To all knowledgeable TPI experts out there ;
I'm trying to understand GM's logic on the TPI dual cooling fans as the research I've done seems contradictary. Most likely from modifications done themselves. Let's assume all sensors, thermostat are stock, (OEM).

Do both cooling fans turn on at the same time ? Or does one turn on at a lower temp than the other ? And witch one turns on first ? (The driver's or pasenger side). And at what temps do the fans come on ?

Appreciate all input...

 

They turn on independantly, at different temps.

The primary fan is controlled by the ECM (car's computer). The ECM gets it's coolant temp reading from the Coolant Tempurature Sensor that is located in the intake at front of engine, two wires, black and yellow. The primary fan turns on first, but I'm unsure at exact temp, probably in the 210 - 220 range.

The secondary fan is controlled by the Fan Switch, located in pass side head, between #6 & #8 spark plugs, single wire, mushroom-looking connector. Not to be confused with the Knock Sensor, which is very close in location AND uses the same one-wire connector, but is in the block rather than in the head. The secondary fan comes on at 238, assuming a stock factory Fan Switch.

In addition, the secondary fan comes on with the A/C - the high pressure switch in the A/C line will activate the secondary fan. So, determining which is which is easy if A/C still works - turn on A/C - whichever fan runs will be secondary fan.

The logic is ... no need to have both fans on at the same time - that creates a huge draw of power electrically to have two fans come on at the same time. Fans draw alot of amperage to first start - a 20 amp fan draws 20 amps when it starts, then after it only draws maybe 10 amps to continue to run. GM (and others) all design two fan setups so that the secondary fan only comes on if/when the primary fan isn't cooling enough (or when the additional cooling from having A/C operating is required).

 

Thanks...That clears up everything for me..