Date:         Sat, 28 Feb 2009 10:31:29 -0700
Reply-To:     Keith Hughes <keithahughes@QWEST.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Keith Hughes <keithahughes@QWEST.NET>
Subject:      Re: Engine stutters, tach nosedives. Then, just as suddenly,
              it's,normal again.
In-Reply-To:  <20090228150500.6FC5B791C12@mpls-mf-02.inet.qwest.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

>
> -----Original Message-----
> From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
> David Beierl
> Sent: Saturday, February 28, 2009 7:48 AM
> To: vanagon@GERRY.VANAGON.COM
> Subject: Re: Engine stutters, tach nosedives. Then, just as suddenly, it's
> normal again.
>
> At 08:38 PM 2/27/2009, Greg Potts wrote:
>
>> I suspect the coil or the hall sensor; though Mark Dearing at Salem
>> Import suggested it could also be the ECU.
>> So what do you'all think? Was it just a loose plug wire, or are there
>> deeper issues lurking?
>>
>
> Deeper issues.  The tach is driven off the coil primary negative
> side, which the ECU ignition drivers pull down toward ground and
> release in order to create the spark.  It is my belief that an open
> circuit in the coil secondary -- from unplugged distributor wire --
> would not alter the primary characteristics enough to prevent the
> tach indicating, though I could be wrong.

No, you are correct here.  The coil doesn't care whether there's a coil
wire in the tower or not, the ICU/ECU still collapses the primary field
and the Tach will see that as a valid signal.  If, however, you have an
intermittent SHORT between the primary and secondary windings in the
coil (BTDT), then you'll have similar symptoms, and the tach will go
nutso whenever it starts missing/crapping out.  Shorts in the coil
*usually* start causing problems when Hot, but not always.  Since,
however, the coil heats and expands as you run the engine (flexing the
primary/secondary coils in the dielectric oil), you should notice a
difference in performance (either better or worse) the longer the engine
runs.
>   I think you're looking at
> a short in the Hall-effect sender plug at the distributor, which is a
> known soft point, or ECU internals or wiring between the two.

Most likely culprit IME. BTDT too!

Keith Hughes
'86 Westy Tiico (Marvin)