SBC low voltage at coil

With the ignition switch on (circuit under load), I read 12.4v at the solenoid, but only 4-5v at the coil + and 8-9v when starting. The engine easily cranks and starts, but misfires, backfires and runs generally poorly.

Hoping to find the "missing" 3v, I measured voltage drop between solenoid "S" terminal and coil + in millivolts. Even though not indicated, I replaced that wire and got the same results, which seems to eliminate it as the problem, but I don't know where to look next. Maybe the ignition switch? The voltage gauge reads 12v with the switch on and 13+ when running. The headlights drop that to 12v.

I'm going to replace the coil, but that's a "hail Mary" rather than a diagnosis.

I was recently very wrong about needing new battery cables as suggested here, so I'm hoping that someone can offer another obvious thing that I'm missing.

Thanks for any and all help.

You don't say what type of ignition system is in the car. If it is a point ignition system then the coil measurement of 5 or 6 volts would be normal. Points systems use a resistor, could be a block style or a resistance wire and some GM's used an internal resistor coil. For testing purposes, make an install a jumper from the battery positive to the coil positive, test run for a short time, NOTE - this many volts can and will eventually cause harm to the points / condensor / coil.. but a short run to help diagnose will be fine. Also, again assuming it's a points system, have you cleaned the point contacts? The 2 faces routinely build up a layer of corrosion which can be removed pretty easily with a points file or even a piece of newspaper placed between the contacts and pulled through. I wouldn't replace the coil yet. The ignition system has many parts and replacing them piece by piece gets boring, time consuming and expensive. Please confirm what type of system you have and if it is a points system, then the voltage drop is normal. (more than likely)

Thanks for this information. My apologies - because my wife and I built this car in the early 90's, I tend to assume that the reader is familiar with it as I am. Yes, the distributor uses contact points. This is the first I've read that my voltage reading is normal for a points ignition system. The last time I replaced the points and condenser was more than eight years ago, so it's highly likely the points have some corrosion, and having cleaned points many times since the 60's, that should have occurred to me. I still don't get how that factors into voltage drop, but my goal is to get the engine running well again.

By the way, I went to my local Advance Auto Parts today to obtain a new points/condenser set and the young guy behind the counter had no idea what I was talking about. Wow. He was very helpful and took the time needed to help me, but it made me feel even older than I am.

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Originally Posted by Dorsey

Thanks for this information. My apologies - Yes, the distributor uses contact points. This is the first I've read that my voltage reading is normal for a points ignition system. I still don't get how that factors into voltage drop, but my goal is to get the engine running well again.

By the way, I went to my local Advance Auto Parts today to obtain a new points/condenser set and the young guy behind the counter had no idea what I was talking about. Wow. He was very helpful and took the time needed to help me, but it made me feel even older than I am.

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Glad you were able to find parts! The easiest way for me to explain the points system is that it is really a controlled short circuit! That's why the lowered voltage. The points control the flow of electricity through the coil and to the condensor. The condensor "bleeds" off the electricity to ground. The ballast resistor controls how much voltage gets into the circuit. To much voltage the resistor gets hotter and chokes the voltage down, I've seen volts as low as 4 / 4.5 and rarely over 6.. That's why I asked you to bypass everything and go from the battery positive side to the coil positive side - in essence replace the ignition circuit - in a short test run, if it got better then we isolated the circuit was an issue. When the car makers went to 12 volt systems, the resistor became even more important, chryslers were notorius for no starts because of resistor failure, once it happened we would sell a second unit to them to keep in the glove box. I'd even train them how to change them and then swing in to put a new one back in the glove box! One last analogy to understand points theory. Similar to water flowing through a hose. The water won't flow at all if the hose end isn't open, the valve needs to be open / closed for control, This is the job of the condensor, if the condensor fails open no electricity can flow and fill the circuit ( coil) so no worky! If the condensor fails closed it's a direct short and the resistor will be the only limiting control, it usually becomes overloaded and hot causing a failure of the circuit. Back to water flow, this is the same reason for the cooling system bypass near the water pump. If there was no flow at all, the water would boil at the joint between the heads and block, the thermostat wouldn't "see" the overheated water because it can't flow.. The points ignition is similar, the electricity needs to flow so the coil can recharge ... I hope that helps (somehow) I hope it wasn't just me rambling. LOL.. I tried to simplify and offer enough info so you can make an informed diagnoses. It's always harder to diagnose (for me) using words / expressions / phrases.. Good Luck and let us know how it works out! :)