I converted an OMC cobra from cable steering to hydraulic, and at the same time added a garmin autopilot. I used a replacement inboard cylinder from sea star as well as a new sea star helm. Everything is installed and working, but when I change direction with the steering wheel quickly, I get a small shake/ vibration. If I steer slowly I do not get the vibration. Here is what I know/have tried so far:
-Replaced U joints
-Checked alignment
-Disconnected the pin for the steering arm to outdrive (problem went away)
-Made sure all air is bleed out
-If a helper puts pressure on the outdrive, the vibration goes away.

Im now leaning towards it being a problem somewhere in the OMC power steering assist system. I did a bunch of reading and found that the OMC cobra had a somewhat common issue like this due to the design and how little force it takes to steer. Someone suggested switching the OMC spool valve to a mercruiser, but I have not found any more information on that.

might be air in the system....

When I went full hydraulic on mine it did some very very strange things until all the air was out.....like turning by itself and ripping the wheel out of my hand a few times lol. I went lock to lock back and forth for at least honestly an hour with the engine off then when I finally fired it I did the same thing for a half hour and still had issues....ended up turning to "lock" in both directions and forcing it to go further which lifted a relief in the system and finally got the last slugs of air out. All is good now and it steers like a dream....now I have to do it all over again since the motor is coming back out this winter ugggghhh hahahaha

Bleeding the system has been kind of a pain for me. Lots of wasted sea star hydraulic fluid at $30/quart.....
I do think I have the all the air out of the system from the helm to steering ram, to auto pilot unit. Even the slightest movement of the steering wheel makes the ram move, and it locks hard at full turn in each direction. On the water the steering acts totally normal, other than the slight vibration when changing directions quickly.

I am less certain about having all the air bleed out of the OMC power assist system. I did take the lines of when I installed the auto pilot and hydraulic steering. So far what I have done to bleed it is just turn lock to lock a bunch of times with the cap off and fill as necessary.

Not super familiar with the auto pilot or OMC setup but is it possible that the auto pilot is slightly fighting you for a split second when you are changing directions and giving you some slight feedback causing the shake you are feeling? I may be way off but just a thought?

Also could be a relief in the factory system slightly sticking? Relief and/or check valve? I'm sure it has to have some kind of valving system somewhere in there? Like I said not too familiar with the OMC stuff....Just trying to spitball some ideas??

You don't need the Sea Star hydraulic oil (no matter what they tell you in the manual). ATF or hydraulic tractor oil is fine. A two-gallon jug of hydraulic oil from Wally World is $38.

Might have some issues with the power steering pump causing feedback. You can bleed the power steering system by moving the steering back and forth without the engine running. If the fluid is frothy or the pump makes noise under load, then it's more than likely air. It should correct itself over time if it is.

I kind of wondered about the oil, thanks for the tip. The power steering pump doesn't make any noise, and the fluid looks good, but I am going to try and bleed both systems again too see if I notice any difference.
I did have someone else tell me that sometimes they need a power bleeder to get the auto pilot system completely bleed. He also said he doesn't generally see people use an engine driven power steering pump, but rather an electric pump.

Two things, may or may not be helpful:

On my buddy's NorTech w/ 3 Mercury outboards, we thought we were having a steering problem. However, deep in some menu on one of the Garmin systems, there was some little auto-pilot thing that was turned on. Turns out, the steering was literally fighting us. Make sure ANYTHING with autopilot is turned off and/or disabled.

When it comes to bleeding, you really can't over do it. I'm talking like 30-50 or more full turns swinging from lock to lock.

Also, please cut off that sharp tail on the zip-tie in the first picture.

Couple updates:

-With the engine running and the power steering pump belt OFF, the shake goes away completely
-It shakes much more when changing directions to the left (hydraulic cylinder retracting)
-Bleeding still hasn't made a difference (havent tried power bleeding yet)


I also remembered when I first installed the hydraulic cylinder, I didnt have it properly adjusted and the steering arm would contact the transom. I found this out when I ran it the first time, and the motor stalled when I turned all the way to the port side. Is it possible I damaged something in the OMC power steering system when I did this?

I'm trying to understand your plumbing....power steering pump is plumbed to the factory power steering arm at the transom and you simply replaced the cable with a cylinder that has hoses running to the helm correct? If that's the case I'm betting you have different retract valving rates on the two cylinders and they temporarily fight each other when switching directions....I'm wondering if there is a better way to plumb everything all together into one system.

That being said...in my mind you have two separate systems working here correct? If so...have you bled both systems independently? As in the helm/ram system and the OEM power steering system?

I could be way off but that's what I'm thinking if that's how it's setup? Just trying to help