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a/c problems

zaruba on Sun October 07, 2007 6:41 PM User is offline

Year: 2000
Make: Mercedes
Model: c280
Engine Size: 2.6L
Refrigerant Type: R134a
Ambient Temp: 90
Pressure Low: see post
Pressure High: see post
Country of Origin: United States

Hi All,

The a/c in my 2000 c280 (with digital climate control) has not worked for about three months now (it stopped working after the car was not used at all for two months). Finally I got myself to take some measurements. As the system does not have a high side port, I took the high side measurements from putting the climate control into diagnostic mode. All measurements were taken at a 90F ambient.
Static pressure: 107psi
Pressures at 900rpm: 98psi-low and 114 psi-high
Pressures at 2000rpm: 82psi-low and 131 psi-high

I find these values strange. To me they do not look as if the expansion valve was stuck in or out. Do these compressors have an internal pressure relief valve that tends to get stuck?

Any ideas?

Thanks,

Greg

-------------------------
Regards,

Greg

(609 cert)

mk378 on Sun October 07, 2007 9:12 PM User is offline

Is it a variable compressor? If so, seems to be stuck on minimum compression.

bohica2xo on Sun October 07, 2007 9:48 PM User is offline

Pretty much an empty system. The TXV is wide open, and the compressor is swishing gas around the system...

The 2000 models still had evaporator temp sensors & TXV's as I recall.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

2POINTautO on Mon October 08, 2007 6:18 AM User is offlineView users profile

107 PSI static says it pretty full.

-------------------------
Give all the dirty details
and dont forget the LO & HI pressures
Year, Make & Model would be nice too

bohica2xo on Mon October 08, 2007 12:42 PM User is offline

NOPE

107 psi static says it has somewhere between one ounce and 2 pounds of refrigerant in the system. With a 90f ambient, the underhood temp will be somewhat higher, so the static should be higher as well - the system is very low on charge.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

zaruba on Mon October 08, 2007 9:04 PM User is offline

The underhood was 90F (the hood was open and the car was running maybe for a couple of minutes). I understand the swishing around but shouldn't the compressor still build up pressure. On other cars if I had an almost empty system, then the compressor would cycle due to an underpressure on the low side. Could you please elaborate more?

Thanks

-------------------------
Regards,

Greg

(609 cert)

iceman2555 on Mon October 08, 2007 10:44 PM User is offlineView users profile

" Ya kant pump what ya aint got".......or so an aged...no not TRB...tech once told me.
First insure that the system is fully charged...this is indeed the first step in a A/C system diagnosis...know how much refrigerant is in the system...
If once the system is fully charged and the pressures remain equal......or near equal...perhaps..it is time for a replacement compressor. Not quite sure...but it seems this is a Denso Scroll type compressor. If this is true....the control valve issue is not relevant.
As bohica states, the static pressures stated are actually meaningless.....static pressures are not an indication of system charge rates......evac and recharge....retest and post results....


-------------------------
The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.
Thomas Jefferson

zaruba on Tue October 09, 2007 1:49 AM User is offline

The car has a thermo-mechanical expansion valve (e.g., the one displayed here:
http://cgi.ebay.com/ebaymotors/Mercedes-A-C-AC-expansion-valve-C230-C280-CLK-E320-E420_W0QQitemZ8013497516QQihZ012QQcategoryZ46094QQrdZ1QQssPageNameZWD1VQQcmdZViewItem
)

So, you are implying that if the valve is wide open and the system is very low on freon (couple of oz), then the compressor will not be able to create suction and pressure as the gas is going to go around way too fast, right? It sounds plausible but strange. Have you guys seen this happen before?

How can one tell just by looking at it if the compressor is a variable compressor? It looks just like:
http://cgi.ebay.com/ebaymotors/1996-2000-Mercedes-Benz-C280-A-C-AC-compressor_W0QQitemZ160103081422QQihZ006QQcategoryZ33543QQrdZ1QQssPageNameZWD1VQQcmdZViewItem


Also, one of you senior guys should work on this wikipedia definition for it not to say stupid things:
http://en.wikipedia.org/wiki/Thermal_expansion_valve



-------------------------
Regards,

Greg

(609 cert)

Edited: Tue October 09, 2007 at 2:03 AM by zaruba

bohica2xo on Tue October 09, 2007 1:59 AM User is offline

I have no control over what "stupid things" wikipedia may say - I do not edit that site.

As Iceman says, such the charge out & weigh it - the static pressure is not an indication of charge.

A TXV does not operate like a fixed orifice. It will open up far enough to pass a fair quantity of gas. With a sub-critical charge in the system, you get all sorts of odd pressures in a TXV system.

Until you can verify the correct charge, you will continue to chase your tail. Evacuate & weigh the removed charge.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

zaruba on Tue October 09, 2007 2:06 AM User is offline

All I am saying is that the wikipedia page on txv is very misleading. Someone who knows enough about it could edit it. Everyone can edit...

I'll evacuate and measure.

Thanks!


-------------------------
Regards,

Greg

(609 cert)

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