How I check powershift clutches




Most of the tractors that I work on are powershift tractors in some form. If a tractor can be shifted through all the gears without clutching it is a full powershift. A tractor that can be shifted two or more speeds without clutching, but must be clutched to shift ranges is a semi-powershift.

Powershift transmissions almost always use multi-plate wet clutch packs (some use servos to shift forks that shift gears, but that is a different discussion). A cluth pack is a stack of clutch discs against metal plates that when pressed together with a piston by hydraulic pressure transfer power. There is a spring or springs to push the piston off when there is no hydraulic pressure. Some clutch packs are opposite, applied by spring pressure and released by hydraulic pressure.

Hydraulic pressure is sent to shift a clutch by some sort of valve. It can be a mechanical valve, but more commonly it will be an electric solenoid valve.

Powershifts with a lot of speeds use multiple packs for each speed. A Case IH 7120 is a mechanically shifted powershift. In first gear forward it will have engaged Odd clutch on the speed input shaft, first clutch on the speed output shaft, master clutch at the range input, and low range pack to drive the differential. By shifting through all speeds to find out which ones don't work a pattern can be developed. Every other gear = odd or even, first six gears = low range. Once a pattern is developed gauges can be used to determine if the valve is not engaging a pack or if the pack has failed. Find a place to check transmission pressure for all the packs. With a gauge in a transmission pressure port, shifting all the gears will usually reveal a pressure drop when you shift to a failed pack. If you can see or hear the inside of the transmission by removing a cover you can pressurize a pack with an air nozzle. A pack that slips generates heat which will melt the piston seals. You will be able to tell by blowing air into a clutch pack to engage it which ones are working and the ones that are leaking. Don't forget the possibility of a mechanical failure like a broken gear or shaft. All the clutch packs can be working fine and if the pinion shaft is broken, the tractor will not move.

Electrically shifted powershifts are a little different to check. They can have electric, hydraulic, and/or mechanical failures. I usually identify the solenoid valve of the speed I want to check first. Then shifting the tractor into that speed, I see if my pocket screwdriver will stick to the end of the solenoid. A magnatized soleniod means electrically it is working. A gauge can then be installed to that pack to see if pressure is leaking. No magnatism means I am searching for and electrical problem. Most electric/hydraulic powershifts use some sort of processor or controller to engage solenoids. A shift switch and maybe some relays tell the processor which gear you have selected. Don't be too quick to replace a processor/controller, they rarely fail and they are very expensive.

If you find that a clutch pack has failed in a transmission, try to determine the cause of the failure before you repair the clutch pack. If you repair the clutch pack first and a stuck valve caused the failure, you could damage the new clutch pack by running it with a bad valve. Always follow after the repair with gauges to make sure everything is right. The most common thing I have seen causing clutch packs to fail is having two packs for different gears engaged at one time. The weaker pack will slip and fail. Make sure after you repair a transmission that the ons are on, and the offs are off.



ęSteve Blankinship 2004