Notes on differences between 12 volt Starter Solenoids

Last Revised: August 11, 2016


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What IS a starter solenoid?

An automotive type starter solenoid is basically an electrical relay that has, usually, one set of high current normally open contacts.
This relay also contains a coil of wire which, when supplied with the proper voltage, creates a magnetic field which pulls an iron core upwards pushing the high-current contacts closed. Closing those contacts connects the starter to the battery to crank the engine.

Here's what it might look like, schematically:

Starter Solenoid Diagram
So, you'd think that this starter solenoid thing is simple, electrically, but I gave up recently trying to characterize them with a simple table. Yes, the big leads are pretty much "IN and "OUT" for the high current to a starter, but from there on, there are many variations.
For instance:
-Assuming a 12 volt solenoid, just to make it easy---
1.-Assuming that there only two internal coil leads and only one of them makes it to the outside world:
One of those leads could be grounded (internally)and the other could be brought out as a single small terminal.
Then that terminal would need +12 VDC applied to activate the solenoid.

2.-However, One of those two leads COULD be connected to the Hot large terminal of the solenoid internally.
In this case the single lead that is brought out would need to be grounded to energize the solenoid.

3.-Okay, so now let's bring both leads from the coil to the outside, a two small-terminal solenoid.
In one case, this is the best kind to find, because you can use it to do either of the things mentioned above.
And sometimes this IS the case.

4.-BBBBUUUTTT!!! Just the other day I called Surplus Center about a nice, cheap 12 volt solenoid that they have. I called because I thought the description was wrong. Well, it wasn't perfect, by my thinking was wrong. This solenoid has two small terminals, but, get this-
One side of the coil is grounded, so one of the small terminals needs 12 VDC to make it operate. BUT, the other terminal gets connected to the 12 volt LARGE HOT terminal ONLY WHEN the engine is cranking.
This lead is connected to the coil to bypass the ballast resistor and the voltage is used to boost ignition voltage during engine starting! That's a new one on me!!