1997 Jeep Laredo Electrical

I noticed a problem with my jeep recently. When I press the a/c on switch my speedometer goes haywire. Usually it works fine. I took it to a mechanic. He is confused as well. I have already put two output speed sensors on it as I thought the first might be bad. I did not realize the a/c part of the problem at the time. The mechanic put the jeep on the rack and says everything works fine up there but it will mess up when he presses the a/c switch going down the road. He could not find any electrical problems.