by Jim Thompson
In reference to a "bad diode", it can happen on these alternators - most alternators manufactured by Bosch (and some by Motorola) during this time frame (to the present day) are built with a "diode trio" pack (actually one a pair of diodes per pack). If even one of these diode pairs go bad, the alternator can appear to be working just fine in your vanagon, but not delivering a full charge to the battery. I remember when these "diode trio" alternators first came out (on mid 70's Volvos if I remember correctly) and it drove my rebuilder nuts when he first encountered them.
If your into rebuilding your own alternators, you can get diode packs, but only thru the dealer (lotsa $$$) or if you have a sympathetic rebuilder, maybe from him, but no guarantees I'm sure (I didn't get any when I bought one 8 years ago to try it myself). In the long run, you may be better off just getting a rebuilt alternator.
When having an alternator tested, I usually go to a local rebuilder or an auto electrician, not a FLAPS. Why? The testing machines the rebuilders have are a little more sophisticated in that a simulated electrical load can be applied on the alternator to determine is true condition. Most FLAPS that have testing machines are less sophisticated and while you can check output and some diode conditions with these machines, they lack the capability to place a load on the alternator and many times will be inaccurate. And a lot of the time the FLAPS employees have very little clue how to operate one of these machines anyway.
The "+" terminal is a plug for the VW Diagnosis Machine. Leave it alone, don't connect anything to it please.