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Date:         Fri, 17 May 2002 14:11:55 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: starter solenoid heads south
Comments: To: Alistair Bell <albell@UVIC.CA>
In-Reply-To:  <B90A8F65.3279%albell@uvic.ca>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 01:43 PM 5/17/2002, Alistair Bell wrote: >1. the starter itself as designed is pretty much a dead short, i.e. it draws >a lot (technical term "butt-load") of current . >I think the power rating for the diesel starter is 2.3kW or is it (1.7kW?), >that equals a current draw (at 10V) of 170 to 230 amps. The contacts in the >solenoid are pretty well taking as much as the battery can give! Plus the >contacts close when solenoid is tested (minus internal spring) and have very >low/no resistance.

This is a quibble, but any decent 12v battery will give you over a thousand amps into a dead short. Briefly. Before it either runs down or throws acid all over the joint. 200 amps at ten volts means an effective total resistance in the circuit of a whole twentieth of an ohm, including the "back-EMF" that the starter generates as it spins (the stall current of the starter will be much higher than the running current). 0.05 ohms doesn't sound like much, but it's quite a bit in this particular scheme of things.

That's why we disconnect the negative lead first, so that when we're undoing the positive and the wrench touches the chassis -- nothing happens, instead of instant welding machine.

david

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


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