Please do not check an alternator by removing the battery from the circuit, there is a reason there is a warning not to do this -- blown diodes almost always result because they cannot withstand 20+ volts when the regulator loses control of the charging circuit from no reference voltage. This can also cook the regulator instantly, so you will then get no current or voltage at all.=.

I sincerely doubt you have gotten three bad alternators in a row. Please take the alternator to a parts store or repair shop and verify output before proceeding.

Once you know you have a good one, make SURE it is actually connected when installed. Corroded connections in the three wire plug or a missing output wire (or misplaced) so you never connect to the positive battery post will result in a "dead" alternator because it has not field current.

And, I will repeat, you MUST verify that the glow plugs aren't staying on. They draw more than enough current to prevent battery charging (on the order of 40 A on a four cylinder engine while the alternator can only produce 35 or so) and the battery will slowly (or quickly at night) go dead. Easy to check with a voltmeter, you should have 0V on the glow plugs with the engine running. If you have 12V, get a new relay, the old one is bad. If the glow plug light flashes once in a while, it may be cycling on and off because there are fragments of contact floating around and shorting things out.

Bad glow plug relay will be very hard on the alternator, the aren't really meant to run a full output continuously!

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!