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Re: Tr7/8 headlight pod problems....

To: David Massey <105671.471@compuserve.com>, triumphs@autox.team.net
Subject: Re: Tr7/8 headlight pod problems....
From: "Michael D. Porter" <mporter@zianet.com>
Date: Sun, 27 Aug 2000 15:05:52 -0600
Delivered-to: alias-outgoing-triumphs@autox.team.net@outgoing
Organization: Barely enough
References: <200008271023_MC2-B131-5D22@compuserve.com>


David Massey wrote:
> 
> Only Superman or Steve Austin (Lee Majors not the wrestler) could turn the
> motor by hand fast enough to generate that kind of back EMF.  Besides the
> diodes are not even in the same circuit as the motors.  The diodes are in
> the circuit with the coils of the relays that run the motors.  What is
> causing the diodes to fail (unless it is an intermittant short to ground)
> is the voltage spike from the relay coil (which is inductive) when the
> limit switch opens.

My problem in deciphering the circuit is that there is no switch diagram
on the schematic I have, and assumed such because I wasn't sure of the
connections in the switch, and because of the warning in the manual not
to turn the motors by hand without first disconnecting them. 

But, I agree with the above--that's more likely. It's a lapse of the
mind--I'm used to contemporary relays with a built-in suppression diode,
which we use, exclusively, at work. Forgot that the relays in this thing
do not have them, and yes, that would be a more likely cause. 

And, yes, a short to ground _before_ the relay coil could also take out
the diode quite easily. 

> Nonetheless, when 400 volt diodes are as cheap as anything you can buy, why
> use 50?

True enough, and Messrs. Lucas probably have the answer, lame as it is.
<smile>

Cheers. 

-- 

Michael D. Porter
Roswell, NM
[mailto: mporter@zianet.com]

`70 GT6+ (being refurbished, slowly)
`71 GT6 Mk. III (organ donor)
`72 GT6 Mk. III (daily driver)
`64 TR4 (awaiting intensive care)
`80 TR7 (3.8 liter Buick-powered)

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