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Please help interpreting sensor readings

MB Man on Thu April 23, 2009 9:08 AM User is offlineView users profile

Year: 2001
Make: Mercedes Benz
Model: E320
Engine Size: 3.2L
Refrigerant Type: R134A
Ambient Temp: 80
Pressure Low: 89
Pressure High: 85
Country of Origin: United States

I'm hoping someone can provide some insight into the problem. My AC quit working and these were the initial symptoms:
a) EC light stayed ON (indicates either low refrigerant, defective pressure sensor, defective temperature sensor)
b) Engine OFF: Low side pressure read 87 PSI
High side pressure read 85 PSI
c) Engine ON: Low side pressure read 89 PSI
High side pressure read 85 PSI

My vehicle has the capability of reading sensors and error codes through the AC display in test mode and these are the Sensor Readings:
1 = 100 (inside Temperature in F)
2 = 80 (Outside Temperature in F)
3 = 93 (Left heater core temperature in F)
4 = 9 (Right heater core temperature in F)???
5 = 94 (Evaporator temperature in F)
6 = 200 (Engine coolant temperature in F)
7 = 05 (Refrigerant pressure in Bar = 72.55 P.S.I)(low refrigerant pressure confirmed by gauge readings)

Error Codes:
Eb1234 (Sun sensor malfunction; not relevant to my problem)
Eb1235 (Emission sensor malfunction)
Eb1241 (Refrigerant Low)

Proceeded to charge AC system with R134A refrigerant with dye to detect leak source. I added 26oz which was probably not enough but it was all I had. I added 16 to the value of P2 to shut the EC light OFF (this increases the pressure sensor (connected to the receiver/dryer) readings to allow the compressor to engage; the compressor has it's own pressure sensor and will not be damaged by making this adjustment). Ran engine and obtained following readings:

Engine ON: Low side pressure read 38 PSI
High side pressure read 225 PSI

Sensor Readings:
1 = 76 (inside Temperature in F)(cabin being cooled down)
2 = 82 (Outside Temperature in F)
3 = 44 (Left heater core temperature in F)
4 = 42 (Right heater core temperature in F)
5 = 55 (Evaporator temperature in F)
6 = 195 (Engine coolant temperature in F)
7 = 12 (Refrigerant pressure in Bar = 174.14 PSI)((is this enough refrigerant pressure? I doubt it....)

Error Codes:


Drove around a while and noticed AC was cooling but not as it should.
Next day, AC quit working and sensor # 7 was down to 5 Bar again.

There is an obvious leak in the system which I should be able to find by tracing the dye. So here are the questions:

1) Since the refrigerant leaked out, does this mean the compressor oil needs to be refilled or does it stay in the system?
2) Do the readings I obtained make sense? There should be an obvious refrigerant leak, correct?
4) Any other suggestions/advice?

I do own a vaccum pump, gauges and the UV light/lenses to trace the dye.


Sergio M.

GM Tech on Thu April 23, 2009 9:15 AM User is offline

Number one failure mode for mobile a./c systems is loss of refrigerant due to a you are doing fine in your analysis..

Yes- find your leak- don't forget- the tracer dye works best with black light (under low light conditions- I prefer night time with lights off) and the UV glasses...Leak will jump out at you--as a bright flourescent green. Also Caddy engines with the premium engine oils (yours may be similar)- the oil has a mauve green look to it with the black light-- don't mistake this for the bright green dye you need to find..

Refrigerant oil will be what you see with the dye in it- so add back a couple of ounces once the leak is fixed should suffice.

I would concentrate on compressor body seals and shaft seals-- as well as condenser---first..

The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Edited: Thu April 23, 2009 at 9:20 AM by GM Tech

NickD on Thu April 23, 2009 9:41 AM User is offline

My 92 DeVille has that same feature, found easier to go out in the morning with the manual after the car sat in the garage all night and check every temperature sensor, they should all read the same within a degree C or two.

With complicated electronic systems, found it easiest to first test the mechanical condition of the AC system, first test out the clutch coil, normally 3-4 amps, and this current should decrease as the coil warms up, also check the pull in and drop out voltage and measure the gap. With it unplugged from the vehicle operate it with a remote starter switch and gauges installed, yep, you want to watch the high side in particular so you don't blow your system apart. Then do whatever is necessary to get the correct readings. But if both gauges read zero, don't even get that far. But if the gauges read low, vehicle is ten years old and only lost 10 ounces, would top it off, but still use an electronic leak detector and a close visual for oil leaks. I don't like finding oil leaks, never know how much was lost. And I know I have my work cut out for me, no dip sticks, no oil sight glasses to look at.

Some tech manuals state you are just looking for one problem, that will be the day, but with the mechanics out of the way, time to plug the vehicle connector in to learn if you have more problems or not, if you do, you already know you have a sound mechanical system, time to start poking into the electronics.

MB Man on Thu April 23, 2009 11:18 AM User is offlineView users profile

Thank you both for the suggestions provided. I will embark on finding the leak source.
I've ordered a receive/dryer, and both temperature and pressure switches. Last time I had a similar problem with my other vehicle, a 1995 E320, the pressure switch had a crack. It's cheap insurance to replace them once I'm tearing up the system anyway.
If the AC compressor is leaking from the front, do I need to replace it or take it apart and replace the seals?
Thanks again.

Sergio M.

Matt L on Sat April 25, 2009 2:29 PM User is offline

I haven't heard of a problem with body leaks in these compressors. M-B gives quantities of oil to add due to slow refrigerant loss (also for fast loss), but I do not recall the quantity. I'll have a look at the manual when I get a chance; you have some time before you need that step anyway.

Also the electronics on this system seems to be working properly. You really did lose refrigerant; if you hadn't, it would be time to suspect sensors or the computer (pushbutton unit). However, I would expect the compressor to start with reading #7 at 5 bar. It's hard to say whether your pressure running is sufficient, but 225 PSI at 80F should be fine, if not a bit overfull. However, note that the reading at #7 while you are running is significantly below the actual pressure recorded at the compressor inlet. This may indicate a restriction in the condenser, or more likely, the dryer. If you do have a restriction there, the head pressure can spike and the computer won't know it, causing the compressor to vent.

One thing to check is the line between the condenser and the dryer. These seem to be problematic for some 210 owners. Also the condenser itself. They are notorious for getting pinhole leaks as they age. Fortunately the condenser is not very expensive and is easy to replace. Not so of the evaporator; it is both fairly expensive and extremely hard to replace, and the 210 platform is not old enough yet to know if we will have the evaporator problems plaguing the 124 and 140 platforms, where a very high percentage (1/3, perhaps) need an evaporator during their lifetime. $3000+ at the dealer.

There is no pressure switch on your car, of course. It is a three-wire transducer (so don't attempt to short any pins). I've also never heard of problems with this. I reused my pressure and temperature sending units when I replaced my dryer last year.

Does your car have an electric fan in front of the condenser? I am not sure what models had those; not all do. If you do, you also need to ensure that it is working properly.

Matt L on Sat April 25, 2009 2:31 PM User is offline

One more thing. If there is a restriction in the condenser or dryer (which are between the pressure sending unit and the compressor), the reading at #7 can actually go down when you activate the compressor, as liquid refrigerant loads up in the high side. Once you have some loss (due to venting), it can drop low enough during operation to shut down the compressor until you reset the error code.

MB Man on Sat April 25, 2009 10:45 PM User is offlineView users profile

Well, last night I traced the dye to the drain hose above the transmission pan. From what I've read, it's the dreaded evaporator leak. Funny how my 14 year old W124 (1995 E320) with 223K miles, still has the original evaporator and the newer W210 needs a new one....possibly. I could not find a single drop of dye anywhere else. I carefully looked everywhere hoping to find another source for the leak but I'm afraid it's the evap.
Today I pulled a vaccum using the low and high pressure ports and a set of R134a gauges. It held past 29 in/Hg for at least two hours. Does the "traditional" vaccum test that I performed NOT take the evaporator into account? Does it stop at the expansion valve (TVX)?

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