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1977 Johnson 85HP Power Tilt and Trim unit

markt

New member
"The power Tilt-N-Trim unit on

"The power Tilt-N-Trim unit on my 1977 85HP Johnson outboard was replaced about ten years ago. I was lucky to find an original brand-new one as "new old stock". This past season a problem developed whereby the engine will not stay raised. The unit is not leaking fluid, it raises the engine without any difficulty; but the engine, regardless of the tilt or trim height chosen, manages to creep back down to a fully lowered position. Any thoughts on what, if anything can be done to correct this? I flushed the hydraulic fluid and the fluid level is okay. I've attached a schematic diagram of the unit. Thank you. Mark
374510.png"" alt=""Tilt and Trim Diagram
 
"i would say,.u need to reseal

"i would say,.u need to reseal,the pistons,and inst a new 'stem valve' while u r in there..# 46,..could be something as simple,as a bad man relief valve,also,i recommend,a manual if u r going to wk on it.."
 
it is your valve body - either check valve or manual release. rubber gets old whether you use it or not. Some would argue that not using it leads to dry rot.

open the MRV manually move engine up/down and few times, and then firmly seat without using torque.
torque can damage mrv tip, aluminum housing (score the valve seat), or the screwdriver slot. If it does not seal with a firm but gentle seat then the MRV is not working properly.

If the MRV is bad, then the system will drift both down (gravity) and up (reverse gear). If the drift is only in one direction then it is a check valve. Check valves are not a DIY item. Or, at least, the success rate on DIY check valve repair is extremely poor. you can get the VB rebuilt through ebay for $90ish. Probably better to live with it for now since you'll miss a couple weeks boating while it gets reapired. I have also heard rumors that one of the domestic marine motors companies like API or ARCO makes a replacement. But I have never actually seen one. May take some digging for you to find it since it will not be an OMC part #.
 
You say that the PTT creeps down to a fully lowered position, meaning that the trim rams are also fully retracted.

However, if this is not the case and the unit stops going down when it contacts the trim rams (the rams stay extended), the most common problem is the relief valves located under the oil pump... rubber tip on the aluminum one... O Rings on the brass one.
 
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