[cunningham]

My '40 Ford has a 350 Chevy. I'm using a new Tuff-Stuff 100 amp 1-wire alternator. At idle, the voltmeter reads 13.9 volts. When the rpm is raised, the voltage starts dropping to around 12 volts. I had the alternator tested and was told it was putting out fine. The guy told me that 1-wire alternators don't always read voltage the way regular alternators do. Anyone have thoughts on this?
Thanks!

[toddalin]

I hope that you recognize that with a "1-wire," you first have to raise the RPM up past ~1,500 to get the field to energize. After that it should work like a regular old alternator/regulator combination until you shut off the engine.

My 1-wire puts out little at idle, but ~13.9 volts when the RPM climbs as measured at the battery.

[cunningham]

Thanks for your reply. I am aware of the rpm requirement. This alternator comes in at 800 rpm, so it shows almost 13.9 v right away. My concern is that when you increase the rpm the voltage drops down to no lower than 12 volts. As soon as it returns to idle, the volts go back to 13.9 or so. What is puzzling, is that I put my old alternator back on and it does the same thing (1-wire also).

[GREENBIRD56]

I'm not an electrical engineer - but.....it sounds like both of the "one wire" regulators are behaving the same - correct? The main lead is also the "sensor" for the regulator - and it controls the output to a fixed offset to the voltage where you are connected into the system. Your system must have very low draw at idle - and sag as the engine runs up - and the regulator is "mirroring" the sag.

The single "one wire" lead is supposed to be a large as possible to make sure the "sense" voltage is as high as possible - any drop due to resistance in the wire subtacts from the output. On mine - I ended up with a #6 fine stranded welding cable (crimped and soldered end lugs) that runs back to where the battery joins the starter solenoid. You can dummy this set-up with a single jumper cable as a test if you think the wire size is an issue - or at least rule it out.