Accessories and ignition on key switch

PMU
1

I have an old style car with a PMU 24.

The key has the old, off / accessories / ignition / start switch.

The accessories and ignition provide 12volts, how ever the accessories gets switched off when the start switch is on.

I would like the key to provide 12 volts to the pmu when the accessories is on so this could be wired to pin 7 ( 12V switched ). When the key moves to ignition I could use an analog input as a switch and turn on the coil so the engine can start. However in this scenario when the key moves to start the accessories ( pin 7! ) will switch off and switch off the PMU while the starter is engaged.

How can I use the accessories and ignition switches like they used to be utilised ?

2

Hi,

You can use the delayed turn-off function.

obraz
obraz356Ă—135 4.15 KB

A configuration like that will always switch off PMU 5 seconds after losing signal on pin 7.

You could also use the ignition signal from the key to make it switch off immediately when the key is turned off, but switch off after a specific time when cranking.

3

But this way you only have 5sec to start the car right?

An additional function that keeps PMU up while cranking (“Pin7” “>OR< “Pin29 starter” for example) with 5sec delayed turnoff gives you unlimited cranking time and then 5sec extra to go from one key position to the next without interruption.

Is that possible?

4

You have as much time as you set. And it’s only for a single cranking event. If you stop cranking and return to the ignition position, the delayed turn-off will be suspended, and the timer will be reset.

You can treat that as a safety timeout for cranking too long. If the engine is tuned correctly, you will never crank for more than 5 seconds at a time.

There is no way to have unlimited cranking time with the electrical connections he has now. The accessories signal is used as a switched 12V signal for PMU. When the key is in the start position, the accessories signal is switched off. That means you lose the switched 12 V signal for the PMU, which in turn will eventually cause the PMU to switch off. You can’t delay that indefinitely.

What is possible is to have a very long delay when cranking and immediately turn off with the key off. You would have to use the middle position (ignition) signal from the key switch as a delayed turn-off channel.

1 Like
5

Thanks for your suggestion. Using a delay will do the job.