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PRM 120 Gearbox - Morse cable adjustment


Col_T

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We recently had the control cable for our PRM 120 gearbox snap, which was replaced. I now find that, when selecting forward drive, the engine picks up revs shortly before the gearbox engages. It's not a huge problem, but doesn't seem right, as selecting tick-over means engaging gear and then immediately throttling back. So the question is, how do I adjust this out, and at which end of the cable?

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49 minutes ago, Col_T said:

We recently had the control cable for our PRM 120 gearbox snap, which was replaced. I now find that, when selecting forward drive, the engine picks up revs shortly before the gearbox engages. It's not a huge problem, but doesn't seem right, as selecting tick-over means engaging gear and then immediately throttling back. So the question is, how do I adjust this out, and at which end of the cable?

There are two possible issues - the gear cable is the wrong length, or the overall travel is insufficient. The differentiator between these two is what happens when you put it into reverse.
 

If reverse engages with less travel, (and engine still at idle) then the gear cable length needs adjusting so that when the control lever is in neutral, the gearbox lever is exactly in the neutral position. I suspect it will in the long term be easier to adjust the cable at the gearbox end - set the control lever to neutral and disconnect the cable at the gearbox. Move the gearbox lever to the neutral position and adjust the cable so that it all lines up.

 

If that is not the problem and in fact the problem is insufficient travel (same late engagement in reverse, after the revs have increased) the effective travel can be increased by moving the anchor point of the cable to a further-out hole on the control lever. But most likely and assuming the new cable was attached to the same hole as before, it will be the first issue -  ie just adjusting the overall cable length.

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To me it sounds as if whoever fitted it has messed with the throttle cable adjustment rather than  a gear cable problem. Do as Nick says, re checking the gear cable. Then put the control in slow ahead or astern and disconnect one end. The attachment "pin" should drop straight into the hole it came from, but I think it will jump as you pull it out. This indicates it was adjusted with the lever in neutral, NOT slow ahead/astern. Screw the end fitting so the pin does just drop into place.

 

If that is not the problem It may be that the gear cable has been put into the wrong hole in the operating lever  inside the control as Nick says.

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  • 6 months later...

Thanks for the info, @nicknorman.

 

Tackled this job to-day and it seems that adjustment at the gearbox end of the cable has improved things. I haven’t taken the boat out or tried any real manoeuvring yet, but it seemed a load better just nudging back and forward on the mooring.

 

That’s it for now.

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  • 4 months later...

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