iwe Rear Ends Only
December 7, 2000
On Thursday December 7 the Club had its year 2000 technical session, held by Ian Wood and son Hayden at iwe Rear Ends Only. Once again, a successful tour, 18 people attended. The Club provided donuts and juice. Those who attended watched the overhaul of a differential in a 1980 GT. This is a subject that I knew virtually nothing about. In all the years I have owned a Volvo and after having worked on many areas of one car or the other at one time or the other I've never had the occasion to do any work on a rear axle.
A number of photos are available for viewing here . There are 19 photos at each about 30 kB. They are small in size.
Once the car has been securely lifted high in the air, the rear inspection cover is cracked open to allow the oil to drain out. In order to remove the differential from the housing the rear axle shafts must be pulled from the car. After removing the rear wheel, disk brake calipers, disk/drum, hand brake shoes and bolts holding the bearings in place, the axles are withdrawn with the aid of a slide hammer. The inner seal stays in the axle housing; the outer seal comes out on the axle shaft. These seals do occasionally start to leak, especially on very hard turns. Replacing the seal on the shaft involves removing the bearing destructively from the shaft. There is a sleeve which locks the bearing on which has to be removed first. This is a bit tricky. It is important to avoid damaging the shaft, especially around the seal surface, otherwise, you'll be replacing the whole shaft. This is a bit of an ugly job. The axle tubes are then cleaned down with the "broom shtick", kind of a mechanic's version of a cotton swab. Now that the oil has been drained the cover is removed completely and the gears marked with a special yellow marking or disclosing grease. This linseed oil based product is supplied by General Motors. Once marked, the gears are turned, leaving a pattern in the marking compound. This pattern tells a lot about how healthy the gears are and what type of contact they are making. These experts know what the pattern should look like and what might be wrong with differential based on the observed pattern.
I should mention about the subject vehicle. This Limited Edition GT has had a history of differential noise dating back to about 1985, at which time the car close to 200000 km on it. It now has 340000 km. It is most pronounced at 60 and 120 km/h as presents itself as a very audible whine. My opinion is that it exists at all speeds but is most pronounced at certain speeds due to the coupling path of the noise from the differential to the body and interior acoustics. The car also suffered from driveshaft-speed vibration which was most noticeable when under load at 30 km/h. The result of the gear marking showed that the gear contact was running off the end of the tooth and the noise was caused from the gears moving away from each other. It makes sense that the gears should contact the central area of the tooth width and not run off the end. The primary cause for this misalignment is wear of the pinion bearings, resulting in the pinion meshing at a different part of the crown gear teeth. The crown gear has a steep side (the driven side) and a sloping side (coast). The swarf collects on the magnetized drain plug. The plug should be inspected for metal whiskers.
The driveshaft is marked and removed. Marking the driveshaft ensures that it goes back in the same orientation relative to the pinion flange. As we will find out, it is also important to make sure that all of the other pieces of the driveshaft are marked to allow them to be reassembled correctly thus minimising the risk of imbalance. Of critical importance is that the planes of the U-joints be aligned with one another as U-joints which are driven off-angle do not maintain angular velocity from one shaft to the other. Luckily, however, two wrongs do make a right and parallel shafts connected between two U-joints do conserve angular velocity as long as the U-joint are correctly aligned. On that very topic, it is possible for excessive undercoating to cause an imbalance. This particular driveshaft features a solid rubber sleeve connecting the two ends of the rear shaft to minimise vibration.
Ian reinforced the importance of replacing the oil on a regular basis. Pennzoil 80-90 works well in axles without limited slip.
The housing is undersized relative to width of the carrier assembly, thus producing an interference fit of about five thousandths of an inch. There are housing spreaders to open the housing but Ian doesn't recommend this. The carrier is then pried out of the housing tenderly with a carefully place bar. Once the carrier assembly is out of the car, the pinion can be turned on it bearings independently of the carrier. This revealed just how noise the pinion bearings were.
The carrier is then carried to the bench to begin cleaning and disassembly. The carrier bearings should be firmly attached to the carrier casting to prevent the bearing from spinning on the housing and causing the shims to wear. The spider gears spin on the cross shaft and mesh with the side gears which drive the axle shafts. The cross shaft is driven by the carrier casing. Since most of your driving tends to be in the same direction (forward) most of these components have pronounced wear in one direction and can be swapped in order to even out the wear.
It is important to not lose any parts, especially the shims, when disassembling the unit. The shims are responsible for adjusting the preload on the carrier bearings. Better yet, don't disassemble it, especially if you intend to have the shop work on it. Nothing worse than a puzzle with a few pieces missing. On that note, make certain that crown and pinions are not mis-matched. They are lapped into each other in the factory and, like the Canada Goose, are mated for life.
A tapered roller bearing can be evaluated by grabbing the rollers in your hand and keeping them from rotating while you spin the whole cage, rollers and all, on the inner race. Pitting and imperfections can be detected by the roughness of the feel. The pinion bearings are normally the first to go.
The carrier bearings are pulled off the carrier with a puller. This is necessary to get at the bolts to take crown gear off to get the cross shaft out to get the spider gears out. Once apart, the whole thing is cleaned in a solvent bath. The cross shaft is polished with fine emery paper.
During reassembly, the four gears in the core are first assembled and the cross shaft temporarily placed through the spiders and right angles to the hole in the carrier. This checks the fit to make sure that the gears are not off by one tooth. My guess is that if they were off by one tooth, it would be impossible to get the cross shaft through the whole stack. The cross shaft is taken out, gears rotated 90 degrees and the cross shaft put back in through the carrier and spider gears by tapping gently. Don't forget the spacer block in the older models. The carrier is whacked with a bunch to keep the pin from coming out. The carrier assembly is completed and then ready for installation. The pinion is removed from the axle by first undoing the pinion nut, pulling the flange and then extracting the pinion through the inside of the housing. the pinion is supported by two bearings. New bearings were fitted on my pinion as the old ones were finished. The pinion longitudinal position was adjusted by replacing the shims. This is normally done in increments of 0.005 inch. The bearing races are installed using "boppers" to drive the bearing shells into place. They look like huge intake valves. The pinion bearing preload is measured by feel and adjusted by removal and replacement of shims. It's a bit of a time consuming task, so best to get it right the first time. Pinion seal is not put in right away in case it has to be taken out again.
Now that the pinion is ready to go, the carrier is placed back in the casing taking care to rotate is as it begins to mesh with the pinion. When the unit is apart, it is obvious that the pinion is lubricated by a spray of oil from the crown fear. The carrier assembly is secured in place by bearing caps, pretty much the same as the main bearings in the engine. The caps must not be moved around as they are lined-bored in much the same way as an engine.
Backlash between the crown and pinion should be 0.005 to 0.010. Backlash may not be consistent around the crown. This measured with a dial gauge. The gears are marked again to check the pattern. Assuming that the backlash is correct, the pinion seal is installed, the pinion nut "staked" to help keep it fast and the rear cover siliconed and replaced. Practically any grade of RTV will be suitable as the all meet the temperature requirements. Ian uses an automotive grade.
While my car was in the air and accessible, I took advantage of this and had the faulty U-joint replaced. You can check the U-joints by alternating the torsion back and forth and also by applying torsion in the drive direction and rocking the joints on both axes. If you are working on the driveshaft, be careful not to damage the drive shaft tube by clamping it in a vise.