Chevrolet Repair: GM 3 wire temp sensor, coolant temp, wire sensor

Question
On my friends 93 GrandAm, I'm looking at a problem where the electric fan doesn't come on, and I've traced it to the ECM not grounding the fan control line.  I'm suspecting the CTS, but now I'm curious why this engine has both a 3-wire coolant temp sensor AND the gauge temp sender.  I thought the 3-wire CTS was designed to eliminate the gauge sender.

The gauge works fine BTW.
Thanks in advance if you can shed some light on this.
Chuck


Answer
Hi Chuck.... you are probably right...if there is no ground at the cts the fan will not come on.the reason for the other sensor is there is one for the ecm and one for the gauge, your gauge sensor can show as ok but ecm sensor be out of wack...if you have a scan tool and a infared temp gauge, point infared gauge at the area of the 3 wire sensor and read temp... it should match or real close to the temp reading on the scan tool.if not replace the sensor... hope this helps