Tilt & Trim on 1984 70hp merc - control valve

[GAMA_BAIT]

I just purchased a 17 ft bass boat with a 1984 70hp Merc. The tilt and trim unit was leaking a little when I got it. The lines were loose so I tightened them up and thought I was good to go (4 line system with 3 wire leads). Before I put the boat in the water, it would not hold the motor in tilted position. It dropped back down very slow. It would drop completely overnight. The first time I took the boat out everything seemed o.k.. To test out the tilt and trim I ran the motor tilted up a little. I notice it didn’t hold the position. A short time later I went back to the launch. As I came in I tried to trim the motor up but it didn’t move. I could hear the pump going but no movement. When I opened the rear hatch to check the pump there was a big puddle of oil under the control valve. When I got home I checked it all out. The leak is coming from a small hole that is in the valve body that the control knob threads into. The hole is machined into the control valve body (maybe an overflow or check valve?) Now as soon as I open the fill screw oil pours out the hole. I can’t get enough oil into it to try to move the unit up or down. I have priced the valve and it list for 771.00 if its still available. Can I rebuild this control valve? Does someone sell a kit? My Merc. Dealer said I had to replace the entire unit that he can’t get parts for something that old. Does anyone know what is wrong here or how to repair it? Thanks - Mark

 

[duncan_ellison]

Re: Tilt & Trim on 1984 70hp merc - control valve

Hi Gama_Bait,<br /><br />I have recently acquired the same system and had a similar problem. In my case it was air in the hoses caused when I took them off to renew the flexible hose.<br /><br />I'd certainly try bleeding before trying to replace the pump (ouch - $$$$$$!).<br /><br />You probably just need to bleed the system, you need to locate the bleed screws on the cylinders and slacken them one at a time whilst you operate the switch until no air comes out. Clymer reckons you should take the cylinders off to do this.<br /><br />BTW, what type of oil are you using ? I'm using red ATF fluid, but the manual reckons ordinary engine type oil is correct.

 

[Dom]

Re: Tilt & Trim on 1984 70hp merc - control valve

Hey there Gama Bait...Had exactly the same problem. Just got it fixed. I rebuilt the rams and bled the system..(some are self bleeding) done by lowering and raising the motor a couple of times. The problem still occurred, though much slower. My mechanic took apart the pump and the lower portion of the pump, and found the pressure relief valve was bad. The fittings were worn and leaky and would not hold pressure. Luckily he had an extra part that he only charged me 50 bucks for. And the problem is fixed. <br />Just a thought...Good luck and let me know. It is a very frustrating situation..I have been dealing with it for this whole season.

 

[GAMA_BAIT]

Re: Tilt & Trim on 1984 70hp merc - control valve

Duncan and Dom, Thanks for the info. This problem is driving me nuts. My problem seems to be in the pump unit control valve. I would try bleeding the system but right now I can't get enough oil into it to get it to work. (I'm using 10w40 - the pump called for that) I have no manual or info for this unit so I'm flying blind on this one. It seems that there is something wrong with the control valve itself. I think the pressure relief valve might be bad but I don't know how to check for that. Why else would fluid leak from the little machined hole in the valve body? I'm thinking that maybe the system had air in it and when I put a load on the system by driving with the motor tilted up a little it cause the sytem to over load. Is there a way to reset the pressure relief valve? I really want to salvage this system but I don't know what to do. Any ideas where I can get a manual and or replacement parts to rebuild the control valve. My pump motor seems good and the rams are newer than the pump, they look pretty good too. Now that it is getting cold here up north, I will have the winter to fix this problem. Right now I would replace the unit with a CMC unit and buy new steering before I put over 700.00 into the existing unit. Thanks for your help. - GAMA_BAIT