Overheating 99 Venture-Electric fan

My 99 Venture overheats after being driven awhile when I stop at a light. I have discovered that the electric radiator fans do not turn on when the engine heats up. However, I know that the fans work, because when I idle with the air conditioning on, the fans are working.
Any suggestions? Thanks.

Define overheating--

Does coolant spew out?
Does gauge go into red?

The gauge is normally at 10 o'clock and goes to 1 O'clock. If I sit there long enough, steam will rise, but normally I just see the gauge go up and hear gurgling from the engine. I have not noticed fluid spewing out, but the steam is a definite symptom.

The gauge is normally at 10 o'clock and goes to 1 O'clock. If I sit there long enough, steam will rise, but normally I just see the gauge go up and hear gurgling from the engine. I have not noticed fluid spewing out, but the steam is a definite symptom.

I haven't seen any steam but it smells like hot metal??? I've also discovered turning on the a/c helps, starts the fans and the temperature drops. Mostly it is the gauge that goes past 12, i.e about 1 o'clock, not in the red. My scan tool tells me its not all that hot i.e. maybe less than 220. I think that is about where the fans are supposed to kick on when no a/c so maybe it is normal. A bit of a worry when stuck in stop/start traffic in the fall/spring. Turning on the a/c works.

I thought it might have been a fan relay but my crude testing, apply volts, get click, suggested they were ok. The wiring around the fan doesn't look brand new :wink: but the fans do run.