The most common cause of failure for clutches, apart from ordinary wear and age, is operating the clutch out of adjustment. Pull-type clutches with mechanical linkage should always have some “free play” in the clutch pedal. That is, you should be able to depress the clutch pedal and inch or so before you feel resistance as it begins to disengage the clutch.
As a clutch wears, the throw out bearing gradually moved forward toward the clutch assembly, and this causes a slow decrease in free play.
Free play indicates that there is a gap between the throw out bearing and the clutch release fork
(which is attached to the clutch release linkage).Once the free play is gone, the clutch release
fork actually begins to prevent the clutch for fully engaging, as it is holding the throw out bearing back.
Eventually, as the free play goes away and the throw out bearing continues to try to move forward, the pressure on the clutch discs is decreased to the point that the clutch begins to
slip. Once the clutch begins slipping, it’s only a matter of time until the heat from the slippage
severely damages the clutch assembly.
A quick regular check for clutch pedal free play can avoid a costly repair down the road.
Fuller’s auto-shift transmissions are a boon to drivers but sometimes a headache for mechanics.
Auto-shift transmissions use what is essentially an ordinary manual transmission with an electric-motor-driven shifter assembly and an on-board electronic control unit (ECU).
Additionally, many have a second device that facilitates communication between the engine ECU and the transmission ECU.
Working on auto-shift transmissions requires extensive troubleshooting procedures before the mechanic begins changing parts, and we often see some new parts on auto-shifts that have made it to our shop after the regular mechanic gives up.
Very often, we’ll find that the mechanic replaced the ECU, the X-Y shifter and sometimes the shifter assembly in the cab, even though the troubleshooting procedures indicate there’s nothing wrong with these parts.
Often the real problem is a bad wiring harness or sensors or worn parts in the transmission.
The other common problem is that the complaint the driver is reporting doesn’t happen consistently. Perhaps it only happens once or twice per week or per month. This makes it difficult to know if the problem has been identified and repaired.
A dragging clutch can cause all kinds of problems, especially in a mid-range truck with a synchronized transmission. But what’s the easiest way to know if a clutch is not disengaging all the way?
The simplest method is to start the truck with the clutch depressed and the transmission in gear — make sure there’s plenty of space both ahead of you and behind you! If the truck lurches when you try to start it, you probably have a sticking clutch.
If the truck doesn’t lurch, put the gear shift lever in neutral with the clutch depressed and shift the transmission into neutral. Do you hear grinding? Do you have toruble getting the transmission into reverse? Then you probably have a clutch that is hanging up.
If the gear shift lever goes into reverse easily or you just hear a quick “chirp,” then the clutch is probably working properly.
Many heavy-duty truck have what is referred to as tandem axles. That is, there are two axles with differential assemblies, one is just an ordinary differential and the other has a power divider attached.
There is a lot of confusion about which differential is working when. We’ve heard arguments from people saying it’s the front, or the rear. The answer is: both of them are working all the time.
Both axles drive the truck, just like both the left and right tires drive a rear-axle pickup truck — as long as there is no loss of traction.
When the differential or power divider lock is engaged, each axle now drives the truck whether the other axle has traction or not.
Later axle models may also include wheel differential locks so that each wheel will turn, regardless of traction conditions.
A not of caution: driving on a surface with good traction (like a road) with the differential lock engaged is a sure-fire way to dust one or both of your differentials.
The power divider contains a nest assembly that compensates for minor variances in tire wear. Locking the axles disables this compensator, which will cause damage to one or both of your axles.
And never lock in your axles when one wheel is free-spinning. In fact, it’s safest to lock in your axles when the truck is stopped.