Vehicle described below in signature. Odometer reads 133K. Original Nippondenso R-12 compressor and R-12 refrigerant.

Until recently, have seen vent temps in the 42F-45F range in 95F - 100F ambient temps. Cooling was good.

Now seeing 60F-65F vent temps. Sight glass is cloudy. Suspected low charge. Hooked up gauges, but readings do not reflect a low charge, at least according to my interpretation of information presented in tech manual mentioned below.

Low Side = 40
High Side = 200

Gauge readings taken inside garage - ambient temp in garage = 95F. Fan set to high - clutch engages - compressor remains on - not cycling off.

I'm referencing Mastercool Automotive Air Conditioning Basic Service Training Manual. I see situations close to, but not quite in line with mine. My current suspicion is the expansion valve is stuck - either closed or open. Not sure at this point.

Knowledgeable opinions welcome.

Thanks for your time.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Those pressures aren't particularly bad. Forty psi should equate to about 40 degrees maybe a little higher. What was the rpm at those pressures?

There are a number of non refrigeration reasons for poor duct temps with proper evap temps. Open recirc doors, heater valve leakings hot water into the heater, and dirty evap are some that come to mind.

If those pressures were at 2000rpm with a good fan on the condenser, then the low side is too high as it would probably be 50-60 at idle. It should go down to around thirty psi at 2000rpms. In a virgin system I'd blame the compressor for not having the horsepower. It also could be an expansion valve stuck open.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Steve:

Readings were taken with car idling in PARK. Rpms were around 600-700.

I'll hook up the gauges again and see if I can enlist an accelerator pedal depressor.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Hooked up gauges again. Current temp. in garage is 90F.

With rpms @ 2000, low side quickly drops to 30; high side gradually climbs to 255. If you believe the theory that high side should be 2.5 x ambient or 2.5 x 90F, I should see no more than 225 on the high side, but am 30 psi over that.

I'll look into a high side restriction.

Have been losing center vent air when accelerating hard - economy gauge pegs. After the needle in the economy gauges falls back down to what would be a good level from a fuel efficiency perspective, I regain center vent air.
That was until the ride home from work Friday night. Now my center vent air is falling off when on a/c, but returns when I depress the e/c button to the right of the a/c activation button.

I may have a combination of plugged/open expansion valve and blend door issues/defective vacuum pod(s).

Add this to a growing list of things to fix.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

Sounds to me like a blend door issue or heater shutoff valve (monovalve) stuck open. I'l bet your low side lines are cold as they should be.

John Roncallo

Just got back from a long highway drive. Did a lot of experimenting with a/c and e/c controls + the recirculation switch.

As reported earlier in this thread, I lose center vent air on acceleration. What I'm also now seeing is that I can reclaim lost center vent air after the reading on the economy gauge more or less bottoms out by switching off the a/c; then swicthing on the e/c. After a few secs., I switch the a/c back on and center vent air continues to pour out until the economy gauge needle goes to the high side.

I have a vacuum pod issue which controls the blend doors.

Thank you all for your interest.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

This will sound crazy, but kinda sounds like you have a vacuum leak in the climate control system. There should be a check valve to help maintain vacuum when engine vacuum goes to zero. Most cars have a default mode of heat to windshield if system failure (think the feds require it). If you are losing vacuum, climate control doors would get wierd. Temp could get wierd too. I'd hook up a vacuum gauge after the check valve and see what kinds of readings it gives you in various modes.

Bob:

Your idea is not at all crazy. There's likely a vacuum leak in the climate system. It's my understanding the pods I mentioned(there are several under the dash) provide vacuum in several places. There's one in particular that controls the center venters. I'll likely start there.

My biggest problem is time or I should say, the lack of it. I'm not trying to beat a tech out of his wage, I just enjoy resolving most of my own automotive problems.

Onward.

__________________
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"