Auto Pilot Problem

Any ideas about this would be greatly appreciated.

My MT 34 DC has what I assume to be the original autopilot that is 30+ years old, but is a very solid unit that I really dont want to have to replace.

It consists of a control head with manual compass card dial where the course is dialed up, the pilot is switched on, and away you go. There are little led lights that indicate when the pilot has engaged port or stb rudder.

Also has the hydraulic drive unit or pump, flux gate compass, and computer.

Okay so here is the problem.

When I bought and picked up the boat, the steering via both helms was not working due to the fact that there had been a hydraulic leak. Once topped up with oil it worked okay but was a little sloppy when left for a period of time. I put this down to 30 year old oil and dirt in the lines and knew I would need to drain the and replace the oil in the system at some point.

When first using the autopilot, the rudder would engage to stb and turn the rudder hard over no problem, but when engaging to port, it would only bring the rudder back to midships and go no further, even though control unit was indicating that the computer was still telling the pump to add port helm. The pump would just stop pumping to port and the rudder stayed midships.

This was during the original 3 day delivery home which meant we needed to manually assist the autopilot by turning the helm to port every time it needed it. Thankfully after about 12 hours under way it miraculously fixed itself and I never had another problem with it for the next 3 months.

However, I did need to replace all the old oil, which was done. After which my helm stations are tight as a drum, but the problem has started again with the auto pilot!

Any comments or suggestions for troubleshooting welcome!

Air in the system.

Could indeed be air due to a continuing leak. Or somthing else. Make & model would be helpful. Did I understand that there is no rudder position sensor?

Hope this works. Love the forum, but hate the way adding pictures works!!!! unless I am doing it wrong.

Anyway, here are a few pics of the set up. Great autopilot when it works, so solid.

I agree that it might be air, I am now thinking it is an airlock in the system. There are no more leaks in the system, I have fixed them. One of the photos show a paper towel on the pump that has been there for a few weeks now to show leaks should there be any.

Thanks for the feedback, I have added some pics

Check to be sure the hydraulic ram that moves the rudders if securely bolted in place. I once had an autopilot/steering issue that drove me to near frustration. A tech at a yard discovered that the ram was loose and was lifting off its mount when the wheel or AP called for a course change. A long shot but worth a look. Good luck.

I have had the problem of the arm coming loose. Could also be a rudder position sensor problem.

Bob

I would say, try and find the manual. It may have a great trouble shooting guide.

Thinking about my last response, I realized that if you can steer manually it can't be a mechanical problem. I would check the rudder angle sensor.

Bob

Hi - these can be difficult to diagnose - first test would be is the pump still running when it fails to operate one way - you may need to put somebody near the pump to check this. I have run a small set of wires up to a small bulb out of the engineroom at times to see if the pump run is the same as what the autopilot indicated. If the pump is still running but not moving the ram - good chance it is air - or a non return valve is not closing - check to see if one of the helms turn backwards . I had problems with mine till I put in new non return valves - now its a straight line astern when running. You can typically force the pump to run to allow quick purging of the system - but you need to locate the bleed screws - search on the web for details of your setup. K

Thanks for the suggestions so far.

BobH said:

Thinking about my last response, I realized that if you can steer manually it can't be a mechanical problem. I would check the rudder angle sensor.

Bob

Click to expand...

I dont have time to take a pic of the ram, but it is securely in place. As is the rudder feedback unit. Because the system is so old and does not have a visual rudder feedback display, I also have a secondary rudder feedback unit which runs to a rudder indicator display.

keo1 said:

Hi - these can be difficult to diagnose - first test would be is the pump still running when it fails to operate one way - you may need to put somebody near the pump to check this. I have run a small set of wires up to a small bulb out of the engineroom at times to see if the pump run is the same as what the autopilot indicated. If the pump is still running but not moving the ram - good chance it is air - or a non return valve is not closing - check to see if one of the helms turn backwards . I had problems with mine till I put in new non return valves - now its a straight line astern when running. You can typically force the pump to run to allow quick purging of the system - but you need to locate the bleed screws - search on the web for details of your setup. K

Click to expand...

Thanks for this advice.

The pump is a continuous running type. And as I have seen the helms move when the a/p is operating, so maybe it is the non-return valve.

Although, strangely it will engage port helm the bring the rudder back from full STB helm to midships. Once it gets to mid ships it stops.....

I thought it might be the rudder indicator, but the problem arose when I changed the oil in the system, so I figured it must be something to do with that.

Good tips for troubleshooting, I know what I am doing this weekend!

Thanks
Tony

How's this for a theory.

When engaging port helm, it appears as though the pump is doing what it should by bringing the rudder back from stb towards port, but does not go beyond midships.

I think the pump is being assisted by the water pressure on the rudder, when it gets to midships, it cant push it beyond because of the water pressure now acting as resistance.

This would mean it is probably a faulty non return valve??

Thoughts?

Thought I would post the fix.

Turned out that the potentiometer was spinning with the rudder feedback arm on the feedback unit.

All fixed now!

Well done - its a great satisfaction to find the problem and correct it. These boats are designed to test our analysis skills. All the best
Keith