Maybe its obvious to you guys but Im not seeing it. In this diagram there is two separate windings in the starter solenoid. This starter is bad intermittently and it is the solenoid. Ive captured the fault with an amp clamp and a scope. Im just trying to figure out why its designed this way with 2 windings, any thoughts?
-Jason

Edit: this is a 2006 hyundai sonata.

 

Here is an amp clamp capture while in crank on the starter sol. circuit. Starter spins the engine a little, then stops has a current ramp then engages again and engine spins normaly.

 

One coil is in series with the motor. That coil likley limits I peak since I can't change instantaneously in an inductor. The other coil moves stuff (the high current contacts and gear engagement).

 

This link provides an extensive explanation of why there are two windings: http://www.mgb-stuff.org.uk/stasol.htm

In a nutshell, when the starter motor is stopped, it behaves a bit like a dead short to ground, meaning that when the ignition is switched on, current flows through BOTH solenoid coils, producing a large magnetic field and closing the relay to the starter motor RAPIDLY.

Once the motor is spinning, the voltage across it approaches 12V, as it is connected across the battery. If the top side of the motor is at 12 V, there is no longer a potential difference across the leftmost solenoid coil, meaning that the magnetic field is smaller. This means that there is just enough magnetic field in place to hold the relay closed and keep the starter spinning.

In a nutshell it's a clever way of getting a large magnetic field for a short space of time to turn the motor on then get rid of the large field before the solenoid burns out!

 

Here's a dismantled one: http://www.ifitjams.com/solenoid.htm

 

Great stuff thank you for the responses and links.

The coil that connects to the motor is the pull in solenoid. The other that simply goes to ground is the hold coil. So looking at my amperage screen shot the two coils do there job and the starter/engine begin to spin. That created a voltage diffrence and the pull in coil is turned off as its connection to the starter motor goes to high voltage. Then its all up to the hold coil, who you can clearly see looses his grip. The starter then no longer has 12 volts (high) feeding it which creates the path to ground for the pull in coil. The pull in coil now grounding through the motor windings builds a field for about 2 seconds. The contacts close and the pull in's ground goes high that leaves the hold in coil to keep the starter/engine spinning. How cool is that? Thanks guys.
-Jason

 

Ths solenoid has two windings in it. The pull in coil is used to do the heavy work. It pulls the plunger in and allows the contact disc to make contact with the the battery and motor terminals. Current flows from the ignition switch, through the pull in winding, the field coil, brushes and armature to ground. As it pulls the contact disc and plunger in, it also pulls the starter drive into engagement with the flywheel.
When the contact disc is in place, the hold in winding takes over and has the light work of holding the contact disc and plunger in place until the key is released. Current flows through the ignition switch, the hold in cojl and then to case or remote ground.
Both coils will have an equal number of turns and the pull in coil will have a heavier gauge of wire.
A starter which chatters can be caused by a weak or open hold in winding or low voltage application to the "S" terminal. A voltage drop test can tell you the whole story.
The pull in coil drops out when the contact disc meets the battery and motor terminals because the potential on both sides of it is equal.

 

Thanks again for the replies. Doing a little more research I found just about every auto manufacturer uses this style solenoid circuit.
-Jason

 

This starter circuit is typical in every vocation and hasn't changed much in a very long time. Newer variations may include relays, thermostatically controlled circuits and ECM controlled circuits.

 

Had another bad starter today. This one had a failed pull in solenoid. Its a 2006 Honda Odyssey and its all fixed now. I figured Id take the starter all apart and see what I could see but didn't have time to cut open the pressed together solenoid.

Now that I know more on this circuit I have the answers to why it wont start in detail other than "oh yea, it needs a starter". Its seems like a trivial detail most don't know or don't care to know. Hold in windings were 1ohm and the pull in were 2.5Mohm.

 

I figured Id take the starter all apart and see what I could see but didn't have time to cut open the pressed together solenoid.

I don't like the new starter solenoids made like that ,pressed together. On the old solenoids you could take the m apart and flip the contact disc over to get them to work again. When the coils weren't burned out, which they very seldom where. Most new car starters I've worked on the solenoid isn't even replaceable now, you have to get a complete new starter.