How to diagnose radiator fan failure?

[sasha055]

I think my radiator fans are acting weird.
Yesterday it was 85 degrees in Seattle (Winter in Texas is warmer than Hot Summer in Seattle) and my coolant temps ramped up to 106C
10 min after parking the car in the garage coolant started boiling and overflowing.

Here's what I tested:
1. Coolant Cap is new, I replaced it 6 month ago.
2. Before yesterday I didn't loose any coolant and there is no condensation on rear trunk lid
3. Radiator fans turn on when I start AC and both of them are working properly (napkin was stuck to the grille on both sides)

Now the weird part:
4. I started the car and waited for it to warm up and fans to start. I watching the temp on AC LCD.
Fans started when temp climbed up to 103C (217F), engine cooled off quickly and fans stopped when engine cooled to 99C (210)

I thought fans should start at 96C (206F)

Is it normal? are my fans broken? if yes what is the fix?

Thanks
Sasha

[sasha055]

Here's an update:

I checked my relays, by putting finger on the and turn AC on/off

When I turn AC on, low speed fan relay is clicking and fans start, so it's not a relay.
However I'm still puzzled why the fans do not start when coolant reaches 96C (206F) and only start when coolant reaches 103C ?

I also run the car on idle until fans kicked in automaticaly while keeping my finger on the relay. When coolant temp reaches 103C low speed relay klicks in and fans start. Why is my car starts fans at low speed at 103C? why not at 96C?

Can it be resistor? if the resistor is bad then fans shouldn't start when low speed relay is clicking, right?

Sasha

[san rensho]

My 2k 2.7 does the same thing. I'm going to change the coolant temp sensor (its only $25) and see if that makes a difference.

[sasha055]

Do you mean thermostat? the one that goes behind the waterpump?

Where does the car reads temperature that is used to decide when to turn the fans on?
PO of the car did a lot of workaround fixes so I think maybe he screwed something up.. like reversing the resistor wires?

Sasha

[san rensho]

No, its a variable resistor that sends an electrical signal to the DME which controls, among other things, the cooling fans.

[sasha055]

I had no ideea there is a resistor that does that.

Can you please teach me where to buy it and how to replace it?
The probles has to be somewhere in electronics, cooling system is working as intended .. only it start 10 degrees slower.

Thanks
Sasha

[san rensho]

Just changed the coloant temp sensor and no change. With the ac off, fans don't turn on until 103C.

Sensor is on the driver's side, on the front of the engine, pointing towards the driver's door. Pelican parts has the part.

[sasha055]

I'm gonna talk to my mechanic ... maybe he know why my low speed fans start at 103C.
I'll keep you posted

Sasha