Anytime you receive a shock around the ignition system, you will be dealing
with either a high tension coil, or a charged capacitor. NEITHER OF WHICH
should be connected to a tach lead. As you found out, high tension can and
does find an easier ground path than the secondary conductors and air gap
at the plug. 50,000+ volts are available there in a hei ignition. I used to
have a 65 kv Sun scope and have seen secondary open circuit voltage go off
the top of the scope a number of times on GM HEI. It will get your
attention if you unwittingly get into the ground path. Sometimes you will
find a loose or poorly plugged in secondary conductor, sometimes carbon
tracking, sometimes cracks in the coil housing or winding insulation.
Anything that presents an easy path to ground will suffice. Even you.
Jim Hupy
Salem, Or
78 GMC ROYALE 403
The aluminum firewall panel is grounded to the body, not the frame. I have
bonded the body to the frame at that corner of the coach. My chassis
battery ground lead is bolted to the side of the engine (not the factory
spot, but better) and I run a cable from that block connection to the
chassis frame. The house battery, which on my '73 is adjacent to the
chassis battery up front, is grounded to the same chassis bolt, with
another cable from that chassis bolt to the aluminum body frame. The body
frame mounting is the bigger challenge, and I will probably move my current
mounting point at the top of the front body tripod to the electrical panel
mounted to the front of the firewall. That should be the primary meeting
point between chassis and body grounds. It is difficult to eliminate
alternate ground paths between the body and the chassis--they mechanically
connect at too many points. But a fat wire close to that firewall point
will minimize the effects of ground loops at least at that location.
I ground the chassis battery directly to the engine block because I don't
ever want starting current to go through those other connections on its way
back to the battery. We think a lot about 12-volt current, but forget to
think about that same current when its on its way back to the battery.
Rick "suspecting a grounding problem is causing an intermittent difficulty
with the combiner" Denney
On Wed, Jun 28, 2017 at 10:23 AM, Steve Southworth
wrote:
> > Oh well, so much for my grounding guess.
> >
> > Do not let Colonel Ken know I was wrong.
>
>
> It was not the problem in this case but still good info.
>
> One area I suspect gives multiple ground paths is the dash grounds and the
> aluminum firewall. Engine items, gauges, lights and controls are grounded
> to the firewall. The firewall is connected to the engine battery.
> However I cannot believe the aluminum firewall is not electrically
> connected to
> the coach frame. Hmm - I'll have to look into this when I get home.
> --
> Steve Southworth
> 1974 Glacier TZE064V100150 (for workin on)
> 1975 Transmode TZE365V100394 (parts & spares)
> Palmyra WI
>
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--
'73 X-Glacier 230 "Jaws"
Northern Virginia
Offlist email: rick at rickdenney dot com
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