Radiator fan doesn't come on when overheating
 

Both radiator fans work when the cabin heater fan and/or AC is on. However, when I get off the highway and sit stopped for ~10 mins, the engine overheats. The fans do not come on, unless I turn on the cabin fans and/or AC.

(I say and/or because I originally thought the AC is what turned it on because I'd stopped with the fan on low and heater on med-high and it overheated all the way; Tested it at home and yes the fans kick on with AC, but then tested again and the fans kicked on with only the internal fan on low, also med-high heat setting).

Any ideas what could be the problem? Is there a separate circuit that turns on the radiator fans when the engine is overheating? Maybe a fuse or relay?

The fans should operate in sync with A/C compressor operation as they are wired up to the A/C magnetic clutch relay. When the compressor clutch locks up to drive the compressor input shaft, the fans should operate. When the clutch unlock, the fans should turn off. If this system works the wiring and relays are OK for this system.

There is another separate circuit for fan operation and coolant temp. There is a temp controlled on/off switch on the bottom of the radiator, engine side. The switch OPENS (not closes) when coolant temp reaches approx 199F.

Find the switch and electrical harness plug wiring. There should be a short harness running from the switch hidden at the bottom of the radiator attached to the radiator fan housing and connecting to the chassis wiring on the side of housing.

This allows unplugging from the chassis wiring and removing the radiator rather then need to unplug the switch end buried at the bottom of the radiator and hard to access.

When unplugged the fans should now operate when ign key is ON. If the fans operate then the radiator switch is bad. If they don't something else is bad.

The above system has 3 relays to work. The No 1 relay is wired to the temp switch. When the temp switch opens, the relay turns OFF to trigger the other relays and power the fans.

Replacing switch typically requires removing the radiator for access or remove the bottom plastic shield and try to gain access.

The system is two speed. High speed for cooling radiator, low speed for A/C operation. The speeds are obtained by connecting the fans 12v powers source in series or parallel.

ok just to be clear,the fans turn OFF when the the switch is closed(continuity) and ON when open?
I tested the sensor via the connector plug and got continuity,tested a new Denso sensor out of the box and got the same result

Yes. For the switch installed on the lower section of the radiator. It should be closed (continuity) until the coolant temp reaches 199F, then open.

If the ign switch is on or engine running and the plug to this switch is disconnected. The fans should turn on.

Fans also turn on when AC compressor is working.

Great news
I had a collapsed upper rad hose and thought it was the cap Replaced it and issue solved.
Replaced the hose too
Went to the store and the radiator blew its top!
My fault,I diddnt burp the system.
Now I have a new radiator,thermostat and will replace the rad sensor. Hope THIS fixes it

Great job!

hey there! I was having a similar problem. I ended up replacing the thermostat (needed to be replaced, now opens), temperature sensor, and now the cooling fan switch. the fans come on when I disconnect the switch from the side of the fan housing and turn the ign on, so I figured it was the switch. after replacing the switch the fans still wont come on and the car still over heats. thank you for explaining the relays to me, I was wondering why there were three. they seemed to test fine, relay one was hot the last time my car over heated. I tested the continuity of the relays with an ohm meter, hooking them up to a battery. one of the relays was at about 1.5 ohms when the switch in the relay closed. could this relay be a problem? is there anything else I should try

Do the fans cycle on/off with the A/C compressor doing so?

Hi again yes they turn on when ac is on. But today when I drove I temp was stable so can it be radiator fan switch?