In the last few days, my engine temperature has been running about 15 degree hoter than before (even though the weather has cool down a bit). Before, even with the triple digit temperature outside, and my AC always on, the engine temperature never go pass 100 (and stay around 80 most the time). Now, with the AC on, when I cruise on the freeway, temperature stay around 85. However, once I stop, or with stop-and-go traffic, the temperature shoot to 100, or pass it.

Further exame of the engine, I notice that the auxiliary fans run at much lower speed. Before, with the AC one, the fan run at low speed, and then once a while, I would see the aux fan run really fast (and loud) for few minutes, then stop, then run again .... (I guess it runs as the temperature rise, and stop when detect cool down). Now, it still runs and stop, but it runs at much lower speed, and much quieter, almost not detectable if you don't open up the hood. With the AC on, and the car at idle, I don't see that fast & powerful running from the aux fan again. Because of this (low speed fan), the fan constantly repeat running for a minute or two, then stop, then run. Also, with the AC on and the car at idle, the engine temperature shoot pass 100, which never happen before.

I've tried to replace the relay (behind the fuse box). Is this car has only one relay for the fan ?, since that's all we found. As some has recommend, I has also disconnect the sensor on top of the thermostat. Nevertheless, I don't detect any improvement. The aux fan still only run at low speed.

My mechanic recommend to replace the entire aux fan system next. He says the fan suppose to have two speeds, and that the high speed for my fan is dead. I don't know if this is true. Since to me, the fan has only one motor, and it's either run at high speed w/ high voltage (or current) supplied, or low speed with low voltage (or current) supplied. If the motor is dead, the fan would not run at all. After all, this's an electric fan. Is my logic correct ???. Beside, the cost of this aux fan work would be costly (~$500), so I'm hesitate to commit large amount of money into it (given my current situation, and not sure if that would cure the problem).

Is there something else that I'm missing. Is there some other test that I sould be conducting in my car ?
My car is a 98 E320. If you have experience dealing with similar like this, would you please let me know. Since I don't know much about Benz, when you give me your advice, can you pls provide location (in the car) of the part that I should be looking at ? .

Thanks so much.

Two things you may want to do:
1. Flush your cooling system
2. Check your fan clutch.
3. If the fan runs, then it's ok and doesn't need replacement like the mechanic said. Those fan last a long time without problems.
4. The fan should run at low speed the minutes you turn the A/C on. When the temp goes high, the fan comes on at high speed.

To me it sounds like your fan clutch is a little week and not engaging properly when the temperature goes up.

Good Luck!

__________________
Meza
1993 190E 2.6
Advanced Diesel Systems Test and Research Engineer

The only thing that puzzle me is that the aux fan is an electric fan, and all people I've talked to so far told me electric fan has no clutch. It either runs, or it doesn't.

On my old car (W124) , the fan clutch wear out once, and was replaced. But that was the main one, and fan clutch was test be rotating the fan, and saw that it spin freely.

However, it's the aux fan that have problem now.

I've also read about the resistor ? Is this for lowspeed fan only.

Also, can anyone tell me if the W210 has only one relay (for both speed), or separete relay ?.

When I unplug the sensor from the thermostat, should I jump them ?.

To take you guys advice, I connect the fan to the 12v source, and the highspeed fan come out (yep, the loud & powerful one), so I'm very hesitate to hand my car to my mechanic to have the whole fan set replaced on Monday. I have the lowspeed fan when the AC on, and highspeed when I jump wire directly to it, so I can't see how the "highspeed" motor fail in the fan set as he said. The repair will cost $500.

Anywhere else I should look at ? should I bring it to another mechanic. My mechanic has been very nice & reasonable to me though (in term of service & price).

Zam2000,
The fan clutch I was talking about is the one bolted to the plastic main fan. The electric fan has no clutch. If your electric fan is working when you apply battery voltage to it, then it's ok and DO NOT REPLACE IT like you mechanic say. I don't much about your specific model. I have a feeling that the problem is related to a switch, relay, or the viscous fan clutch. The viscous fan clutch should have slight resistance when the engine is cold and higher resistance to spin when the engine is hot as the viscous fluid inside of it get thicker when temperature goes up. If you don't see a difference when you spin it hot or cold then it's bad. The fact that you have normal temperature reading while you drive make me think the viscous clutch is a possibility. The eletric cooling fan is related to the coolant temperature. It comes one at low and high speed. The high speed comes on for a few minutes only and when the temperature drops it either stops or spin at low speed. I have a lot of experience with BMWs and most of what wrote is based on experience with BMWs. However, MB and BMWs are not much different or even other cars when it comes to cooling. Also, one thing I strongly suggest is to check the overflow tank cap. take it to a shop and have them test it for leaks. If it doesn't hold presuure, then it's bad. You may just go to any parts store and get one for about $5 and see what happens. In my 190e, I couldn't figure out my heating problem for days and finally it was resolved when I changed the cap. From what you have described, I believe your system is working fine and it's just not holding pressure.
Good luck!
Meza

__________________
Meza
1993 190E 2.6
Advanced Diesel Systems Test and Research Engineer

So I had the fan control unit replaced ($160 part, my labor), the problem still exist.

So far, I have tested by unplug the sensor on top of the thermostat, the fan runs, at low speed. Unplug the sensor on top of the AC valve, the fan stop, and when I plug it back, the fan runs, at low speed. On both case, the fan runs for couple min, then stops, then runs again.... If I apply 12v to the fan, it runs at high speed. Replaced the fan control unit, nothing change.

I measure the voltage that the fan receives when it runs, and I saw it goes from 5.4v then slowly drop to zero (corespond to the fan go from slow to stop). Never had I saw it goes beyond 6v. This explain why the fan run at low speed.

Is there anything I need to look at ?

I think it's the aux fan temp switch.

I believe your temp switch has 2 temp ratings.At lower temp, the fan kicks in low speed, and at higher temps the fan kicks in high speed.

I think it is controlled by a resistor in the switch.

If this is so, your switch is not kicking in the aux fan at high speed at high temps.

I may be wrong, but I have an Euro model car and it has only 1 temp switch rating at which the fan kicks in - HIGH SPEED.

I have seen on Fastlane that the temp switches in the US have 2 temp ratings - 1 for low speed and 1 for high speed.

Wonder if anyone else can help confirm this?

My 2 cents.

Oops.

Checked Fastlane again.

The aux fan temp sw is only single rated - 100 deg C.

So I guess I'm wrong.

Unless there is an external dropping resistor connected to this circuit for low speed?

Anyone else?