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I'm happy to report that the shuddering on slowdown that my 1994 940 always exhibited has now been solved. The problem has been there ever since I bought the car a year ago.
At the start, I suspected rear disc runout, so I removed the wheels and checked runout on both discs with a dial gauge, and it was within spec. The discs were only moderately worn, but not excessively scored or damaged, and the pads were still approx 3/4 of original thickness. I tightened the rear wheels with a torque wrench afterwards. The rear shocks and springs were renewed,
I concluded that the driveshaft must be the problem, after being assured by an auto transmission service place that my transmission was in good condition. I renewed all 3 universal joints, the centre bearing and the centre bearing support. But still it shuddered on slowdown. (As an aside, I had to do the centre bearing twice, as the no name bearing that FCP Groton sold me (Chinese ??) was really noisy, FCP Groton were fine, and replaced it no problem with another, but for the effort involved, I found it better to buy an SKF bearing and definitely get it right second time around)
Clutching at straws, I went ahead and replaced the rear discs, leaving the pads as is, and the shuddering disappeared. At last I can stop my 940 smoothly - it was quite jittery slowing down suddenly from 100 km/hr (60 mph) with the old rear discs.
What I've learned from this exercise is that apparently, you can get a shuddering from rear discs that are within runout specifications and look OK as regards wear / scoring. As to the hows and whys of this, I'd appreciate some enlightenment. I would have saved quite a few hours of drivetrain work (and $$) if I'd just acted on my original hunch and changed the rear discs.
John Marshall
Christchurch New Zealand
1994 940GL 165,000 km
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