[CA] [CA11] EV-1, P70 Heading issues

[Jim Beck]

I have an Evolution autopilot with P70 Control Head on my sailboat (Marlow Hunter 40). When I am making way with the autopilot in Standby mode, the heading indicated on the Control Head agrees with the heading showing on my onboard magnetic compass within a few degrees. This is true, regardless of the direction in which I may be heading. However, when I engage the autopilot (Auto mode), the heading may change dramatically at times, causing the vessel to make a turn of up to 20 degrees. The heading shown on the Control Head and that shown on my compass then do not agree.

I have been dealing with this issue for quite sometime and have tried numerous different things, unsuccessfully, to rectify. I have recalibrated multiple times, I have downloaded the latest software and recommissioned. I have checked the rudder position vs the indicator, which seems to be accurate. The compass deviation indicator on the Control Head shows 7° deviation.

When out on the water a few days ago, I performed the following tests: I did a slow circle in Standby mode (another compass linearization) and noted that my Control Head heading reading generally agreed with my compass throughout 360 degrees. Then I motored on a straight course and engaged the autopilot in Auto mode. The first heading happened to be 69°. I recorded the Control Head display heading and my compass heading. I then initiated turns using the +10 button. I did a couple of 20° turns, but mostly 40 ° turns until I had completed 360 degrees and recorded the readings of the headings shown on both the Control Head and my onboard compass. The following table shows the results. All numbers are in degrees. [I have been unable to get this chart to format properly in this message..can't get the data to line up with the column headings...hope it is understandable.]

P70 Heading Compass Heading Difference
69 70 +1
89 85 -4
129 116 -13
149 131 -16
189 170 -19
229 220 -9
269 269 0
309 321 +12
349 05 +16
29 41 +12

It is perplexing to me that the P70 reading while in Standby agrees with my compass, but can differ significantly when the autopilot is engaged. Doesn’t the Control Head always get heading information from the EV-1 computer? Also, it is interesting that the P70 reading tends to be closest to the compass reading when I am on a heading that is on the east-west axis and varies the most when north-south. I think that is significant, but I don’t know how to interpret. Any help you can give would be appreciated as to what the issue may be and how to resolve.