Stupid generator question

mcorrell

mcorrell

Senior Member
What should the system voltage read with a good generator measured at cutout? I get about 34 volts on gen side of cutout and 6.4 volts on battery side. This is at 1800 rpm and 3rd brush rotated about 3/4 down. Battery voltage not running is 6.25 volts. Ammeter indicates small discharge w /o lights and 15 amp discharge w/lights on.
 
What Bob said, appears that these is an open going to or in the cut out. This assumes a stock Model A generator. The voltage will go up if there is no battery load on the output. This is why a stock generator will "work" on a 12 volt battery. Not optimum but will work, sort of like driving your modern car 110, it will do it but it won't be real happy.
 
The battery is the 'voltage regulater' for the Model A electrical system.

Any poor connection between the battery and the generator will cause an elevated voltage for all the electrical devices on the car. The typical symptom is the light bulbs will burn out fast. This is why it is critical that all electrical connections on the A be inspected and cleaned. You must also look out for broken strands at the connectors that will increase resistance and cause odd things to happen.

In your case it appears the cut out is not closing. It works by opposing magnetic fields. If the voltage increases enough from the generator side then the contacts will close and allow current to flow to the battery.
Is it a reproduction cutout? They are not so good from what I have read.

If it is an original then you can grind the spot welds on the bottom inside and pop the cover. Then you can see what is happening.
 
My generator had a reproduction cutout where the contacts became stuck multiple times. Got a restored original cutout from Berts (talked to Steve) which is now working well. The coil of the repo was much smaller.
 
Cut out is OE type and converted to diode. I have current flow throught the cutout with the VOM connected negative to gen side and positive to battery side. I have no flow when I reverse the connections so this tells me the cutout is working. Maybe it needs to be turned around? Generator is early type with long taper armature. I was careful to follow diode kit instructions to be sure I kept the gen and battery sides correct, but this cutout wasn't on my car. My generator also has the terminal near the pulley end, not near the brush end.
I've not tested output on the battery side with the battery disconnected for fear of burning up the cutout or generator. I orignally had only 1/2 volt at the gen terminal prior to diving in and finding a bad ground inside of it, so I am on the right track, I think. My question is, assuming the system is functioning what should my voltage readings be on the gen side and the battery side of the cutout?
 
A diode cutout should have battery voltage on the battery side and be about .6 volt higher on the generator side.

A silicon diode will have about .6 volt drop when conducting.

The problem is if you have the cut out reversed then the current will be flowing from the battery to the generator and cook the generator. If you take the belt off the generator will motor. You will also see voltage on both side of the cutout when the engine is off if the cutout is backwards.
 
I get no voltage on the gen side of the cutout when the engine is not running. Is it possible I have a bad diode, even though I have current flow in one direction only?
I assume the correct polarity for the cutout is negative on the gen side and positive on the battery side since it is a positive ground system. The kit instructions did not state any polarity.

I have a repop, points style cutout I can install for test purposes.
 
That is correct you should not get voltage on the gen side of the cutout when the engine is off.

Do you have a meter with a diode test function on it?

With the cutout not in the circuit the diode should show .6 volt in one direction and none in the other direction.
 
I'm not sure about the diode tester. I've always checked them by reversing polarity to see if they had continuity in one direction only. What other test can be performed on a diode?

I can bypass the cutout/diode by connecting a suitable wire across its terminals with the engine running, correct? If I do so I should show a charge at the ammeter, correct? Would the system voltage increase. I assume that to charge the battery the voltage must be higher with the engine running than with the engine off.
 
Watch out for the screw on the output side of the cut out----Sometimes it is TOO LONG & may ground out on the generator case ! ! ! MANY terminal pliers have a facility to CUT OFF/SHORTEN common sizes of screws.
AND, remember, the stock type cut out is just an OFF/ON switch.
Bill W.
 
Thanks for the advice. I've checked for grounded conditions and am OK there. My cutout is the OE type with the screws that connect on the sides. It seems I have power to the diode/cutout and I have the diode polarity correct and no issues with continuity, therefore there must be an issue with the diode itself, even though it flows current in one directuion only. I will try another sutout that is not solid state.
 
My ammeter also indicates small discharge w /o lights and 15 amp discharge w/lights on.
I have driven with lights on and have seen no ill effects, car starts everytime, battery always has a good charge.
All wires new, repop cut out, original 6V gen., no sign of grounding.
Bad ammeter?
 
mcorrell said:
I'm not sure about the diode tester. I've always checked them by reversing polarity to see if they had continuity in one direction only. What other test can be performed on a diode?

I can bypass the cutout/diode by connecting a suitable wire across its terminals with the engine running, correct? If I do so I should show a charge at the ammeter, correct? Would the system voltage increase. I assume that to charge the battery the voltage must be higher with the engine running than with the engine off.
Click to expand...

You've got the diode test right and the bypass test right.
Yes you should show a charge at the ammeter and yes the voltage must be higher than battery voltage to charge it, usually 1 to 1 1/2 volts higher.
From your previous tests it sounds like the cutout is okay.
Bill
 
mcorrell said:
I get no voltage on the gen side of the cutout when the engine is not running. Is it possible I have a bad diode, even though I have current flow in one direction only?
I assume the correct polarity for the cutout is negative on the gen side and positive on the battery side since it is a positive ground system. The kit instructions did not state any polarity.

I have a repop, points style cutout I can install for test purposes.
Click to expand...

You should have no voltage on the gen side when not running, that means the diode is installed correctly and doing its job. A diode should flow in one direction and not the other. If it is defective it will not flow at all or it will flow in both directions.
Bill
 
burner31 said:
My ammeter also indicates small discharge w /o lights and 15 amp discharge w/lights on.
I have driven with lights on and have seen no ill effects, car starts everytime, battery always has a good charge.
All wires new, repop cut out, original 6V gen., no sign of grounding.
Bad ammeter?
Click to expand...

Sounds like we have the same set-up other than I have 34 volts at the gen side of the cutout. I've never known an ammeter to show only a discharge and no charge but I suppose it is possible. I'll do some more testing this weekend and post the results.

I sure wish I could get a USA made repop ammeter. The one that was in my car when I got it was a Chinese repop and it failed (physically broke) creating a no start condition. I bought an older used USA made repop that looked to be in good condition but maybe it is not working on the charge side?
 
mcorrell,

I would suggest that you re-read the replies to your original post as they contain the info you need to find the problem.

High Voltage from the Generator will cause it to destroy itself as the field windings will have a much higher than normal amount of power through them.

Marc
 
Your ohm meter tests on the cutout show it to be OK, but the voltage readings with the engine running show the cutout to be bad. I wonder if you have a cold solder joint inside the cutout. Something isn't letting the current flow across the two terminals. Bypass the cutout with a jumper wire and see how everything works.

As Marc said, the generator will burn up in minutes if you run it as is.

Bert's in Denver has good restored original cutouts for sale, and that would be a good choice. 800-321-1931
 
Turns out I had a bad diode in the kit I got from Brattons. I checked it with the diode test on my multimeter and got zero voltage drop with the correct polarity. I dug out a 70 year old cut out, cleaned it up, installed it and I now have 15A charge at about 1000 rpm and up w/o lights. Since I run my lights for visibility for other drivers I will leave it there.this gives me a slight charge on the road.
 
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