TheSamba.com :: View topic - 64 Beetle original 6-volt generator tests good but has NO output

Thu May 31, 2018 7:48 pm

64 Beetle original 6-volt generator tests good but has NO output (0 volts)
I'm reluctant to convert to 12v because this is an all-original, one family 105,000 mile car.
Routine tests have been run, but there are other measurements possible that I can't find specs for (see below)
With regulator DISconnected:
- Polarizes with correct spin (clockwise...fairly slow but spins)

- Resistance from DF (field) -to- D+ (armature +) is 1 ohm (seems low but verifies field IS connected to D+).....What should DF to D+ ohms be?
(Note: only 1 wire from gen to D+ lug)..there must be an internal DF-to-D+ connection for '64)

- Resistance from D+ armature brush to ground brush is about 20 ohms ...shows that both brushes contact the armature.....(armature and brushes show little wear....105,000 original miles)..
...what should D+-to-ground be ??

-Resistance from DF (field) to ground is about 20 ohms....this makes sense because DF is connected to D+ (1 ohm) and then D+ is connected thru armature brush to ground brush (20+/- ohms. If field coil is shorted to ground, one would expect way less than 20 ohms.

Any ideas on what exact problem might be....or additional tests to run....would be appreciated.......I would like to 'fix what's broke'.

Thu May 31, 2018 11:59 pm

DF resistance seems OK

But D+ towards ground you should read only a few ohms, 20 seems an high value

Try to measure while hand rotating the rotor, D+ reaches ground via both collector brushes and one of the rotor winding.

You said that the "spin" test was OK? the generator runs as a motor?

Anyway I do not understand your thought about converting to 12V.

If the problem is in the generator the solution should be around the generator, not to change all devices to 12V.

Fri Jun 01, 2018 1:27 pm

Thanks for very good 'spin-by-hand' idea........by Herbie1200

Problem MAY be worn brushes, resulting in too little pressure on commutator.

Pressing the brushes down and spinning by hand resulted in DF (field)-to-ground of 2 ohms (stopped) and 20 ohms spinning..
...Pressure also made commutator more shinny.

Voltage output on D+, spinning by HAND, is 50 MILI-volts (0.05 volts)

Brushes are 11/16 tall (17.5 mm).....What is wear-limit on brushes ??
Bently manual only says "if too worn to protrude from holders, they should be replaced..."

These brushes protrude appx. 1/16 inch above holders. There appears to be reasonable spring pressure (maybe)

Looks like next step is try new brushes...
- What is as-new correct length (height) of 1200 cc Beetle brushes ??..
..(make sure new is right ones)
- Is there any good way to change the lower (ground) brush while on car
(pull bumper ?.....remove fanhousing screws and rotate generator w/fan around ?...other ?)
(TIA) Lou

Fri Jun 01, 2018 3:49 pm

Check for generator output by grounding the DF to the generator case as outlined in the following link: http://www.speedyjim.net/htm/gen.htm

Performing the test outlined above will help narrow down if the problem lies with the generator or the voltage regulator (for a six volt system susbstitute in 6.7v for battery voltage and 18v for max generator output during the grounded field test).

Sun Jun 03, 2018 8:21 pm

'64 Beetle 6v all original 105,000 miles.....
Still looking for problem causing generator to test good EXCEPT no voltage out:.D+ volts=zero

All standard tests run, and rerun (4 times):....(regulator removed)

Spins OK as motor...in correct clockwise direction (Battery 6v to D+, w/DF(field) grounded.)

My understanding is:..'Motor test OK' verifies brushes contact armature and armature works, plus field carries current (??)...... Clockwise rotation verifies polarizaiton is OK ??
- Corrections /comments on motor test (???)

But, spinning with DF (field) grounded produces no voltage at D+ (from armature). Is it maybe necessary to have a load (resistance) of some sort on D+....other than just the voltmeter ??

Field(DF) has connection to D+ ...measures about 1/2 ohm...connection also verified by motor test (??)
But, 1/2 ohm seems like very low resistance... anybody know what field should measure ??
(TIA) Lou

Sun Jun 03, 2018 8:27 pm

If grounding the field (DF) while the engine is running does not make the generator put out any voltage at the D+ terminal, the generator is bad. The fact that it motors when polarizing it makes no difference, the generator isn't working and needs to be rebuilt by a competent electric shop or replaced.

Sun Jun 03, 2018 10:34 pm

The generator and voltage regulator function together as follows: The generator's DF terminal is continually switched by the regulator between 3 points: 1. circuit ground (min generator rpm or max generator load), 2. through a power resistor to ground (med rpm / med load), or 3. D+ (min field current / max generator rpm / min gen load). By adjusting the generator's field current, the regulator maintains the generator's voltage output at D+, keeping it in the Goldilocks zone of say, 6-7.5 volts.

You can test the generator/regulator system by connecting
- DF to ground (for maximum field current)
- D+ to ground or open
- hand-spin the rotor. Feel the difference in resistance when spinning the rotor with D+ grounded versus D+ left unconnected. With D+ grounded, you should feel significant resistance as the generator attempts to supply current to the zero ohm (short circuit) load. With D+ unconnected, you should feel no resistance as the generator is doing no work.
- Do Not spin the generator with any method other than by hand. Grounding D+ forces it to try to provide infinite current. Spinning the generator with a drill, etc. with D+ shorted to ground would of course damage it via over-heating.