Troubleshooting Starter Solenoid - Old School

I have a 12v solenoid with the standard two large terminals and it has TWO small terminals.

When I jump across the two large terminals with a screwdriver, the starter motor just spins but does not engage the motor.

Can anyone tell me how to troubleshoot it?

I’d also like to know how to manually jump this solenoid so I can crank the motor.

You jump the small terminal that has the wire from the ignition switch on it to the large terminal that the battery cable is attached to.

In the olden days we would hit the starter motor pos stud with a jumper cable connected to the pos of the battery. My thought your starter motor needs to get bench tested.

The two small terminals are energized by a relay when you turn the ignition key. That makes the solenoid close the contacts between the two big terminals and supply power to the starter motor. So, you have already effectively jumped the solenoid by shorting the large terminals with the screwdriver.

If the starter motor just spins, you either have a) a trashed flex plate/flywheel with several missing teeth b) a very low battery or poor connections causing low power to the solenoid/starter motor c) a worn out starter motor that’s not spinning fast enough to extend the bendix gear or d) bendix is sticking and not extending properly.

A load test can tell you if the battery is low.
Next would be to check connections at the battery and starter motor.
Next I’d pull the starter and inspect the bendix gear and assembly. I’d probably bench test it too. If you’ve never done this, it has to be seriously restrained, there is tremendous torque when it is powered up. If the gear looks chewed up, that could mean the flexplate/flywheel teeth have been taking a beating too and need inspection. An auto parts store may be able to test the starter motor for you and make sure the gear is extending under power. Call around.

The solenoid serves TWO functions. It pulls the Bendix drive to engage the flex plate AND when it’s pulled all the way back a copper disc inside the solenoid makes contact with the two large terminals on the back of the solenoid. When you simply jump the two large terminals all it should do is spin but not engage the flex plate. What you’re seeing is normal.

What kind of car is this and can you describe the problem?

Battery shows 14v at the solenoid.
Connections are clean.
Flywheel is clean and no damage.

I think I remember years ago that on some starters, just jumping the large terminals would just spin the starter motor - as I have here.
If memory serves me, on some solenoids you have to jump from a small terminal to to a large terminal.
I was curious why any solenoid would have TWO small terminals on it. I get that one comes from the switch. What purpose might the other small terminal serve?

OK, I’m going to assume this is an old car. If so, the extra terminal serves to bypass the ignition ballast resistor to provide full voltage to to the ignition coil while starting.

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Looking at solenoids on some auto parts sites there are many kinds for sure. Lots with 1 small terminal and some with 2.

With the two small terminals, does it sound likely that I should jump from one of them to one of the large terminals as someone mentioned above?

Yes, that was @keith advice and I agree with it.

Still need to know what the symptoms are. Are you getting a hefty click when you try starting it or no sound at all?

getting a hefty click

just went through something similar on my own car, even though ive been an auto mechanic for many years it brought me a new understanding, some have explained it already but ill do it in my own words. the starter has either three or four connections. it may have one or two small connections if you supply power just to one of the small connections it should engage the arm on the starter and cause the loud click noise. that small jumper also causes the power between the two large terminals to connect together, and the one large terminal with the strap going to the starter will cause the gear to spin. so for instance my car would always click, but if i then wedged a screwdriver just right to hit both large terminals and the small one at once it would work. so after a 15 dollar ebay solenoid it works great, rather than a 200 dollar junk rebuilt starter. hope thats understandable

Edit: i see yours definetly has four terminals, i am not 100% sure but i believe the extra small terminal is like an accessory connection and not necissary, the one that should cause the starter to engage should have an S near it.

If it’s a Ford, jumping the battery lead to the starter lead at the solenoid near the battery should crank the engine. If you jump the solenoid and the starter spins freely the bendix is likely shot.

if you engage the two large terminals on your solenoid it will just spin, you must also engage the small one to make the gear move forward and engage the engine. I do not believe you have to engage both small ones.

Sorry just trying to make my message easier to understand ha

Also rod knox is correct on the ford, although i beleive it still has some sort of activation wire and solenoid on the starter to move the gear forward? i dont remember exactly.

I guess this might be a little more complicated that I thought with that 4th wire. Mine is a kit car and I have no schematic. The solenoid is on the firewall.

A clue might be found at

If this link works, scroll down about 1/4 way down the page to see the first line drawing or sketch - a 4 wire sketch. For the setup pictured it seems that there are two windings in that solenoid - one ‘pull in’ winding and one ‘hold in’ winding. That may explain my 4 wires.

As ‘2 cents’ mentioned there may be another setup using 4 wires. Some Fords seem to have two small terminals labeled ‘I’ and 'S" . The S goes to the ignition coil. I don;t think I have this setup.

in the diagram on that page the G is going to ground. the S and the M are the only two that would actually need power to make it work. im not sure if that helps much ha

what does your starter look like? i cant picture any starter ive ever seen (even fords with solenoid on the firewal) that dont Also have a solenoid on the starter to move the gear forward. that part could be your problem.

Edit: always trying to clarify. The S and M need power. The starter needs to be grounded. That G terminal may need to be grounded but i dont think so. but according to that diagram, it is going to a ground.

If you’re getting a hefty click then the large contacts inside the solenoid are probably in rough shape. In the old days we used to actually pull the solenoid apart and clean up the copper disc.
Then we would pull out the two main contacts (they’re just copper bolts). You should find they’re badly eroded partway across the tops of their heads then file them on the un-eroded surface just enough to clean them up but not even with the eroded part. then we would re-install them 180 degrees out of rotation so the high part would become the new contact area. Money was tight in those days.

Of course it would be better to just replace it with a new solenoid if you can find one but you still haven’t mentioned what kind of car it is.

cents is correct. although on my pg260 starter solenoid the housing was like crimped on so i would have had to cut it off somehow and like put a clamp on it. im gonna play like tom and ray and through a wild guess, its a shelby cobra kit car with a 4.6 ford set up

Edit: seriousness, if you tell us the car and engine set up we or you can determine the exact starter that is on it, and what that starter uses to engage the gear forward into the flywheel which seems to be the motion you are lacking.

As was said above if the starter motor spins the bendix should engage on the flywheel. The bendix is strictly a mechanical unit that relies on rotation of the starter.

I should clarify my last post was for GM starters. As I recall Ford starters were quite different.

If it’s a GM starter try this: Have somebody turn the key to the start position. As soon as you get the hefty click jump the two large terminals on the solenoid. I’ll bet it will start right up. Just be sure to release whatever you’re using to jump it immediately.

The “extra” small terminal on the starter goes to the terminal with 2 wires to it. One is the wire from the ignition switch that goes thru the ballast resistor or thru resistance wire to supply 6 volts to the coil when the car is running, The wire from the starter supplys 12 volts while the car is cranking for a hotter spark to start the car.
If you supplied 12 volts to the coil all the time, you would burn up the points in the distributor and they would turn bright blue.
When I had old cars of this type with a bad solenoid I would start them by shorting between the 2 large terminals with the key on, using a large screwdriver I didn’t care much about.
People might have more helpful answers if you to;d them what you are driving. For example, I know very little about GM cars.