Vanagon EuroVan
Previous messageNext messagePrevious in topicNext in topicPrevious by same authorNext by same authorPrevious page (February 2000, week 4)Back to main VANAGON pageJoin or leave VANAGON (or change settings)ReplyPost a new messageSearchProportional fontNon-proportional font
Date:         Tue, 22 Feb 2000 21:00:33 -0500
Reply-To:     David Beierl <dbeierl@IBM.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@IBM.NET>
Subject:      Re: Alternator not charging/ solenoid meltdown
Comments: To: Steve Green <sgreen@WELL.COM>
In-Reply-To:  <qyazOM8JIl=EiFn+p08Vk3ozrfDB@4ax.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 07:43 PM 2/22/2000 , Steve Green wrote: >David, I think you are confusing the Vanagon alternator D+ terminal with >the field winding terminal used on some other alternators. D+ is an >output from the alternator, not an input to it. It runs the idiot light >and the fridge relay. My van's alternator charges normally with the D+ >wire unplugged.

Steve, on the '80-'84s there is a 150 ohm, half-watt resistor paralleling the idiot light LED and resistor. This is labeled "series resistor for alternator pre-exciter circuit" in Bentley, and allows some large fraction of 80 ma (in addition to the 15 ma or so through the LED) to flow from the 15-circuit (ignition-switched power) to ground through the D+ terminal. This gives a very small magnetic field from the field windings, but it's enough to start the alternator charging enough to supply its own field current which makes more output which makes more field current until output is very rapidly up to normal. (which drives the D+ terminal high which makes the light go out, so it's both an input and an output at different times). They could get the current from somewhere else, but since they need the identical function to drive the idiot light it makes sense not to duplicate it. And it would have to be somewhere else on the 15-circuit, otherwise the battery would slowly drain through the field coils while the van was sitting.

If the iron core of the rotor (pretty sure the field winding would be on the rotor, that way the slip rings and brushes don't have to carry heavy current) happened to retain a little magnetism then it would probably start up on its own, but the tickler current makes it a sure thing. And I'm guessing that the core material is chosen for very low magnetic resistance, which would mean that it would retain very little magnetism. Also, (I'm trying to visualize the magnetic fields in the alternator while it's spinning down after you kill the motor, and not succeeding) if the spin-down phase exerts an alternating and constantly weakening field on the rotor -- well, that's the recipe for a demagnetizer.

Back in the days of DC generators in cars I remember that after taking one apart you had to "polarize" it by passing a heavy current through it briefly, so that it would charge in the right direction, and maybe so that it would start at all -- obviously those puppies retained some magnetism in the pole pieces. Of course I was working on <gag> Lucas stuff, maybe the Americans did it differently. Them was the days of mechanical voltage regulators, too, big as your fist. Ecchh. Worse than ignition points.

Anyhow, according to the circuit diagram for the '89s or whatever year he has, by then they had two resistors paralleling the light circuit instead of one, no doubt to spread the heat load -- 80 ma through 150 ohms is just about one watt. The field circuit has its own internal resistance, so the actual current would be less than 80 ma, but I don't know how much less.

cheers david David Beierl - Providence, RI http://pws.prserv.net/synergy/Vanagon/ '84 Westy "Dutiful Passage" '85 GL "Poor Relation"


Back to: Top of message | Previous page | Main VANAGON page

Please note - During the past 17 years of operation, several gigabytes of Vanagon mail messages have been archived. Searching the entire collection will take up to five minutes to complete. Please be patient!


Return to the archives @ gerry.vanagon.com


The vanagon mailing list archives are copyright (c) 1994-2011, and may not be reproduced without the express written permission of the list administrators. Posting messages to this mailing list grants a license to the mailing list administrators to reproduce the message in a compilation, either printed or electronic. All compilations will be not-for-profit, with any excess proceeds going to the Vanagon mailing list.

Any profits from list compilations go exclusively towards the management and operation of the Vanagon mailing list and vanagon mailing list web site.