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More Power To Ya: Wiring Upgrades for Older Cars

If you are running a 60 amp (or higher) alternator in your car, you should probably add special wiring to handle the extra load. I noticed recently that some of the engine compartment wiring harness connectors on my 1973 Dodge Dart (225 slant 6) were looking a little melted. After studying the wiring diagrams, I discovered that (per the Dodge Factory Service Manual) a 35 amp was standard, while a 60 amp “heavy duty” alternator was optional - and the wiring was different for that option. Nowadays, 60 amps would be small and 100 amps is common. I have been running a 60 amp alternator for about 8 years now.

I got the parts for this modification at NAPA and used crimp-type connectors so there was no soldering. The hardest part was hooking onto the ammeter - I tried to do it from below the dash but finally gave up and pulled the instrument panel.

After adding the extra wiring, my car charges much better! This wiring basically bypasses the normal engine compartment connectors to give a more direct, low-resistance path from the alternator, through the ammeter, to the battery. If you use a 60 amp (or larger) alternator without this wiring, you are in danger of overloading the factory harness.

Note: Rick Ehrenberg recommended the following wiring sizes:

Alternator output rating Wire gauge Fusible Link
Under 50 Ampere   12 16
50-65 Amps 10 14
85 Amps 8 12
100 - 120 Amps 6 10

I am offering this as a helpful suggestion for fellow auto enthusiasts and hobbyists. Nothing in this description should be interpreted as offering engineering services to the public. All responsibility for validity of wiring and vehicle safety rests with the vehicle owner. Do not undertake this project if you are not comfortable with assuming that responsibility.


Alternatively, you may prefer solder-type terminals, in which case you'll need those, also solder and electrical tape or heat-shrink tubing.


The alternator replacement process

Before you begin: you must disconnect the positive (red) lead from the battery. Failure to do so may result in injury!!

Basically, the extra wiring is:

1) A black wire from the alternator to the "-" terminal of the ammeter. Feed the wire directly through the firewall (I fed it through the grommet for the speedometer cable).

2) Then a red wire from the + ammeter terminal, back through the firewall, through a fusible link, to the "+" terminal of the starter relay.

I used a squeeze-tap connector on the alternator feed just below the firewall connector, where it wasn't obvious, to tap into the alternator current before it passed through the offending connector. Then a bullet-type terminal for the black wire (plugs into the socket on the squeeze-tap), so it's still possible to remove the engine-side part of the harness without cutting anything. A ring-type connector was used at the ammeter.

The fusible link was conveniently wired into the system at the starter relay. A ring-type connector was used to fasten one end of the fuse link to the large nut on the starter relay which connects to the positive battery terminal. A butt connector was used to connect the fuse link to the red wire. The red wire was then fed through the firewall to the ammeter, where a ring connector was used.

The fusible link is a safety factor and was present in the optional 60-amp factory wiring. This is to prevent dangerous overheating and possible fire if a short develops in this wiring, since this wiring connects directly to the battery. Do NOT add this wiring without it!

This duplicates the factory stock wiring (per the 1973 Dodge Dart factory wiring diagram) as close as reasonably possible. A purist will note that this might cost you show points, if the judges notice the non-stock modifications. However, those astute judges would also likely notice that your car doesn't have the option code for the 60-amp alternator you've installed. If you're in this for pure show, put a stock 35-amp alternator on there! But if you're using your Mopar for daily use and want it to charge "better than new" with a high-output alternator, make your wiring harness like they made 'em at the factory!

Also see Rick Ehrenberg’s electrical tips.

Stan Martin's 1973 Dart
Stan Martin's 1973 Dart Swinger with slant six automatic - 201,355 miles and still going strong!


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