Dick, you are always so helpful on this forum and I thank you for pointing me to this manual. I spoke again with the person that found the wiring problem and learned that I didn't quite get the facts straight. Here's a corrected story:
1. the fitter asked me about a strange sound which I identified as the Yanmar engine panel alarm accompanied by red lights
2. he had seen a red wire connected to the data box. When he reconnected it to the new instrument, presumably 12V positive, the alarm sounded
3. it turns out that he then disconnected the mysterious red wire and the alarm stopped.
In summary, the red wire is no longer connected and all is well. Now I will refer to the manual and try to work out what happened.
Thanks again - Ken