Replacing points distributor in 66 C20

[MrK5]

This weekend I swapped out the points distributor for an MSD. The previous owner had the truck since 69 and kept almost all the receipts. I looked like the last time he'd done a tune up was in '90 when the motor was rebuilt 12,000 miles ago.

I found a few interesting things I wondered if they were completely unusual.

First, the 12V feed to the coil was a full 12V verified by multimeter. The wire did not look like the resistance wire descriptions I've read. Also what first tipped me off was the wire to the starter was not present; just the 1 wire. It looked pretty factory, not home-brew like the other wiring examples I've seen from the previous owner. Unfortunately this is the best image I have of it. The taped wiring harness looks pretty original too.



So I was curious if any of these trucks came factory with full 12V to the coil? Or was this something people used to do to "hop-up" the points ignition?

The other thing I found was the coil wired backwards. The 12V feed was on the negative post and the distributor wire and tach were on the positive post. I did some reading and found wiring it this way will reduce the output voltage by 15%.

The truck sure idles a lot better with the new distributor. I just wish I'd thought to order a black coil instead of the bright red.



In case anyone is worried, I didn't trust that small guage 12V wire for the coil so I'm using it to trigger a relay with coil power being pulled straight off the battery.

[ray_mcavoy]

The pic of your wiring is not showing up.

Most of the GM coils from this era had a primary resistance of around 1.8Ω. If these are run at full battery voltage (no resistance wire or ballast resistor) it will severely shorten the life of the points.

But there are some coils that have a higher primary resistance (around 3Ω or so) that can be run off a 12V system without needing a resistance wire or ballast resistor to limit the current. Of course, with the higher resistance being built into the coil itself, a bypass circuit can't be used to momentarily bypass the ballast resistor for a hotter spark while cranking.

Many aftermarket / replacement coils will often be labeled as to whether or not they require an additional resistor.

[MrK5]

I will see if the old coil has any info on it. Attached a picture of the 12v wire for the coil.

[ray_mcavoy]

That yellow wire feeding the coil appears to be stock.

The factory configuration in 64-66 trucks had the resistance wire running from the firewall connector down to the "R" terminal on the starter solenoid. Then the yellow wire (which is regular stranded copper wire) running from the "R" terminal up to the coil. Some other GM vehicles have the resistance wire running directly to the coil and then the bypass wire going to the solenoid "R" terminal. Both configurations work the same electrically.

Note that even with a resistance wire in the circuit, it is possible to measure full battery voltage on the coil feed wire with a multimeter. That is because the resistance will only drop voltage when there is current flowing through the circuit (Ohm's Law). The multimeter only draws a miniscule amount of current when measuring voltage so it won't create a measurable voltage drop. Try connecting the yellow wire to the coil + terminal and grounding the coil - terminal (to simulate a closed set of points). That will draw some current through the circuit and you should see less than full battery voltage at the coil + terminal if there is a resistance wire in the circuit.

[MrK5]

Okay, I got it. I connected that wire to trigger a relay to feed the coil from the battery. I'll see what the voltage on that wire is with the key in RUN. The relay might be enough draw to drop the voltage.

[ray_mcavoy]

Relay coils usually don't draw a lot of current (typically around 150 to 200 milliamps) and the stock resistance wire is usually around 1.8Ω so the voltage drop should be less than half a volt. So your relay setup should work okay whether there is a stock resistance wire still in the harness or not.