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Thornycroft engine fault


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First thought is starter motor.

Its almost as if it spinning but not engaging - is the dog getting thrown forward ?

 

Take the starter off and connect it to 12v and see if it is working (is the dog getting thrown forward ? and check the teeth and see if they are damaged / stripped)

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Wot they say above.

The engine looked a bit mucky and lots of rust around which - if the boat has been 'standing around'  for a while could gum up the starter and stop the cog engaging with the flywheel ring.

The batteries sound great - so that's one item out of the way.

If  you're not familiar with engne starters - there's a gear wheel on the starter shaft which spins when you run the starter motor.

The shaft has a helical spline - so that when the motor shaft turns the gear 'slides' up the shaft , engages with the engine flywheel and rotates the crankshaft.

If there's rust / gunge / w.h.y on the shaft the gear wont slide - and nowt happens - other than the noise as heard in the video.

It doesn't help if the weather's cold as that add to the gumming up situation.

 

Not a good explanation but it migh help in the understanding of the process.....

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May be a slipping one way clutch located behind the pinion, try to turn the armature in both directions by the pinion, you may find it won't turn the armature in either direction. A better test is to clamp the motor in a decent vice and wire up, then try to stop the pinion with a length of 2 x 3 or so timber.

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4 hours ago, Alan de Enfield said:

Take the starter off and connect it to 12v and see if it is working (is the dog getting thrown forward ? and check the teeth and see if they are damaged / stripped)

For the OP, if you do do this note the following.

With the starter off the engine, you'll need to bolt the battery black earth lead that goes somewhere on the engine to the starter motor body via one of the bolt holes as well as the big battery 12V+ lead to the main tag and any other wires. The kick when the starter turns is considerable. Be ready for it, preferably by having the starter held in a vice. Be careful with the wires. Any short circuits will flow very high currents and produce big fat sparks.

Jen

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  • 4 months later...

Hi all,

 

So update on this issue (finally got to it due to lockdown up here). I have removed the starter and had it reconditioned, fitted it today and still nothing. The starter is working fine as can feel it.

 

The next step is to try and move the drive wheel (i think thats what its called?) with a 26mm ring spanner and hope that they both just need to align.

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Is this not a pre-engaged starter? If so it will not power up if the pinion is not in full mesh with the flywheel gear ring.

It is obviously powering up so either the clutch behind the pinion is duff [the reconditioner missed it?] as said in post #8,    or there are no teeth on the flywheel.

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On 14/02/2020 at 17:58, OldGoat said:

Wot they say above.

The engine looked a bit mucky and lots of rust around which - if the boat has been 'standing around'  for a while could gum up the starter and stop the cog engaging with the flywheel ring.

The batteries sound great - so that's one item out of the way.

If  you're not familiar with engne starters - there's a gear wheel on the starter shaft which spins when you run the starter motor.

The shaft has a helical spline - so that when the motor shaft turns the gear 'slides' up the shaft , engages with the engine flywheel and rotates the crankshaft.

If there's rust / gunge / w.h.y on the shaft the gear wont slide - and nowt happens - other than the noise as heard in the video.

It doesn't help if the weather's cold as that add to the gumming up situation.

 

Not a good explanation but it migh help in the understanding of the process.....

 

I very much doubt Mitsubishi would use an inertia starter as you describe. Yes there is likely to be  a helix on the shaft but t is to slightly twist the pinion as the pre-engage solenoid pushes the pinion along the shaft.

 

As this is almost certainly a pre-engage starter the fact that it energises makes me less likely to suspect the pinion is stuck to the shaft. My guess is that the pinion over-speed free wheel/sprag clutch has failed so the shaft rotates but the pinion does not. Anyway the motor almost certainly has to come off for further diagnosis.

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5 minutes ago, Tony Brooks said:

As this is almost certainly a pre-engage starter the fact that it energises makes me less likely to suspect the pinion is stuck to the shaft. My guess is that the pinion over-speed free wheel/sprag clutch has failed so the shaft rotates but the pinion does not. Anyway the motor almost certainly has to come off for further diagnosis.

The OP has had the Starter overhauled and the problem still exists

 

1 hour ago, scotsdgpiper said:

I have removed the starter and had it reconditioned, fitted it today and still nothing.

 

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Just now, Alan de Enfield said:

The OP has had the Starter overhauled and the problem still exists

 

 

Did they do a lock torque test or just fire it up in a vice?

 

If they did a lock torque test and its a new problem then I fear burred flywheel teeth at least - I have also known a very few ring gears to spin on the flywheel but that is so rare it can probably be discounted.

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I have had supposedly overhauled starters where they have not tested the clutch properly. With diesel engines high compressions that small sprag clutch has a hard life. 

When we used to tune petrol engines, we often had to uprate this clutch to prevent them spinning free. 

  • Greenie 1
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4 minutes ago, Tracy D'arth said:

I have had supposedly overhauled starters where they have not tested the clutch properly. With diesel engines high compressions that small sprag clutch has a hard life. 

When we used to tune petrol engines, we often had to uprate this clutch to prevent them spinning free. 

The place i got it reconditioned is very reputable and have a "designated fixer", i can only assume that he did all the correct tests but will call tomorrow and find out.  If its not one thing, its another with this boat!!! lol

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Just now, scotsdgpiper said:

The place i got it reconditioned is very reputable and have a "designated fixer", i can only assume that he did all the correct tests but will call tomorrow and find out.  If its not one thing, its another with this boat!!! lol

That's boats for you!

TD'

Just now, Tony Brooks said:

At least the sprag clutches are more reliable then the multi-plate clutches of yesteryear

And pre-engaged is vastly better than the old inertia bendix starter pinion.  Remember Mk 1 Cortinas with their constantly loosening 2 bolt starter motors? I got tired of putting ring gears on those week after week.

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