Why does a starter "bounce"?

Question

Custom 4-cylinder boxer, based on a Volkswagen/Porshe of some kind (not entirely sure what), with the starter from an '85 Nissan Sentra or thereabouts. (generic part number 16816)

Video of two attempts with fuel and spark disabled, and oscilloscope captures of the solenoid and motor terminals:
https://www.youtube.com/watch?v=xhyJddXH4es

The video shows a single engagement for each attempt, and then the gear drops back out and grinds, but it has also "bounced" for several successful engagements before finally grinding as shown in the video. The 'scope shows some wiggle, but no dropouts.

Same two screenshots from the 'scope:

Yellow is the solenoid tab.
Blue is the motor terminal, after the contacts.

The best I can come up with is that the compression release (what would have been a power stroke) is enough to turn the engine faster than the starter does, and so the helix pushes the gear back down as if the engine were running. But because it's not running, it needs to re-engage, which leads to the "bounce". Is that reasonable?

It's been shimmed in various ways, asymmetrically as an attempt to either fine-tune the mesh, or use the angle to try and drive the gear out, and the mounting holes have been wallowed out to allow direct adjustment before tightening down again. During all that, it's been obviously too close (never popping out, just grinding) and too far (popping out and still grinding), and in between it seems to work...if it would just stay engaged, which it never does.

Going off of the "engine-too-fast pushing the helix down" theory, I found that if I disable the motor and only power the solenoid, I can easily push the gear back down with my finger. Does that indicate a problem?

A professional starter repair shop saw that video, kept it for a week, and ended up saying it was fine. So they didn't do anything, and it still has the same problem.

Any ideas?

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More testing, same result:

A small auxiliary battery that only runs the solenoid, does this, measured at the solenoid tab:

That battery plus a booster pack:

Without the booster pack again, but used the second channel (blue) to watch the battery itself:

Looks like these test leads are too small, and the auxiliary battery could be a bit bigger as well. But I don't have bigger clip leads at the moment.

The regular wiring is about that size too, so maybe that's part of the problem?

Also noticed that the aux battery still has the same wiggle on the solenoid tab. It's less pronounced, but still there. I thought it inherited that entirely from the motor, but there's no connection to that anymore, and it's still there.

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Got some 4AWG jumper cables to replace the 22AWG clip leads...plus about 2 feet of 10AWG to snake into where the solenoid tab is and have the right connector on it. Clipped the jumper cable onto the starter mounting bracket, and the other end of the 10AWG, and then about 5 feet from there to a standard car battery on the ground.

No change. Still saw about 10 volts between the solenoid tab and the mounting bracket, and 11 volts across the freshly charged full-size lead-acid car battery. (a smart charger had just clicked off)

An ohmmeter across the solenoid measures a dead-short circuit - 0.0 ohms - which is slightly odd because holding the same leads together reads about 0.2 ohms. Bad solenoid?

Answer 1

I can’t answer your question definitively, but I’d like to give you evidence that your theory of the engine overrunning the speed of the starter gear via compression release is not correct. (Don’t take it personally, I’m trying to help you find the true cause of the bounce.)

I should mention up front that being able to force the gear back with your finger against the force of the solenoid sounds completely wrong to me. I think you should measure with your scope the actual voltage delivered to the solenoid terminal while the starter is running to see if it is reasonably close to 12V. If it is dropping very low due to the starter’s current consumption, your fix may be as simple as changing the wiring to the solenoid.

Now to shoot down the theory of the engine overrunning the gear, if you will forgive me –

This is a scope trace spanning 380 ms and showing the beginning of a starting cycle on a 5.9L inline-6 diesel engine with a 19.5 : 1 compression ratio. (Image credit: me.) I was developing a method to automate engine starting and needed to sense engine rotation. This engine is from the brand’s pre-ECM era, 1989, and there are no crank or cam sensors so I added a magnetic pickup to sense the teeth of the ring gear.

The upper yellow trace is the signal from the teeth on the ring gear and the lower blue trace is from the + terminal on the starter solenoid. Each dotted box division is 20ms. (Click the image to see it more clearly.)

As you can see on the blue trace, it takes about 30ms for the solenoid to move far enough to mesh the starter gear with the ring gear and close the starter contacts. When it does, there is a voltage drop as the starter begins consuming amps. The teeth on the ring gear (yellow trace) start moving promptly, slowly at first, but within about 50ms the ring gear has reached a steady speed.

Magnetic pickups give a bigger signal when a passing gear tooth is moving faster. You can see that the overall upper yellow waveform has two “swells” and these correspond to compression and release of cylinder pressure. (Fuel injection was inhibited during this test.)

You will note, however, that the spacing between peaks – inversely proportional to the rotational speed of the engine – changes only modestly, roughly in proportion to the “swell” of the overall waveform. This is a high compression engine, certainly exceeding the compression ratio of any Otto cycle engine. So you see that the boost in rotation speed from release of compression is minor indeed, and likely nowhere near enough to overrun a starter motor that would run at thousands of RPM if not for the work of turning the engine.

Back to your problem: If you find a severe voltage drop at the starter solenoid, perhaps you can temporarily hook up a jumper cable directly from the battery to the starter solenoid terminal to see if the starter will crank without the bounce. If so, you’ll know that the solenoid is not getting sufficient voltage through the ignition switch to keep the gear engaged. If it still bounces while fed directly from the battery, you may have a defective solenoid or an inappropriately strong return spring.

Answer 2

I have just tried adding power to just the solenoid on a spare starter motor that I have on hand (this is one off a Jaguar, but I would have thought it would behave the same).

There is no way I would be able to push the pinion back while the power is still connected to the solenoid, so I think there is an issue there.

Have you got sufficiently thick cables for the 12v supply and ground.

Can you try using a separate 12v supply for powering the solenoid to the one that the starter motor uses?

Answer 3

Thanks @MTA for debunking the compression theory.
https://mechanics.stackexchange.com/a/91092/10721
I don't have to tools to measure that.

And @HandyHowie almost got it right.
https://mechanics.stackexchange.com/a/91091/10721

Upvoted both.

The problem turned out to be a weak solenoid. As @Pᴀᴜʟsᴛᴇʀ2 and @MTA pointed out in their comments to the question, and @HandyHowie said in the answer, it was too easy to push the gear back in. A different rebuilt starter (they're not made new anymore) is much harder to push in (though still possible by hand), even with the tiny 'gator-clip wires for testing, and doesn't disengage from the ring when installed. It still bounces, in time with the characteristic sound of a starter (is that where that sound comes from?!?), but it engages every time, stays engaged, and never grinds.

So I guess if you need this starter, don't be surprised if the one you get from the parts store is defective out of the box. A weak solenoid may not be caught during the rebuild, or even a bench test, and you can't get new. Keep exchanging until you get a good one. :-/