# Where is the resistance in a starter motor circuit?

In a car or motorcycle, the starter motor is switched by a relay. The relay coil is switched by a key switch or small pushbutton. All of it is a 12 volt DC system.

Is it correct to think of the relay coil as having a very large resistance, and the starter motor as having very low resistance?

I'm trying to build my (so far nonexistent) electronic intuition. I know that there must be many, many more amps running through the starter motor and its large wires than through the starter button and its tiny wires. I'm struggling to understand (like deep down inside me) why the amps aren't running willy nilly through the starter button as soon as its closed, rather than through the motor which is doing a lot of work. Does the coil in the relay have a lot of resistance to lower the amperage, or is there a better way to understand what's going on?

You are correct.

The coil in the relay has many turns of thin wire and, as such, has higher resistance and draws much less current than the motor which has a few turns of fat wire with low resistance.

The relay is used so the switch can be smaller and won't burn out due to arcing. ALso so you don't have to run 1/4 inch cables all the way up to the steering wheel.

The current consumed by the motor does however drop off as it runs up to speed. It's initial in-rush current is very large though. Since starter motors are only ever really engaged for a brief second or three, it's almost always operating in starting mode. (Pun intended)

• @scottSaunders: Note that in most relays, there is no electrical connection between the relay's coil and its contacts. Oct 3, 2017 at 19:29
• Some cars had a manual starter button that operated the contacts directly... early Landrovers for one. Oct 3, 2017 at 20:24
• Thank you! That helps. I think that years ago I conflated the concepts of Load and Resistance - as though resistance somehow "used up" the electricity. Now, I think I understand more correctly and I'm trying to clear my head of the remnants of that mistake. Oct 4, 2017 at 13:40
• @ScottSaunders you have to think of power and electricity like water help behind a dam. If there is a small hole in the dam you have high resistance and not much water escapes. A big hole.... Oct 4, 2017 at 15:53
• Another way to think about it is that the magnetic field produced by the relay coil closes the relay contacts without making electrical connection. So the relay coil current and the starter motor current have no direct connection. Jan 11, 2019 at 6:02

The electrical circuit which energizes the relay isn't the same electrical circuit which feeds the starter motor. The starter relay closes the contacts which complete the circuit which sends electricity to the starter solenoid. The starter solenoid is the bigger relay which closes contacts which feed electricity to the starter motor. So, it's a small relay, which energizes a larger relay (the starter solenoid) which has the contacts which connects electric power to the starter motor. The electrical resistance (ohms) is in the copper windings which are in the starter motor. The total resistance (ohms) of the windings is under 0.1 ohms, usually.

I can confirm that everything Snow is saying is correct. I am currently working on a no start issue on a 2015 Kenworth T880 with the Cummins X15 and Eaton Ultrashift automatic. The issue presented itself as would begin to crank, but stop after 1/4 second. I didn't have a load tester with me, ran Vdrop tests across entire starter system. Full 12v across starter supply, batteries, and alternator connections. 10v was indicated on the dashboard with the Low Voltage Disconnect warning constantly on. Disconnected batteries for 30 minutes and they held full 12v, no internal failures. Checked starter signal, functions normally. Checked starter motor resistance, and boom, over 90 K-ohms. Cross referenced the resistance with a brand new starter off the shelf and motor resistance is 0.1 ohms. The high resistance was not allowing the starter to draw enough power to turn the engine through it's compression stroke, thus the partial crank and sudden stop. Replaced starter, cleaned battery and starter terminals, all issues resolved. Any starter motor should have no resistance, much greater than .3 ohms and you will probably start to see issues.