PRM 120 Gearbox Initially Fails To Engage

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Much as the title suggests really!

 

Started the boat, which had laid idle for a couple of months, engaged forward gear ........... and nothing happened! Gradually increased the throttle until suddenly, with a big clunk (more a bang actually) forward gear finally engaged. As I had to manoeuvre the boat, I moved the combined throttle/selector lever from the forward to reverse position several times and on about the first half dozen times that I selected forward gear, the same thing happened before the 'box seemed to settle down into normal operation. There were no issues when reverse was selected.

 

Anyone any ideas please?

[Tony Brooks]

What is the oil level in the box?

[cuthound]

4 minutes ago, Tony Brooks said:

What is the oil level in the box?

 

^^^^ This. Used to happen to our shareboats when the gearbox oil.was low.

 

You will need to check the colour of the oil on the dipstick. Should be a pinky red colour if it is ATF, or a golden colour if it is engine oil.

 

PRM120's were originally filled with engine oil but the manufacturer changed the specification to ATF some years ago IIRC.

Edited November 7, 2019 by cuthound
Spillung

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Thank you both for your prompt replies. The oil level is up to the mark, it is ATF (I had the conversation with PRM fairly recently) and was changed about 10 engine hours ago.

[Tony Brooks]

In that case as long as the cable adjustment is correct and not throwing the gearbox lever a bit short I fear it may be the clutches.

 

Disconnect the cable on the gearbox and try operating it by hand so you get the full throw.

[Stilllearning]

It may be worth checking that the cable to the gearbox is adjusted so that the gears engage fully before the throttle cable increases the revs.

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Thank you both for further prompt replies. Cable adjustment spot on as witnessed by the operation once forward engagement finally stops clunking and engages normally. I was hoping for an altogether less ominous diagnosis than clutches ........... but it wasn't far from the front of my mind. Given that the problem seems to go away once things have warmed up and settled down, what sort of life has the box got before replacing/reconditioning?

 

N.B. Your proof reading is spot on today Tony!

[StephenA]

Ours does that sometimes when its not been used for a while and its cold.....

 

 

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14 minutes ago, StephenA said:

Ours does that sometimes when its not been used for a while and its cold.....

 

 

That's encouraging then ......... it was definitely cold.

[Boater Sam]

Its a bronze cone clutch, hence the snatch on engagement. I high engine tickover exacerbates the snatch. Try letting the box warm up by running the engine for several minutes longer before engaging a gear. 

 

The forward cone gets more wear obviously so if it is better in reverse then it may well be wear and tear issues.

 

Save up for a change to a hydraulic box, say a PRM150 when the moment comes.

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12 minutes ago, Boater Sam said:

Its a bronze cone clutch, hence the snatch on engagement. I high engine tickover exacerbates the snatch. Try letting the box warm up by running the engine for several minutes longer before engaging a gear. 

 

The forward cone gets more wear obviously so if it is better in reverse then it may well be wear and tear issues.

 

Save up for a change to a hydraulic box, say a PRM150 when the moment comes.

Now that strikes a chord. In the back of my mind there was a feeling that there was a basic difference between a 120 and its larger cousins. It's also making me feel a lot better about the prognosis as this gearbox has done an excessive amount of running in reverse gear (don't ask unless you're wanting a long – and somewhat unbelievable) – story. Something that logically shouldn't have any effect on it's current behaviour when forward gear is engaged.

[Tom766]

I had the same problem with my brand new beta marine 10 engine, although new it sat in a lock up for @ 5 years. Turns out the transmission oil being old gummed up the cones in the gearbox. Clever engineer ran the gearbox VERY BRIEFLY on diesel. This cleaned everything up and fixed the problem.. 

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2 minutes ago, Tom766 said:

I had the same problem with my brand new beta marine 10 engine, although new it sat in a lock up for @ 5 years. Turns out the transmission oil being old gummed up the cones in the gearbox. Clever engineer ran the gearbox VERY BRIEFLY on diesel. This cleaned everything up and fixed the problem.. 

Thanks for that tip. Maybe I'll just have to pluck up the courage .........

[Tom766]

It was really simple and worth a try before chucking lots of money at it. This new beta is a gem ?