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Date:         Sat, 16 Aug 2003 17:02:11 -0400
Reply-To:     "Daniel L. Katz" <katzd54@YAHOO.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         "Daniel L. Katz" <katzd54@YAHOO.COM>
Subject:      Re: Of Vanagon Demons and other Ailments...
Comments: To: JD Foster <jidd@JIDDWARE.COM>

good advice, and also there can be contact resistance where the terminal crimps onto the wire.

note also that even a small contact resistance on a low voltage, high amperage circuit, can be serious. for example, suppose the current needed to crank the engine was 200 A. this means that at 12 V the entire starting circuit, including the internal resistance of the battery, the dc resistance of the motor, and the reactive impedence of the motor, would have to have an impedence of less than 12 V/200 A = 0.06 ohm. a 0.01 ohm contact resistance could not be measured directly with most meters, but the 200 A x 0.01 ohm = 2 V potential drop across the contact resistance would be easy to measure.

i remember one time watching a guy trying to start his wife's car, which had a big V-8. it wouldn't crank, but you could hear the solenoid clicking. as it happened, i had my hand on one of the battery terminals, leaning on the car. that terminal was warm. the other was cold. i made an off hand comment to the effect: if this were a quiz, id say there was a contact resistance on the warm terminal. but wait, this IS a quiz! i put the a voltmeter across the contact surface where the terminal clamps onto the battery (top post) - one probe on the top surface of the battery post, the other on the cable terminal, and measured several volts when he cranked the car. cleaning the battery post and contact area of the terminal fixed the problem.

on the ground strap that bonds the body (just under the coil) to the port side head on an '84 vanagon, i recently measured an 8 ohm !!! resistance between the braided strap and the terminal, on the engine side. that was a factory crimp gone bad, and would have made the ground strap essentially useless. test was conducted with strap off the vehicle - concievable that with snug bolt crimp was squeezed down and made better contact, but i measured 0.25 V from body to head before repair, only a few mV after.


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