Shell Valley voltage drop when cranking
 

Hey All, I have a Shell Valley cobra and can't figure out why I have 12 volts to the pos. side of the coil with the ignition on but only 9 volts when cranking the motor over. I need 12 volts when cranking over don't I? I have an MSD setup and it just seems like a weak spark while starting. Once I get it started it runs good. Is there possibly a ballast resistor somewhere in the harness or what do you think. Thanks for your thoughts, Russ

First, please measure the voltage at the battery when you crank.

If you have 12 volts at the coil with the key turned on, you do not have a ballast resistor.

Depending upon a number of variables, 9 volts at the coil during cranking might not be unexpected. The massive load of the starter motor cranking the engine will always reduce the overall voltage available in the system. The voltage drop will be aggravated by a weak or undersized battery, small or poorly connected cables, a poor engine ground, or a "tired" starter motor. This reduction can be minimized with very large, short battery cables with heavy, fully soldered ends. You can also reduce the voltage drop by using a massive (read: heavy!) battery, so the starter uses less of the total battery capacity.

Ignition systems are designed to accommodate the reduced voltage when cranking. If you are having starting problems that you can attribute to low voltage, then the battery, cables, and starter should be checked first. Don't forget to check the battery-to-frame and frame-to-engine grounds. Paint at these connections can reduce their conductivity.

The old breaker-point systems used a ballast resistor to reduce the voltage across the points, but bypassed the resistor when cranking via the "I" post on the starter solenoid. The MSD system should already get its power directly from the positive battery cable, so using the bypass circuit wouldn't help. If the MSD box is getting its power through the ignition switch, that could be the problem.

Hope you get it worked out.

Hey thank you for the replys. Strictlypersonl I Put the voltmeter on the starter batt. terminal and got the same number as on the coil pos. so i guess all is well within the harness itself. Barnsnake, you gave me a lot of helpful information that made a lot of sence to me and I will double check grounds and connections and I'm pretty confident that I really don't have a major problem after all. Life is good and thanks again to you guys. Russ