Hello,

I've got an '98 C240 Elegance V6, and I've got a failure that my local dealer seems not to believe : when the engine is very hot (after half an hour of traffic jam, or 3 mn of 3000 rev's with the car stopped), the outside temp indication on the dashboard disappears, and after a few seconds (I can hear a 'clong' as if something's closing) the A/C send very hot air (like on the max heating position). If I stop the car engine to cool it down for a few minutes, the outside temp comes back. If then I start the engine, the A/C sends normal cool air until the temp disappears again, etc. All along this, the engine temp. indicator stays on normal.
I repeated it a few times to be sure.

I thought that could be an outside temp. probe failure, so that the A/C think the outside temp is very cold and therefore send a very hot air, but my local dealer said that there is no relation between the outside temp and the air conditioning system.

Who is right ?

Thanks by advance,
Richard.

. . . no relationship
 

Richard,

Not sure about your car, but on my S500, the external temperature probe has no relationship with the A/C temperature and/or its operation. It's only gives an indication of the outside temperature to the LCD panel in the instrument cluster.

My probe is mounted in the left front bumper below the main section. Because the probe is mounted so close to the ground, it tends to read high because it 'reads' the hot pavement temperature.

The A/C uses a separate coolant sensor (B10/8 on the top of the manifold) to control its operation. I believe your dealer is correct.

It sounds from your description as though the LCD panel has an intermittent connection.

I don't have vehicle specific info on your non-US model, but the temp display function is described in the WIS for models 129, 140 as of 1.6.96, 170, 163,202 as of 1.8.96, 168, 208 as of 31.7.99, and 210 as of 1.6.96 up to 30.6.99.

The first paragraph reads:

"To display the temperature, the outside temperature indicator A1p4 requires: the iutside temperature, the speed signal, the engine running time and the coolant temp."

It goes on to say:

"Since the outside temperature display depends on the speed , engine running time and coolant temp, high temperatures (influenced by the heat generated by the engine, for example) are not displayed when the vehicle is stationary or moving slowly."

There is a whole page of conditions and exceptions of temp indication but my fingers grow weary.

I am not sure about whether the A/C uses the outside temp from this sensor. If it did it would be communicated by the CAN from the inst cluster to the Pushbutton controller. I presume it would leave a code if the data was interrupted.

Thanks to both of you for your answers.

According to my MB user's manual, when the ext. temp. grows rapidly, the indication stays on the last low measure. The user's manual doesn't says that the indication will disappear in that case.
But, if Steve is right, I hope that MB have think of this case. I'm still shure that an A/C sending suddenly VERY HOT air is not a normal situation on a so technicaly elaborated car. And I'm still shure also that there is a relation at least with the ext. temp. indicator and the A/C, because I repeated almost 20 times the failure, and EVERY TIME the A/C send hot air as soon as the indication disappeared.

If one of you can be of further help ...

Thanks by advance,
Richard.

P.S. : My C240 is the french version, I'm living in Paris.

Its lunch time and I continue reading about the temp display. The other conditions vary by year and the ones for the later car are simpler so I will repeat them to continue the answer about testing the reading for accuracy.

"When the ignition is turned on or off and with an engine temp less than 60deg C, the current temp is always stored and displayed.

When the ignition is turned off and with an engine temp less than 60deg C the temperature last measured is stored and displayed.

When the ignition is turned on and when an engine temperature is less than 60(C The stored temperature is displayed."

There are two reasons to write this. One is to show the poor nature of the text that MB techs must decifer. The 60(C is not a misprint on my part. It might lead one to believe that the "less than" sign (< ) which I paraphrased might also be misused in this quite contradictory text.

The second reason is to say that they are pointing out that the actual temp might not be displayed in the interest of not reading the radiator heat after a period of inactivity.

For those who can be interested buy this problem, here is the end !

My local MB dealer finaly fixed the A/C. He confirmed that THERE IS a relation between the external temp and the temp of the air send by the A/C. The hotter is the outside air, the colder is the air send by the A/C, to improve its action inside the car.

So, the failure came from a bad connection behind the instrument panel, that appeared only when the back of the instrument panel was very hot. MB changed the entire instrument panel (at no cost for me, great !), and now everything's all right.

Long life to MB !

Richard.

outside temp
 

rich i dont know but something just doesnt sound right.i mean what if the system for the outside temp totally failed.does that mean you wouldnt get any cold air?right now iam having a bit of a tough time getting cold air from my ac.my outside temp is totally messed up and reads in excess of 130 degrees.the system in mb cars seems to be like a house with central heating and cooling much different that an american car.wouldnt the outside temp probe have an effect on heating also.i mean if it affects cooling it must affect heating right?iam totally confused now.some one who knows please tell me.:confused:

Until the split systems and later model CAN cars, the outside temp display had no relationship to the output ait temp. One the later cars useing CAN for information swap the AAC control unit (n73 i think) uses the outside ambient temp, sunsensor input and in the case of the 203 sunangle input to determine output temps. That neat little ball sticking out of the cowl is the sun posistion sensor on the 203, 220's use a similar system. 210's got the sunsenso and I think they got the ambient input via CAN from IC in 2000 or 2001.

Joe