GMCforum: GMCnet » [GMCnet] Found the Source of My Problems

Home » Public Forums » GMCnet » [GMCnet] Found the Source of My Problems

Show: Today's Messages :: Polls :: Message Navigator

[GMCnet] Found the Source of My Problems [message #227277]

Sun, 27 October 2013 20:32

Ken Henderson

Messages: 8726
Registered: March 2004
Location: Americus, GA

Karma: 9

Senior Member

Those who followed my 5 month summer odyssey know that I had repeated
"vapor lock" problems, once in Death Valley. You also know that other
instances were just as peculiar as that below sea level occurrence.
Because of those anomalies, I finally decided that it wasn't vapor
lockbut an ignition problem. So, I checked the ignition coil,
cleaning and
tightening all of the connections to it. When that didn't work, I replaced
the ignition module. When I STILL experienced "Hot Hill = Stop", I finally
replaced the distributor with my spare, keeping that checked out ignition
cap and coil. STILL the problem persisted. As one last, final, desperate
fix, I replaced the ignition cap and coil with the spare.

That was in Gunnison, CO, where you don't want to be tinkering with your
ignition timing unless you already have a timing light -- ''cause the
garages there don't have any. I wound up guessing about 5* too much
initial advance -- hopefully the EFI still pulled the extra out -- I never
heard pinging (nor much of anything else).

The route out of Gunnison in over Monarch Pass, 11,312'. I ran up that hot
hill at 45-55 mph, slowing for curves, NOT the grade. And made it all the
way across with no "vapor lock".

Today, I finally got around to bench checking the coil which I SHOULD have
replaced FIRST thing. With an ohmmeter connected to the secondary winding,
I aimed a heat gun and an IR thermometer at the coil. During 8 test runs,
at about 250*-275*F, the resistance of the coil would suddenly climb from
about 15K Ohms to infinity. AH-HA!

During all my beside the road episodes, I was usually able to rest for only
5-15 minutes, then crank up and proceed. That's hardly what I'd expect
from a heat related problem. Things don't cool off that fast. In fact,
normally, the temperature of everything under the hood continues to rise
for 20-30 minutes before beginning to slowly cool off.

So, I was not very surprised when during each of my 8 bench tests, if I
continued to heat the coil after it opened, The resistance would drop back
to 17K Ohms or so!!! Even when I heated it to 400*F, the coil stayed
"healed".

Very peculiar, but those test results correlate exactly with the symptoms I
had on the road. Sure wish it was practical to open the coil and see what
sort of break is in there. A broken wire in a dense bundle, with 25K+ volts
applied to it, can do funny things.

For those who may be concerned about that 250*-275*F failure temperature,
remember that the under-hood temperature has been reported by several with
extensive instrumentation as reaching 250*F. Add to that the constant
current flow through the coil and 275*+ for the coil does not seem
unreasonable. And arcing across the break would probably add even more
heat.

Am I 110% convinced I've found and fixed the problem? No, but I sure ain't
gonna re-install that coil!

Ken "Hair Pulled Out" H.
Americus, GA
'76 X-Birchaven w/Cad500/Howell EFI & EBL
www.gmcwipersetc.com
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist

Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach