Ed,
Yes, there is an adjustment. The aft half of the A/P donut (the part that attaches to the wheel) snaps off. I've attached a photo of what the inside of the A/P looks like. In the center of the picture is the sprocket which is attached to the motor, which makes everything turn. On either side are excentric cams. One of the cams is attached to the engaging lever. The other cam is an adjusting cam. It is held in place by a nut, which is covered by a small round plastic tab on the forward side. Take off the plastic tab, and you will see the brass nut. You will need very small needle nose pliers to adjust it. By adjusting the cam, you can make the belt tighter or looser. You'll have to experiment to see which works better for you. Another possibility is that the small roll pin which attaches the engaging lever to the excentric cam is sheared. Those are very hard to find, and you might have to jury-rig something else.

After shearing a couple of the pins I slightlty oversized the hole and used a small stainless nut and bolt to hold the handle on. Worked great for a couple seasons, then the control head died due to water intrusion. Sold the wheel unit to a guy on the great lakes, still working as far as I know, 5-6 years after pin replacement. I never got that kind of life out of the roll pins.

I had the 1st generation Autohelm wheel pilot on my 88 C30, then an S4000+ on my current boat. I went through wheel drives like donuts, same clutch problem you're having. Autohelm, (later Raymarine) repaired or replaced the drives more times than I can count. I got at least 3 complete replacements on the C36, under warranty. Each and every time, the same clutch problem would reoccur. The final straw was when the wheel unit literally self destructed, in a busy channel of course. The plastic ring that attaches to the wheel separated from the backing plate with one of the idler wheels broken off of the backing plate and dropping to the deck. I remembered reading an article in Sail mag about a guy crossing the Atlantic who got so fed up with his that he (in his own words) " beat it to death with a stainless steel winch handle and threw it over the side". With my wheel jammed, I followed that approach; most satisfying, though my wife wasn't as amused as you might think.

I then purchased an S1 unit. The wheel drive looks to be completely redesigned. I've had no problems with it in several seasons, something I could never say about the older unit.

I had a st 4000 and started shearing pins and small nails causing the auto to stay engaged usually when I was approaching the harbor sea wall. I up graded a couple years ago to the st 4000+ which is a much beefier unit and have had no problems. It is fairly easy to attach the to the wheel and it plugs right into the existing wiring and also can be used with the same head. The cost of the kit was about $400.