Hyundai Repair: overheating but not really, negative battery cable, 2003 hyundai sonata

Question
i have a 2003 Hyundai sonata 4 cylinder i have replaced the top and bottom radiator hoes thermostat water pump flushed the radiator the temp gauge says my car is running hot but my mechanic said its not and he doesn't know why the gauge is reading what it is reading has anyone ever run into this problem and the temp sensor has been replaced as well

Answer
I have seen this occur primarily in hot weather when the air conditioning is running.  If there is excessive resistance in the negative battery cable, the large current for the compressor clutch and fans partially feeds back through the coolant temperature sender unit, causing the gauge to show an overheating (or near overheating) condition even though the coolant temperature is normal.  (A scan tool capable of reading data from the engine control module (ECM) will show whether this is the case.  I presume this is how your mechanic knows it is not overheating.)

To test for this, use a digital voltmeter to test the voltage drop in the negative battery cable.  Start the engine and turn on the a/c.  Place one lead on the negative battery cable and place the other on the bare metal of the engine or transmission.  If the reading is greater than 0.2V, you'll need to replace the battery cable assembly.  While doing this, ensure all points to which the negative cable attaches are clean and have good electrical contact.