Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

? about 'magnetically operated' switches and properties of electricity

I’m sitting here studying my MPI control relay and have a theoretical question at this point. (my lingo, no doubt, is bad, but hopefully it’s understandable.)

I’m looking at a simple switch held open by a ‘very lightly-loaded leaf-spring.’ This switch get’s closed by way of a coil that - when ‘energized’ - produces magnetism that pulls the switch shut.

Once such a switch is pulled shut, however, and a current is established through it’s contact points, is there some kind of electrical property that ALONE would HOLD it shut if the coil were then ‘de-energized’ ?? thanks…!


The device you are describing is a simple relay. Unless it is specifically designed to be self-holding once energized it will release when coil current is interrupted.

Yes if you will call residual magnetism an electrical property. The steel in the relay core is formulated to provide no residual magnetism when coil power is removed. A steel core material can be chosen to provide enough residual magnetism which can hold the relay switch in the closed position with coil power removed but that would be a defect in the relay that you describe.

Sounds exactly like the toaster I just fixed. Solenoid or magneto or your preference of terms holds down a metal bar till the thermostat kicks it out.

The relay that you describe would be called a latching relay. Vehicles don’t normally use that type. Just a simple on-off coil type.

If the relay latched closed even though the current was turned off, how would you ever get it open? As Cougar says, latching relays do exist, but the few that I’ve encountered (in military ground equipment) have an unlatching mechanism.

There actually are latching relay setups in some cars. As I recall, my Camry has one in the ignition that latches the fuel pump on when the starter is activated and is unlatched when the ignition is turned off. Apparently they did that to keep the fuel pump from running if the ignition is on, but no attempt has been made to start the car. They use two ordinary relays to do that. Or maybe I didn’t understand the circuit. Relay logic tends to be mindbending.