Sprag clutch could be cause of tranny trouble

Hey there, time traveller!
This article was published 21/12/2007 (6523 days ago), so information in it may no longer be current.

QUESTION — I have a 2002 Chevrolet Venture Van with a 3.4 V6 in it. For the past two winters, including this one, first thing in the morning when I give the van a shot of gas to take off from a complete stop, the transmission slips for just a quick second then catches. It doesn’t happen for the rest of the day, it only seems to happen in the winter. I had the tranny fluid changed last summer thinking that was the problem and maybe it was low, but no. Any thoughts before I start worrying about a rebuild? I’d like to keep the van for another year or two. Thanks.

ANSWER — It sounds like there may be a problem with one of the sprag clutches in your transmission or the forward band apply. A sprag clutch is a mechanical unit that allows rotation freely between parts one way but prevents the rotation when turned the other way. You could compare the operation to that of a ratchet wrench, which also allows rotation one way but holds the other way. The difference is that the ratchet wrench has many small teeth that catch a spring-loaded tooth to hold it from rotating, while a sprag clutch uses only friction and the wedge action of many small metal segments between two rings to lock or prevent rotation.

In your transmission, there are two sprag clutches and one roller clutch. The roller clutch operates the same as a sprag clutch but uses rollers instead of sprag segments. When you start out in first gear, the input clutch is applied to direct powerflow through the input sprag. The sprag locks and transfers the force into the gearsets. The roller clutch is also locked in first gear to hold parts of the gearset stationary.

The forward band is wrapped around the roller clutch housing to hold the roller clutch and this band is applied by hydraulic pressure from the forward servo (piston). When the transmission shifts to second gear, the second gear clutch is applied and the input sprag "over runs" or freewheels. The input clutch doesn’t need to be released because the sprag automatically releases when the speeds increase. This may sound complicated, but it is typical of many automatic transmissions.

Your transmission problems could be caused by a slipping input sprag, a slipping roller clutch, or a forward band that is not applying completely. Before overhauling the transmission to replace the sprag and/or roller clutch, I would have the forward band inspected. The band can be gauged with a special tool to determine if it is adjusted correctly. A different length pin that pushes the band can be installed to correct it if necessary.

The servo piston and seals that apply the band should also be inspected. I suspect this is where the problem will be found. The piston may be sticking when the transmission is cold and allowing the band to slip. Repairing the servo and piston can be done without taking the transmission out of the vehicle, so the repairs will be relatively cheap compared to an overhaul.

QUESTION — I have a 1996 Ford Explorer AWD. I have new winter tires, studded on rear and excellent winters on the front — not studded. My boys were stopped in a road side check. The constable said the combination was very dangerous. Is it? My thoughts were that studded on rear would enhance stopping power and perhaps assist in getting going. I do not think I have a problem, but since my wife drives 25 kilometres to work and back, using the Explorer, I want to be sure…

ANSWER — Preferably, winter tires should be installed on all four wheels and have equal traction capabilities to keep vehicle handling balanced. If you do install two tires that have more traction, always install them on the rear of the vehicle, regardless of whether it is front-wheel or rear-wheel drive. A vehicle that has the rear end slipping around is very unstable and difficult to drive.

It would appear that you are following the best procedures for winter traction and safety. The only problem I could see here and perhaps what the constable was looking at was the use of different tires on the front and rear of an all-wheel drive vehicle. If the tires have different circumferences, then they travel different distances per revolution and this causes the all-wheel drive transfer case to react to drive all the wheels at the same speed. This will cause the tires with the least traction to slip even more on the road surface and could cause it to spin out easily. If the tires are all the same size and circumference, then there should be no problem with using the combination you have on your Explorer.

Jim Kerr is an experienced mechanic, instructor of automotive technology and member of the Automobile Journalists’ Association of Canada. You can e-mail questions to Jim at the address below.

kerr.jim@sasktel.net