I've got a similar problem with the '87. The vehicle produces full heat all the time. So, I change the CCU and replace the monovalve diaphragm (looked OK) and it seems to operate correctly for about 15 miles or so.

Then, without warning, it decides to go to full heat. Shut it down and restarted it and it's working fine again.

Two bad CCU's..........unlikely.

So, I'm thinking a monovalve that's intermittent...........we'll see.........I don't drive it much so I'm not overly concerned with it.

Unplugging the monovalve should provide some insight into your situation with no heat..........