1996 Jeep Cherokee Temperature Sending Unit/Switch

I noticed when turning my key to the Accessory position before you start the vehicle all the lights on the dash board would come on exept for the over heating bulb. I read in the my Haynes manual that it should come on with the rest of the lights. Worried my engine could overheat with out me knowing it I started to look into the problem. I did like the book said and grounded the single wire going to the temperature unit and the light on the dash did come on. So I was sure I had a bad unit. After I went to the store and bought one it still worked the same, no dash light before start. So I took it out and returned it to the store for another one telling them it was bad. I am now on my second Temperature Unit from the origonal and it still works just the way It did when I bought it. I asked a Mechanic and he said some models dont have the overheat light come on before you start the vehichle but I wanted to get a second opinion. Im not sure about these "Haynes Manuals" another example is the book said I had front wheel bearings. When I went to grease them. There weren't any.

Thanks for any help in advance!