charging system hell

[v8volvo]

I have been having some pretty strange problems with my charging system lately and wanted to get some input on one thing I have noticed. The symptoms are that, when my alternator does charge, it charges only at about 13.4v, and most of the time it does not charge at all. Not just really low charge voltage, but no charging at all. The alternator tested good on the bench.

Now, here's the weird part. Sometimes, when I turn the key on, the battery light comes on, then when I start the engine the light goes away as normal. But then sometimes it comes back on, and I check and find that it has stopped charging. Other times, I start the engine and the light just stays on no matter what I do, even if I rev the motor pretty hard to try to get it to self-excite. And still other times, I'll turn the key on, and the battery light will never come on. I start the engine, the light is still off, but the alternator is not charging. These symptoms have now occurred on two different alternators, my old original Motorola (which still tested good as of two weeks ago) and the Bosch unit I initially replaced it with (a NAPA rebuild that self-destructed later on because the fools hadn't fully tightened the through-bolts). It is possible that in both cases I just had bad alternators, so I'm going to get a warranty replacement from NAPA and throw that in just to see what happens, but I think it must be more than just coincidence that these two alternators had the same problems, and both charged at only 13.5 volts regardless of how high the RPMs were.

Here are my questions: is it normal for a VW diesel to charge at 13.4-13.5v, or should they be up closer to the 14 range like most cars? Also, how hot should the alternator get when running? Both of these have run hotter than I would think they should -- hotter to the touch than the top of the cylinder head.

And finally, most importantly, how should the wire to the D+ terminal of the alternator behave? This is the circuit that is supposed to signal to the alternator that the ignition is on so that the field will excite and the alt will start charging. It should be 12v through the small blue wire at the alternator, run through the battery warning light's LED. I do get 12v at the end of that wire with the ignition on, measured with a voltmeter, but there is not even enough power in the circuit to light up a test light. The bulb filament glows so dimly you have to put your eye right up to the light to barely see it. I checked how that circuit behaves on my '76 Volvo, which also uses a Bosch alternator, and the wire to the D+ terminal lights the test light up bright with the ignition on. However, that car has to light up a bunch of lightbulbs on the Volvo's instrument cluster, whereas the Rabbit only has an LED, which needs much less power to light up. Is it normal for a VW diesel to have only a very slight 12v in that circuit, or should it be generously powered? Does anyone remember how their car behaves?

The last piece of data I have in this investigation is that, of the two fat red positive wires that go to the B+ terminal of the alternator and are supposed to take charge voltage to the battery, only one of them has battery voltage. The wiring diagram in my Bentley manual says that those two wires join up later on so I figure I should not be concerned...but could this have some significance?

So far, my possible suspects include:
--bad ignition switch, causing intermittent power supply to D+ circuit
--bad connections in instrument cluster to battery light, messing up D+ circuit
--bad grounds (already checked and cleaned all frame and engine grounds but will do so again)
--bad wiring harness to alternator

OK if you read all that, thank you very much for your patience. Sometimes I can be a bit long-winded. Please let me know your thoughts, I'm gonna go nuts if I don't get a break on this soon. All these problems started to occur, unfortunately, right in the middle of a long cross-country road trip. Broke down in Denver, CO, Kanab, UT, and Berkeley, CA. Gotta love old all-mechanical Rabbit diesels though -- I ran just off the battery, with the alternator unplugged, for a total of over 1000 miles! All the way from Utah to Los Angeles and then also from Berkeley to Seattle. As long as you don't use your lights much and always park facing downhill so you can start it on the clutch, it'll go almost forever without an alternator. These old VW diesels may break, but chances are they'll keep on plugging regardless!

[rwest1]

All you mention sounds like bad alt. And or including the built brush holder/regulator (2 screws and this pops out for inspection). Check for bad solder joints on regulator also if you remove. But you say same problem with 2 alternators, which points to other circuit areas, most of which you mention as suspects (agree you should swap out again since its warranty part…just to be sure). Charging V in normal system is a function of the batteries state of charge, High V for low charge, low V for high charge. Also you say the Alt. Is getting hot which along w/low charge V might indicate its producing to much current due problems elsewhere in electrical system. A fully charged battery should read 12.6 V or better, usually closer to 13 V if it hasn’t been setting to long since last time ran. You might compare time & temp of alt. heating to other car(s) you have to see if you’re on to something! Through the ign. Sw. 12V is applied to cathode of LED, anode is wired to D+ terminal on alt. This provides the exciter V for the alternator to start producing power, and when its working 12V is on both sides of LED and the light goes out (same potential on both sides). There are some good articles in Bentley on testing light; alt. etc. might want to look at those. Since the problem is intermittent you can thump around to try and find loose/bad connections. An ammeter (clamp-on type is great) would be a big help if you can borrow one that can handle the amount of current in a charging circuit. May need to make test cables to bridge around suspect wiring. These old electrical systems are prone to corrosion after all the years of service, particularly I’ve had problems with fusible links by battery corroding so bad they fall apart when you pull all the electrical tape off to inspect. Don’t see the link as particularly suspect, but if you have problems in electrical system overloading alternator it could be about anywhere. Think I covered all your questions if not let me know, also let us know what you find out. Regards

[rwest1]

In garage this morning so I measured D+ V. With ign. Sw. on, Alt. lite on, the D+ V. @ Alt. is .7VDC. Regards

[v8volvo]

Thanks for the great response. I hadn't thought of those fusible links but I will definitely check them out, as well as getting into all the grounds and batt connections with sandpaper and really cleaning them up. And I'll get another fresh alternator and see what that does. Will report back. Thanks.

[v8volvo]

Alright, after a hiatus due to schoolwork I'm back on this problem, trying to get the Rabbit on the road again.

Someone that knows charging systems could really help me out here: what kind of power do I need to see at the end of that wire going to the alternator's D+ terminal?

On the Rabbit, I get battery voltage shown on the voltmeter, but if I hook it to a test light (with wire disconnected from alternator, of course), the bulb lights so dimly and slowly that I almost can't see it.

On my '76 Volvo, which uses an externally-regulated Bosch alternator, with the D+ wire unhooked from the alt I also get batt voltage at the end of the wire showing on the voltmeter. But when I hook the wire to a test light, it lights the bulb up bright right away.

So, could my charging system issues be related to some problem in this circuit? Like that exciter wire getting shorted out somewhere before the alternator?

Also, trying to understand past symptoms: with both of the alternators I've had installed in the car (my old, original Motorola, and the NAPA-rebuilt Bosch type that later failed), the car's charging voltage was 13.5v. Is that normal for an A1 VW based on what others have seen, or not? Besides bad brushes in the alternator, what external problems could cause low charging voltage to occur, assuming that this car SHOULD be getting 14+ volts like every other car I've seen??

Current situation is the old Motorola (which tested good on the bench) is installed in the car. The batt light comes on with key on, engine off. When the engine is started, the light remains on and does not go off ever. There is no charging at all. I am completely STUMPED.

[vixentd]

Clean your battery posts and the ground and charging leads. Had the same problem and dirty ends were the culprit.

[Op-Ivy]

I agree with vixentd. Give all your connections a proper checking/cleaning!

I had some starting problems a while ago so I thought my battery or alt was dead. Turned out the terminal on the starter was a bit corroded

[v8volvo]

OK, I cleaned the battery terminals, and actually replaced the entire negative cable with a new one because the old one was stiff and corroded. No change, still not charging at all.

Re-reading rwest1's earlier post, I realized that my low charging voltage might actually have been normal, because on a VW diesel, you have a big strong battery and almost nothing other than the starter to ever draw it down if you are mostly running during the daytime (not using your lights). Since most of my driving is long-distance highway, it would make sense that the battery would be so fully charged all the time that it would only want 13.5v or so of charging.

At this point I'm leaning towards either just a bad alternator, or some kind of weird problem in the ignition switch or instrument cluster circuits. My warranty replacement alternator is coming in today and I'll give that a try before I go further. Will relate the results...

[888]

I don't know if this is any help at all but I had the POST (power on self test)LED's and gauges on my cluster go out on my 84 Rabbit 1.6 NA one time, including the alt LED. Everything else on the cluster worked. I won't bore you with what all I changed out to try and find the problem but it took weeks. I finally drove it regardless and one night I had to work late and of course it died because I had to use the lights. I changed the alt with another but I knew the alt wasn't really the problem but I thought I killed it. I pulled the fuse block and the carpets to check grounds, etc etc. I went all through the Bentley and the only thing I could find was a 5 amp fuse that could possibly affect the circuit but I didn't know which fuse it was.

Long story only slightly shorter, the radio fuse controls the cluster, the LED's, and the ability of the circuit to charge. If you checked the grounding cables in the engine compartment and behind the fuse block and they are good, you might just replace the radio fuse or be sure there's no corrosion in there. The windshields are known to leak when they rust at the flange. hope this is some help.

[A1-2-A3]

After reading your posts I would say you do not really have any sort of problem. Is your battery going dead? Your alt. will only charge to as much of load it has on it and RPM will change that amoount as well when it is charging. My Bat light always came on when starting on my old rabbit and as soon as I got over a certain RPM it when out and stayed out for the reminder of the trip. I would leave the alt on the car. Get your alt. load tested and see how many AMPS it puts out. This is done on the car. They hook up the tester to the battery and a amp probe on the positive off the ALT. Then they load your batteries and see how your alt responds with how many amps and volts it throws out. Varying the RPM will make this value higher. If its under the rated AMPS then its a faulty ALT. If its good then your charging system is OK.

[v8volvo]

Well, I replaced the alternator with another NAPA rebuild, and the problem seems basically solved for now. NAPA does a horrible job on these rebuilds -- I'm going to have to replace this one again I can tell, because the bearings are already squeaking. But it does charge, reading about 13.4v when running with a well-charged battery.

The only thing I don't get is that it seems like the alt has a lot of load on it. Even at that low rate of charge, with no accessories running at all it drags the motor's idle down a fair bit compared to if I just start it and let it run without revving it to excite the alt and make it start charging. I don't remember my old Motorola alternator doing this so maybe it's just that it's a crappy alternator. But I also noticed it got pretty hot pretty fast.

Maybe the person who said there is something that's overloading the alternators, causing them to die and also making for the low voltage, is onto something. When there was a lot of electrical load on the system right after cold starting (I have the long-term glowplugs that stay on for a few minutes after the engine starts), the charge voltage was LOWER, not higher (about 12.3v).

I'm lost when it comes to electronics. How would I test the alt loading? Would I need an ammeter?

I'm wondering if something hasn't shorted out somewhere or something...