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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:

enter image description here

enter image description here

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?


More testing, same result:

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

enter image description here

That battery plus a booster pack:

enter image description here

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

enter image description here

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.


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?

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  • I agree with both MTA's and HandieHowie's answers stating the problem seems to be lying in the solenoid. You should not be able to push it back by hand when it is engaged. Feb 12, 2023 at 16:34
  • It appears that the solenoid for this starter is available as a separate item, and since all indications point toward a weak solenoid that's consuming far too many amps, replacement of the solenoid may be the best option.
    – MTA
    Feb 14, 2023 at 14:09

3 Answers 3

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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.

enter image description here

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.

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  • I attached the 'scope screenshots from the video, directly into the question. I think they answer the question of voltage drop. From the battery to the starter is about 10 feet of double 4AWG. One set goes direct, and the other goes to some "convenience posts" about halfway in between, that the rest of the system treats as "the battery". To power the solenoid, a single 18AWG (maybe 22AWG?) comes off of the solenoid's battery post, goes through a relay contact about 2 feet away, and comes back to the tab. (the start button is tiny, so it runs that relay instead)
    – AaronD
    Feb 13, 2023 at 16:00
  • @AaronD It looks like sufficient voltage at the solenoid terminal and you can clearly see the voltage rise (from fewer amps) when the gear disengages. Is there some clue in the wavering voltage within the first dip? The frequency of the waver is about 100 Hz. The waver doesn't happen where the second dip should be. Instead, the gear disengages. What's that about? Clue or red herring? Hoping to see results from the test of a heavy gauge wire direct from battery to solenoid terminal.
    – MTA
    Feb 13, 2023 at 17:34
  • @AaronD Just a thought: have you checked the starter's overrunning clutch? From the view in the video, the gear should turn freely counterclockwise but should be much harder to turn clockwise. Clockwise will turn the motor's rotor, counterclockwise will turn only the gear shaft up to the overrunning clutch while the motor's rotor remains still.
    – MTA
    Feb 13, 2023 at 18:24
  • Pushed the gear teeth by hand. Turns easy forward, not at all reverse. So the clutch works.
    – AaronD
    Feb 13, 2023 at 20:14
  • I don't know what the 100Hz wiggle is about either. I think parasitic resonance in the wiring ought to be much faster than that. (maybe it is much faster and my auto-sample-rate scope is aliasing?)
    – AaronD
    Feb 13, 2023 at 20:15
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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?

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  • That would be the next thing to try. Haven't done it yet. In the meantime, I put the 'scope screenshots directly into the question, so you can see from there what the voltage is doing.
    – AaronD
    Feb 13, 2023 at 16:05
  • About 10 feet of double 4AWG from the battery to the starter. One set goes direct, and the other goes to some "convenience posts" about halfway in between, that the rest of the system treats as "the battery". To power the solenoid, a single 18AWG (maybe 22AWG?) comes off of the solenoid's battery post, goes through a relay contact about 2 feet away, and comes back to the tab. (the start button is tiny, so it runs that relay instead)
    – AaronD
    Feb 13, 2023 at 16:06
  • It looks like the starter is mounted vertically, does it work any different when mounted horizontally? Can you still manually push the pinion in?
    – HandyHowie
    Feb 13, 2023 at 17:22
  • Even if that did give an interesting result, how would it help? The engine is mounted vertically, and everything else is built around that. So the starter kinda has to work vertically too, whatever it takes to do that, even if it ends up being non-standard.
    – AaronD
    Feb 13, 2023 at 20:21
  • Plus, it's kind of a pain to take out and put in. Lots of things all crammed into a small space...
    – AaronD
    Feb 13, 2023 at 20:22
1

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. :-/

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