Aircon not kicking in immediately

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KLee

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Nov 8, 2004
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to No.1 brand in reliability
Hi I need pointers on what could be causing my aircon woes. I had my whole diavia compressor changed and the drier was replaced and pipings checked etc in May this year. Now the aircon doesn't come on immediately - only after say 10 to 15 mins on the road. When it does come on, the coldness is the same as before so this must rule out refrigerant gas leak issues. What could be my problem so I am not taken for a ride by some of these 'change everything' mechanics. Any advise appreciated.
 
The aircon does not come on at all now. Since I had the car in for an annual service I asked to have the system diagnosed. It appears according to the mech that there is no power going to the compressor so it isn't engaging at all. He has testing and confirms the compressor is still working, fuses still good and system gas pressure is still intact. I have to take it back to the guy that overhauled the aircon system.

Pls anyone have a clue as to what could lead to ' no power ' going to the compressor?
 
Something as simple as a bad connection somewhere ?
Have you tried playing with the A/C EC button ?
 
Yup, of course toggled that switch to death. i just got off the phone with the aircon electrics mech who's looking at the car n who also overhauled the aircon compressor. He confirms no power going to compressor n has traced it to the black control box behind the whole panel of aircon/fan speed/fan direction/recirculate !! What the heck is happening here?! I havent heard off this control unit going bad ... could it be over 10 mths since the new compressor went in, that the control system is falling apart - could he have overloaded it with a badly installed compressor/aircon system? I'm disillusioned w MB
 
temperature sensor?
 
Y'know I'm no mechanic, but have had thoughts it might be something to do with a temperature sensor failing. BUt I don't know enough - there must be thermostat somewhere in the system right so it will regulate coldness (if it does that at all in MB without electronic climate control - mine is all manual knobs, which I actually prefer). Is the thermostat inside the box which sits after the blower - in this box I'm guessing is it where cold refrigerant is forced in from the compressor and the heat to cool exchange occurs. Ah found picture suggesting this box is called the Evaporator.

I found some EPC schematics on the net for the bunch of AC EC / fanspeed / fandirection switches and dials and cannot see any special complex control unit behind these switches as alerted by the mech. Am I wrong? What is behind these switches? Mine model does not have electronic climate control.

Dare I ask, if indeed there is some control box, what roughly is the price new and secondhand?

Help
 
I was quoted £110 for a scond hand part.
There is a temperature sensor in the panel above the rear view mirror. There is a little vent up there. The sensor has a little fan on it to pull air in. It might be worth cleaning it off as they do get crudded up. Didn't make any difference on mine, but my fault was in the duovalve
 
DrNick, wow I never knew there was a temp sensor up there!! This is under the roof near the cabin lights you are referring to right. And a tiny fan too? Geez. Come to think of it, I remember when I switch to Recirculate air within the cabin, that setting never lasts more than 5 minutes, it switches back to letting in external air. Does this sound like I must be having some temperature sensor issues all along?
 
KLee said:
DrNick, wow I never knew there was a temp sensor up there!! This is under the roof near the cabin lights you are referring to right. And a tiny fan too? Geez. Come to think of it, I remember when I switch to Recirculate air within the cabin, that setting never lasts more than 5 minutes, it switches back to letting in external air. Does this sound like I must be having some temperature sensor issues all along?

MBs switch back in most models to external air after 5 mins or so. Normal

Bazzle
 
The aircon control will be pretty simple.

There will be a pressure sensor, a thermostat on the evaporator and the power switch and possible a switch on the fan control.
These will work so that when the power is fed via the switch, the fan is on, the pressure sensor detects pressure in the aircon system and the evaporator is above freezing, the compressor clutch will be fed with power.

Any of the above will cause the compressor relay to drop out.
 
Thanks v much for the description dieselman. I'm trying to get a picture of all these components mentioned and its whereabouts in relation to where the mech is investigating - Is all that you described hosted on or around the evaporator unit? And is the evaporator directly behind the centre vents?

So you are saying the sequence of events must occur for power to be sent to the compressor ... is there circuits that would stop power being sent if any of the above fail or will the problem usually manifest itself visually or components just blowing out? The compressor is the newest component in the system being replaced with a reconditioned unit almost a year ago.
 
Bazzle said:
MBs switch back in most models to external air after 5 mins or so. Normal

Bazzle
I thought I read in the manual depending on external temperature it would decide to let air recirculate up to a max of 30 mins. Lemme recheck the manual.
 
Check the operation of the ventilation fan - does it come on immediately when you turn it on?
If the fan doesn't run, the A/C compressor won't cut in.

There can be a problem with the brushes in the fan motor on older cars.
 
KLee said:
Thanks v much for the description dieselman. I'm trying to get a picture of all these components mentioned and its whereabouts in relation to where the mech is investigating - Is all that you described hosted on or around the evaporator unit? And is the evaporator directly behind the centre vents?

So you are saying the sequence of events must occur for power to be sent to the compressor ... is there circuits that would stop power being sent if any of the above fail or will the problem usually manifest itself visually or components just blowing out? The compressor is the newest component in the system being replaced with a reconditioned unit almost a year ago.


The problem isn't the compressor, your A/C says there is no power to it.

The pressure sensor will be in the engine bay, the thermostat will be on the evaporator behind the dash.
 
Guys, the whole morning back and forth getting info from anyone I could regarding this issue including MB and the independant a/c mech. The mech says that he is very certain it is the a/c control unit box behind the EC button that is gone. I pressed him for what he has tested and he confirms he looked at every component in the a/c loop. He then also says that the compressor does kick in for a few seconds then cuts out. Based on this I felt a bit suspicious - its either the box is goner or its not, how can it momentarily supply power ... unless the control unit is receiving a reading from another faulty component and supplies power intermittently to that effect. Just my 2 pence logic as I know little of the whole system.

Calling around confirmed the a/c control unit cannot be repaired only replaced. He can source secondhand (GBP230 part n GBP125 labour) and assures the history of the second unit is v good and whilst the part comes with a 6 month warranty he is giving me 12 months (?). I asked for the part number but he said he cannot get it because it is still on the car and cannot see the part number. (??) This made me more sus because the car has been with him for a whole day. I have seen a schematics online somehwere for the whole centre dash assembly - I cannot see any control box there, only a climate control unit which I don't have since mine is manual dials.

I called 2 different MB parts depts here in Sydney - to get some sort of sanity check. Of course MB will only fit new but with my VIN they could not conclude if it came with 'factory aircon'. This brings me to my other investigations I did whilst back in UK - with MB Park Royal we sort of concluded this car didn't came with 'factory aircon', this was gleaned from looking at a) VIN based information, b) the compressor is Diavia (italian) , c) the shape and size of the radiator - apparently mine is half tank smaller.

Regarding Diavia - someone here has confirmed was fitted to many MBs in UK - I really would like to know why and the history of it - anyone?

Anyway, MB won't give out the part numbers for the 2 they see in the system for the Behr a/c control unit (yes managed to get the brand out of them! grr). One unit for 'factory aircon' and the other 'after factory' . Not much help they are even though I said labour needs to be incurred just to open up the dash to get the part number! Anyway indicative price is GBP500 for new unit n if fitted by MB at approx GBP180 labour I get 2 yr warranty.

I'm deciding to pull the car off the mech. I don't have a good feeling. Tho I really am no better :eek: Need to consider options. Esp thinking from reading other posts, pressure switch, cabin temperator sensor ... sigh.

* hot off the press * I wrote but didn't send the above whilst investigating around the net, its now at the end of the day and the mech calls me up and says he talked to MB and now says from the information he got from them, he can build his own relay and power distributor system outside of the control unit to send power to the a/c compressor!

HelloOO! This confirmed by initial suspicions that the control unit might not be the problem yet! Holy cow, I am no mech but I am already getting good at this!

If anyone has any suggestions pls drop me a line - even if it is a link to your own post on the same subject to give me hope and hints!

btw where is this window where I can see bubbles? Is this my drier?
 
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OK, so my car is a W210 E class, but has the manual dual zone aircon (with the two wheels). If the C class is anything like then its very easy to change the control unit, if we are talking about the unit with the AC switch and the wheels in the dashboard. Its literally a 15minute job to get the old one out and put in new (I had to do it to get the part number for mine).
Remove radio, two screws to remove surrounding trim, now accessible with the radio out, unclip unit and pull. Disconnect wires.
A bit trickier going back in because you have to line up the mechanical controls but still not very exciting. Certainly not GBP125 labour.
Its even easy to open the unit and check the PCB for obvious faults/overheating etc.

It may be different in the C class, but I would ask what is actually requiring all the labour.

I point this out as it then becomes a sensible option to try a new/alternative unit and see if it fixes it, if you can find a dealer willing to try this. It does sound like its not the controller though.
 
The post earlier about the ventilation fan is an easy on to check as well. Make sure the fan is going, and responds to the control knob. If the fan is off or below a certain threhold the AC doesn't come on. How the system knows the fan is working is another matter. If it just reads the position of the switch, then this could cause issues. Work the fan control knob vigorously and see if theres a change!
 
DrNick said:
OK, so my car is a W210 E class, but has the manual dual zone aircon (with the two wheels). If the C class is anything like then its very easy to change the control unit, if we are talking about the unit with the AC switch and the wheels in the dashboard. Its literally a 15minute job to get the old one out and put in new (I had to do it to get the part number for mine). Remove radio, two screws to remove surrounding trim, now accessible with the radio out, unclip unit and pull. Disconnect wires.
A bit trickier going back in because you have to line up the mechanical controls but still not very exciting. Certainly not GBP125 labour.
Its even easy to open the unit and check the PCB for obvious faults/overheating etc.

Regarding the removal procedure is this overkill? Also, once you got the part number, is the replacement just a straight swap, does it require any resetting for the new unit to learn/integrate with the rest of the a/c components?

With regards to the cabin temperature sensor up in the roof, I seem to have 1 opening nearer the windscreen (so between cabin lights and windscreen) and the another opening after the cabin lights facing the rear. Which one has the temp sensor?
 
KLee said:
Regarding the removal procedure is this overkill?
Not if you are considering a quick comand install while the dash is in bits ;)
KLee said:
With regards to the cabin temperature sensor up in the roof, I seem to have 1 opening nearer the windscreen (so between cabin lights and windscreen) and the another opening after the cabin lights facing the rear. Which one has the temp sensor?
If you take the dome light out you can look at the thing.
http://mbclub.co.uk/forums/showthread.php?t=306

I think it's the black box on the pic showing all the sockets, you can make out some little blades inside.
 
Looks very similar to the E class, so yes it probably is overkill. Skip to the bit about removing the radio and start from there, and stop at the bit when the AC control unit is in your hand.
 

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