Thread: 1966 2a FFR - 24v system with 12 volt starter motor - help!

Originally Posted by nzpathy

The starter solenoid has failed, and I am thinking this is because it is a 24volt solenoid, supplying a load of 12v to the starter motor, and receiving a 24v switching voltage (from the starter button). The rest of the ignition system is 24v.

The only reason a 24V solenoid switching 12V would fail is if the 12V starter current is higher than the rated switching current of the solenoid. The 12V and 24V elements are completely isolated.

You haven't mentioned the failure mode. Contacts cooked or solenoid burned out?

Provided a replacement solenoid is rated for the starter current there's no reason to change what you have already. When the starter needs an R&R it might be worth having it re-wound as a 24V machine. We did this with an anchor winch motor on a boat a few years ago and it greatly simplified the whole shebang. 24V boat, 12V anchor winch. The idiot (for that's the nicest thing I I could call him) that wired the boat derived 12V from splitting the 24V banks. It was a battery destroyer.

Is the solenoid attached to the starter motor or stand alone in your system, as stand alone makes things easier?

I agree with Brad, stick with what you have as it's worked since 1966, If you're struggling to find a 24 V solenoid try a truck auto electric place as a lot of trucks especially the older ones run a 24 volt system, it seems a lot of boats and golf carts are 24V as well. In your application a 12V coil/12V contact solenoid would require a volt drop resistor and that just turns power into heat, another way to do it would be with an 24vdc/24vac inverter and just use one half of the cycle to get 12V, providing you can find one with a centre tap, but that introduces another point of failure as you mentioned, and AC ripple which might need a filter.

Better than a resistor or inverter would be a master/slave relay/solenoid system, especially if using a relay with a withdrawable base as the 24V master to a slave 12 V solenoid, as if it failed it would just be a case of pulling the faulty relay out of the base, and plugging a spare in. Although depending on current a contactor might be required.

Thanks chaps. Those look like useful ideas. The 12volt battery is dedicated to the starter motor; and that battery is charged via a BCDC charger from the 24volt battery bank. The existing solenoid failure mode...not sure, but there is insufficient current to the starter motor and the wiring to and from it is robust; I'm presuming the contacts are burnt. I'll hacksaw it open once I've a replacement. I hadn't thought of looking at truck part suppliers...so will do that as that should open up other 24v solenoid choices.