[BrendanP]

My 2015 1.6 diesel has recently begun to not start first time. The engine isn't cranking but there is a whirring sound as though the starter motor is spinning. If I stop and try again it usually starts as normal. I know the solenoid should push the starter pinion along the shaft splines to engage with the teeth on the flywheel, and the contact in the solenoid should close to energise the motor. Does anyone have experience in overhauling the starter motor? My thoughts are that it could be the pinion is sticking on the motor shaft, or the motor brushes are worn down. Any ideas?

[The Gonz]

That little bit of boost to get the solenoid to engage the pinion could be missing from an otherwise healthy battery.
Have you checked your battery voltage at rest and under load?

[BrendanP]

The battery is a Yuasa YBX9096 AGM, and it's 3 years old. The engine cranks over fast enough so I don't think the battery is the problem. I can measure the volts when I have a helper.

[The Gonz]

I'd still be betting on electromechanical. If the voltage is good, is it actually getting to the solenoid at full potential or is there a voltage drop along the way?
Weak termination? Compromised cable? Can the solenoid be dismantled and cleaned?

[BrendanP]

It did it a couple of times this morning during auto-start. That's a real pain when moving off from traffic lights with cars behind you.

[The Gonz]

Ah, new information, you have autostart! That complicates it somewhat but also clarifies it. The autostart logic is also a player now.

[Greyhound]

... I can measure the volts when I have a helper.

Did you get to do a battery voltage check?

I assume you can switch off auto-start for journeys for the time being to avoid slow getaways.

[BrendanP]

Update. I bought a new starter motor, pretty easy to fit once you've removed the air filter box and the battery. The new starter seems to have more oomph when spinning the engine over. The supplier didn't require me to exchange the old motor so I took it apart to see what might be wrong with it. What I found was that the plastic moulding that acts as the gearbox housing (the epicyclic gears inside the starter) was broken. The photos show the damage. The edges where the piece had broken off were quite worn, indicating this wasn't a fresh break. This housing supports the rear bearing of the output shaft, so maybe it was moving around enough so that the pinion wasn't engaging with the flywheel ring. Anyhow, I guess this is a part that is impossible to obtain on it's own, so I don't think the motor is repairable.

[The Gonz]

I'm intrigued to know what kind of plastic that is and what temperatures it has to endure ...

[BrendanP]

I had a look on the moulding for material identification marks but it has nothing on it. I expect it's some variant of glass-filled nylon. The ring with grooves around the edge is, by the look of it, a bronze plain bearing. I can't pull it apart any further because of the stop ring on the shaft, it looks like it's held on by an internal snap ring so once it's on, it won't come off. The plastic will get a hammering every time the solenoid throws the pinion forward, a bit like using a slide hammer.