Battery Disconnect Switch

[1795]

As Morris states, if you do not wire the battery disconnect with correctly, when you engage the kill switch on a running engine, it will keep running. The first time that I wired the kill switch I tested it on a non-running engine and the engine wouldn't start and I thought that I had done it right. Then when the SCCA tech session came and I had to use the battery disconnect to stop a running engine it did not, as the alternator kept the engine running. Back to the paddock and time to re-wire. Not making that mistake again.

Jim

[twobjshelbys]

Kill switch aside, we always disconnected the positive side. That was just the way it was. I still do. All other power systems disconnect the positive side. But the owners manuals for most new vehicles say to disconnect ground. I don't know that it really matters since even with positive connected, nothing can complete the circuit to the ground side of the battery.

[patrickt]

Originally Posted by twobjshelbys

I don't know that it really matters since even with positive connected, nothing can complete the circuit to the ground side of the battery.

As kids, another one of out brilliant tests was to disconnect the battery while the car was running. If the car kept running, we knew the alternator was working. Yes, geniuses that we were. We only burned out the charging system half the time doing that. Of course, when that happened we would then proclaim that that was the problem all along and slap each other on the back for being such good automotive diagnosticians. So yes, a car will run (sometimes) with the battery disconnected... just don't do it.

[HTM101]

This battery kill switch will shut down a running engine...if wired per the diagram.

https://www.pegasusautoracing.com/do...ocID=TECH00109

[patrickt]

Originally Posted by HTM101

This battery kill switch will shut down a running engine...if wired per the diagram.

https://www.pegasusautoracing.com/do...ocID=TECH00109

True. You know the funny thing about old Fords, and the way a lot of us have our solenoids and ignition wired in our "historically correct (kinda)" Cobras, is that some of these starter solenoids are prone to sticking in the closed position. This causes the starter to run continuously even while the engine itself is running. However, turning the ignition key “OFF” will not stop the engine because the stuck solenoid is feeding the ignition circuit via the second wire off the solenoid (which was originally used to bypass the ballast resistor). I've seen a zillion Fords wired this way, and a fair number of Cobras (it's even ERA's default method of wiring even with MSD boxes). Since MSD ignitions do not use a ballast resistor, the second wire connection to the starter solenoid can be safely removed and taped off. But if you have an old points type/ballast resistor ignition, you still need it. Somebody's bound to follow this post up with a memory of standing over an idling Ford engine, that has the starter motor running constantly, and they've pulled the key out of the ignition switch and are holding it in their hand thinking. "OK, just what the f*** do I do now?"