I have personally experienced this problem. Connected the tach wire to the double spade on cluster. Basically fried the taxi output of the 2115 chip. Symptoms of this were cruise control no longer worked and strange idle symptoms. I also had the 311 code thrown on the ECU. The idle symptoms might only be noticeable to the most discerning driver. Basically when coming to a stop, you could feel the engine idle drop down real low - almost would die - before the idle circuit kicks in. This was driving an automatic. It never died on me but always seemed like it might.

I had no other symptoms other than those listed above. Speedometer & odometer all worked fine. I was able to pull a 2115 chip from another cluster to replace mine. That solved the idle, cruise control and ECU code.