Alternator Giving Very High Volts

[smitty1976bus]

Hi all,

I have a 78' Westy with a 70 amp alternator, been having charging issues for a few months now, and just thought it was my VR on its way out. So I bought a spare one to keep in my bus for when I'd need it. One day, my ALT. light comes on, so I swap over the new VR. Start the bus up, and boom, over 16 volts!

Great.... I ended up buying another VR.. same problem, super high volts. Ok.. now I'm thinking my alternator bit the dust. I tested the Alt -> VR connectors first, using RatWells handy chart:

I got as far as measuring the D+ terminal, with the ignition on, but engine off. The volts were at 11.6. So straight from the battery. So thinking one of the diodes blew in the alternator, and caused it to push out max volts all the time. Bought a rebuilt unit off of BusDepot:

Old

New

Went through the painstaking task of putting it in, and just fired up the engine.... Super high volts still! Has anyone seen this issue before? Is there a ground I'm missing or something?? Any help is appreciated.

[Tram]

Ever think the gauge might be the problem? Double check it with a VOM
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Немає виправдання для війни! Я з Україною.

[smitty1976bus]

[Tram]

[sjbartnik]

Also check transaxle ground strap; alternator/regulator needs to see battery ground; if there is a poor ground, it will not see battery voltage properly and will overcharge as a result.

Quick easy way to check this on a Bus would be to use a jumper to connect alternator housing to battery negative while engine is running and see what happens at the voltmeter. If it drops down to ~14v then you have a ground issue; if it stays high then the issue lies elsewhere.
_________________
1965 Volkswagen 1500 Variant S
2000 Kawasaki W650

[timvw7476]

DO NOT TRUST that black plastic block that plugs most of your alternator
wires into your voltage regulator, make sure to 'seat' every copper prong
in it's proper place. Those blocks have been known to seat while letting
one connection run loose. Also, if a diode fails, you lose volts, never gain,
if I understand & recall rectification schematics properly.

[mikedjames]

A battery charging at 17 volts is pretty well dead too.
The 11.7 volts with ignition on is another sign.
Unless there is a loose terminal clamp or earth straps.

I once left a battery clamp loose amd the bus ran OK for 14 miles then the warning light came on and the voltage measured at the alternator output went to about 18 volts.
Retightened the clamp by hand and pushd it down on the battery and the voltage went back to normal when I started it again.
_________________
Ancient vehicles and vessels

1974 VW T2 : Devon Eurovette camper with 1641 DP T1 engine, Progressive carb, full flow oil cooler, EDIS crank timed ignition.
Engine 1: 40k miles (rocker shaft clip fell off), Engine 2: 30k miles (rebuild, dropped valve). Engine 3: a JK Preservation Parts "new" engine, aluminium case: 26k miles: new top end.
Gearbox rebuild 2021 by Bears.

1979 Westerly GK24 24 foot racer/cruiser yacht Forethought of Gosport.
1973 wooden Pacer sailing dinghy

[telford dorr]

If you're getting 17 volts, it's because the alternator is running wide open, unregulated. Unplug the regulator and retry. There should be no output at all. If there is, then there is either a short in the alternator harness, or there is a failure inside the alternator. If the output falls to zero, then it's likely a regulator issue.

Side notes: never run an alternator without a battery load connected (make sure the battery has water above the plates). Otherwise, the alternator output voltage can rise high enough to cook the alternator diodes. If you must test, turn the headlight on to load down the alternator as much as possible. High voltages are really hard on the FI controller, and other bus electronics.
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[smitty1976bus]

Thanks for the replies everyone, I'm working on it right now.. hopefully solving it soon, ha

[telford dorr]

Here's a general schematic for what you're working on:

To prove that the problem is the regulator and not the alternator:

1 - disconnect the regulator.
2 - at the regulator connector, jumper the red wire to the green wire. Leave the brown wire disconnected. Run the engine. There should be no alternator output.
3 - at the regulator connector, jumper the brown wire to ground. Leave the green wire and red wire disconnected. Run the engine. There should be no alternator output.

If either tests fail (e.g. the alternator runs wide open), then the problem is either a shorted harness or a short inside the alternator. If both test pass, the problem is the regulator.

This is based on the assumption that your voltmeter is reading accurately. To check, measure a different car and see if it reads as expected.

How it works (in case anybody is interested):

An alternator is like the power amplifier on your stereo. The output current from the B+ terminal into the battery is approximately equal to the current flowing between alternator terminals D+ and Df (the rotor current) times the engine speed times the alternator gain constant (a fixed number). This current causes the battery voltage to rise. When this voltage reaches between 13.8 to 14.4 volts (the setpoint), the regulator turns off the rotor current, which causes the alternator output current to fall. The battery voltage falls below the setpoint, the regulator restores the rotor current. This cycle repeats hundreds of times a second, resulting in a constant battery voltage. The battery is what converts the charging current to a voltage, and is why an alternator won't work properly without a battery. (Note: a really big capacitor, like 50,000uf or so, will work in place of the battery, but it's not likely one would have one of these handy...)
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[skills@eurocarsplus]

at minimum that battery should have 12.2 "at rest" in my opinion....you're battery sounds weak. ideally a healthy one would be 12.6

that said. we use to do the 15-15 test.....put the battery on a 30amp charge for 15 minutes. if the battery hit 15v, it was junk. sometimes you could monitor it for just a few minutes and watch the voltage climb to 20v

just because a battery seems to "work" doesn't mean it's good. swap a known good battery and see if that does anything for you
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[smitty1976bus]

Great schematic and explanation telford dorr, thank you!

I did the test you recomended

[telford dorr]

[smitty1976bus]

[sjbartnik]

That brown (ground) wire looks like it's hanging on by a thread. I would think that could be your problem all by itself.
_________________
1965 Volkswagen 1500 Variant S
2000 Kawasaki W650

[telford dorr]

Final tests:

- measure the resistance of all wires, end-to-end. The resistance should be zero (same as meter probes shorted together).

- measure the resistance between all wires. The resistance should be infinite (e.g. open circuit).

From your symptoms, there is a short from the brown wire to ground somewhere. We're assuming it's in the harness (the most likely location).

As a sanity check, measure the resistance from the alternator Df and D- terminals to ground (the alternator case). It should be infinite (no connection).
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[Chochobeef]

With as short as those wires are, I would just make my own new harness. New connectors with soldered ends. Then get some new sheathing and some new plug ends and call it good. Just match the wire gauge as best you can, and always go bigger if you can't find an exact size match.

[smitty1976bus]

There just isn't enough hours in the day.. Tackled as much as I could this evening.

[telford dorr]

New harness looks nice.

[sjbartnik]