Cessna ignition switch
 

My 54 C170 ignition switch lost a mag check function and had key wobble like a worn 915. The new switch A510-2 has a diode for the start relay to ground. Question is the old switch did not have a diode so is it necessary?

The diode should not effect the normal operation of the switch. It's like a one-way gate in the circuit, allowing the current to flow from positive to negative. It has a value when you switch off allowing the current stored in the relay coil flow harmlessly to ground. I know it's like this in automotive use, I can't imagine your vintage Cessna is any different. If it were my C170 I would run this question by your A&P, just to be sure.

My IA is going to check my work Saturday but is not aware why mine needs a diode hence it did not have one out of the factory. I called the switch maker and their answer was some don't have them but it would not hurt to install the diode.

The diode is for surge protection, its a safety device. Check the AD's you may have to change over anyway