not charging, 91 Jetta TD

[ringram]

For the past while, my alternator does charge. I've also noticed at the same time, my tach does not work.

I turn on the ignition switch, the battery light turns on. I start the engine, bring up the revs a bit, and the light goes out. looks good. When the engine starts up, the tach goes up to about 1500 RPM, then drops down to about 500, and stays there. If I put a charger on the battery and charge it up, I noticed the tach will sometimes work, but most often not work the next time I use the car.

Bringing up the revs does not get it to charge. I see about 13.2 on a meter when checking at the battery posts.

It did work prior to doing some transmission work. I will check voltage drops across the ground and positive leads.

There are three connection on the alternator: 1 is the heavy lead going to the starter connection and then to the battery; the second is I believe the W terminal for the tach, and the third is the exciter. Should the exciter have 12V on it all the time?

I don't think it's the regulator as the voltage doesn't get too high.

Any other suggestions?

thanks,
Roger

[TylerDurden]

If the belts and connections are sound, the regulator is most likely failure... but 13.2 sounds like a working alternator.

I will check on that 12V at the exciter lead.

[Fatmobile]

It should be higher voltage than that for the 90 amp alternator, the ones I put in the Rabbits read over 14, just under 14 with headlights and fan on.

Could be a bad diode, allowing some reverse voltage to bring down the total.
Check the AC voltage.

[TylerDurden]

I measure at the exciter lead:

~1.5V when the engine is not running, key on
~12.2V when the engine starts and afterglow is active
~14V when the engine is running and afterglow is finished

I would swap in a known-good regulator and check.

Also check for a sticking-closed glow relay, I've had that happen.

[ringram]

engine off, battery voltage 12.5
ignition on, exciter lead, 12.5, battery 12.5
engine running, exciter lead 12.5
glow plug relay is off

cycled engine speed, no change in voltage - measured between battery post negative and alternator output post (12.5 V)

used a jumper wire and jumped 12V from battery post to exciter lead - no change in output, cycled engine speed, still no change

removed jumper from exciter and moved it to add extra ground to alternator casing to battery negative post, no change in output, cycled engine speed, still no change

I noticed that when the engine first starts, the tach jumps up, and the headlights glow bright - both for a second. Then the light dim again, and the tach falls down to 500 RPM and stays there.


Where do I check the AC voltage? On the alternator output? No AC voltage. The belt seems OK, the alternator is definitely spinning during the testing. The voltage regulator is easy to get at. Is there a way to test it? Can't find a supplier in the city yet. May have to break down and go to VW. What about the brushes? Where can I access them? Do I have to pull apart the alternator?

[Fatmobile]

That doesn't sound like it's working well at all, not even the 13.2 volts mentioned earlier.
I've heard of AC coming from messed-up alternators but I don't know what levels to look for and the battery might hold the voltage too steady to read.
I'd read it at the alternator post.

[ringram]

I did read it at the alternator post, same value as at the battery.

Can I re-build it myself? The voltage regulator pack has brushes on it. Are these the only ones or are there others? Is there a way to test the diodes? I don't think they're blown. I'm leaning towards the voltage regulator, but from reading the repair manual, it says if the voltage is too high, it's the regulator, but if it's too low, it's the alternator.

I thinking I'm better off pulling the alternator and getting it rebuilt. Maybe I can get lucky and find one at the wrecker.

[82vdub]

You may be able to find a starter/alternator rebuilder in your town to rebuild it for you. I do this for all my starters and alternators then I know it fits and I know who serviced it if there's an issue.

[Dakotakid]

If we are talking Bosch, you can replace the brushes for less that $15 here in USA.
All you need is a soldering gun, some patience, a little bit of solder, maybe a small round file, and a steady hand. You can also carefully dress the slip rings with some very fine sandpaper and a couple of your fingers....provided there is still life left in the slip rings.

I always carry at least one spare regulator in each car because the brushes do go eventually on the Bosch's. Usually at night when it is at least 15 below out. Motorolas used to go pretty much forever.

You can order new brushes at many automotive places. They used to come two in a pack....many do not anymore....only one per box a lot of the time. Also, they no longer supply the little springs with them either. Friggin' modern world.

[TylerDurden]

This guy has a decent primer on the VW systems.
http://www.ratwell.com/technical/Chargi ... html#vrhow

[ringram]

I'm thinking it may be a bad voltage regulator. Correct me if I'm wrong, but the brushes on the alternator are part of the voltage regulator. So, I can kill two birds with one stone by replacing that unit. I'm thinking that the regulator is bad and is preventing the alternator from putting out power.

I found this link on tearing down an alternator. Not exactly what I have, but close.
http://www.peachparts.com/shopforum/die ... om617.html

Roger

[ringram]

Here's another good link:
http://www.ratwell.com/technical/Alternator.html

[Dakotakid]

Wow, that first article is a work of art (although I did not actually go through it as such). This guy should have been paid something for all his work with photos, etc. The only difference I could see by skimming is that this alt. is the older Mk. I (for the most part) 3-prong hook-up.

Yes, you can replace the regulator and already have new brushes on it. But, don't be so quick to throw away the original as I have only seen two regulators legitimately poop the bed in my life. Usually, it is a matter of the brushes and non-smooth slip rings.

You can keep the old one and, maybe, some day become a little more competent and simply replace the brushes and then have a spare. There are non-oem regulators available as well as a good ol' Bosch one.

If you decide to buy a whole new alternator, I would suggest you buy from a place which offers a legitimate "lifetime warranty" and you put the receipt in a safe place. More often than not, these "rebuilds" are rebuilt in Malaysia or somewhere with non-oem parts. They generally do not last as long as oem. But, at least for now, this might be a good solution.

I once purchased a new Mk. II starter from O'Reilly's (many years ago). So, any time I feel it is getting the least bit weak, I simply call them up and ask them to get me another and then go down (taking my receipt) and get the "new" one without any sort of hassle. Sometimes, I even get confused and take back one they did not rebuild......yuk! Hey, confusion can be a wonderful thing!!!!!

[ringram]

ya, I was thinking it might just be the brushes as well, but they look long enough. I'll take them out and measure them and see what the manual says. I have no problem replacing them.

While they're out, I'll inspect the slip rings and measure the resistance. Looks like this unit was rebuilt at some time. Maybe if I polish up the slip rings and brushes, I MIGHT get lucky.

Any way to test the voltage regulator other than swapping in a good one to compare?

[Fatmobile]

Found this:
Ripple voltage or AC voltage can leak past the rectifier bridge diodes and actually cause the battery to discharge. It can be measured by switching your DMM to AC and connecting the black lead to a good ground and the red lead to the BAT terminal on the back of the alternator. Do not connect the leads to the battery, as the battery will absorb or dampen some of the AC. Run the engine at 2,000 rpm and read the meter's display.

You may want to also load the system by turning on the rear window defogger and headlights. A good alternator should measure less than 500 mV (.5 VAC). A higher reading indicates damaged alternator diodes and may cause problems in the ECU.

Another way to check the integrity of the diodes on a Bosch regulator is to check the voltage readings at the D+ terminal and B+ terminal. The voltage reading should be the same at both terminals. A difference of more than one volt would indicate faulty diodes and the need to replace the regulator.
Here:
http://www.volvoclub.org.uk/faq/Electri ... atorDiodes