Late Model Alternator Conversion

[srduster340]

I just installed a 1990 Dodge Ram 120 amp alternator on my '71 Duster and it's not charging. The vehicle has been completely rewired with a Ron Francis harness. Previously on the vehicle was a dual field square back alternator. One of the field connections was grounded to the alternator case. Got 13.8 volts at idle. Since I have an electric fuel pump and electric fans I decided to upgrade. Instead of grounding one of the field wires as on the old square back alternator, I have one connected to a 12V switched source as the factory would have done for fear of burning out the alternator. Should I ground the second field wire?

 

[67Dart273]

I believe these should wire up just like the older Mopar "dual field" I hate that term, "isolated field" is the correct description.

I would test this to make sure the alternator is OK

Ground one field terminal, hook the other to 12V and slowly bring up RPM while watching a voltmeter. It should increase. Be careful, don't let voltage climb above 12V

What are you using for a regulator, that is, 69/ earlier Mopar, 70/ later Mopar, or Ron Francis?

 

[srduster340]

I'm using a Ron Francis regulator. New style gauges installed too. It's after 10:30 where I live so I will have to wait till tomorrow to crank the car up to check charging. Thanks.

 

[67Dart273]

I'm pretty sure the Francis setup wires up like the 69 Mopar. That is, ground one brush, wire the regulator to the remaining one.

 

[BillGrissom]

I have one connected to a 12V switched source. ... Should I ground the second field wire?

Yes, that would make it charge, but at full output. You can stop every 5 min and disconnect/reconnect to manually manage battery charging, but that gets old fast. Better to let the voltage regulator do that job.

If your original 1971 voltage reg, do as post #4 - connect one brush to "fld" of Vreg and ground the other brush. This mode is called "high-side control".

Alternatively, use a later "isolated field" Vreg, in which case you leave constant 12 V on one brush (as you wired) and connect the other brush to Vreg. This mode is termed "low-side control".

My 1996 Plymouth still has the "isolated field", with "external regulation". Instead of a Vreg box, I think that is inside the "engine controller", still using "low-side control". I expect a 1990 RAM alternator is the same. I don't know if Chrysler ever changed to an internal Vreg. The external type is easier to trouble-shoot. The internal type can be an easier fix since you swap the entire brush/Vreg assembly, i.e. the "throw parts at it" approach.

 

[srduster340]

I grounded the second field connection, crank the car up, yep she's charging! Thanks for all the help.