Rewiring Ammeter to Voltmeter?

I replaced my Generator with an Alternator and it works wonderfully, but have recently noticed the hot wire from the Alt is starting to get hot while driving. I’m going to replace the wire for now, but would rather put in a Voltmeter and rewire it.

What is the new way to wire it? Main output of the Alternator straight to the solenoid, and then a hot 12v to the voltmeter? I assume the “gen” light will no longer work.

Theo; I am no electrical expert, but I wouldn’t just replace that wire without investigating. You could have a substantial short to ground somewhere, or excessive resistance. That is a 12 awg wire, shouldn’t get hot. Second, I believe the gen light has to work for that alternator to start generating current.
Good luck, David

It can (and should be done quite easily), it is needed to retain the warning light circuit, it acts as an exciter to make the alternator work properly.

there are excellent instructions in the “UK E-type Forum…technical mods”…also in the archives here.

it is important to be done properly…ammeter takes all the current thru it and has fat wires, the voltmeter does not need that, and they need to be safely dealt with.

a voltmeter is correct for an alternator, and reduces in cabin electrical fire risk

in my personal opinion, a digital voltmeter is the best thing, as they are predictive on your battery status, anything else is completely useless for that important job

The original dynamo provided a maximum of 25amps, whereas even a small alternater will now provide about 45amps. Hence if the battery is a bit flat the alternater will be pushing a lot more current through the ampmeter than was originally intended until the battery is fully charged up. The same might go for the wiring.

it most certainly does

consider the worst case scenario, flat battery, jump start, the alt will INSTANTLY pump its max output straight down the wire to battery, so that may be 90amps,

wire size from battery to alt must equal max alt output

I checked all the rest of the wiring and all seems fine except for the bit right off the alternator which has a few sketchy solders, so i’ll replace that and then test.

What I don’t quite understand about going from Amm to Volt is “not wanting that much amperage going behind the dash” but to remove the Ammeter you connect the two wires together, meaning the same amperage is still behind the dash, just not going through the gauge. Does that really make that much of a difference?

When I replaced my dynamo I got the smallest alternator I could find, which was 45 Amps. Since it was a single-wire alternator (regulator built-in) I connected a single 10-gauge wire from the alternator directly to the battery. That is the path of high current. As you say, I connected all the wires on the back of the Ammeter together and installed a Voltmeter instead. There is a wire to the alternator for the IGN light, so that still works.

Mike Eck

New Jersey, USA

www.jaguarclock.com

'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2

I and others have given our thoughts on this in other threads…just search for “ammeter shunt.”

Briefly, I like ammeters, regardless of whether generator or alternator. They show you the discharge when starting, the time required to go back to charge, and whether charging or not with various accessories switched on. You get a good idea of battery condition, too. They were replaced IMO to save the money (particularly on wire), as were other useful gauges. Ideally one should have both ammeter and voltmeter.

Regarding the current through the bulkhead–yes it would still be there if you merely shorted the ammeter wires. You should run a wire directly from the alternator to the battery (or suitable terminal nearby, like a bulkhead terminal on some saloons or the starter).

If you fit such a shunt AND leave the ammeter connected, the ammeter will still work as before, but will indicate fewer amps–like 5 when it’s really 15. You can determine this ratio via choice of wire gauge and length for the direct connection (shunt). The result is that the ammeter still works, and there is less current through the bulkhead into the cabin.

If you still choose to short the ammeter wires together, you can substitute a voltmeter just as without the shunt. But there will be far less current. Or you can remove the ammeter wires altogether and just connect the voltmeter to any ignition-switched voltage in the cabin. That wire need only be 18 or 20 gauge.

When the Jaguar 420 came out it was fitted with an alternator. At this point a 60 amp ampmeter was also fitted instead of the 30 amp one as fitted to the mk2 and S types. The alternater however only delievered about 35 amps. Eventually a Voltmeter was fitted instead.

my '69 420G still has the original ammeter,

havent got around to replacing it,its on my long “to do” list

know that I should, now that fitted a 70A alt to replace the 43A, to run A/C

have Smiths S1 XJ voltmeters which look suitable

(our club rego bizarely disallows non-standard instruments)

did upgrade wire from Alt to battery

bypassed external regulators, 2 wire internal reg Universal Lucas alt, wire to warning light

looks original

future “improvements”… relays for headlights and ignition switch