Dicky gearbox

[Guest 000]

Really keen on some advice about my misbehaving gearbox.

It is a simple mechanical cone type PRM 80 gearbox and when cold it slips for a few seconds when 'ahead' gear is selected. The when-cold slipping has been getting progressively worse. Once warmed up, it performs faultlessly across the whole engine rpm range. I have done all the usual linkage and gearbox checks and everything is as it should be. I suspected that the cones are a little polished and a simple lap job would solve the problem.

Now the box is sitting on my workbench and again, I can't fault it. I have put some serious leverage across the input and output shafts with the box in gear and I can't get it to slip.

I don't understand what's going on, and I'm a bit reluctant to pull the box to bits if I can't make it misbehave.

[ballystick 74]

If it is like car syncros then it sounds like the oil is too thick especially in cold conditions and won't allow the oil film to be easily overcome when cold but ok when hot, I would check the oil specifications and maybe try a thinner modern multi grade if you can't find a problem.

[Guest 000]

Thanks for that. The box runs with ATF which is pretty thin stuff. The oil looks pristine and I have always been careful to do the regular oil changes. And the darned box only has 150 hours on it but its three years old and from the suppliers viewpoint, conveniently outside it guarantee.

[Steve Pope 251]

No friction modifiers in the oil??

[Guest 000]

No, using the Basic ATF with all the right numbers specified in the manual. Think I'm going to have to strip it..

[Island Time 1,252]

Is there a plate on the gearbox listing the lube specs?

 

Some of the Volvo ones that originally specified ATF went back to oil. Many had issues with the ATF similar to yours. I've also seen another gearbox that the Nz agent Stated used one oil spec, and it had gear shift issues- changed to the oil on the gb plate, fixed. Make absolutely certain you have the correct lubricant. Ask the manufacturer - not just the model number, but the version as well. The main model number might not be specific enough. For example, my ones (Volvo 120s) come in versions A thru D, all with slightly different specs.

[John B 106]

ring James at moon's. Theyre prm agents and likely know of any idiosyncrasies

[wheels 543]

Hi Chris. Unless you a confidently  mechanically minded, don't go pulling the box apart. It is unlikely you will get the bearings out without damaging them and you would then need to replace them and they require setting up with shims. Not hard, but not simple either.

The 80 is a mechanical box and you are correct in using ATF. Dexron ll or lll is required. Although in saying that, these boxes are such that wrong oil type is not detrimental to them. But ATF has a better overall performance. 
They are usually a bullet proof box. Simple and rugged and unlike a hydraulic box, these can withstand high temp operation with no real detrimental issues.
The most important and first point to check is that the gear select lever is shifting it's full range. The cone should slip ever so slightly, which results in a smooth engagement. They don't really "glaze" as such, so scuffing it up most likely won't solve the issue. In saying that, I am not saying it is impossible, because when ever you say that, someone comes up with the impossible.
BUT! are you absolutely sure it is the box that is slipping? What about a possibility of Prop key shorn and prop is slipping, or drive coupling is slipping?

[wheels 543]

I should add. Make sure you get an ATF of Common brand like Castrol. Be careful of the uncommon Brands that advertise some kind of super special "modified" performance. They often put in tricks for friction reduction and that could stuff up a box like this.
Also for others with these boxes. There has always been confusion with what is written in some of the manuals. Some manuals say to use the same oil as your engine, another says a 10W30 engine oil. As I said, it is not going to harm the box, but it is now recommended to use AFT Dexron ll or lll in both the PRM 80 and the PRM 120. These are the only boxes in their range that are fully Mechanical. All their other boxes are Hydraulic and they require the use of 10W30 Engine Oil,

[erice 732]

if you are not 100% sure the box is slipping

 

ie as wheels says above it could possibly be shift lever alignment or prop key etc

 

how easy would it be to put the box back in and double check everything right at the input and output of the box?

 

3 years of light use sounds far, far too early for problems

 

to eliminate shift lever issues you could try shifting in and out of gear directly at the lever on the box, photo or mark the positions when slip occurs, doesn't occur etc 

 

same with the prop shaft, mark with a piece of tape and video at high resolution then view on computer etc

 

does your box have a cush? drive

 

have had slipping bike alternators before where the fault was broken alloy bits and chewed up rubbers inside the damper housing etc

[Guest 000]

Thank you for the replies.

I have talked with James at Moon Engines - in fact he supplied the box in the first place. In our initial conversation about the fault there was a bit of fancy footwork going on from which I gather, rightly or wrongly, that this slipping issue is a 'known issue.' He offered to sell me a new box at a super special reduced rate which unfortunately is a lot more than what I paid for the gearbox in the first place. Failing that, he advised me to pull the box apart and lap the cones which he described as not very difficult.

Regarding the slipping, I am certain it is the gearbox. With a cold gearbox in ahead gear, I can hold the output flanges in my hand and stop the rotation for a few seconds until it 'bites'.

[erice 732]

workshop manual for the prm100

 

http://boatinfo.no/lib/gear/manuals/prm100-140-175-250-265.html#/0

 

 to the extent of having lost their pattern of ' criss cross ' grooves will tend to slip

[Guest 000]

In the interests of fairness, I should add that through further research, the slipping when cold issue is not exclusive to PRM gearboxes but is a known problem across the range of small mechanically activated cone type boxes.

Still a plain in the butt, though.

[John B 106]

Thats disappointing.

[wheels 543]

I agree, it is disappointing, because that was a problem in the very much older boxes and I thought the problem had been solved.
Chris, i wonder if it is worth Emailing the manufacturer and telling them how little hrs and that it is only just out of warranty. Also tell them that you have talked to Moon, being their Distributor and maybe, fingers crossed, they may help in some way.
The other thing is, you could try pointing the Barrel of the Consumer Guarantee Act button at James and see where that gets you. Three years and only 150hrs is not acceptable, certainly for what they cost, in my view.

[Guest 000]

Did that, Wheels. Got a lot of waffle from PRM England about how wonderful their boxes are, they'd never had a fault etc etc and basically, between the lines, I could go sit on it.

The box is now opened up and sitting on my bench.I did this together with a friend who is a gifted engineer and probably NZ's leading restorer of vintage motorcycles.

He was not very impressed.

The cone has some sort of black coating on it, or did have. Most of it is now lying in the bottom of the box. Interestingly, the cup into which the cone meshes has almost zero wear which would indicate the cone coating is suspect.

But, another interesting point, according to my friend. The box and the end plate were assembled using silicone as a gasket. This caused his eyebrows to raise so that they almost disappeared into his receding hairline. He says its use is forbidden in gearbox applications. As he pointed out, as you bring the parts together, the silicone exudes out of the joint. The stuff on the outside you wipe off with your finger, but what about the stuff on the inside? According to him, oil and heat and vibration causes bits of it to break off where it remains in suspension in the oil and goes where it shouldn't, interfering with the correct operation of the box. There were two loose bits of silicone inside when we opened it.

We made photos and sent then to the local supplier. The ball's in his court now.

[ballystick 74]

I'm not sure about that being a problem, getting back to automobile technology. Nissan uses a special silicone type of sealant made only for manual gearboxes without problem, there is even types for automatic transmissions too, so you can't just group sealant into a one use and one type bucket.

[Guest 000]

I'm sure you are correct but, as with everything the proviso 'if used correctly' applies.

I don't know how to post photos - if I did I'd put the one up showing enormous gobs of silicone that have squeezed out of the joint into the gearbox's internal spaces. Now whether that's the right type of silicone or not I have no way of telling. But I can see that it's badly applied and a problem in the making. Paper gaskets any day!

[erice 732]

for pics

 

use the 

 

More Reply Options

 

box at bottom right of the normal reply box

 

then attach the pic and keep attaching it...

[Guest 000]

Still can't attach pics. Can you do it on a tablet running android?