I'm scratching my head here. The car is a 55 Chevy Bel Air, pretty much all original. I just upgraded to a Pertronix, and while doing so, I decided to feed my ignition a full 12 volts via a relay. So I wired up the old + lead to be the signal wire for the relay, I wired 12 volts off the battery, then fed the high current lead to the coil, and lastly grounded out the other lead (negative side) of the signal coil. With the engine off, this works perfectly. Turn the key on, I see 12.4 at the coil. Turn the key off, I have 0 volts. When I run the engine, then turn the key off, the engine does not shut off and the voltage remains at like 13.1 or something. I have to physically disconnect the coil lead to kill the engine, at which time the voltage will show 0 again. Any suggestions?
old positive to pin 85 pin 86 to ground pin30 to 12v hot pin87 to ignition coil Is this how you have it? I couldn't tell from your post. this should work, although I don't think its necessary to use the relay at all.
Sounds like you have voltage back-feeding in from somewhere... Is this the only thing you changed, or was there any charging system or other rewiring done also?
This was the only change, and I agree, voltage back feeding - but only when the engine is running.. that is very strange. This is the way I wired it. 30 - battery 85 - ground 86 - incoming voltage from original coil lead (+ from switch) 87 - Hot side of coil Looks like I switched the signal and ground wires, maybe that's the problem.
Well mystery solved, although problem not solved yet. When I connect the ignition lead directly to the coil (as original), I only get about 8.5 volts (this is why I wanted the relay, to give the system a full 12 volts +). When I turn the key off, the engine takes approx 1 second to die, during which time the voltage goes from 8.5 yo 6.5 to 4 to 0, etc. So with the generator backfeed, lets say still at 6.5 volts, or even at 4 volts, its not low enough to open the relay, so it keeps it closed and providing 12 volts from the battery. So now to figure out how to make this work the way I want..
with your ignition lead disconnected from the coil,do you get 12v? This would be with the key on of coarse.Then when it is connected to the coil,,you get 8.5 this is correct,right?
Stop the back feed with a diode, or figure out how it's feeding the ignition wire. That wire should go directly from ignition switch to the coil with no detours or piggy backed wires. With the engine running and key off it should be zero immediately. Not quite sure how the relay is going to like a 8.x volt trigger? That resistor wire feeds the stock ignition coil the correct voltage without a ballast resistor.
Yes, exactly. With the wire disconnected I get 12v + and with it connected to the coil and engine running I get about 8.5v. I found the specs for the relay I used, its drop out voltage is 1.2v so there is no way it is going to disconnect if there is residual voltage on the line.
Interesting idea about the diode, I was playing with the idea of connecting something like a small light, etc, to draw off the voltage. Similar to your idea. Cars with points don't typically feed full 12 volts the coil because it could burn up the points, they like to run them around 8 or 9. My 66 Mustang was this way and so was my 69 Ford truck, although upgrading to electronic ignition and feeding them 12v via a relay was no problem in either of those cases.
Yes, your GM car has a 12 volts start - 9 volts run system. The I terminal on the starter solenoid feeds the coil 12 volts while cranking only, thru the starter solenoid. In run the resistor wire feeds 9 to the coil and the I terminal should be dead. Unless its all been changed -lol maybe the difference is that there is something in this particular car, a problem or alteration causing the back feed? I'm not there, but I'd check the coil feed wire for being dedicated and the ignition switch or starter solenoid for a leak or problem . There should be no back feeding thru that wire.
Dump the relay totally. There's no load on the ignition to speak of, and a relay is just complicating things. You could use the old ignition wire and just eliminate the resistor, or jumper it out. I always use the KISS method.
With a more modern electronic ignition you can run a full 12 volts and take advantage of the improved spark performance, at least that was my goal. For now I am skipping the relay though.
If you use a zener diode, that should do it. These have a 'range' where they pass power, below that they stop it. Or find a relay with a higher 'holding' voltage.
A good modern electronic ignition is good for 5 times that. There is a lot of reasons to modern the igntion, there's been many decades of advancement since 1955
I think you misunderstood 1971 and I hope that I get what he is saying. The wire attached to your coil has a specific length of special "resistor wire" soldered into it. It should be quite easy to spot in the harness, the insulation coating on this section is very different. Find that section and bypass it with regular wire. Doing that will eliminate the voltage drop and send full voltage to the ignition- therefore eliminate the need for a relay. You could cut the resistor wire out, or leave it there and jump over it. I'd jump over it incase you ever wanted/needed to go back to a points ignition, everything is still there. That would solve one issue- but you still need to find the back feed source into the ignition wire
55's did not have a resistance wire in the circuit. They had a ceramic block with the resistance wire mounted on the firewall.
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