There are normally two temp sensor switches.
One is for the temp gauge. Its resistance chanes with temp so that the current in the meter changes to show the temp. It is normally mounted on the engined block.
The second switch is just an on-off switch normally mounted somewhere near the thermostat. It make earth connection for the auxilliary fan relay when the coolant temp has reached a specified temp. It comes with different temp specifications like 72°C, 85°C etc.
The location of these switches may be different in your car.
Try to locate the second switch. follow the wire (single wire only) that leads to the fan relay. Replace the switch with one of lower temp spec if you want the aux fans to come one earlier.
It is not an expensive item.
Shan.

 

hi all ...did you guys check the thermostat ...they have 2 kind of temp.
100 c is nothing in mebe ...is normal

 

Hi Shan!!!

Thank You for your respond...

I was busy with school work lately, therefore it took a while for me to read and to respond to your message...

I found two switches...(both located in front of the engine...One is to read the outside temp. and the other is for the aux. fan.. I think)...For my car the aux. fan starts to kick in around 105 deg. C (So can I change it to a lower...like 72 or 85 ... like you said???) I don't know why Mercedes put in the "105 deg. C aux. fan sensor " for an E420...anywayz...I called a local MB dealership...and one of the guy told me that all E420 have the 105 deg. C aux. fan sensor...

I guess it's normal for a 4.2L --8 cycl. to operate at around 100 deg. C ...I just hope that it not going to screw up the engine over a long period of time...

-Long

 

what i did on my E320 1994 model was simple. Check the thermostat it was 87 deg centigrade, have it changed to 71. check belt was in good condition and the fan coupling too by trying to stop the fan while engine is running (but with care), on replacing the thermostat, the engine temp stays around 80 and still climbs to 100 with AC on. I unplugged the Aux. fan, Got a 12Volts, 30Amps Bosch Relay connected the input live to the Battery, the other input line to the ignition, then connected the output live to the Fan and the other line to Ground then back to the Fan (The fan turns in a clockwise direction when the ignition is on). My temp stays at 80 with AC on even on Hold UP

 

SAME THING HAPPENS ON MY CLK 500. THE REASON IS THAT THE MOTOR WILL NEED THE HEAT ( WHAT THE THE DEALER SAYS) I THINK THAT MINE IS A BIT FUNNY BECASUE IT GOES TO 100 WHEN I DRIVE AROUND THE CITY AND THEN FALL DOWN TO 80 WHEN I AM ON THE HIGHWAY. ITS NORMAL FOR A BENBZ TO DOUE THAT. HAPPENS TO MY BROTHERS E430 OFTEN WHILE HES IN THE CITY.

DONT WORRY ABOUT IT IF ITS TOO HIGH OF A TEMP THE ENGINE WOULD OF BROKE BY NOW. BUT IT ALIVE DONT WORRY ABOUT YOUR E 420.

 

Looks like this is a common problem. My 88 560SL does the same thing. At 70MPH, it runs about 80. In traffic, it creeps up to 90-95. I've flushed the radiator, changed the temp sender, changed the thermostat....nothing seems to make any difference. Doesn't overheat, just creeps up. The fans seem to be working OK.

 

The engine is a lot cooler on the highway, since the car is moving with air cooling down the engine.

 

I'm not going to sweat it....if it just had an idiot light, I'd never know the temp was changing! Even th emanual addresses this phenomina, so it must be common.

 

What you guys are discussing is normal. There is no problem.

If the temp rises but doesn't come back down when the aux fan kicks in or when the car moves at highway speeds, then you have a problem.

On newer cars, if you're in traffic and the temp rises to a point that scares you, turn the A/C on. This forces the aux fan to turn on and the temp will come down.

 

I would say that the problem described indicated a failed fan clutch. It's job is to freewheel the fan until the teperature comes up . Once heat from the radiator heats the fan clutch it is supposed to progressively tighten the connection of the fan to the pully until the fan is fully engaged. If your car heats up in traffic it's b/c the fan is continuing to freewheel when the car is at a standstill not pulling in the air necesary to cool the engine. Needs a new part to reapir. Side note the aux electric fans shouldn't come on until 110 degree C to act as a last ditch effort to prevent an engine damaging overheat, not to regularly regulate the temps. It will aslo come on low speed due to a/c pressures. Mercedes are the best cars on the road. They should regulate temps at 87 degrees C without compromise. Any other behavior indicates a fault in need of remedy.