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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Diesel
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Diesel This is a forum dedicated to diesel engines and their applicable accessories.


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Old 11-10-2009
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Starter motor running backwards?

I've got a diesel Yanmar 3QM30. Recently I've been having trouble starting; there have been a few issues that have come up and been resolved.

About two weeks ago, I was having trouble with the starter... the solenoid wasn't kicking the Bendix forward, and the interior connections seemed unreliable... had to jump it to get the motor firing. Pulled it, brought it into the shop my company uses regularly. Get it back a few days later, all good to go.

Still no dice starting, but no reason to suspect the starter at this point; it was spinning fine with no compression on the engine.

I bled the lines, there's a steady stream of diesel on all three injectors.

Pulled apart the lift pump, cleaned a bit of gunk, and achieved some more fuel pressure to the injectors. Bled the lines again.

Finally it seemed like the battery bank just didn't have the cranking amps needed. I swapped in a high-cranking starter battery, isolated it, and the engine now cranks like a champ, so there's certainly plenty of power going to where it needs to be. However: the crankshaft is spinning in the wrong direction.

It was baffling for a little while, because the engine would show all signs of being able to turn over, and in fact did on one cylinder (which I've read 1-cylinder diesels can run backwards), but the other 2 never fired, presumably because nothing was spinning along with the shaft, and when they hit the compression cycle, they prevented the shaft from spinning any further.

I've checked the wiring and the battery itself. Nothing seems to have reverse polarity. I swapped the connections leading into the starter; it wouldn't fire at all, so I swapped them back. The house bank doesn't have the amps to crank the engine, so I can't tell which direction the crankshaft would spin powered off them. I could swap the house bank and the starter battery and see what happens, but I figured I'd ask around here and see if what I'm worried about may be the case:

Is it possible that, when my starter got rebuilt, it was rebuilt to spin in reverse? Maybe the shop thought it was a starter for a second engine in a twin-screw, going the other way, or an automotive setup. Is there something obvious I may have missed? Everything else about the engine seems fine... muffler isn't backed up, no signs of water or blockage, plenty of diesel running through the system, plenty of power. It's got air, compression on at least the 1st cylinder is happening, it's got fuel.

Just when I crank, it attempts to run backwards, which is amusing but frustrating.

Any thoughts?
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Old 11-11-2009
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I'm not sure how you are determining that the crank is turning backwards, but you can probably tell which way the starter is supposed to turn the motor when you pull it out again and inspect the fly wheel teeth. They will be more rounded off on the driven side. Now you can attach your cables to your starter while it's lying at your feet and jump the solenoid and see which way it spins. That will tell you whether the starter is spinning the wrong way. I think it's very unlikely that shop changed something to make it run backwards.
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Last edited by erps; 11-11-2009 at 11:21 PM.
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Old 11-11-2009
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I'm thinking that the drive has a one way clutch so if the starter motor turns backwards the clutch will not engage and turn the motor over at all.
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Old 11-11-2009
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Is this boat new to you?

Has this motor ever ran?
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Old 11-11-2009
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Do yourself a favor, save your money, and CALL A MECHANIC, while you still have something worth trying to repair. Unless of course money is of no object to you, then take pictures, and write us some more funny stories.
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Old 11-11-2009
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I'm thinking that the drive has a one way clutch so if the starter motor turns backwards the clutch will not engage and turn the motor over at all.
Bingo! I've yet to see a starter that when spun backwards would throw the gear onto the flywheel. Perhaps some will do it in either direction but I've yet to come across one and I have re-built a fair number of starters over the years.

I agree with CaPBlu that you need to call a mechanic..
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Old 11-11-2009
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I've yet to see a starter that when spun backwards would throw the gear onto the flywheel.
unless they gave you a rebuilt starter off the shelf that is designed to spin the other way (and kept yours for a rebuild later).
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Old 11-11-2009
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unless they gave you a rebuilt starter off the shelf that is designed to spin the other way (and kept yours for a rebuild later).
At this point anything is possible...!
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Old 11-12-2009
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In my younger (water-skiing) days I owned a Ski Nautique tournament tow boat that had a Ford V8 in it that ran counter-clockwise. So counter clockwise starters do exist.

But to rebuild one that works opposite to the way it originally turned is bizzarre. The bendix would also have to be changed. The teeth on the bendix would have the chamfer on the wrong side and so would the teeth on the ring gear be incompatible making it really hard to go into mesh.

So unless there was a great deal of effort put into reversing the rotation, an accidental reversal probably would have no chance of actually turning the engine and is highly unlikely.

But not impossible.
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Old 11-13-2009
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I don't think an engine with two valves driven from a camshaft will run backwards.

I am tired so I might be wro---wrrrr---wr0nnnnnn
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