November 12, 2011

Let's get this started! Part 2...of several.

Quite a bit of time spent today getting the wiring sorted out. What was throwing me for a loop was the one-position ignition switch that wouldn't send power to the START and "RUN" (IGN2 here) tabs on the ignition switch - and therefore getting power to the coil, which is needed to get spark to the plugs.

So I "bench-tested" the switch - which just means pulling it out of the truck and hooking it up to a battery and a tester on the work bench. There I found no power was going to the IGN2 or START tabs on the switch, and therefore no spark. The key, unlike modern switches, only moves one position on this one. It powers the ACC (accesory) terminal, but doesn't send power to the START terminal and, as I mentioned, doesn't have a second notch to light up those terminals. Like so many of these issues, it's partly due to the fact I didn't take this truck apart, so I wasn't sure if GM IGN switches should have 2 positions, or if all terminals should light up at first/only position, or was it somehting more sinister?

After much head scratching and a call to one of the owners, I ended up jumping power from the ACC terminal across to the IGN2 and START terminals to get power to all terminals. I then had spark at the plugs, but I knew that set-up means there's 12V to points all the time....which can burn them up in short order. Good enough to set the TDC today, but it'd have to be figured out before getting it running for anything longer than a few minutes.

GM's 6 cylinder ignition switch - terminal view.

So, after pouring some gas down the carb to get fire (it did!), I decided to leave it for a fresh day and a fresh brain. That decision was made after a fine-sounding backfire - one that reverberated off the big garage doors and seemed to yell: "I'm alive!"
Today, after a few hours of research, I started a thread on the HAMB (a great classic car and truck forum) and someone told me what I needed to know: that 6 cylinder ignition switches only have the 2 positions (off and on). There is no key third (or "start") position because it used a foot starter. Apaprenlty only a V8 switch will have 3 positions.

I also found that the IGN2 and START tabs don't even get power from the switch in a 6 cylinder truck, due to the foot actuated starter motor. They get it from that starter motors switch when the foot pedal is depressed. This circuit is used to bypass the ballast resistor while starting the engine. Therefore you get 12V for starting, but somewhat less for running. Very unique.

After some more research, I also found a better schematic diagram to use - key tools for wiring up old vehicles. As with so many things, I sure wish now I had spent more time concentrating in Grade 9 electronics class!!

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