Trim issue with '77 Merc 120- drive will not raise a second time

[mfelthousen]

I have a 1977 Starcraft Holiday with an original Merc. 120 I/O. I just replaced the rams, and followed the procedure for bleeding. (while rams compressed, fill reservoir. disconnect OUT hoses at ram, hold 'up' until steady stream of oil comes out. Reconnect hoses. Refill pump. Disconnect DOWN hoses at gimbal manifold, plug holes. Raise drive, then remove plugs. Briefly run drive in 'DOWN' until oil comes out. Reconnect hoses, refill pump.) I am reasonably sure this is the original pump, based on the parts I see in online parts lookup guides (high pressure Prestolite system without a 'clear' reservoir).

The issue is this: when I go to cycle the drive up/down to purge the remaining air I can raise the drive, and lower it once after this procedure. There's no indication of a problem- the drive moves smoothly, if slightly slow. After that initial cycle, the drive will not raise more than a couple inches and the pump labors. I can duplicate this scenario repeatedly if I relieve the pressure on the system by cracking open the lines at the rams, then tightening them up again.

The bushings were all replaced, the pins are straight, and when the rams are off the drive I can lift the drive easily by hand. The gimbal assembly was serviced just before winterization last year. The rams also work fine unloaded. All of this also happened with recently rebuilt rams, so I don't think it is a problem with the rams themselves.

Starting at the battery, I started going through electrical connections again to see if a corroded connection was causing a problem. Unfortunately the main terminal on the 'up' solenoid snapped off while I was snugging that connection down, so I won't know how successful I was for another day or so. My question is... how likely is it that this issue is a poor ground or power connection? I've been told that this style pump has an internal check valve that could stick, and that is the more likely culprit. I know that rebuild kits are no longer available.

Thanks in advance...

 

[Alumarine]

Are you sure you have the lines on properly?
Forward ports on rams to forward ports on manifold, rearward ports on rams to rearward on manifold.

Are the replacement rams new?

 

[wellcraft-classic210]

Have you been able to monitor the oil level in the reservoir when this happens // Could it be getting low as the cylinder fill ?

 

[mfelthousen]

Thanks for the replies. Yes, these are new rams, and the lines are connected correctly- forward position on the manifold goes to forward position on the rams, etc. The reservoir is not clear, but I’m reasonably sure it didn’t run out based on the sound of the pump.

 

[Alumarine]

Why are you replacing the old ones or did you rebuild the old ones?

 

[mfelthousen]

I’ve had the boat about a year. The original rams needed to be rebuilt because they would not hold the drive down in reverse, and sometimes needed assistance in going down. They also started leaking out of the end of the ram. In cold weather they acted as if binding at the lower end of travel, but that was also when I was storing the boat for the winter so I didn’t get back to this until spring. Given the leak, I had them rebuilt by a mercruiser tech this spring. They worked for a couple engine hours, then literally overnight the binding issue started again. When I started troubleshooting the pivot housing on one of the rams split while raising the drive. My guess is there was a stress fracture caused by opening the rams for servicing, as the tubes had heavy gouging afterwards. I replaced the split outer tube with a brand new one, and the ram still bound up after a complete cycle. By this point, after fighting the issue for this long, I decided to replace the rams completely. But, the symptoms are the same with the new rams also.

 

[Alumarine]

I was going to suggest you disconnect one ram and see if the drive would raise and lower repeatedly and then try the other one.
But, if the problem is the same with new rams then I doubt it's a ram problem.
Hopefully it is an electrical problem that will be resolved when you get the wiring sorted out.
Make sure your battery is good and fully charged.

 

[mfelthousen]

Thanks all for the suggestions. I'm now reasonably sure that the remaining issue was electrical. I took the time to restore the wiring of all of the circuits involved back to the original configuration (repairing 40 years of previous owner mods), and found a few areas that could have caused issues. I won't know for sure until I have a lower unit on the boat in another week or so. The weight of the upper unit may not be enough to recreate the problem.

 

[kenny nunez]

Your problem is related to the reverse lock valve. Sometime in the early-mid 70s Mercruiser had a micro switch mounted on the bracket with the reverse lock valve. The wires were blue. Later models modified the pump where the the switch was eliminated. The switch would prevent the pump from operating when the drive was shifted to reverse. If the switch was by-passed or not adjusted then the it causes one of the rams to rupture. The cause of your problem is the previous owner.
While your boat is a 77 it could have a much earlier drive. Try to post a picture of your inner transom plate where the reverse lock valve is located .

 

[kenny nunez]

If your engine/drive is a 77 then the problem is in the valve block under the pump. When I had my shop I had a Mercruiser hydraulic test kit. The valve block has a 1/8” socket head pipe plug and similar socket head set screw inside that regulated the “down” pressure. Without the test kit there is no way you will be able to set the pressure. Order a new valve block for your model, that is the only thing that you will be able to do.

 

[mfelthousen]

Thanks for the responses. It's took longer to get back to a point where I had enough weight on the rams to test, and the issue did end up being electrical. Now that I have gone through all connections in the circuit between the battery to the pump, and either cleaned or crimped on new ends, the drive now raises and lowers repeatedly without an issue. Which is good, because individual parts for this model are no longer available. My only option going forward is a complete pump assembly.

Some of the electrical safeties were bypassed or removed, so as I went through everything I restored them to original specs (circuit breakers, etc).