Date:         Sat, 14 May 2016 05:04:07 -0400
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Miss at idle?
Comments: To: Scott <SCOTTDANIEL@TURBOVANS.COM>
In-Reply-To:  <57363DD1.5080402@TURBOVANS.COM>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 04:49 PM 5/13/2016, Scott wrote:
>while there, look for an insulated spark-plug wire pulling pliers ...
>so you can pull off one plug wire at a time while it's idling.

The problem with doing that is it violates the Bentley instruction
that it's ok to ground the high voltage, but never let it run open
circuit (because the resulting high voltage puts too much stress on
insulating components in the system, possibly including the internal
structures in the driver transistor**).  And damage of that sort to a
transistor can be cumulative, so it shows up as a failure down the
road and not immediately.  This is a hallowed old technique but it's
not safe with modern ignition systems.

**Using a scope I've measured roughly 450 volts for a few
microseconds per firing pulse on the coil negative terminal -- that's
why the tach bites you if you touch the connection.  If the high
voltage were not allowed to arc to ground I'd expect the primary
voltage to go much higher, which would directly stress the extremely
thin insulating layer inside the driver FET (field-effect transistor).

Yrs,
d