TR4A: Differential Damage

The classic problem with a TR4A diff is the mountings on the chassis, these corrode away until the diff teaqrs itself loose under acceleration. Mine broke free as I pulled away at a road junction, within seconds the car was completely immobilised as driveshafts fouled on bodywork and chassis. The TR4a was specified with a "soft engagement" version of the Laycock "A" type overdrive, possibly to prevent problems of strain on the rear suspension during engagement at full power. My car had (for some unknown reason) the earlier TR4 type overdrive, which appears identical in all but this engagement feature. This probably accelerated the rate of failure of the diff mountings. When the mountings break, I imagine they do so one at a time, in fact there was clear evidence that at least one of the ones on my car had been broken for some time. This means that the diff is forced into an odd position when under power, and this may put odd forces onto other components, especially drive shaft UJs and trailing arms. I would advise all IRS owners to inspect their car's rear trailing arms for cracking, as a breakage here could ruin your whole day. During an examination of my car's rear suspension about a year after the diff mounts were fixed I noticed 25+mm cracks in both trailing arms. These cracks were in the outermost web of metal connecting the mass of the arm to the cylindical section containing the hub mounting. This is viewed most easily with the car on a ramp, the web runs longditudinally with respect to the car, and is adjacent to the brake back-plate. Check carefully for cracks here. I replaced both arms with ones from a scrapped 4A. More recently this differential suffered what I understand to be a very unusual failure. Under quiet low strees conditions, driving up a medium hill, a pinion tooth snapped. The car could be nursed back home (yds not miles) but made a noise like hitting a brick against your skull once a revolution. Some research suggested that the more common problem with high mileage diffs is loosening of the crown wheel bolts to the point that one falls into the mechanism. I imagine my problem may be related to unwittingly driving with a seized propshaft UJ (described eslewhere), this can put massive stress on the diff.

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