PRM 280

[hughc]

We have been having trouble with a PRM 280 which, although supplied in 2019 has had virtually no use until last year. The box is of course out of warranty.After only 50 hours of use the box started overheating. The gearchange was becoming very crunchy and the oil was coming out black. The oil cooler was checked and cleaned although it needed very little attention. Using a PRM pressure gauge the oil pressure was within specified limits. PRM suggested an oil change and said that the black colour of the oil was because, 'the phosphate coating was wearing off.' Hm.. The engine, a Ford BSD 332, has had a PRM 160 fitted for 30 odd years without any overheating problems so the cooling system parameters seem to be O.K. All the checks suggested by the makers have been carried out without finding any reason for the problem. Has any one else on the forum had this problem and suggest a reason or cure. I will be grateful for any pointers as to the way forward.

 

[Tony Brooks]

Please, will you be more specific as to what you mean by the crunchy gear change.  The gear change valve is just a semi-rotary spool valve that is bathed in oil, so the movement on the single lever control should be all but silky smooth  unless the cables are maladjusted, the cable is worn or bent, or the detent ball that is supposed to give you a positive ahead neutral, and astern position on the gearbox has lost its ball, so the spring is pushing on the spool valve and scraping.

 

I wonder if the valve is not going fully into ahead/astern, so pressure is lost and the cutches are slipping when in gear.

[Bee]

Logically its either not being cooled sufficiently or its generating too much heat. The black oil sounds like burnt clutch plates but to the best of my knowledge these boxes are either fully in gear or not, being halfway in or out is difficult to achieve. The only thing I can think of is to try and narrow down the cause, try and check oil and water flow through the cooler by holding hoses and pipes, disconnect operating cable and operate the lever by hand and see if the prop shaft is spinning at the speed you'd expect. Apart from that it sounds like something inside the box might have failed. Sorry, not a lot of help I'm afraid. Good luck

[blackrose]

When was the oil first changed? On my PRM 150 it's supposed to be changed after 50 hours and not doing so can seriously damage it as someone I know found to their considerable cost when their PRM 150 required a complete rebuild. I assume it's the same with any PRM hydraulic gearbox and that you did this initial oil change, but since you haven't actually mentioned it I thought it's worth asking.

Edited February 4 by blackrose

[hughc]

Thank you for the replies.We isolated the cooling circuit and dropped a coil in the cut to act as the gearbox cooler. It stayed cold on both the ingoing and the outgoing. The pressure relief valve is in good condition as was the spool. The gearchange cable was changed when the box was installed and is operating as indicated in the manual and the detent is as it should be. 'Crunchy' is a definite jolt going into gear with a horrible metallic sound. The oil was changed shortly after the box was fitted to remove any manufacturing detritus. It has then had , because of the overheating, two more complete oil changes. What I don't understand is why, with correct gearbox oil pressure, there is no oil flow through the cooler. Before the oil heats up the change is as Tony says silky smooth.

Having phoned PRM for advice I have to say that they were of no use whatsoever.

[Tracy D'arth]

AFAIK the oil cooler is in the bypass pressure release circuit of the gearbox so if there is a massive internal leak or the pump is not delivering enough oil you may not get any flow through the cooler. It is very odd though.

I have fitted a few PRM 240 and 280 boxes over the years and have generally found that on the first oil change there is a substantial amount of muck in the oil. They are substantially the same box as a 160 but with friction clutch linings of composite sintered material instead of cardboard. 

The only cause that I can think of is if there is a leak across the spool valve it is possible for both clutches to be engaging at the same time. The gears simply churning in the oil will not quickly overheat the box if at all.

I would take the spool valve out and dismantle it, inspecting for a possible leak across the 2 outlet ports.

[Eeyore]

There is precedent for this, some 150 boxes suffered from similar leakage. Quite a difficult diagnosis.

 

Edited February 4 by Eeyore

[hughc]

Thank you for the suggestions. To bring the story up to date we swopped spools with the original one from the 160 which proved to be no better. However looking for any way to avoid changing gearboxes we decided to fit the valve block from the 160 to see if we could narrow the fault down to the 280 valve block. Now the 160 was fitted with a trolling valve, which has worked well for twenty five years or so and was fitted to the 280 with the trolling valve spool. Every thing seemed to be working properly so we ran Wyrd up and down the yard for 1/2 an hour giving it some stick when winding at the end of each leg. The cooler appears to be doing its job and we checked  oil pressure and flow. At the end of the test the box had not overheated and the change was smooth and silent. The trolling valve spool is a different design from the standard 160 and 280 having extra porting. We have yet to try the trolling valve on the 280.   I can only think that the original 280 valve block is internally faulty but neither I nor Tom can see any cracks or manufacturing faults. If any one has experienced a similar fault on a 280 it would be interesting to know if changing the valve block effected a cure. Thank you all again for your input.