Coil voltage high engine running

I have points and the resistance wire in place and a 10si alternator. The resistance wire is red, though, and is marked "resistance do not cut", it's terminated properly at the bulkhead connector and looks factory. I read just over 6 volts at the coil with ignition on, but with the engine running read 12 volts at the coil, 14.5 volts at the battery. Weird.
From what I've read should be 9 or 10 volts at the coil?

Its a 1971 GMC 2500, 350

[dmjlambert]

Others who have more experience may chime in. My 2 cents is I think the voltage you read with the engine running may be skewed by noise or spikes caused by the ignition system, and it may depend on what type of meter you use. Do you happen to know how many miles you get from your ignition points before you have to replace them? If you're getting suitable lifetime out of the points then your resistance wire should be good to go.

Originally Posted by dmjlambert

Others who have more experience may chime in. My 2 cents is I think the voltage you read with the engine running may be skewed by noise or spikes caused by the ignition system, and it may depend on what type of meter you use. Do you happen to know how many miles you get from your ignition points before you have to replace them? If you're getting suitable lifetime out of the points then your resistance wire should be good to go.

I only have a couple of miles on the truck, it's pretty fresh out of a field
Haven't even replaced the points yet, just cleaned and adjusted.

I have a yellow wire by itself on the positive side of the coil. Red and purple down at the starter where the diagram shows yellow and purple, apparently someone has been in here before.

I'll dig a little deeper after dinner

[Accelo]

You voltage is a little low. I would pull the main bulkhead connector and confirm the pins are not corroded.

You should have 12V ONLY during the starting phase but the voltage supply from that wire should disappear when the switch returns to "run".
Their are two wires at the coil. One is the resistance wire and the other provides 12V during starting.
This by-pass wire is necessary because without it the normally full voltage at the input side of the resistor would drop due to starter load during engine cranking and would result in even lower voltage on the out-put side of the resistor going to the coil, basically defeating the purpose of using the resistor in the first place.

So remove the second wire and check them both with the key on. One should read close to 8V and the other should not read any voltage. It has 12 volts only while the starter it running.
Let us know what you find.

[dmjlambert]

The only purpose the resistor wire serves is to make your points last longer. So you can decide if the truck still runs but could use new points every 15K miles or so, you have enough resistance. The resistor wire probably goes from the firewall connector to the yellow wire where the engine harness goes down to the transmission bell housing bolt on the passenger side where there is a wire holder. There it probably connects to a yellow wire that goes between the starter and coil.

I checked the firewall connector, it could definitely use a cleaning. A set of points would probably last me a long time, this truck won't be a daily driver, I'll keep an eye on them.

[Mike C]

I’m a huge fan of the $59 Chinese HEI on Amazon or eBay. One of the first mods that I do on any pre ‘75 vehicle that I am doing a tuneup on. Drive it as is and when you are ready to throw parts at it, do the HEI conversion which wants full battery voltage for best operation anyhow.

I have been reading on the different types of ignition upgrades, I may go that way in the future

[RichardJ]

>> I read just over 6 volts at the coil with ignition on,<<

The engine just happened to stop with the points "closed". The next time you turn the engine off, it may stop with the points open. You will then read battery voltage on the "+" and "-" post on the coil.

>>engine running read 12 volts at the coil,<<

A DC volt meter is used to measure a DC Voltage. A constant DC voltage.

The ignition points is a switch. A V8 ignition fires 4 plugs per engine revolution.

At idle, the DC meter is trying to measure the voltage on a switch circuit that is opening and closing 4 X 650 = 2600 times a minute.

That "resistance do not cut" wire doesn't look factory to me, but what do I know. It should measure about 1.6 Ohms end-to-end.

>>A set of points would probably last me a long time, this truck won't be a daily driver,<<

Points will generally last 10k - 15k miles, depending. Sitting unused, corrosion will shorten the life somewhat.

Originally Posted by RichardJ

>> I read just over 6 volts at the coil with ignition on,<<

The engine just happened to stop with the points "closed". The next time you turn the engine off, it may stop with the points open. You will then read battery voltage on the "+" and "-" post on the coil.

>>engine running read 12 volts at the coil,<<

A DC volt meter is used to measure a DC Voltage. A constant DC voltage.

The ignition points is a switch. A V8 ignition fires 4 plugs per engine revolution.

At idle, the DC meter is trying to measure the voltage on a switch circuit that is opening and closing 4 X 650 = 2600 times a minute.

That "resistance do not cut" wire doesn't look factory to me, but what do I know. It should measure about 1.6 Ohms end-to-end.

>>A set of points would probably last me a long time, this truck won't be a daily driver,<<

Points will generally last 10k - 15k miles, depending. Sitting unused, corrosion will shorten the life somewhat.

I see what you're saying about trying to measure at the coil with the engine running, makes sense. That resistance wire has definitely been in there a long time, if not factory.

This truck sat for so long , though, it's going to need the whole system gone through... headlight switch (and I'll probably go to relays), wiper switch, fuse panel is corroded, fuel gauge actually works though..

Luckily, under the dash looks mostly stock, and the engine bay except for some wires being extended to have the alternator on the right hand side.

It had 1984 plates when I bought it, I don't really belive that it sat for that long maybe just around the farm after 84..

Thanks for everyone's tips and input

[Accelo]

[I]I’m a huge fan of the $59 Chinese HEI on Amazon or eBay. One of the first mods that I do on any pre ‘75 vehicle that I am doing a tuneup on.

I'm not a fan of the Chinese one but any HEI conversion is a move in the right direction.
Dealing with the different spark plug is the hardest part. I haven't found the correct length wires to go under the exhaust manifolds like the stock ones do. I just cut the resistance wire at the bulkhead connector to about an inch in length. Then solder a 14g wire to the stub. Voltage drop is less than .2V.. If you do it this way I don't have to mess with the connector. It works great and looks stock after you re-wrap the wire.

[Mike C]

I'm not a fan of the Chinese one but any HEI conversion is a move in the right direction.
Dealing with the different spark plug is the hardest part. I haven't found the correct length wires to go under the exhaust manifolds like the stock ones do. I just cut the resistance wire at the bulkhead connector to about an inch in length. Then solder a 14g wire to the stub. Voltage drop is less than .2V.. If you do it this way I don't have to mess with the connector. It works great and looks stock after you re-wrap the wire.

If you had as many cars as I do (mostly old farm stuff) you too would learn the merits of the cheap HEI. And I have found them to be machined well and really reliable for the price. And for your plug wires, 75-82 small block Corvette wires route exactly the same as the 67-72 truck wires around the back of the block and from underneath. I like the Made For You vertical looms designed for valve covers just mounted along the oil pan rail.