When is a U-Joint Bad

How do you know when a U-Joint has gone bad? What's the symptom?

It will begin to vibrate. When its really a long ways gone it will clack and have alot of slack in it. What symptoms is your car showing?

Walt is correct. There can be several failure modes. In either case there will be vibration. The joint could simply wear out (the crosses become unround). In this case you could grab each side with your hands and feel some slack; none is allowed. A second failure, preceding the first, would be lack of lubrication, which would simply give a tight spot, felt as a vibration. All this assumes the joint was properly assembled to begin with.
You can calculate the number of turns a u-joint will make at any given speed, and then using a lawn mower reed tachometer, measure the vibration frequency to see if it is the joint or something else. Lot easier though just to crawl under and look for slack.

Paul's advice is excellent. I've seen the end reults on a TR that had a U joint let go at highway speed. It's not pretty. I'd recommend that if you're replacing one, you should replace all of them (6 on a IRS car) if you're not sure when the others were redone. It's cheap insurance compared to the catastrophic failure costs.

Mark

Are there any tricks to getting to the alemites to get grease in my replacement U-Joints? The OE were of course not lubeable but all aftermarket U-Joints have grease fittings. The outboard ones are virtually impossible to get a grease gun to, even a needle-nosed variety.

Bill

I found a zerk fitting that has a shaft about 3/4" long that I substitute for the zerk on the U-Joint just long enough to grease it. This is on the driveshaft Ujoints on TR3/4 so I do not know if this would work or help on the 6.

I got a u joint when I replaced the ones on my spitfire, that has the zerk on the end of one of the caps. Works great for greasing them, of course you don't leave it on when you drive.