Why is the ACC circuit shut-off during start?

D

DavidM

New member
I'm going to re-do the ignition switch circuitry in the car. In looking through the FSM for the key switch workings, I noticed that it shows the ACC circuit is disconnected when the key is rotated to the start position. The ON circuit is still connected. What's the reason for this? Is it so that all the stuff on the ACC circuit isn't drawing power while the starter is being powered? Or some other reason?

My plan was to have a toggle switch for the ACC circuit and one for the ON circuit. Can the ACC circuit be on when the starter is engaged?

Thanks.

David
 
I am no automotive engineer, (and I don't play one on TV), but my best guess is to turn off all the stuff unnecessary to start the engine (radio, headlights, blower fan etc) to give the starter the most juice available.
 
ACC vs ON circuits

As I recall from early Ford stuff...
The actual ON circuit (thru the ignition switch) was bypassed, while the ignition current was taken directly from the starter solenoid (on the fender well). Lights were separate (old cars), but all accessories were off during the starting mode. Therefore, the ignition (at the coil) was hot only when the starter was spinning.

This made it easy to wire in a remote starter box that had both an ignition switch, and a starter button.

Good racing.

Bill:024:
 
To prevent voltage spikes generated during the high current flow from damaging equipment connected to the acc circuit.
 
jhooten said:
To prevent voltage spikes generated during the high current flow from damaging equipment connected to the acc circuit.
Click to expand...

That's something I was wondering. I was planning on wiring my gauges and Traqmate/Chasecam to the ACC circuit. With a separate switch for the ACC circuit I could leave it off until the car was started to protect that equipment. Of course, I think in the old car I had everything wired to the ON circuit so it stayed on and I didn't fry anything.

David
 
Modern starting/charging systems are better designed and less likely to generate destructive pulses but it can still occur.
 
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