Voltmeter (to replace the ammeter) Wiring Changes 

Chassis 636-on

See the wiring diagram here.  Note that the wiring diagram is for the later car, but the relevant wires are the same.

Procedure:

1. Separate the siamesed large twin plugs, 28/53 (red) and 27/54 (blue), by pushing out the pin between the halves: 
2. Connect the #53 red-wire plug with the #54 brown-wire plug
3. Connect the #28 yellow-wire plug with the #53 yellow-wire plug.
   #28-red and 27-brown are not used.
4. Connect a jumper from either the tachometer power (bat) or the + terminal of the fuel gauge to the signal side of the voltmeter.
5. If the alternator output is more than 80A, run an additional 8 gauge wire (minimum) directly from the alternator to the battery or starter solenoid.