Thought my a/c was fixed but is not. This is the problem: no cold air, even though clutch cycles on. Checked low side pressure and it was at 80psi with car off. Brought it down to about 45psi with car off. Once I turn on the car and a/c I get 0. No pressure at all on the outside though the clutch still cycles on when I turn a/c on.

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1990 190E 3.0L

Low side pressure should be the same as high side with car off.

80-90 psi at rest sounds like it's got a charge.

Where did you get the 45 psi?
If the car is off, that's not good.

Low side should pull down to 20-30 psi when compressor is running, high side should be 200 psi +.

Pressures depend upon outside temp and humidity, plus airflow thru the condenser.

A good indicator of the system working is when the return to the compressor line is cold.

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles

Any idea why the drop in the low side? I just checked the pressure again with the engine off and the low side is 80psi, but when I turn on the car and turn the a/c on the compressor turns on but the low side drops to 0 and I have no cold air.

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1990 190E 3.0L

When the AC is on and the compressor is cycling, does the aux fan come on? If not, you may have too low of a freon charge. Don't worry about what the pressures are when the car is off, that's not when your AC works. Either way, should be around 90-100 psi off as pressures are equalizing between low and high side. As mentioned before, Low pressure side should be in between 30-40 psi with the AC compressor on and high side should be near 180-220 or thereabouts.

How much freon did you put in the system?

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Allen Kroliczek
Oak Grove Autosport | Oak Grove Autosport
01 G500, 82 300TD, quite a few more.....

Yes, the fans are on. I can turn low speed and hi speed on with a switch from inside car as well. I added 2.2 pounds of Freon. R134a.

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1990 190E 3.0L

Static pressure is pointless to read...

Force the ac compressor to run if you are sure the refrigerant has not leaked out then check pressures from there.

Either way it sounds like you have a major leak. Start by replacing all o rings.

In your climate I'd expect about 40psi low and 240 high.

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2016 Monsoon Gray Audi Allroad - 21k
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2003 Gray Mercedes ML350 - 122k

Disagree.

Static pressure is an indicator of system status.
If you have 80 psi, and you still have 80 psi a week later, that's a sign the system is intact.

Again, depending upon ambient conditions, 80 psi could be a bit low for a static pressure, but you're close enough to get the system to engage.

I getting the impression the OP doesn't have a manifold gage set, since the only info we have is for the low side. We really need to know the high side pressure to tell what is going on.

If the low side pressure is going to 0 when the compressor engages, that tells me that the system is still low on Freon.

The limited info begs me ask the following questions:

Was the system open for any length of time before the recharge?

Was the system evacuated and kept under vacuum for at least one hour before recharge?

Was the system able to hold the vacuum reading overnight?

Jim

__________________
14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles

I replaced all the o-rings about a week ago. I also vacuum tested the system and had no leaks.I would think that with with the same pressure in the system that that would also indicate no leaks which is why I checked the static pressure because that at least shows me the system does have some pressure in it. When you say "force" the compressor to turn on, what do you mean? My compressor turns on when I press the a/c button. I can see the clutch turning. Also I can hear the faint noise the compressor makes from inside the cabin through the vents.

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1990 190E 3.0L

When you say vacuum tested it does not answer the questions I asked in post #7.

Does the compressor come on, then cycle on and off?

It's possible you have sufficient Freon to get the compressor to turn on, but not enough to blow cold.

The characteristics of Freon 134a cause the cooling to act more like a step function vice a sinusoidal curve. Freon 134a will cool adequately if a sufficient amount is present, but as soon as you drop below the "sufficient" amount the vent temps drops off to ambient. R12, by comparison, will have a much more gradual temp rise as the Freon amount drops.

Again, if your return line is not cold (and you have good hi side pressure), then you do not have enough R134a in the system.

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles

Sorry, I missed that post. Hi side was just at 200 maybe a bit above. Vacuum tested for 2 hours and had no leak with manifold gauge. Line does get somewhat cold. When I recharged the system it was frosting.

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1990 190E 3.0L

If you are getting a cold return (low pressure) line, then you should be getting a cool discharge from your vent.

Check your heater hoses and see if the monovalve is shutting hot water flow off to the heater core.

Jim

__________________
14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles

No hot water is getting to the heater core. Heater valve and this ball valve is in place instead of aux water pump. I know for a fact that one or both of these devices are working because I am unable to get heat also. This is why this is such a mystery to me. I know there is no leak, the system was vacuumed and tested for 2 hours (1 hour vacuum , tested for 2 hours) so 3 hours total. I'm taking it to a mechanic tomorrow. Hopefully it's just low on refrigerant. Unless anyone has anymore ideas.

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1990 190E 3.0L

Odd indeed.

The ball valve is pretty good evidence that the water flow is stopped.

The cold return line is a pretty reliable tell that the freon level is high enough you should be seeing some cool air at the vents.

You may be very close to enough freon but not quite there.

Remember, R134a acts like a step function as you bleed in the freon... nothing, nothing, nothing, then bam, cool air......

Jim

__________________
14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles

So I took the car to my mechanic and he hooked up the manifold gauges and said that the compressor is not compressing. He said I need a new one. This would make since as I have tried and tested pretty much everything else. Compressor was replaced 4/17/08. I guess 7 years is a pretty good run for a compressor, what do you think?

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1990 190E 3.0L

From your post 10, you are seeing 200 PSI on the high while running, this is evidence that the compressor is working. Are you still getting 0 PSI on the low side while running?

Is so, the expansion valve is stuck shut and not allowing freon to enter the evap coil.

Some compressors ( my 97 C280 ) are variable displacement and volume is regulated by looking at low / high side pressure. ( maybe just one pressure I don't recall the details )