Today I was driving it intown for a few hours and everything was find (it was about 85) until I turned on the a/c, then my tempreture gauge started moving up to 110c, so I turned the a/c off and the temp started going down....

Later, I found that the auxlilary fan (in front of the radiator) does not come on at all, please help me troubleshoot that or let me know if I need to open another thread.?

I drained the cooling system and refilled it, water pump appears to work perfectly.

I checked the #2 fuse and that was good. Gave external power to the fan and the fan was operational.
So it has to be some relay or the tempreture sensor somewhere??

Found the switch on the thermostat housing and unplugged it, according to the test procedures the fan should come on when you unplug the switch, but didn't.

Should the car overheat if this fan doesne't come on when ac is running?

From the wiring diagram, it looks like you would need to ground the wire to the coolant temp sensor to make the fan come on, not just pull it off. The other switch that can activate the fan is the receiver dryer temp switch - blue wire. The aux fan should come on 100 degrees coolant temp.

Both of these feed into a relay which turns the fan on. This has feeds from Fuses 2 & 10. The relay is located above the fuse box. It has Black/Red, Red, Brown/black, Black, and Blue wires. The Black wire actually powers the fan.

When you turn the AC on, you put additional load on the engine, and the condensor, which sits in front of the radiator, heats up. So yes, you would expect to see a temperature increase.

__________________
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

I grounded the switch on the thermostat housing and fan didn't come on. I apparently cannot find the other switch on the reciever dryer, at least not with the blue wire, can you give me a description?

The fan didn't come on when the car hit 110.

Worse yet I cannot find the relay, I pulled of the glove box, and the passenger side panel that covers all the relays, but don't see a relay with the wires you described, so perhaps some clarificaion on that as well? For those of us who are not familiar with these damn cars.