Jumpy volt gauge

This problem can have a number of causes

1--Bad regulator, usually caused by the old mechanical relay type

2--Voltage drop in the harness, something I continually preach "around here." This is the circuit path "in a stocker" from battery -- fuse link -- bulkhead connector -- ammeter -- ignition switch connector -- through the switch -- back out the switch connector -- back out the bulkhead -- to the regulator IGN terminal

3--Ground loop or other ground problems. The VR MUST have the SAME connection as the battery negative terminal. PERIOD.

4--Modern problems. It is possible in some cases that some elecrical interferance from systems can affect other systems, IE high output ignition systems can generate interferance into the regulator.

There is also a wierd problem that can only be described as a kind of oscillation or feedback loop. I've seen this a couple of times, and didn't understand until the second time around. What CAN happen is that some alternators can "accidently" or by poor design have a poor cut--in characteristic. The engine slows down a little, the regulator says, "Hell, we ain't chargin" and ramps the field current way up. The engine speeds up a little, and suddently the alternator bangs in there and the voltage shoots way up, and the regulator drops back TOO FAR, then has trouble deciding how to compensate.

The only two times I "saw" this, seemed to be a combo of "try a different regulator" and a different alternator.

(I'm non an engineer, not paid to be, nor do I play one on TV)

5--"Easy stuff" like slipping fan belts. Take nothing for granted.