Auto-Park Brake Questions
A: Recently we’ve received several questions regarding the auto-park brake system used on some Workhorse P Series gasoline-powered motorhome chassis. To help owners understand how the system works, what could cause it to fail, and the results of such a failure, we contacted the company for an explanation. A product service technician at Workhorse provided the following information.
The auto-park brake system (option J71) is used on all P Series chassis equipped with Hydra-Matic 4L80-E and 4L85-E (the latter starting with 2003 model year) transmissions and with gross vehicle weight ratings (GVWR) above 15,000 pounds. This brake replaces the foot parking brake and parking pawl used on similar units rated at less than 15,000 pounds GVWR. (The parking pawl on the 4L80E transmission is rated at a maximum of 15,000 pounds GVWR.) Workhorse chassis (W Series) equipped with Allison 1000 MH transmissions do not have auto-park brakes; they have parking pawls. However, the new W24 (2004 model) chassis with an Allison 2100 MH transmission does have an auto-park brake system, as this transmission has no parking pawl. Allison transmissions are not available on P Series chassis.
The foundation for the auto-park brake system is a drum-type brake mounted on the rear of the transmission. When the brake shoes are applied against the drum, they stop rotation of the prop shaft, which holds the rear wheels from rotating. A spring-loaded chamber mechanically applies the brake. A hydraulic cylinder that obtains pressure and flow from an electric-powered pump releases the brake.
The brake is applied when the shift lever is put in the “park” position. The brake is activated when the park neutral switch and relay remove a 12-volt-DC power supply to a normally open solenoid valve. The valve dumps the oil pressure that holds the spring compressed, and the spring mechanically applies the parking brake. When the shift lever is moved to any gear position, 12-volt-DC power goes to the solenoid valve and it closes. At the same time, an electric-powered hydraulic pump provides pressure and flow to a cylinder that compresses the spring and releases the brake. The operation is similar to that of spring-activated parking brakes on air-brake-equipped units.
For a parking brake to meet Department of Transportation (DOT) standards, it must fail in a safe mode so that the parking brake is activated or remains activated in the event of any failure within the system. This means that any failure will result in the parking brake being applied or not being released. This is a desirable feature and is required to prevent a parked vehicle from rolling in the event of a system failure. For example, any loss of power to the solenoid valve, including a dead battery or a loose battery connection, can cause the brake to apply. Similar designs are used throughout the industry on motorhomes, buses, and trucks. Other failures that could cause the brake to apply include loss of fluid pressure resulting from leaks in the system, or contaminated fluid.
Owners also should be aware that connecting add-on electrical devices to the same circuit or wiring as the auto-brake have been known to cause problems. If you decide to add an aftermarket backup camera, obstacle sensing device, and/or a backup warning device, make sure it is fused separately if wired to the backup lights. The backup lights are most often wired through the neutral safety switch that is necessarily fused on the same circuit as the auto-park brake, as this is the switch that actuates it.
One concern owners have expressed is, what would happen if the brake should apply at high speed? The vehicle will come to a slow stop, as it does not have sufficient power to lock and skid the rear wheels, unless the vehicle is traveling on an icy or a slippery surface. However, severe damage can occur to the brake and it may not hold the unit in a parked position. In this case, the brake immediately should be checked, repaired, and adjusted as necessary. The prop shaft will not be damaged.
Should the brake become locked in a no-release condition due to a failure, it cannot be mechanically released. However, a qualified technician can remove the cable pin. This will release the brake, but the unit will not have a parking brake and cannot be parked without blocking the wheels.
To reduce the possibility of having an auto-brake failure, Workhorse recommends a yearly inspection of the system for loose connections, corrosion of components, adjustments, reservoir fluid level, fluid condition, and system operation. The cable should be adjusted at least once every year. The system has a light and buzzer that warns the operator if an adjustment is needed. The adjustment is required to provide proper holding power and is not related to failure to release. The system also should be inspected to make sure that added items such as reverse bells/lights or any other added equipment is not installed on the same circuit or wiring as the auto-park brake. An electrical overload resulting from such added items has caused the parking brake to apply or not release.
In May 2001 Workhorse issued technical bulletin 80101-T that described chassis wiring improvements for 2001 model-year chassis (after VIN# 5B4LP57G51333218 on March 12, 2001). One of the changes was to separate the circuits used for the auto-park brake and signal lights so that a shorted or improperly wired towed vehicle would not interfere with the auto-park brake.
Some owners have asked whether a replacement kit is available to convert the auto-park brake system into a pedal- or lever-type system as is used on some commercial trucks. The answer is no. All chassis equipped with automatic transmissions and hydraulic brakes without parking pawls have the auto-brake system to comply with DOT standards. This includes all Workhorse gasoline-powered chassis with Allison transmissions other than the 1000 MH transmission, which has a parking pawl.
Q: The August 2003 edition of Family Motor Coaching contained two terrific articles about fire safety and various types of fire extinguishers. Specifically, one article touted the advantages of “designer foam” fire extinguishers. I currently own several of Kidde’s “Fire-Out” foam extinguishers. Are these the same as designer foam? I called my local fire department and was told that they had never heard the term designer foam, so I called Kidde and got the same response. If my current extinguishers are not designer foam, what’s the difference?
Roger Berndt, F261848
A: To answer this question we contacted Mac McCoy, who wrote the “Fire Extinguishers” article that appeared in the August 2003 issue of FMC. Here is his response:
“The term designer foam is used to show that there is a difference between the old AFFF, FFFP foam, which is what the fire service, military, and industry use. There are new foams now available to the general public. The new foam is non-toxic, non-hazardous, biodegradable, and has an emulsifier that helps break down the hydrocarbon. I picked up the term from a supplier. It’s just a term used to show the difference between new foam and older foams.
“If you have a Kidde foam extinguisher, you have one of the best. Kidde may not call it ‘designer,’ but based on what it has done during the past three years that I’ve shown it to RVers, it sure fills the bill. In 1999 Kidde introduced one of the best foam extinguishers for RVs, and there are now several other smaller manufacturers of foam extinguishers in the United States, and many in the United Kingdom as well.”