Engine temperature and Aircon.....

[PJayUK]

Excuse my ignorance on this but this is puzzling me...

Why does the temperature on my W210 seem to run slightly hotter when the AC is running? I have noticed that it sits just above 80 with the AC on but stays virtually bang on in the centre of 80 when its on "EC" mode...

Same journey, same traffic conditions etc....

No problems with overheating as was fully tested on the M25 last night where I sat in stop start traffic for a good hour! Temp only just crept above 80 but stayed there quite happily until I eventually got moving......

 

[NormanC]

Because the engine is generating power to drive the aircon thus it needs more cooling. To get more cooling more water has to go through the radiator. To get more water through the radiator the thermostat has to open a bit more. For the thermostat to open a bit more the temperature has to rise slightly.

Simples!!

It's all due to the fact that the thermostat opening is dependent upon temperature. A typical thermostat may start opening at say 78 deg C but it won't be fully open until around 92 deg C.

 

[PJayUK]

So I am assuming therefore that when EC mode is selected the AC compressor is fully disengaged and hence the load on the engine is reduced? From what I understand the fans for the AC are electrically operated so they wouldn't have any impact on the engine.

Sounds pretty logical!

 

[NormanC]

So I am assuming therefore that when EC mode is selected the AC compressor is fully disengaged and hence the load on the engine is reduced? From what I understand the fans for the AC are electrically operated so they wouldn't have any impact on the engine.

Sounds pretty logical!

Yes, in EC mode the compressor clutch is de-activated hence there is very little load on the engine from the compressor. As far as the fans go, their electrical load through the alternator is negligible compared to the load from the compressor. Fan load is measured in watts, compressor load in kilowatts!!

A couple of years ago I had the pleasure of driving a Fiat Seicento with air conditioning right across the middle of the Peloponnese. You'd be amazed how little power is left at an altitude of 2000m when the air con is switched on - just about enough to keep it moving!!

 

[Number_Cruncher]

As already mentioned, the engine needs to do more work to drive the AC compressor, and more work again to produce the electrical power to drive the fans.

As well as these, the heat from the car's interior is rejected in the condensor which sits in front of the car's coolant radiator - therefore, the air flowing through the coolant radiator becomes pre-heated by the airconditiong condesor, and this causes a direct rise in coolant temperature - the cool water returning from the radiator to the engine isn't quite so cool any more!

 

[Rory]

In order to keep the air moving through the condenser, many cars run the engine cooling fan all the time when the a/c is on so the car actually runs a bit cooler - I don't think MB's do this, though.

 

[Number_Cruncher]

In order to keep the air moving through the condenser, many cars run the engine cooling fan all the time when the a/c is on so the car actually runs a bit cooler - I don't think MB's do this, though.

The system on my W124 uses a pressure switch to indirectly indicate the temperature of the refrigerant in the condensor, and this turns the electric A/C fans on, so, as you say, while the A/C is cool, the fans don't run.