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  #1  
Old 09-22-2009, 06:20 PM
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A/C recharge

I'm going to be recharging the A/C in my '89 300CE. Essentially the A/C works well when the car is first started and the interior is cool ie. car has not been parked in the sun. However, once I hit traffic in the city it starts to get hot and not cool at all.

At highway speeds the performance is notably better. I'm thinking that I'm just a bit low on refrigerant. Would it be better to evacuate and test the system for leaks? Should I consider replacing the receiver dryer and some seals and O-rings? I don't want to have to do the dreaded evaporator replacement so I'm leaning towards having the dryer replaced and the system leak tested as opposed to just topping the system up a bit with R12.

Is there a chart somewhere that I can use to see what pressures and performance (vent temperatures) I'm getting depending on ambient temperature? Thanks

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  #2  
Old 09-22-2009, 07:09 PM
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How long since it was last filled? If it has been several years, your leak may be too small to find, in which case there is no reason to open the system or replace the dryer.

The chart should be in the service manual. I don't have the one for your car though. You're better off recovering and recharging by weight in any case.
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  #3  
Old 09-22-2009, 08:05 PM
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I don't know when it was last recharged. Given the low mileage, I'm thinking it has never been recharged. There are no stickers anywhere in the engine compartment to indicate that the A/C has ever been serviced or opened and my service records don't show any servicing.

Last edited by mbzman; 09-22-2009 at 08:29 PM.
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  #4  
Old 09-23-2009, 09:42 AM
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I would look and make sure your condenser fan is coming on. If it isn't, that would explain why it only cools when you're moving. Assuming you have a low-pressure safety cut-out in the system, cooling would actually get worse the faster the compressor spins, as the pressures & temps would get lower.

Yes, it's called a pressure-temperature chart or PT chart. You should be able to google one.

If it's currently holding gas, you should just add more & not crack the system open.
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  #5  
Old 09-23-2009, 03:21 PM
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What's happening to your engine coolant temp gauge when the A/C stops blowing cold in city traffic? Does it get dramatically hot or only moderately warmer? Both your clutch fan and your auxiliary fan must be operating correctly to keep engine temps under control, and that helps make the A/C more effective, too. That's not to say that you may not need to add refrigerant, only that proper airflow is needed for proper A/C function.
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  #6  
Old 09-24-2009, 07:08 AM
LarryBible
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The aux fan will not play into this issue. It is there to bring down excessive high side pressure which is obviously not a problem when it is not cooling well in traffic. If it is an air flow problem leading to this issue, then check the engine driven fan clutch.

You should NOT take corrective action until you have completed a proper diagnosis. That is Troubleshooting 101. You need to connect a set of manifold gauges, turn on the a/c wide open, block the throttle to 1500 to 2000 RPM, place a high flow fan in front of the vehicle, let the pressures stabilize for five minutes. You then need to write down the ambient temperature, low side pressure and high side pressure. At that point either post this information here or look up a chart and analyze.

Look before you leap.
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  #7  
Old 09-24-2009, 09:45 AM
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Quote:
Originally Posted by LarryBible View Post
The aux fan will not play into this issue. It is there to bring down excessive high side pressure which is obviously not a problem when it is not cooling well in traffic. If it is an air flow problem leading to this issue, then check the engine driven fan clutch.



Really? That's what that fan is for - the condenser on the A/C system; you know, for when you're car is idling or stuck in traffic. In fact, on most vehicles it is indeed called the "condenser fan".

Excessive high side pressure "obviously" IS a problem if the vehicle does not cool well in traffic or idling, because if the coolant temperature is OK, the fan clutch is obviously working - and since it's controlled by engine temp ONLY, it will not pull enough air to cool both the coolant and the refrigerant.

Since the car does not overheat in traffic and the A/C works while moving, then it probably isn't an air flow/obstruction problem or a fan clutch related issue. It's a low-speed/no-speed idling air-flow issue that a bad condenser fan would absolutely cause.

Excessive high side pressure trips the high side cut-out, killing power to the compressor at low/no speeds; no compressor, no cooling. This is A/C 101.

Just google "the refrigerant cycle".
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Last edited by dhjenkins; 09-24-2009 at 10:00 AM.
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  #8  
Old 09-24-2009, 02:31 PM
LarryBible
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The aux fan cuts on in the event of EXCESSIVE high side pressure. Excessive high side pressure is not a good thing, but even with it, you will still have cooling IF the engine driven fan is doing IT's job.

Larry Bible
ASE Master
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  #9  
Old 09-24-2009, 03:02 PM
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You're confusing the pressure switch W0133-1628267 with the temperature switch W0133-1626582. The first is for low or excessive pressure (30psi low, 435psi high), which cuts out the compressor, while the other activates at normal operating pressure to turn the aux fan on. It does not wait until the pressure is "excessive" to activate; that would result in a very short compressor life and overall decreased efficiency. The lower the condenser temp/pressure, the cooler the compressor and the lower/cooler the evaporator temperatures (to a point).
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Last edited by dhjenkins; 09-24-2009 at 03:30 PM.
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  #10  
Old 09-24-2009, 03:13 PM
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Hi guys,,,BTW...how do I wire the ac condensor fan to come on when the ac is on?...it appears that my ac-fans only turns on when engine coolant reach upper limits?...I d like to modify the fan so it comes on when ac is turned on full time...also where is the ac condensor fan thermo-switch located..??

my car is a 1991 300SE (w126)
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  #11  
Old 09-24-2009, 03:20 PM
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At least on the W126 the aux fans are not switched on based on pressure, they are turned on based on temperature.... The two are very related though

You need to get a set of gauges and see what is going on. If the expansion valve is blocked up or sticking the high side pressures could spiral out of control.

In general 35*F below ambient is doing fairly good.

-Jason
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2013 Fiat 500E. 20,000 miles, 80 miles at a time.
2012 Passat TDI. 95,000 miles. More space, power, and fuel economy than the Benz
2004 Touareg V10 TDI. 150,000 miles. One of 450.
1999 Jetta TDI. 310,000 miles.
1992 Jetta ECOdiesel. 156,000 miles. 1 of 48. Sold.
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  #12  
Old 09-24-2009, 03:28 PM
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Quote:
Originally Posted by eatlead View Post
Hi guys,,,BTW...how do I wire the ac condensor fan to come on when the ac is on?...it appears that my ac-fans only turns on when engine coolant reach upper limits?...I d like to modify the fan so it comes on when ac is turned on full time...also where is the ac condensor fan thermo-switch located..??

my car is a 1991 300SE (w126)
Easiest way would be to run a wire from the + line @ the compressor to a simple 12V relay setup at the condenser fan.

A simple 4 pole switch will have contacts labeled 85, 86, 87 & 30.

30 is the power supply to the thing you want to run +12V
87 is the thing you want to run (the fan) which is then wired in series to ground
85 is ground
86 is where you'd run the wire from the compressor to.

Thermoswitch is on the reciever drier.

The problem is, at highway speeds, that spinning fans are probably going to cause an airway obstruction, as the wind will be spinning them faster than the motor allows. It's better to buy a fan controller that shuts off at a certain speed; of course, you could always just give it a try - a simple relay like that costs about $5, and if you mainly sit in traffic you probably won't have a problem.
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  #13  
Old 09-24-2009, 03:33 PM
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Quote:
Originally Posted by compu_85 View Post
At least on the W126 the aux fans are not switched on based on pressure, they are turned on based on temperature.... The two are very related though

You need to get a set of gauges and see what is going on. If the expansion valve is blocked up or sticking the high side pressures could spiral out of control.

In general 35*F below ambient is doing fairly good.

-Jason
Point taken. The P/T relationship is so close as to almost be the same thing in a working system.

I'd echo the gauge suggestion. If you're going to borrow some that have touched 134 & PAG oil, make sure to buy some flush first and clean those puppies out. Of course, you can probably get a used set on ebay for around $30; just make sure the needles aren't pegged (too much pressure on low side).
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  #14  
Old 09-25-2009, 07:18 AM
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Quote:
Originally Posted by compu_85 View Post
At least on the W126 the aux fans are not switched on based on pressure, they are turned on based on temperature.... The two are very related though
That's only partly true. Low aux fan speed is triggered by pressure, as measured by a pressure switch on the receiver/drier. High aux fan speed is activated by high engine temperature, as measured by a coolant temp sensor.
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Owned since new and still going strong and smooth
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Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

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  #15  
Old 09-25-2009, 10:01 AM
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Hm, according to the documentation that I had read the low speed of the aux fans was triggered by the temp sensor on the receiver / dryer (and that this is actually a temp switch not a normal pressure switch) and that high speed is activated by coolant temp like you said.

I wish they had made the temp switch at the R/D dual stage so it could activate the fan's high speed too... it seems like on some hot days it could do some good. But I tell ya what those fans are LOUD on high speed... move a lot of air too! You can feel them running in the steering wheel!

-Jason

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000).

2013 Fiat 500E. 20,000 miles, 80 miles at a time.
2012 Passat TDI. 95,000 miles. More space, power, and fuel economy than the Benz
2004 Touareg V10 TDI. 150,000 miles. One of 450.
1999 Jetta TDI. 310,000 miles.
1992 Jetta ECOdiesel. 156,000 miles. 1 of 48. Sold.
1991 Jetta ECOdiesel. 430,000 miles. 1 of 700. Sold to VeeDubTDI, totaled in front of our house
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