The aux fan on my dad's 1987 300E doesn't come on when the engine gets hot. There is a sensor on the head which I assume is there to turn on the fan. It has 2 plugs on it, a single pole and a two pole. When I jump the two pole together on the wiring harness, thereby bypassing the sensor the fan comes on high. Can I assume the sensor is bad and just replace it ?

Pete

Depends on what you call hot
Jumping the fan sensor verifies fan circuit as Ok, but remember that the sensors cut-in spec is 105C...so, if the temp is not that high , a new sensor will not solve your problem , if you even have one.
If gage reads above 105C , then a sensor is needed.

The guage was showing about 110 in my garage after idling about 15-20 minutes. The other day while stuck in traffic on a fairly cool day, about 80, it got up to about 115 before I started to get rolling. Once at speed the temp came right down to 85.

Then the sensor is suspect...also make sure you are not low on coolant.