Intermittent A/C problem on W203

[KLP 92]

I'm having a problem on a late 2002 C270 CDI. The A/C has been playing up for a few weeks and will work intermittently.

So far i've had the system regassed twice, first to top it up and second time to ensure they correct amount of gas was in there. The system is losing any gas, after 2 weeks the same amount of gas was still in the system.

I checked on MB Star diag to see if there are any falut codes. The onky one on there was low refrigerant pressure, which wasn't a current fault but a stored one. Deleted the faults codes and the sytem was fine for a day.

The A/c worked all day yesterday but stopped working again today in the morning and is working fine now. The worst possible type of fault when it comes to diagnosing!

Now i've phoned a few chaps for advice, i've been told it could be a sensor on the side of the heater box behind the dash, resluting in a dash out job. Is this a possibility?

I've rung MB today and they can't find any pressure sensors for the A/c, only a temperature sensor located at the bottom left of the condensor.

Any help/suggestions will be more than welcome.

 

[jaymanek]

sounds a nightmare, ill be watching this space!

I cant think of anything plausible other than loose/rusty connections etc... you need to figure out if the pumps etc are running when its not working...

 

[KLP 92]

sounds a nightmare, ill be watching this space!

I cant think of anything plausible other than loose/rusty connections etc... you need to figure out if the pumps etc are running when its not working...

I'm hoping its something simple. The car is covered under an independant warranty but for electrical/AC there is a stipulation saying parts only. So i don't mind if the parts expensive ie new compressor but as long as the labour is low, ie not taking the dash out!

A good warranty company though, they shelled out for a Recon gearbox direct from MB! Don't build these modern MBs like the old ones.

 

[Will]

Don't know for certain about the W203, but on other models (eg W202), there is a pressure sensor fitted to the receiver/drier. Surely for the fault code 'low refrigerant pressure' to be generated, the sensor that monitors this (I'm assuming this one?) will have detected a drop in pressure. If the system pressure is fine and there are no other issues then the sensor or associated wiring sounds faulty?

That would be my course of action, check and/or swap this sensor

Will

 

[BlackC55]

What pressure is it when its all runing ok? It should be 12-16 bar. Also look at the live data on the star when its faulting.
Look at the evaporator temp sensor on the live data. It should be air con temp 7-12 degrees roughly if its working ok. If its not you should get a strange reading.

Have you thought about a very slightly low referigerant level?

 

[Andy W]

In theory under charged or overcharged should bring up a fault code probably stored but dont dismiss a fault such as moisture in the system, can be a real pig to diagnose and to correct and is usually caused by the pipe work on the system been left open for an amount of time, poor working practices or reclaimed gas. Also a blocked condenser can trip the system on high pressure but once again a fault code should be evident.

 

[KLP 92]

Don't know for certain about the W203, but on other models (eg W202), there is a pressure sensor fitted to the receiver/drier. Surely for the fault code 'low refrigerant pressure' to be generated, the sensor that monitors this (I'm assuming this one?) will have detected a drop in pressure. If the system pressure is fine and there are no other issues then the sensor or associated wiring sounds faulty?

That would be my course of action, check and/or swap this sensor

Will

Can't seem to find this sensor though! Spoke to MB and they only seem to think there is a temperature sensor on the condensor, and thats it!

Spoke to an AC specialist and he seemed to think the sensor is on the heater box behind the dash, and that the dash had to be removed to change the sensor

 

[KLP 92]

What pressure is it when its all runing ok? It should be 12-16 bar. Also look at the live data on the star when its faulting.
Look at the evaporator temp sensor on the live data. It should be air con temp 7-12 degrees roughly if its working ok. If its not you should get a strange reading.

Have you thought about a very slightly low referigerant level?

I'll put it back on diagnostics and see what it says. The level is dead on 750g, checked twice and is whats written on the bulkhead.

Will check the readings and get back to you. The only prob is that the AC is working now!

 

[BlackC55]

Can't seem to find this sensor though! Spoke to MB and they only seem to think there is a temperature sensor on the condensor, and thats it!

Spoke to an AC specialist and he seemed to think the sensor is on the heater box behind the dash, and that the dash had to be removed to change the sensor

The dash does not have to be removed from memory. It is awkward but do-able. Look up by the parking brake lever; its up there somewhere. It is in the evaporator.

 

[miro]

I'm having a problem on a late 2002 C270 CDI. The A/C has been playing up for a few weeks and will work intermittently.

So far i've had the system regassed twice, first to top it up and second time to ensure they correct amount of gas was in there. The system is losing any gas, after 2 weeks the same amount of gas was still in the system.

I checked on MB Star diag to see if there are any falut codes. The onky one on there was low refrigerant pressure, which wasn't a current fault but a stored one. Deleted the faults codes and the sytem was fine for a day.

The A/c worked all day yesterday but stopped working again today in the morning and is working fine now. The worst possible type of fault when it comes to diagnosing!

Now i've phoned a few chaps for advice, i've been told it could be a sensor on the side of the heater box behind the dash, resluting in a dash out job. Is this a possibility?

I've rung MB today and they can't find any pressure sensors for the A/c, only a temperature sensor located at the bottom left of the condensor.

Any help/suggestions will be more than welcome.

The TX valve is under the wiper assembly (IIRC). When that plays up it refuses to send cold gas into the heater box and this gives the impression that the cooling is not working.

It may also be a compressor fault.

 

[jaymanek]

Nav, what warranty company is it?

 

[Andy W]

The TX valve is under the wiper assembly (IIRC). When that plays up it refuses to send cold gas into the heater box and this gives the impression that the cooling is not working.

It may also be a compressor fault.

Moisture will cause exactly the same but in theory it will flag up a low pressure fault because the expansion valve will close down and not allow liquid refrigerant to pass through the valve.

 

[KLP 92]

Had the car put back onto diagnostics and its reading a fault code stating: Refrigerant pressure low.

The car has enough gas in there, its leak free so i'm presuming sensor or valve issue.

Does anyone know where the pressure sensor is located?

 

[KLP 92]

Nav, what warranty company is it?

Not sure, the chaps i purchased the car from sorted it out for me. Had the car collected and returned to me complete with invoice/paperwork for work done.

 

[Andy W]

With dirt or moisture, moisture in particular, it travels around the system with the gas, because of the temperature drop across the expansion valve any moisture present will freeze at this point, once frozen liquid refrigerant can not pass through the valve so the compressor still trying to pull vapour runs out of vapour and trips out on low pressure.

When the valve gets warm the moisture melts and allows flow of the liquid refrigerant again which now makes the pressure switch as they are make on rise, this moisture will keep recirculating around the system and every now and then will freeze in the expansion valve again.

The only way you will catch it doing this is by fitting a low side gauge and running and observing the system, as the valve blocks, the suction pressure will drop to the low pressure switch cut off point and obviously cut off.

Now the remedy is quite simple but has to be done right.


I dont suppose you have had any parts fitted in the last few months either new or secondhand and the gas it has been charged with is new "virgin" gas and NOT reclaimed or old gas supposedly cleaned, there is some rubbish gas about.

By your symptons I am convinced it is moisture.

 

[KLP 92]

With dirt or moisture, moisture in particular, it travels around the system with the gas, because of the temperature drop across the expansion valve any moisture present will freeze at this point, once frozen liquid refrigerant can not pass through the valve so the compressor still trying to pull vapour runs out of vapour and trips out on low pressure.

When the valve gets warm the moisture melts and allows flow of the liquid refrigerant again which now makes the pressure switch as they are make on rise, this moisture will keep recirculating around the system and every now and then will freeze in the expansion valve again.

The only way you will catch it doing this is by fitting a low side gauge and running and observing the system, as the valve blocks, the suction pressure will drop to the low pressure switch cut off point and obviously cut off.

Now the remedy is quite simple but has to be done right.


I dont suppose you have had any parts fitted in the last few months either new or secondhand and the gas it has been charged with is new "virgin" gas and NOT reclaimed or old gas supposedly cleaned, there is some rubbish gas about.

By your symptons I am convinced it is moisture.

When i start the car in the mornigns it doesn't work most times. But during the day it normally switches itself back mon and then will opeartate fine for the rest of the day.

I did have the system regassed a few weeks ago but after then i had the gas changed again as i had a small leak on the bolt at the top of the condensor.

Well they did say they changed it, and charged it. Do you reckon i need to have it emptied and regassed?

 

[Andy W]

You must eliminate the gas charge problem first, if it was me I would be doing it myself anyway but I would remove the gas, replace the filter drier, purge the system with oxygen free nitrogen, dehydrate with a vacuum pump down to 1 mbar, break vacuum with OFN three times, after final vacuum recharge with new gas and accurate scales, I would then know that there was no moisture or contamination in the system as purging through with nitrogen would of proved this and I would know that the system was completely dehydrated, if the low pressure switch was a cheap item I would probably change that as well to save going through all of the above again.

Just a quick point, I would never use one of these new fangled leak sealers that garages seem to be going crazy for as a cheap fix, the stuff is like sludge and I predicted a while ago it is going to cause some damage, particular to engineers reclaim machines and vac pumps.

Also, I am sorry to keep going on, these charging stations that the motor trade use are only as good as the bloke who is using it and the reclaim cylinder on board which is used for every ones car can be full of all kinds of crap, with my reclaim cylinders, when I empty them I leave them upside down in the workshop and the gunge which comes out of them, I have seen garages reclaim gas, carry out a repair and put the same gas back in + any contaminents already in the cylinder, scary stuff.

Another thing which goes wrong is when the garage adds 25ml of oil every time the system is recharged, over 3 or 4 years you can have so much oil in the system been pumped around it does not travel as a mist as it is supposed to, it is thick and starts to fill the filter drier and expansion valve gauze, there are many variables which can cause a system to fail which only a refrigeration engineer could possibly understand.

 

[KLP 92]

Thanks mate for the advice. Will try to have that all done tomorrow!

Coming to think of it the problem did start after the regassing, that chap is going to get a size 12 boot up his backside if it is the cause of my grief!

Is the filter the one in the condensor? There's a replacable one in there according to MB.

 

[Andy W]

The filter is usually the receiver / drier, the item with a sight glass on top of it, as I said, I would reclaim the gas, attach gauges disconnect suction pipe from compressor depending on accessabilty or at a point where just the suction can be disconnected, put OFN in the system, it should come straight out through the pipes you now have open, the pipe closest to the compressor should remain blocked off or else it will drive all the oil out of the compressor, observe the open pipe for what is coming out, reattach pipe and repeat with the liquid line, (small pipe from the receiver drier) doing exactly the same, if you are getting gunge coming out or lots of oil then that is your problem. Replace the filter drier as this is a sponge for moisture and when moisture is trapped in it when it becomes full it is released back into the system.

You may have to remove the compressor to drain it successfully, flush the entire system with a circulating pump before adding oil in the compressor, dehydrating the system and recharging with virgin gas.

When I started in refrigeration 29 years ago the first thing I was told was dirt and moisture is the biggest enemy in refrigeration and it is usually incompetence or negligence that causes this. I have had to scrap refrigeration systems worth 10's of thousand pounds due to moisture contamination, at least on a small system like a car it is easier to remove but can be time consuming.

 

[Andy W]

Attached picture of the ML air con system just to give you an idea, unfortunately I dont have a diagram for a W203, I have W202 but not 203, the filter drier / receiver on this car is item 15.

Sorry it turned out so big, if a mod wants to resize or thumbnail it please do, I am hopeless at computers.