is it the thermostat sensor?

 

Yes I think so. The one on the back of the head is for the gauge.

 

depends on the year xj

 

its a 91

 

is there a sensor on the drivers side rad tank

 

The thermostat is what you're looking for.

 

91 changed to chrylser the sensor will be the coolat tem sensor on thermostat housing

 

What year?

1987-1990 has a sensor in the driver's side radiator tank - it's the Thermal Fan Switch (TFS.)

1991-up has the sensor in the thermostat housing - it turns the fan on by way of the ECU (OBD - it reads the signal and makes a decision.)

It's rather more common to lose the electric fan relay - early models have it on the driver's side fender liner, while I think it's usually in the PDC for the later models (should be labelled "Cooling Fan," "Radiator Fan," or similar.) It's either a standard DIN relay (about 1" cube, plus connector prongs down bottom) or a mini-DIN relay (later models, about 1"x1"x1/2", plus prongs.) Both are readily available in the aftermarket.

Also, you may want to check the motor to make sure the coils haven't shorted through or burned out. Disconnect and test with an ohmmeter - zero resistance is bad (shorted through) and infinite resistance is bad (burned out.) I don't know what a "good" spec is, but it's fairly low.

 

Eh not jackin, quick related question...

Would a shorted electric fan cause an increase of voltage to 19V at the battery (20V alternator to ground)?

 

No. You're overcharging - either a regulator fault, a failed/failing battery, or the field coils are getting more than the 0.5-4.0VDC nominal they're supposed to get.

Check the alternator and look for wiring faults first, and check the battery. Look for the writeup I've posted on testing the primary electrical system (starter/alternator/battery) using a DMM and some easy techniques - if it's not here, I know I've posted it at JeepForum (same moniker there.)