driver s side of the radiator core support. L. The PARKING BRAKE PUMP ASSEMBLY includes the pump, motor and hydraulic reservoir. Usually it has an unmarked yellow cap on the translucent reservoir. The reservoir uses ATF fluid, not brake fluid or power steering fluid. You should carry spare ATF fluid for it. When energized, the pump builds up enough pressure to overcome the actuator spring and release the parking break. After it builds up the initial pressure needed, it might cycle on and off occasionally while driving as governed by the pressure switch I. As mentioned above in section G, when the pump is running you will see 5

6 the auto brake light G on the dash light up, and if that light stays lit for more than a mere flicker while you are driving, there are probably serious problems, and aapb failure may be imminent. A pressure relief valve in the pump limits system pressure to 1800 psi. The pump and motor are very durable and don't generally need replacing. The pump and motor and tank assembly is located in a component box on the passengers side of the vehicle on 1999 and 2000 model year chassis. The component box is on the inside of the right frame rail behind the transmission. On 2001 through 2004 P Series models the pump assembly was moved to the front driver s side radiator core support. M. The Fusible Link receives power directly from the battery, and supplies the pump L when the relay J-K is energized.. 6