The MGA With An Attitude
In-Car AMP METER With Alternator -- ET-221A

This is a very unusual request, but it has come up a couple of times. What needs to be done to install an ammeter in a car with an alternator? The first issue is that the alternator puts out a lot more current than the original generator, so you need a higher capacity ammeter, like with 60-0-60 face scale rather then 30-0-30. Then you need to install wires large enough to carry all that current through the ammeter. Then there is the issue that these high current wires will run through the firewall into the passenger compartment behind the dash, and any possible short in these wares has the potential to pass perhaps more then 100 amps, melt insulation and burn the wiring harness. But if you insist on doing this, consider the following diagram.
ammeter with alternator
This diagram shows the original control box to identify the wire terminals. The control box can be removed by simply tying the "A" and "A1" wires together. Without the ammeter the new 12-ga wire can run directly from the alternator output to the main battery cable at the starter switch. With the ammeter, all of the output current from the alternator has to pass through the ammeter, so the wire between the ammeter and the battery cable must be large enough to handle the new maximum current. You can do this with two 12-ga wires or one 9-ga wire, or possibly even larger wires if the alternator can put out more than 60 amps.

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