Starter solenoid operation

[Bitsyncmaster]

All these problems with starter replacement got me thinking why they have a copper jumper going from the solenoid to the starter motor. My thought was the solenoid was just a coil of wire, powered by the starter relay to ground. Yes it has that setup but it also has another coil of wire but that one goes to the starter motor.

The starter motor is basically a very low resistance to ground. So using two coils lets the solenoid have high power pulling the gear into the ring gear and then the other coil gets shut off when the starter motor gets power to rotate.

The result is less current to start the car.

Learn something new every day.

[David T]

The solenoid does a few things. First it pushes the bendix of the starter out into contact with the flywheel. Secondly there is a switch inside it that closes a circuit between the battery and the starter motor. And for a third function it also powers an auxiliary contact which goes to the Thermo Time switch to power the Cold Start Valve so it can only run during cranking. That contact also powers the bypass relay for the ignition ballast coil again, only during cranking.
David Teitelbaum

[Bitsyncmaster]

You missed the "few" thing I just posted. It also uses high current to get the ring gear engaged. It then reduces the current in the solenoid because holding the ring gear requires much less power than moving the gear into the ring.

So to measure the current required of the solenoid you would need to disconnect the +12 volts to the starter and measure the current. That would give you the peak current required of the starter relay. Then if you measure that same current with +12 volts turning the engine you would read a lower current.

The two coil currents are not matched. The "hold" coil current is less than the "pull" coil current. So if that copper jumper were put on the wrong coil it would still pull the same since both coils are on, but the hold current would be higher than need be.

[David T]

It is more complicated than that. There is "inrush current" and there is "holding current". When the solenoid is first energized the core is not inside and the coil is not "saturated" with current. The initial current draw is very high when you first energize the coil. The coil is just an inductor as far as electrical is concerned. Once it pulls the core inside and the coil becomes "saturated" with current, the current draw is greatly reduced. Similar to a motor starting current vs running current. Not easy to measure accurately without sophisticated equipment. Initially for an instant the current draw is infinite, limited only by the battery's capacity and the wire gauge. It will look like a dead short with a quickly diminishing amplitude.
David Teitelbaum

[Bitsyncmaster]

So if that copper jumper were put on the wrong coil it would still pull the same since both coils are on, but the hold current would be higher than need be.

I take this back. If the jumper were on the wrong side then one coil would be energized to pull and if the solenoid pulled in it would stay in and keep the starter turning.

[DniveK]

My jumper was on wrong...and at it did nothing but heat up the solenoid and blow out my digital starter relay. The starter never engaged at all. Changed to OEM relay, no starter engagement. After changing the jumper, started right up. Also retried the digital relay to see if it was indeed fried and it started the car when I put it in without the key even being in the iginition. There was clearly way too much current for the relay when the jumper was reversed.

[Ron]

Actually, you had the "wrong two" strapped together...it got hot because the hold in coil was trying to pull in by itself AND feed the starter motor, yep, "clearly way too much current" :wink:

Was like this, me thinks:

6781

[Elvis]

...And for a third function it also powers an auxiliary contact which goes to the Thermo Time switch to power the Cold Start Valve so it can only run during cranking. That contact also powers the bypass relay for the ignition ballast coil again, only during cranking.
David Teitelbaum

David, the bypass relay - YES, the cold start valve is not energized by the switch - or is my schematic wrong here ?
Are you definitely sure ?



It is more complicated than that. There is "inrush current" and there is "holding current". When the solenoid is first energized the core is not inside and the coil is not "saturated" with current.

strange theory



The initial current draw is very high when you first energize the coil. The coil is just an inductor as far as electrical is concerned. Once it pulls the core inside and the coil becomes "saturated" with current, the current draw is greatly reduced.

very strange theory !

the coil is an inductor and the current therefore rises "slowly".
once the current is at its maximum, the coil only works as a resistor.

*teacher mode off*