Which guage or both ?
I know Chrysler has used 100 amp alternators in some M-bodies. So how'd they do it and keep the ammeter?
External-shunt ammeter rather than internal-shunt. Virtually all the current stays on the engine side of the firewall. All ammeters are really milliammeters; only a tiny amount of current actually swings the needle. The rest goes through a shunt. If the shunt is internal to the ammeter, all the system current goes through the gauge. If the shunt is external, only the tiny current needed to operate the gauge goes through the gauge.
The stock internal-shunt ammeter is not "dangerous", and it's perfectly adequate for stock charging systems...especially if the connections are cleaned and tightened once every couple of decades whether they need it or not. Throw a high-output alternator into an otherwise-stock system, and yeah, things are going to burn up. That's not because the stock system is "dangerous', it's because tossing a high-output alternator into a stock system is halfaѕѕed. The '73-'76 A-bodies with heated backglass got 65-amp alternators and 8-gauge main charging cables separate from the rest of the engine wiring harness...right from the factory. And yet we see joker after joker toss a big alternator in his car without changing anything else, and then blaming the predictable result on the stock system being "dangerous". Horseapples!
Go take a look at
R/T Engineering. They can rework stock ammeters to handle up to 70 amps efficiently, and they can convert stock ammeters into voltmeters (complete with a new gauge face that looks like factory equipment). Their website's got a bunch of really good info on it, but you have to mouse around awhile to find it; the organisation isn't completely obvious. For example, I know there's a pic of one of their voltmeter conversions
somewhere on the site, but I can't find it at the moment! Be sure to see
this page. They actually have intelligent, Mopar-owning and -driving people who actually answer the phone, so if you get fed up trying to find what you want on the site, call them.
I used their electronic limiter on my '71 Dart, and now since I need to go in the dash of my '73 to replace a dead temp gauge, I figured I'd beef up the ammeter in prep for swapping on a late-model alternator. Should've called RTE before I bought a good used ammeter off eBay intending to send it into them for upgrade then swap it in, for two reasons:
1. The eBay seller (finally) sent me the wrong ammeter. It's a '76-only A-body ammeter, which is an external-shunt type unit. I could swap it into my '73, but I'd have to create an external shunt on the engine side of the firewall, of the correct capacity to make the ammeter read correctly (depending on the output of my alternator). Certainly doable and not too difficult, but:
2. RTE informed me when I did call them that they have upgraded A-body ammeters for sale on the shelf, either with good used/cleaned-up faces or with newly-restored/reprinted faces. I picked the "good used/cleaned up face" option and the one that arrived is really nice.