The Stovebolt Site Forums The Shop Area The Electrical Bay New 6v alternator discharging

Hello,

I have a 1931 Chevrolet with the original 194 straight 6. I just changed the generator to a 6 volt alternator. As per the instructions all I did to wire it was run a 10 gauge wire from the stud on the alternator (labeled bat) straight to the stud on the starter where the battery cable connects. When I fired the engine I noticed that the amp gauge was showing a 5 amp discharge. I turned the headlights on and it went to 10 amp discharge. I tried revving the engine to excite the alternator but it made no difference.

What could be causing this? I'm unsure of how to check the alternator to see if it is working properly. The gauge worked correctly when I had the generator in place.

Thanks

The alt. is not wired through the ammeter. Put a volt meter across the bat or alt. If it is charging the voltage will be 7-7.5 volts.

It's feeding the current to the ammeter backward from the normal circuit with the jumper directly connecting the alternator to the battery. Check the output from the terminal on the alternator directly to ground using your multi-meter and it'll likely show normal charging. My '38 does the same thing.

Why not wire it correctly? There's no reason not to use the ammeter in the circuit- - - -it doesn't care what's providing the charge current- - - -a generator, an alternator, or even a couple of hamsters running in a cage!
Jerry

Hi Jerry, it's been a long time.

OK, thanks guys. I haven't looked at a schematic for the car and I was unsure how it was wired. I was just told to wire it straight to the power source and I didn't know any better to wire it through the amp gauge. I thought perhaps the amp gauge worked with another set a wiring and I was afraid if I wired the amp gauge into the circuit it might damage it.

Thanks for the info, I'll wire the amp gauge into the circuit.

In this new setup I'll have 10 gauge wire going from the alternator to the amp gauge and out of the amp gauge to the starter (where the battery connects). Should I put an inline fuse in there and if so, what size fuse should I put?

Thanks

That circuit will be carrying the full output of the alternator after cranking the engine, so if you choose to fuse the circuit, go about 10% over the max the alternator can produce. That will protect the wiring harness from a short, but it won't blow a fuse if you have an extended cranking session (flooded, etc.) and the alternator goes to full output for a while.

You could also put a 10 gauge fusible link in the circuit instead of a fuse.
Jerry

The alternator is a 90 amp. I just looked it up, I didn't know they made 100 amp blade fuses, but sure enough they do.

What's the difference between a fuseable link and inline fuse? I've been using the terms interchangeably. I had planned on buying a 10 gauge wire with a fuse holder that you splice in between.

A fusible link is a "wire" that melts like a fuse if its current capacity is exceeded. One of the big-blade fuse holders might be a better option, but I don't think a 10 gauge wire will carry 90-100 amps safely. I'd choose about a 6 or 8 gauge wire for that kind of current capacity, and install a horseshoe-shaped stainless steel shunt between the ammeter terminals to keep the ammeter from deflecting too far during heavy charge situations. A piece of a worm-drive radiator hose clamp shaped into a semicircle and drilled to fit the posts on the ammeter makes an excellent shunt. It allows enough current to bypass the ammeter to protect it, while still letting the meter needle deflect to indicate charge and discharge. Dash-mounted ammeters aren't terribly accurate anyway, so as long as the needle deflects in the + or - direction, it's doing its job. The old stovebolt generators only had about a 35 amp capacity, so a 90 amp alternator will peg the ammeter just after startup or if you have to do some extended cranking to clear a flooded condition, etc.
Jerry

Thanks for the info. Shouldn't the internal regulator keep the amp gauge from pegging out under normal operations (not including spikes or at starting under long cranks)

The regulator controls the voltage not the amps. An alt will charge up to its rated capacity if the load demands it. Those old cutout systems only had a 15 amp gauge. You will fry it if you run that alt. through it.

You can damage your original ammeter as well. I removed the 10 gauge jumper from my '38 after a while & let the current flow through the original wiring. It seems that the current after a few difficult starts was more than the original ammeter could handle because it stopped functioning correctly. It now shows a discharge all the time. I found a 6v voltmeter and connected it since I can't depend on my ammeter any longer. I put a stem light to illuminate it. This picture was taken with the ignition off. <CLICK>

I did some research after Jerry mentioned the shunt. It turns out you can use an original amp meter but it takes more effort. You have to play with finding the right shunt for your application so that it flows most of the dangerous amperage around the gauge while allowing the right amount for your gauge to show. This is done by changing the length or gauge of the wire you use as a shunt.

The other issues seems to be the increased hazard of fire by running the wire into the cab, where as the volt meters just have a small wire that enters the cab and is not the entire amount carried from the alternator. This is simpler and apparently less risky, however I am very inclined to want to keep as much original in the car as I can and as little extra out.