I'm running a sbc, 12V Coil wired to the ignition switch, and while running the coil becomes extremely hot. What could be the cause of this? Thanks!
Do you have a ballast resistor in series with the 12V source from the ignition switch? Almost all coils need the resistor to limit the current draw. Don't ask me how I found this out....
Sounds like the coil is not letting the stored energy out, check your coil when the key is on, when it is off, and when its running. If its wired to recieve power when the key is off, its gona blow up. NO JOKE, it will blow the fuck up.
The original coils in the sbc were ran with points and condensor. They were designed to have a resistor wire in the harness. The ignition switch sent 12volts through the resistor wire dropping the voltage down to just over 6 volts so the coil and the pints would not be damaged. If you are still running the points, then you need to install a ceramic resistor before the coil. Tuner
IF, you don't have an IGNITION - called a ballast - resistor wired in series between the ignition switch and the coil; the coil WILL get DAMN HOT! The ONLY exception is an "INTERNALLY" resisted Coil. If your using the stock starter, the small terminal marked "I" will ALSO be connected to the "+" terminal of the coil. (assuming "-" ground) (it allows for a full 12 volts when the starter is cranking)
its funny this came up...and i HATE to steal someone else's post. But, i'm also running a SBC with points. what ohm resistor is right? how much does it matter if its the wrong ohm?
Resistor it is, thanks guys. The coil is an MSD coil, and no it doesn't have juice when the key is off, only when the key is in the on position. Thanks again.
Each system is calibrated for proper voltage drop and current flow. Example with points or contacts. System would commonly use a primary resistance wire ( loom resistor) or ceramic resistor of 1.50 ohms and igntion coil with a resistance of 1.50 ohms across the positive and negative coil connections. This would give us a current flow of 4 amps and a voltage drop of approx 6 volts per resistor allowing a small voltage to cross the contacts ( allowing a usefull life on contacts). Now if we changed this to say a msd or accell low resistance coil of say .5 ohms ( pretty common with todays electronic coils) then the current flow would jump to 6 amps and the voltage across the contacts would increse causing a short contact life. Using this example of a .5 coil and no ballast this would be 24 amps and a very mad coil ( hot ). Just some thoughts What distributor are you using and how is the current controlled on your car ????
You said it gets hot when the key is in the "on" position...is this running, or while you're testing stuff out and working on the car. Worse burn I ever had was from a coil that was charged because the ignition switch was in the "ON" position while I was working on something, and then I forgot about it. Grabbed the coil, and almost instantly blistered the palm of my hand. 3am, I had to call my wife to come back to our work's shop, because I couldn't drive the stick-shift truck home. If key wasn't in "on" position, or engine was running, no problems. This was with a Pertronix ignition; no ballast, but has a coil. -Brad
gmc bubba, I'm running an early points distributor, not hei, and as far as how the current is controlled I have no idea. Bit of an electrical idiot. The truck started off with a 351C in it from a previous owner. The Cleveland blew up so I put the SBC in there and kept the original wiring. Ford to Chevy though things seem to be quite a bit different under the hood from an electrical standpoint. The way I understand it I should check the resistance of the coil and then run a ceramic resister that is the same ohm rating? Thanks
Your coil should have a primary resistance across the terminals (pos to neg) of about 1.25-1.50 ohms . Anything less than 1 ohm shouldnt be ran with points. You can buy the 1.25 resistor at any parts store ( 55-58 chevy works well). That should solve your hot coil issue. And dont park with key on for long periods of time.
I ran a resistor from the local auot parts store between the ignition switch and the coil and now the truck won't run. I ran a wire from the starter to the coil so it fires up when the starter is turning but cuts right off when I release the starter button. The coil is a .5 ohms msd coil so I am going to change it to a stock replacement, but why won't the truck run with the resistor in there? The spark is extremely weak as well, so I'm guessing the resistor is too much, so how do I fix the hot coil issue? Thanks guys!
The MSD .5 is designed to run with a MSD box or electronic control modules with current control. Lets go back to a basic point ignition , resistor, 12 volt coil ( 70 chevy pickup would work just fine) and a good set of contact points. I am working on a inexpensive coil amp that could run the lower resistnace coils AND use contact points. Stay tuned and i will let ya test one. For now let me know when shes running good with the stock ignition system. Get the car running as good as it can run awith this set up.
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