Twin disc MG502 no forward or reverse

[Windwaves50]

Got towed in recently during a fishing trip. I have 26 ft fishing boat powered by a 200 hp volvo diesel, twin disc mg502 2:1 transmission, pss drip less, straight shaft.
I have approximately 3500 on boat motor and transmission. Both were rebuilt at the same time about 9 years ago. Never had a problem till this day.
All of a sudden out of nowhere my forward and reverse stopped working.
Here is a list of what I tried so far to troubleshoot the problem after reading the Twin disc MG502 manual.
1. Make sure selector is fully engaging.
2. Changed oil and cleaned filter screen.
3. Attach psi guage to check for oil psi. I have no psi at all. Although I only have neutral, I still should be getting psi from what I understand.
4.Went ahead and ordered and installed a new oil pump. Still no psi , or engagement of gears.
5.Removed and visually inspected selector valve rod and springs under it. Looked fine.

I am currently getting ready to pull the marine gear out of the boat and getting it serviced.
Anyone have any idea what may be broken ? Thanks for reading!
Happy New Years! Windwaves

 

[jgwinks]

Had that happen last summer with one of my Perkins 6.354's. Ended being just the damper plate (it's what connects the engine crankshaft to the trans input shaft). $150 part but you have to pull the trans to get at it. Not an easy job.

 

[Windwaves50]

Jgwinks, I greatly appreciate your response and experience ! I am not a mechanic by any means but it makes sense now. Thanks !!! I will look at that when I pull the gear this week. Is there anyway to identify that the damper is bad just by looking at it?

 

[Windwaves50]

Had that happen last summer with one of my Perkins 6.354's. Ended being just the damper plate (it's what connects the engine crankshaft to the trans input shaft). $150 part but you have to pull the trans to get at it. Not an easy job.

Jgwinks, I greatly appreciate your response and experience ! I am not a mechanic by any means but it makes sense now. Thanks !!! I will look at that when I pull the gear this week. Is there anyway to identify that the damper is bad just by looking at it?

 

[jgwinks]

It's a disc with several springs around it. Some of the springs or the part behind that connects them to the shaft spline may be broken. The first thing I did after pulling the trans was to reach in and try to turn the spline by hand. You shouldn't be able to but mine turned pretty easy so I knew I found the problem. Four of the eight springs and the connecting piece were broken and there were pieces of them laying in the bottom of the bell housing. It was pretty obvious.

 

[jgwinks]

Some damper plates have a rubber disc instead of the springs. That will also be pretty obvious if it's bad, the rubber would be torn. In any case if the center spline can be turned independent of the rest of the plate, it's bad.

 

[heysteve]

If you don't have it already, go on eBay and get the service manual (here). I have the one for my 503 and it's beautifully illustrated and informative.

 

[SIBERNUT]

Im sorry to be ignorant, but the linkage is not connected. I must assume you did that ?

 

[Windwaves50]

It's a disc with several springs around it. Some of the springs or the part behind that connects them to the shaft spline may be broken. The first thing I did after pulling the trans was to reach in and try to turn the spline by hand. You shouldn't be able to but mine turned pretty easy so I knew I found the problem. Four of the eight springs and the connecting piece were broken and there were pieces of them laying in the bottom of the bell housing. It was pretty obvious.

Thank you for the explanation jgwinks! I will be see what I find.

 

[Windwaves50]

Im sorry to be ignorant, but the linkage is not connected. I must assume you did that ?

Yes it's the first thing I checked while adrift and stuck in the ocean.

 

[Windwaves50]

Thank you for the explanation jgwinks! I will be see what I find.

Jgwinks, you were spot on! This is what I found when I pulled the transmission apart from the engine today. I found the damper plate had no teeth left, it was completely rounded. Also the transmission input shaft also has severely worn teeth. I hope this don't cost too much too get repaired. We'll see.

 

[Mischief Managed]

There's a very similar common problem with stern drive systems and it's caused by inadequate lubrication combined with extensive idling. The reason they fail during idling, is that the engine does not turn at a steady enough RPM at idle, it slows a little between power pulses and during the compression stroke, and with no load on the prop, the momentum left over from the last power pulse keeps the stern drive internals spinning slightly faster than the engine when it slows. This allows the stern drive to take up any forward play in the splines. During the next power pulse, the engine catches up and then slightly exceeds the speed of the stern drive internals until the spline play is all taken up again. This tiny action slowly hammers the crap out of whatever spline materiel is softest. It looks like the same thing happened to your coupler and splines (which are probably made of equally soft materials). Oddly enough, stern drives boats that are used on plane for the most part never have this problem because under load, all the pressure is in just one direction, there's no hammering. If you operate at idle a lot, that may be the problem. Is there any way to grease your splines periodically?

 

[Windwaves50]

Thank you for your knowledge! Yes, I do alot of fishing at idle! , wow I didn't know this was occurring. Slowly eating away!... till failure...
Will grease slow it down? It does take alot of work and realigning if I plan to grease it periodically.

 

[Portage_Bay]

Would bumping RPM in idle up till it smooths out help?

 

[Mischief Managed]

Thank you for your knowledge! Yes, I do alot of fishing at idle! , wow I didn't know this was occurring. Slowly eating away!... till failure...
Will grease slow it down? It does take alot of work and realigning if I plan to grease it periodically.

Grease would definitely help a lot. I would use grease with the highest pressure rating you can find. Molybdenum disulfide grease is probably the best bet.

 

[Windwaves50]

Thank you. I'll use that.

 

[normandawn1]

Get a polyflex damper plate if you can. Did mine last year and they are much quieter than the spring plated. And if they do fail you still have drive
Fun job.

 

[johnma]

Get a service manual before you pull the trans. I’m not sure on a unit this small, but larger Twin Disc units as well as most larger transmissions are hydraulic shift. If you have fluid but no pressure it could be something as simple as an oring or broken spring in a relief valve. If you’re on the US east coast you can call TransAxle in Pennsauken NJ or TA Powertrain (same company) in Garner NC, their techs will help you troubleshoot, supple parts or do a repair or rebuild if required. Before I retired 6 years ago I was the general manager of their off highway and hydraulic division and troubleshooting over the phone is a standard service.