89 open conversion aux. fan issues

I converted to an open cooling system and continue to have issues with the fan switch. The switch is a Napa fs113, wired to a relay. Switch will work for a short period of time and then die. When I test the relay, by-passing the switch grounding with jumper fan kicks on. On the relay when I go to ground with a circuit tester, I have a small amount of voltage? I want to keep the automatic switch. Any help is greatly appreciated.

Have you checked the resistance of the switch and the voltage drop?

No I have not. I have read several write ups on converting and several recommend this switch. Normally open and when temp is reached, goes to ground, relay then reacts.

checked the voltage drop, and resistance. Nothing, another bad switch, number 4. Have a new one on the way, last try with this brand of switch!.

Thanks again.