Alternator/Generator

The following is mostly based on Dave's experience
with the replacement of the alternator in his '73 Super
Beetle, with input from Rob and others, and pertinent
questions and answers added as appropriate.

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The material is subdivided as follows -

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Alternator/Generator/Dynamo
-- General Discussion --

A technical point to begin with to clear any confusion -- generators and dynamos are the same thing - direct current generators. An alternator is different -- it produces alternating current, which must be 'rectified' to direct current before it's used to charge the battery etc. The word 'dynamo' is often used in the UK, but not so much elsewhere -- most other countries call it a 'generator.'

The generator has the same diameter down its entire length, between the pulley and fan housing. The alternator is the same diameter to the generator in the area where it sits on the support stand, but it is larger in diameter at the pulley end, where it overhangs the support stand.

A common problem -- the battery goes dead. It has no charge, but when the battery is 'jumped' the engine will start and continue to run.

A problem almost everyone has experienced -

Yesterday I left my parking lights on all day (it was raining on the way to work) and the battery was 90% flat when I got in the car (dim dash lights - no cranking). Luckily the car park is on a slight slope so I was able to push it, jump in and pop the clutch, and away I went. I don't think the battery really had enough volts for the ignition, but with generators, you only need a couple of volts for it to self-energise and so if the clutch popping gives you enough rpm the generator immediately runs up to it's 14.5 volts and the coil says "great - sparks are on the way". The alternators won't do that - they need almost the full 12 reference volts (via the blue wire and the dash light) before they can start generating power. So you are more likely to need a jump start to get 12 volts from a good battery. In other words, generators are more tolerant of the sick/flat battery.

A new battery may solve the problem of the battery going dead for a while, but then a few months later the car won't start again. If jumped it works fine, but the battery doesn't seem to be getting a charge.

When faced with this situation, take an electrical meter (see our discussion of VOM operation) and first measure the voltage across the battery. With the engine off, set the meter to "DC volts" (15 volt range), then simply apply the two leads from the meter to the two battery terminals. The reading for a fully-charged battery is about 12.6 volts. A dead battery will read nine volts or less.

To check the output of the generator/alternator (that is, to make sure a charging current is going to the battery), start the engine and read the voltage across the battery terminals as before. As the engine speeds up, the voltage should increase to about 14.2-14.5 volts. If it does, then the generator/alterator is working properly and charging the battery as it should.

If you don't have a meter you can try the following (less effective) test: remove the positive terminal from of the battery with the engine running. If the engine continues to run, the generator/alternator is okay. If the engine dies, there is a problem in the electrical charging system.

Note: Some have suggested disconnecting the wires from the (D+) and (DF) terminals on the alternator/generator, then connecting an ammeter across the terminals to check the operation of the generator/alternator. To do this you would need to put a high-amp ampmeter in series with the main red wire (D+) to read the amperage going into the battery. The simple tests above are the easiest.

If the alternator/generator test shows an output of less than 14 volts or so, check the voltage regulator before writing off the generator/alternator. If you have an external voltage regulator it with be under the rear seat on the left.

Note: Up until 1967 the voltage regulator was located on top of the generator. From 1967 until 1974 it was located underneath the rear seat on the left. From 1974 on the voltage regulator is internal to the alternator. If the 6-volt Bug has been converted to 12 volts, the 12-volt regulator is usually screwed to the fan shroud just above the generator so the same wiring can be used.

The function of a voltage regulator is to maintain a precise voltage regardless of the current drawn by the load. With meter switched to DC volts and probes on positive and negative terminals of the battery, read the voltage as before (again, should be a bit more than 12 volts). Then start the engine but don't rev it up. Slowly increase the engine speed to maybe 2000 rpm. The voltage should climb up to 14.2-14.5 volts and remain level there. If so, the alternator/generator and voltage regulator are good.

If the voltage does not go up as the engine speed is increased, check to see that the generator brushes are not worn excessivly. If you can, apply a little pressure on the brushes while the engine is running and see if the generator lights goes out or the voltmeter reading increases. If it does, the brushes may be worn to the point that they don't exert enough pressure on the commutator.

Speedy Jim gives a good method for testing a generator on his Web site.

Now, if the alternator/generator and voltage regulator are both okay, it is likely that there is a short somewhere in the system that is draining the battery. Test for this as follows -

  1. Take the positive cable off the battery.
  2. Connect the VOM meter between the positive battery cable and the positive post on the battery.
  3. If set to DC Volts, the meter will respond to the slightest drain. If it shows a drain, switch to DC milliamps. Any reading over, say, 5 milliamps is suspect.
  4. From that point you must disconnect things to find what is causing the electrical drain. Start by removing the fuses one by one until you find the culprit.

  5. Trace the circuits associated with the offending fuse to find the short.

If the car has been left out and not run for a period of time, electrical problems could result. VW used fairly simple connectors in the electrical system, and quite often just shining up the contacts can fix problems. Connections to consider include the following -

  • In the case of a generator or alternator, clean the contacts on the generator/alternator itself, and then the regulator contacts under the back seat, left-hand side.
  • There is a heavy red wire from the regulator to the battery on the other side. Brighten up the connectors on both ends of this wire.
  • If your car has no regulator under the seat, it's a later alternator model which has the regulator inside the alternator itself. On these models, there should still be a heavy red wire tracking from the left side across to the battery, and this might have some connectors on it -- clean them up. This is the wire which charges the battery, and also supplies the car with power from the battery.
  • Also look at the heavy red wires in the luggage compartment. The red wire from the wiring loom on the left side leading to the headlight switch is the main power lead, then two red wires come from the headlight switch -- one to the ignition switch, and one straight to the fuse block (this one works those electrical things which operate even with the ignition off, such as interior light, radio etc). Make sure all these connectors are clean so the power can get where it needs to.
  • There is one additional wire (a thin one) from the alternator to the "Gen/Alt" indicator light in the instrument cluster. It is essential that this wire be in place -- otherwise the alternator cannot charge. On alternator cars, even if the indicator light burns out, the alternator can't charge -- it uses a tiny signal current through this bulb to activate the alternator -- even when the light isn't actually glowing it has a tiny current through it. It's not the same situation with the older generator models -- if the indicator light blows it can still charge.

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Generator Polarization

Questions are often asked regarding the need to polarize a generator. If the generator is charging (the battery stays charged) then you don't have to polarize it. When a generator has been left unused for a long time (months or years), the metal core looses it's magnetism, and it NEEDS a little residual magnetism to start the charging process. Polarizing just gives it enough "boost" in magnetism to start it charging properly. When it's being used, it keeps its core polarized core so no need to do it again.

Note: Why do generators need to be "polarized"? Automobile generators need some magnetism to get started. This "residual" magnetism remains in the field pole pieces even after the engine has stopped. The next time the generator starts up, the residual magnetism creates a small voltage in the armature windings. Not enough to charge the battery, but enough to allow the field windings to draw current. As the field current increases, the pole pieces create even more magnetism. That makes even more voltage in the armature, and the cycle continues until the generator is capable of producing maximum output.

What happens though to a generator which has been stored a long time or is freshly rebuilt? The residual magnetism may have decreased to the point where it can no longer get the generator started producing voltage. In the case of a new generator or one which has been mis-treated, the residual may even be of the wrong direction (North and South poles reversed). Polarization is a simple process used to restore the field pole residual magnetism and ensure the magnetic direction is correct.

To polarize a generator, connect a jumper wire from the (DF) terminal on the generator to the generator frame. Remove the fan belt, then connect a wire from the positive terminal on the battery to the (D+) terminal on the generator. The generator shaft should start to spin.

Note: Don't run the generator this way for more than a few seconds to avoid overheating.

The generator will now be properly polarized. If the generator did not spin during this process, the generator is most likely defective.

Put the fan belt back on and re-test the generator voltage with the (DF) terminal grounded. If the output voltage is still low, the generator is defective.

Note: Voltage regulators do not need to be polarized -- they are not polarity sensitive. Even if voltage regulator came with instructions to polarize it -- these instructions actually polarize the generator, not the regulator. The regulator manufacturer simply wants to make sure that your generator will work properly so you don't blame the voltage regulator.

Neither to alternators need to be polarized. Alternators use a trigger current to start the process of charging - electromagnetism, so don't need any internal magnetism to begin with, like a generator does.

That trigger current is supplied via the alternator dash light (it's a tiny current and won't make the light glow when the engine is running) so if that bulb blows - the battery won't get charged and it will go flat. Curious circuitry VW used, but it works just fine. So that means that with an alternator equipped VW, you should always glance at the dash as you turn the key and make sure the Alt light does glow with just the key on, just to make sure that that circuit is intact.

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Miscellaneous Questions and Answers

Note: Our Generator Troubleshooting article may be useful.

Question - I have a 1972 Beetle with a red generator light permanently on. I have changed the brushes and regulator, and still a red light. A problem with further fault finding is the wiring isn't all original, lots of do-it-yourself stuff. I have checked continuity though and all seems fine. But could you please help deciphering which contacts are which -- in my manual (D+), (D-) and (DF) are all close together, yet well labelled. But unfortunately the connections on my generator are positioned differently. Two towards the pulley and one towards the front (fan). Do you know which are which?

Rob responded - Does the battery go flat, or is it keeping its charge? If it's going flat, then the red light is telling the true story; but if the battery is being charged, then the light is telling lies and IT'S at fault.

On my '71 engine, there are only (D+) and (DF) connections - there is a black plastic connection block with two connectors for wires. Some models do have a separate (D-) connection -- this is the one nearest the fan, and should be a brown wire.

The (D+) connection has the thick red (or red/black) wire attached to it -- it should be a large spade connector on the left side of the connector block (in earlier years is has a brass bolt/nut type connection). This is the main charging wire.

The (DF) connection (usually a thinner green wire with a smaller spade connector) is on the right side of the connector block. The (DF) connection supplies power to the field windings to produce the magnetic field inside the generator.

The (D+) red (or red/black) wire goes to the upper regulator (under the back seat) connection on the engine (rearmost) side of the regulator. The (DF) green wire goes to the lower connector on the same side of the regulator.

On the front end of the regulator, the upper connection (#61) should have a thin blue wire attached. This blue wire goes to the "Gen" warning light in the instrument cluster. The lower connector, (B+), has two thick red wires -- one to the battery and one to the headlight switch up front (which is used as a junction box for the main power supply up front). There may also be a thinner red wire connected there too -- I think that one goes to the headlight dipping relay up front.

Question - I want so much to have the generator of my Volkswagen reconstructed or have the rewinding done.

Rob responded - The VW generator can be rewound by any competent automobile electrician -- it works the same as any older style generator, and most auto electricians can still rebuild generators.

Question - I've heard that generators needed a higher rpm to charge the battery, like cruising speed, but that alternators need only the idling speed to charge the battery.

Rob responded - That's partly true. Alternators can charge the battery at idle rpm because they usually have a smaller pulley, so in fact they spin faster than a generator for the same engine rpm. That means they will charge the battery at lower ENGINE rpm but the alternator itself is spinning faster than a generator does.

The VW generator is called an "early cut-in" generator, so it starts to charge the battery at speeds just above idle - from about 1000-1200 engine rpm. It will provide more current at higher rpm of course, up to it's rated maximum of 30 amps (for the 12-volt generators).

Question - In the case of a failing generator, would it be wise to convert to an alternator instead of having the generator repaired?

Rob responded - You can do that if you want to, but it requires a little alteration to the car's wiring. Speedy Jim has the necessary changes to the circuits on his web site.

Question - Does a few Amperes make a difference?

Rob responded - The higher amperage of the alternators makes no difference at all if you have stock VW electrics -- it's only useful if you have spot lights, a big stereo and stuff like that added.

Question - The alternator that by Beetle has is rated at 45 amperes, for Toyota cars. There's one available, specifically made for the Beetle, rated at 51 amperes. Physically, the 51 amp alternator looks about a third bigger and much heavier than the 45 amp alternator. Maybe it gives more power? Will it put less strain on the battery?

Rob responded - The beetle's wiring will handle the VW 50-amp alternator OK, but if you use a higher power alternator than that ('75 VW alternators are available) then you might need to increase the wire size for the charging circuit to the battery.

A bigger size alternator won't reduce the load on the battery. It's main load comes from using the starter motor, and that won't change. A bigger alternator will recharge a flat battery faster of course, but the 30 amp generator does work just fine for most old VWs. The 50-amp alternator was added in 1973 when they added fresh air fans and other extra electrical items to the cars.

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