I have a dead alternator on a new (to me) 2000 Cat 36. 

The original equipment is a Mando type 51A alternator and I can't find a direct replacement. I have a replacement from WAI that is supposed to fit the Universal 35MBC engine. The posts are different and I've consulted the Universal schematics and other sources.

The Motorola type 51A alternator that was in some of these engines apparently had 4 terminals - AC tap (tach), Battery, Sense (jumped to battery post), and Exciter.

The Mando has the posts labelled differently (e.g. R for exciter and P for the AC tap to tach) but also has a Grey wire going to the "L" post on the alternator. The schematic in the universal manual shows the Grey Wire going to "L" but doesn't say what the other end goes to. The manual does show that for the higher output alternator option, the grey wire isn't used.

The WAI replacement I bought has the 4 leads that I seem to need (AC tap for tach, Exciter to the pink and lt blue wires, Positive post to battery, and Voltage sensor that can go to the jumper to the battery post). There is also a "Reg" post.

Does anyone know what the grey wire going to the L post on the Mando is for? Does it need to be attached to anything on the new alternator? Any problems with replacing the Mando alternator with the WAI that I should be aware of?

Any help greatly appreciated.