I would check the coolant ratio..you want 60/40, h20/AF...too much a/f will not take the heat away.

I like Behr thermos and no other. Also check for debries between the rad/cond...
Does the viscous coast when you turn engine off ????

The viscous fan is tight and seems to spin O.K in concert with the engine. Doesn't carry on spinning after engine turned off. The revs are a bit low in drive - 550 rpm. However even when in Park (700rpm) the system will overheat - quite promptly.

I will order a Behr thermostat and install it before doing anything else to the cooling system.

Bye the bye. The P.O. installed a Satnav system. He has done something to the hall sensor on the speedo to get a required signal to the satnav GPS system. I can do without SatNav - I have a street directory in my head.
Another assumption - this will be the reason that the cruise control is not working. I have seen previous posts where the hall sensor also affects the KE injection system - I don't know if this is a fact.

Try a thermo..

If you can get the temp down w/heater, that is thermo indication.
If you wire that low fan up w/cabin sw and that holds the temp at idle, then the viscous is suspect.

Hi Arthur,
I have 2 " new" thermostats. 1 Erzatz, - Made in ????. The other supposed to be OEM, labeled made in Germany, cost US$18.00.

Both don't work properly.:mad:

Went to Stealership bought a third thermostat with the 3 pointed star, the part number and made in Germany stamped on it. Cost US $55.00.:eek:
It works perfectly.;)
On freeway with ambient outside temperature of 12 degrees C engine temperature sits at 87 degrees C . 15minutes at idle - temperature never gets close to the 100degree C mark.:D

Curious, I bought all three into the house popped them into a saucepan and heated them in water to boiling point. All reacted the same in this static test.:rolleyes:
In the car it's a different matter.

The only way to truely test a thermo is in the car and that is always the first step on an over-heat conditrion on a Benz..and I prefer the Berh b/c of many years experience with them..
Most think a thermo is a thermo is a thermo....I commend you in not brushing off the changeout on the assumption that it could not be the proper diagnosis just b/c you had changed yours.

I would still do the toggle mod..just for that extra manual convienience..
..and you can add a bridging resistor on the blu sensor to have the high fan cut=in at a lower temp for a Buck. That is a good idea if you are in a tropical type climate. I prefer the cabin switch cuz I am then in control and I am one of those guys who actually watch the gauges ...Imagine that ...:)

Update.

Had a warm day today. Temperature hit 22 degrees C. Engine overheating again.:mad:

Checked viscous fan when engine very hot - fan blades spin freely. Checked viscous fan when cold - fan blades are hard to turn. Is it possible that the fan unit can be installed backwards?:confused:

I obtained another auxiliary fan temperature sensor I specified one to operate at 100 degrees C. Unfortunately it does not work. as required. The high speed fan runs continuously wether the engine is hot or cold.


As Arthur has pointed out, removing the wiring from the sensor should cause the fan to operate at high speed - correct. This would infer that the sensor initially starts at a lower resistance when cold and gets to a higher resistance when very hot. There must be a balance circuit that operates the high speed fan relay.

The red unit has a resistance to ground on both terminals. As it gets hotter the resistance decreases.

here is a foto of the two units. The blue one is the original.

http://farm4.static.flickr.com/3215/...b7d992507d.jpghttp://farm4.static.flickr.com/3292/...72dc3c688b.jpg

>>As Arthur has pointed out, removing the wiring from the sensor should cause the fan to operate at high speed - correct. This would infer that the sensor initially starts at a lower resistance when cold and gets to a higher resistance when very hot. There must be a balance circuit that operates the high speed fan relay.
>>


Incorrect.. but close ..you just have it backwards.
The sensor is a NTC ..Negative Temp Coefficient Thermistor...
That simply means that as the temp INCREASES, the resistance Decreases..
So, when that sensor gets to approx 250 Ohm, it triggers the fan driver circuit in the A/C Control Panel Module N/22. And that N22 triggers the relay for the fan motor.
The reason my test for high fan circuit is to remove the blu sensor plug is b/c that same circuit has a DEFAULT trigger in case the sensor or wiring fails/opens, the fan comes ON by default in order to protect the engine in an overheat condition coupled with a bad sensor..just a safety circuit thay have added in there ..and knowing that, I use that as a test for the fan circuit b/c it is so simple to do. By unplugging the sensor . I am forcing the DEFAULT ciruit in the N22 module ..that's all. It is a Tech Trick for immediate diagnosis of high fan circuit integrity.
The RED sensor you have will not work, but as I stated earlier , you can bridge the blu sesnor with a resistor to lower the cut-in temp of that sensor.
There is a member here that sells them as a plug-in, but you can make one for a Buck. By parallel bridging the sesnors thermistor, you are just changing the calibration factor relationship of temp/Resistance , which will make the fan come on quicker [ lower coolant temp]...I prefer the cabin sw I discussed in an earlier post.
If you have an ohm meter , I will post the temp/Ohms chart for the sensor to check it.
If the fan does not come ON @ 105/107 C , but does come On with the unplug test, you need a blu sensor or you have an out of calibrated trigger circuit in N22.
A general test I use is I have a couple of resistor plugs I made up..one is a 100 ohm amd one is a 500 ohm. With the 500 ohm across the plug in place of the sensor, you should have no fan, but when you replace that with the 100 ohm, you should have fan. That is just a general ON/OFF N/22 test.

The "blue" sensor, call a varistor, should have a resistance of about 5K ohms "cold". It will decrease resistance as the coolant warms and about 300 ohms, that should trigger the fan ckt to come on.

The failure mechanism of a varistor is that it creeps "UP" in resistance value, so instead attaining 300 ohms @ 100C, it only gets to, say 500 ohms. This value will NOT trigger N22 to turn on the Aux Fans.

I sell a device (Cool Harness) that will allow the Aux Fans to come on at designated temperature values. For it to work properly, you must have a properly operating varistor so test it using an ohmmeter and see what value it is. At 100C (or there abouts), it should be around 300 ohms, if not, replace it with a new "blue" one.

The blue sensor measures above 5K ohms cold, but not much less when hot.

I went to the Stealership to see about a new one. After consulting the computer with the part numbers as seen above on the picture, They informed me that one needs to be sourced from Singapore (MB Asia's warehouse.) - a weeks wait.
In the mean time I've installed a switch inside that bridges the low speed fan switch at the AC dryer.
The AC is a R12 unit. It's low on gas. So some buckeroos need to be spent to change out the required components to R34 etc.:rolleyes:

Highly recomend staying w/R12 if at all possible...might just need some topping off.

The sensor is definately bad. You can also put a toggle on that circuit until you get the new part, but in that circuit, you want the sw to OPEN for high fan...should be able to get by untiil the parts arrives with the low fan cabin anyway...

Sensor testing chart:
Temp C - Ohms
20 ......... 5-8K
60 ....... 900-180
80 ....... 460-650
100 ...... 300-400

*Approx , as there was a production change on the sensors on that chassis, so use your part#

Those sensors don't fail often but when it does, the resistance rises.

Agree w/ AD; wouldn't add any switches. Get a new sensor from Phil; pn here is: R1075-26553 - about $30.

AD: don't agree with the tolerance numbers. At 100C, for example, the sensor needs to have a tighter tolerance than 100 ohms (300 to 400 ohms). Needs to be 20 - 30 ohms max.

For example: the stated 'trigger' values for a 3 stage system is:
Switching Point (deg C): 100 ------- 107 ------ 115 degs C
CTS resistance ( ohms): 310 ------- 250 ------ 200 ohms

If the sensor is 100degs "high" then the engine coolant temp could be close to 120C . . . not a good thing!

He has single stage sytem and those tol are taken from the Benz CD Chart for THAT chassis. And the tol are much closer as one approaches the ohm values at 105/107C.
That may be why there was a production change for that sensor.

And the added note for approx values for simple
approx sensor testing was added cuz I knew you were coming..................
after the fact.

No sir . . . it's the same part for MY car and Ivan's car(!) and a ton of other MB cars. Your 'explanation' doesn't hold water . . .

It makes no difference what car it's installed into . . the varistor has the same specs b/c it's the same part.

Of course, are you saying that Fastane is incorrect?? Don't think so. . .

Didn't say the sensor was different..I said the CHART was from the BENZ chart. You want to argue with the CHART ???? ...be my quest.

And I listed the approx value so he could test the sensor for approx value just to see if his sensor was even working . as part of my diagnosis.
And the diagnosis is done and his problem is fixed.

Here is the rest of the chart from THAT BENZ CHARTfor THAT sensor. I merely listed up to 100C for his info to test the sensors approx ohms,which we now know does not change regarless of temp..which was the purpose of the info to begin with.

110C- 230/290 ohm
120C -180/220
130 -135/175.

Not my Chart or specs ..Benzs Chart....
Stop your barking ..

If there was, the PN would have to change . . it didn't!. It would effect all MB cars using it.

Since I've tested a lot of these in boiling water (100C) and they all were within 20 ohms. The readling may have been even better b/c of experimental errors.

Keep a civil tongue in your head. :mad: