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.

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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.

Any info on the realy would be good? Picture would be best, can't find one on-line for some reason.
Also, where is the switch on the dryer reciever?
Also testing procedure for the relay would be good? I would jumper the black wire to which other one for the fan to come on?

I have given you every piece of information I have on this.

It's not likely to be a part you find online, probably a dealer item.

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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 have ordered the realy, just want to make sure it's not a break in the wire, so I was wanting to test the other switch on the reciever dryer, but there is two there and I am not sure which or how to test those.

Okay, I bought and replace the relay, had to remove the panel as it was about 2 inches above the fuse box. AND THE FAN NOW WORKS when I ground the switch on the thermostat housing and when I short the two wire on the dryer reciever. But my overheating problem still exists.

Afte replacing the relay I turned the car on with A/C on in about 90deg weather. Watched the tempreture climb to 105 (on the gauge) the aux fan didn't come on, but I started to HEAR BOILING coming from the radiator, looked over at the overfill canister and that was full and boling and there was coolant on the floor of the garage. What ta heck is going on?

Welcome to the I-fixed-one-but-here's-another world of old Mercedes.

The first thing you should check is the radiator cap, probably best done simply by spending 14 bucks for a new one. Liquids boil at higher temperatures under pressure, so if your cap can't hold pressure, you will boil at 100.

Then I would make sure that your thermostat is working right. Again, most easily checked by replacing. Or you can throw it in a pan of water on the stove and see if it opens.

Be sure that the engine fan is turning at a good clip when the engine warms up.

Next is to make sure that you are getting ground from the temp sensor. It may not be working. You can do it easily by connecting a test lamp to a positive source, touch the probe end to the tip of the sensor with the engine hot, and see if the light comes on. Or you can use a multimeter on the ohms setting between the tip of the sensor and the housing. Hope I am not insulting your knowledge here.

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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

Thanks for the response. The thing overheats only with the a/c on, and will not overheat with a/c off, thermostat was changed two years ago by a merc dealer in CA. You are correct that I need to replace the radiator cap since it's original. The fan clutch is working good.

I will test the ground on the sensor, however what doesn't make sense is when I ground the wire going to the switch on the thermostat housing it comes on, so that switch is not working or I don't understand something about it. Shouldn't it only ground at 100deg C? And why is the dryer/reciever not making the fan come on??

All that happens when the AC is that the engine has to work harder, and the condensor adds heat to the air being pulled through the radiator. Maybe your cooling system is marginal and that's all it takes to push it over the edge.

A couple of things might be going on with the sensor. First, it may not be working and making the connection to ground. Second, the thermostat housing may not be a ground if it has a gasket and maybe the bolts are corroded. Easy to check. I dimly remember having seen ground wire running to the thermostat housing bolts from the frame or other ground points.

Another cause of boil-over is an air pocket in the coolant. You might try filling it by disconnecting the top radiator hose at the radiator end and adding coolant through the hose.

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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 assume you're using 50/50 coolant/water mix.

The cooling systems on these cars are quite marginal. In hotter climates, they tend to get very unhappy (ask me how I know).

Your radiator may be clogged. It could be time to pull it and have it boiled and/or rodded.

Your water pump may be corroded. (You know what to do here.)

However, none of this addresses the boiling problem. Often, the expansion tank itself is the culprit for leaking/losing pressure. So after all the advice I gave you, perhaps a new tank is in order.

Any way you slice it, you're going to have many new cooling system components before you're done. But consider this to be an excellent long-term investment in your car's health -- because you will be replacing this stuff either now or later!

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'01 SLK320, '79 450SL & '01 C320 -- What? 3 Mercedes? I am DEFINITELY crazy!!!

Replaced the radiator cap and now replacing the switch for the aux fan on the thermo housing. My mechanic told me it was normal for the car to boil at about 105 to 110??? Does that sound right to anyone?

Dmitri