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Posted by: worn Oct 1 2014, 08:30 PM

After returning from the ockteenerfest we were able to regroup. I ran a potted voltage regulator for much of the trip,but it charged low. I substituted a more old school metal box vr only to find that the power transistor appears to have shorted running full current to field windings al the the time and cooking the battery. Tired of that.

So I took a collection of 5 VRs to test with variable voltage and found that they all stopped sending current to the field windings at approximately 14 volts. Great! The solid state units did so in silence, the oem stuff emits a buzzing sound. So they work. However when installed on a new alternator and new battery I tend to see charginging at only around 12 volts. I am puzzled and wonder how normal this is. Any thoughts?

Posted by: 914Sixer Oct 2 2014, 06:05 AM

Battery and alternator all put voltage load on them when they are working.

Posted by: Spoke Oct 2 2014, 08:31 AM

Why the new alternator? Seems like your VR testing showed the alternator was working.

Is the GEN light coming on with the 12V output? Where are you measuring the 12V?

If with the new alternator you are seeing low voltage, then time to start over again with testing. Check the alternator for output. With the VR removed, short the D+ and DF spades on the relay board. You should have 16V or so.

Posted by: worn Oct 2 2014, 01:38 PM

New alternator because the other one melted probably immediately upon first start due to internal short. Bosche rebuilt with little metal globules all over.
Current alternator will do at least 16 volts. Reading from a panel vdo gauge calibrated against reasonably good dmm.

When I put them on the bench they all stop sending current to the field windings at 14 volts but in practice they run a charge rate of a little over 12. This is higher than the battery at rest, but lower than the 12.6 that I have been told a healthy battery will be at and lower than the 13.4 i have been told that I should be charginging at. All components new, as far as you can trust that, including the harness. That is I have swapped apparently good harnesses with no difference.
So the thing that puzzles me is the mismatch between the bench test and the outcome, knowing that the alternator is capable of French frying the battery. I guess first to test is my panel gauge. If it works, that will go against what I have learned about vdo, but the adjustment screw gave me a potentially false sense of control.
Thanks!

Posted by: Dave_Darling Oct 3 2014, 01:15 PM

The panel VDO gauge is just there for entertainment purposes. Don't trust it to check the health of your charging system. Use your DMM on the battery POSTS--not the clamps, not the wires, not the body of the car and a hot wire, the battery posts themselves.

You'll probably find that the 12V you were seeing is closer to 13V, which is OK but not stellar.

--DD