Extra fuse box

[kevin s]

If you have used a relay to switch the ignition controlled circuits there is one potential pitfall, the tiny current feeding back through the ignition warning light can be enough to hold the relay engaged and keep the engine running, I've had this happen on 2 cars (minor and TR7V8). They don't do it all the time and some never do it (seemed to be worse with a fully charged battery) if it does don't panic, the solution is very simple just add a diode in ign warning lamp circuit.

[geoberni]

If you have used a relay to switch the ignition controlled circuits there is one potential pitfall, the tiny current feeding back through the ignition warning light can be enough to hold the relay engaged and keep the engine running, I've had this happen on 2 cars (minor and TR7V8). They don't do it all the time and some never do it (seemed to be worse with a fully charged battery) if it does don't panic, the solution is very simple just add a diode in ign warning lamp circuit.

Interesting problem.
A good point, especially as relays aren't necessarily polarity sensitive.
The minimum hold in voltage will also always be less than the required pull in voltage.
I won't bore everyone with the full story, but decades ago I encountered a similar 'Doesn't turn off' snag on an aircraft, which was down to circuit design being affected by poor earthing, causing a solenoid valve to stay on after it had been turned off. Confused the hell out of the pilots