How to Run a Clark Powershift by Hand When the Pneumatics Fail
The Clark Powershift Transmission Manual Transmission procedure is something almost nobody talks about because in forty years of working on fleet trucks, I can count the number of times someone actually needed it on one hand. Most technicians never learn it. They swap the unit, order a new control valve assembly, and write off the problem as expensive. That is not always necessary. The Powershift is a semiautomatic, fully synchronized, dual-clutch transmission that was widely fitted to medium-duty trucks and school buses from 1965 through the early 1970s. It uses air-over-hydraulic pressure to shift two clutch packs, and each clutch pack handles half the gear range. When the pneumatic control circuit loses pressure or the shift valve sticks, the transmission locks into whatever gear it was last in. There is no mechanical lever connected directly to the clutch forks. The only way to move the truck is to manually actuate the clutch packs or drop the tail housing and turn the input shaft by hand. I learned this the hard way on a 1969 Ford F-700 that had been sitting for three weeks after the air compressor died on a highway pullout. The Powershift had settled into third gear and the driver had no way to get it out of neutral when we reconnected the air system because the shift valve had already sealed off internally. Cold weather had thickened the fluid inside the control cartridge. I could not get the truck to engage first gear through the normal air shift cycle. So I removed the upper cover on the control housing, found the manual shift solenoid block, and used a 3/16 Allen key to press the shift pin directly. It took maybe four tries with small increments of pressure before the clutch pack slipped in and the transmission dropped into first. That worked every time after that.
The basic manual shift sequence goes like this. Park the truck, chock the wheels, and relieve all air pressure from the system. Then remove the top cover of the Powershift control housing. Underneath, you will see the shift valve manifold with small cylindrical pins corresponding to each gear range. You push the appropriate pin with a long alignment tool or a sturdy piece of hardened rod while someone turns the engine over slowly. The goal is to let the clutch pack disengage and reengage under zero air pressure. Do not force the pin. If it resists, tap it lightly with a mallet. Forcing it will strip the sleeve inside the control valve body, and that is a $400 part you did not need to break. Here is the part that nobody puts in the service manual. The manual override only works reliably when the transmission fluid is warm. Cold ATF in a Powershift control cartridge is essentially hydraulic cement at below freezing temperatures. If you are doing this in winter and the pin will not move, pour a small amount of warm transmission fluid over the control housing. Not hot. Warm. Ten minutes is enough to drop the viscosity enough for the pin to slide freely. I have seen people use propane torches on the housing and warp the aluminum casting. That cracked the seal and ruined the entire unit. Do not do that. Another thing to know is that the manual shift procedure is not a repair. It is an emergency exit strategy. Once you have the truck moving and back at a shop, the real issue has to be diagnosed. The most common failure point is the shift solenoid coil on the control module, followed by a clogged orifice in the air distribution block. If you are resetting the transmission manually because the solenoid failed, replacing the solenoid and cleaning the orifice with brake cleaner and compressed air will restore normal operation in about an hour. If you skip the diagnosis and just keep using the manual override every time the air system fluctuates, you will eventually destroy the clutch packs because you are slipping them without the proper friction material cooling cycle.
There is a workaround for the cold weather issue that I developed after dealing with this on a fleet of twelve school buses in upstate New York during a particularly brutal January. I fabricated a small heating pad from a 12-volt silicone heater mat and routed a thermocouple wire through the control housing insulation. The mat draws about 15 watts and keeps the fluid above 40 degrees Fahrenheit even when the truck has been sitting overnight at -10 degrees. The installation takes about twenty minutes and costs roughly $35 in parts. It eliminates the need for the warm fluid pour and makes the manual shift procedure viable any time of year. The biggest limitation of this whole setup is that the Clark Powershift was designed around a clean air supply. Dust, moisture, and oil vapor in the supply line will foul the shift valve passages within two to three years if the air filter is not changed regularly. I have pulled control cartridges out of units with less than 50,000 miles where the inside looked like it had been painted with a thin layer of varnish. That is from unchecked compressor blowby. The fix is to install an external spinning disc air filter between the compressor and the Powershift control system, and replace the desiccant in the air dryer annually. That adds maybe an hour of labor but prevents the kind of internal buildup that makes the manual shift procedure irrelevant because the passages are already clogged solid. If your Powershift control housing is already beyond the point of manual override, you have two real options. The first is a complete control cartridge rebuild kit, which runs about $280 and covers most of the worn seals and orifice plates. The second is swapping to a later model Clark R200 or R225 control assembly if the truck is old enough that parts are available. Neither is particularly cheap, but both are cheaper than replacing the entire transmission case. The manual shift procedure buys you time to decide which route to take.
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I do not recommend attempting this on a Powershift that has been subjected to heavy towing abuse or repeated overheating events. In those cases the clutch packs are already glazed or the drum surfaces are scored, and shifting manually will not fix the underlying damage. The truck will move, but it will not stay in gear under load. If the transmission slips after you get it rolling, the problem is internal mechanical wear, not the control system, and you need to plan for a full overhaul. The manual shift method is not elegant. It is not fast. It does not look impressive to anyone watching from the side of the road. But it gets the truck home when nothing else will, and that is the only reason it matters.
