Alternator amperage selection

[Sonny]

i want to add a Holley Mighty Mite electric fuel pump to my car. The only power requirements I have now are:
Water temp gauge
Air fuel ratio gauge
Stock am radio
Typical stock car lights
Electric pusher fan 19 amps (only comes on in traffic)
MSD ready to run dizzy.
Is my 60 amp alternator enough? Should I upgrade and to how big?
Should I use a relay like I have on the fan?

 

[493 Mike]

i want to add a Holley Mighty Mite electric fuel pump to my car. The only power requirements I have now are:
Water temp gauge
Air fuel ratio gauge
Stock am radio
Typical stock car lights
Electric pusher fan 19 amps (only comes on in traffic)
MSD ready to run dizzy.
Is my 60 amp alternator enough? Should I upgrade and to how big?
Should I use a relay like I have on the fan?

From your menu of needs I think you will be OK with the 60 amp unit. Make sure the bulkhead connector is in good shape!
Mike

 

[Sonny]

From your menu of needs I think you will be OK with the 60 amp unit. Make sure the bulkhead connector is in good shape!
Mike

Thanks Mike. All my harnesses are new.

 

[72RoadrunnerGTX]

What you are looking for in a newer upgraded alternator is enough low speed output current to cover all loads while at idle. A factory 60amp alt will output around 15-20 amps at idle at best. With most loads turned-on, at idle, if the ammeter shows a significant discharge, you need a bigger alternator.

If by new harnesses you mean new reproductions of the original harnesses, with the original ammeter, highly recommend by-passing the charge circuit wires at the bulkhead connector. Also, any aftermarket loads added to the car need to connect to the alternator or at some point on the alternator side of the ammeter, no loads should be connected to the battery directly or the battery side of the ammeter.

 

[Photon440]

The specs in the FSM show that the rated amperage is measured at 1250 rpm. That is at the alternator, it is expected to be about 5 amps less when measured at the battery, if that helps.

 

[72RoadrunnerGTX]

Some quality aftermarket high output alternators are shipped with bench test results that show both a total output measurement and a low speed or idle output number. With additional loads added to the vehicle, the benefit comes from the added low speed output capacity.

 

[Sonny]

What you are looking for in a newer upgraded alternator is enough low speed output current to cover all loads while at idle. A factory 60amp alt will output around 15-20 amps at idle at best. With most loads turned-on, at idle, if the ammeter shows a significant discharge, you need a bigger alternator.

If by new harnesses you mean new reproductions of the original harnesses, with the original ammeter, highly recommend by-passing the charge circuit wires at the bulkhead connector. Also, any aftermarket loads added to the car need to connect to the alternator or at some point on the alternator side of the ammeter, no loads should be connected to the battery directly or the battery side of the ammeter.

I hooked the ammeter wires together some time ago. I hooked the fan power to the battery, with the provided circuit breaker and relay harness per instructions. Is that ok?

 

[72RoadrunnerGTX]

Every aftermarket electrical accessory made comes with instructions that say the power needs to be connected directly to the battery. Biggest reason for that is the average installer can’t identify a proper alternate power source connection. For some audio equipment it’s to address noise concerns. For 98% of vehicles on the road this does not create any issues at all. On a car designed with an ammeter, under sized wires, and under rated quick connects between the battery and alternator any loads added to the battery are in fact also added to this charging circuit when the car is running. When running, all loads are getting power from the alternator not the battery ideally.

Assuming the new harness is a reproduction of the originals, using original specs, 12ga charge circuit wires, even with the ammeter by-passed, you still are running the added loads through the Packard terminals in the bulkhead connector and under sized wires. I would not recommend a larger alternator wired this way with any loads added at the battery.

The fans would be better off connected to the alternator stud if you want to run the factory speced wiring and not over stress them.

BTW, it’s a myth that Mopar passenger car ammeters spontaneously combust. The weak link is the bulkhead connectors, always was. Add heavy loads to the battery on an old Mopar, you will quickly find the weak connections in the original wiring. Sometimes it's ammeter connections.

 

[artr8]

Assuming the new harness is a reproduction of the originals, using original specs, 12ga charge circuit wires, even with the ammeter by-passed, you still are running the added loads through the Packard terminals in the bulkhead connector and under sized wires. I would not recommend a larger alternator wired this way with loads added at the battery.

The fans would be better off connected to the alternator stud if you want to run the factory speced wiring and not over stress them.

Taken from experience, I left the original charging circuit intact. Then added a 10 gauge wire attached to the alternator stud. Ran the 10 gauge wire to the front of the car for primary power on the fans through relays and temp switches. A 40 amp circuit breaker was added after the connector for the fans and then tied back in to the battery. Does not overload the original circuitry or pull power for the fans from the bulkhead connector.

In your case, running the aftermarket fuel pump, set up with an switch on powered relay (not the ignition circuit). Pull the primary power from the 10 gauge circuit mentioned in the paragraph above, and enjoy trouble free service. AND don't forget to chassis ground well by sanding a small area where you attach with a little di-electric grease.

 

[Sonny]

Every aftermarket electrical accessory made comes with instructions that say the power needs to be connected directly to the battery. Biggest reason for that is the average installer can’t identify a proper alternate power source connection. For some audio equipment it’s to address noise concerns. For 98% of vehicles on the road this does not create any issues at all. On a car designed with an ammeter, under sized wires, and under rated quick connects between the battery and alternator any loads added to the battery are in fact also added to this charging circuit when the car is running. When running, all loads are getting power from the alternator not the battery ideally.

Assuming the new harness is a reproduction of the originals, using original specs, 12ga charge circuit wires, even with the ammeter by-passed, you still are running the added loads through the Packard terminals in the bulkhead connector and under sized wires. I would not recommend a larger alternator wired this way with any loads added at the battery.

The fans would be better off connected to the alternator stud if you want to run the factory speced wiring and not over stress them.

BTW, it’s a myth that Mopar passenger car ammeters spontaneously combust. The weak link is the bulkhead connectors, always was. Add heavy loads to the battery on an old Mopar, you will quickly find the weak connections in the original wiring. Sometimes it's ammeter connections.

Thanks. So just move my battery connection over to the alternator stud for my fan? I’m going to hold off on the electric pump for now.

 

[72RoadrunnerGTX]

Yep, connect the fans to the alternator output directly. The current to run the fans is coming from the alternator as you have it now, just added to your new OE sized wiring, no need for that. Sounds like you have already by-passed the ammeter but are still running the charge wires through the bulkhead terminals on that circuit. You likely will have a bulkhead issue down the road sometime with reproductions made now to the original specs, adding more aftermarket loads won’t help prolong that.