Similar to the Europa, the early Plus 2 came stock with an ammeter. Lotus switched to a voltmeter with the introduction of alternators. The Plus 2 Workshop Manual actually has a section detailing how to retrofit a voltmeter if upgrading a car to an alternator, which I scanned below.
As described in several of the earlier posts, the voltmeter wiring is easy. One terminal to ground, and the other terminal to a suitable "Hot in Run or Start, Unfused" terminal. The 12v voltage stabilizer input terminal (typically mounted to the tachometer) is used in several of the Plus 2 diagrams. The wire gauge can be very light as the circuit sees very little current. With a bit of luck you may not need to remove the dash to install and wire the voltmeter.
If you are looking for a "period correct" voltmeter, it is sometimes referred to as a "Smiths Battery Condition Indicator" or similar. I bought one on eBay UK that was similar to the gauge shown in the Plus 2 Workshop Manual. Strangely, the bezel cover over the needle was different; I switched the piece from my existing ammeter so the gauges match better.
Here is a listing for the period correct gauge (at least as shown in the Plus 2 WSM). Note the rounded shape of the needle cover and the green and red colours on the gauge face.
- This is the 'period corect' Battery Condition Indicator shown in thew Plus 2 WSM.
http://www.ebay.co.uk/itm/classic-smith ... SwjDZYd-WaThis listing shows a needle cover with the angled sides. The gauge face is slightly different, with just red on this one. Looking around eBay, it appears gauges similar to this one are available new.
- This is the voltmeter version. The needle cover matches the other gauges better. Looks like this one is available as a new gauge.
http://www.ebay.co.uk/itm/Smiths-Voltme ... SwCU1YtG5pFor my 1969 Federal Plus 2, the stock ammeter was wired as follows:
- Brown / White wire, size 44/0.010, to the RB340 voltage regulator (the terminal has another Brown/White attached). The wiring diagram is not very clear, but appears to be the A terminal?
- Brown wire, size 44/0.012, to the main solenoid terminal (where the large black cable to the battery is attached).
I am not familiar with the S2 wiring layout, but the first thing to check is if the large gauge wire from the control box / voltage regulator is mounted to the dashboard or to the engine bay firewall. If it presently runs across the firewall, installing the ammeter becomes more difficult and problematic as the wire has to be relocated. If the wire runs across the dash, it still becomes a dash out job.
As mentioned above, the general ammeter wiring configuration places a "Hot at All Times, Unfused" wire and ring connectors next to the wooden dash. Typically you will want additional connectors in the circuit to allow dash removal. All of these spots provide a risk of fire from a short circuit.
HTH
Stu