300sd ac problem - PeachParts Mercedes-Benz Forum

jimandsuzy · #1 06-02-2007, 08:48 PM

I bought an 85 300sd last winter. It was converted to R134 in '92. System had pressure but low on freon so I put in some R134. Ac worked but only put out 60 degrees with mid 80's outside. A friend evacuated the system and refilled with 4 cans of R134 and got pressure readings of 40 psi low side and 220 high side. He also clamped off the heater water line but air temp at hiway speeds is still low 60's in high 80's weather. Where would I look for the problem? TIA

bustedbenz · #2 06-02-2007, 11:25 PM

Quote:

Originally Posted by jimandsuzy

It was converted to R134 in '92.

Quote:

Originally Posted by jimandsuzy

Where would I look for the problem? TIA

Right here

134 isn't as efficient a cooling system, the systems in these cars are fairly small anyway, and running them with a less cold refrigerant, effectively, causes less cooling effect. Does it normally work better? If so then I'm wrong... but I've seen several cars through our family, some benzes and some not, and the ones that got converted never cooled right afterwards.

bustedbenz · #3 06-02-2007, 11:41 PM

I should have said that 134 wasn't as good "in my opinion"'; there will be those (i was just reading through some more a/c threads) that find it plenty good. I'll just revise my previous comment: in my PERSONAL experience, 134 was useless in 12-designed cars. I use freeze-12, it's an in-between product that gives me good results.

jimandsuzy · #4 06-06-2007, 05:54 PM

It should do better than 20 degree drop from amb though.

arew264 · #5 06-06-2007, 06:40 PM

As I recall, you generally get 30-50 degree air, it varies... Another member would be much better at a diagnosis, though.

dannym · #6 06-06-2007, 06:43 PM

You should have asked your friend to flush the system while he was at it.

Danny

vstech · #7 06-06-2007, 06:44 PM

I understand that in the older cars, the baffles don't properly switch the outside air, and you end up getting hot underhood air mixed in with the cooled air. especially if your mechanic pinched off the heater hose to prevent it... you may need to find some of the threads on here that explain how to bypass the vacuum system for controlling the baffles.

Kuhlrover · #8 06-07-2007, 12:20 AM

From the pressures you provide, there appears to be a nice pressure differential. This would indicate to me that the compressor is doing it's job. How cold is the A/C hose coming out of the evaporator (from the inside of the car) and how hot is it coming out of the compressor (high pressure side)? If temperature of the A/C hose coming out of the evaporator is a lot colder then the air coming out the vents then you have some hot air mixing in the duct work that should not be happening. If they seem to be the same temperature, it is possible you have a clogged expansion valve. There is a very small orifice in the expansion valve and if it get clogged you don't get enough coolant to flow through it.

TimK

Bill Ladd · #9 06-07-2007, 07:05 AM

I recently rebuilt my system -- full evacuation, flush, new compressor, dryer, expansion valve, and proper charge of 134a.

I'm surprised at how well it cools. It's not freezing cold. On super hot, humid southern days, it takes a while to cool the cabin, but for now, it'll do.

Based on what you have going on, I also suspect the expansion valve . Not that I'm any kind of AC expert here, just based on my research here and my hands-on with my car. The valve has a very small orafice and seems that any foreign debris will clog it. Also, one cannot properly flush they system without dealing with the expansion valve. I removed mine, drilled it out, re-installed it, flushed the system and then replaced the valve with new.

There is also a lot of information here about parallel-flow condensers. Seems to be the missing link in getting our older systems to work well with 134a. Do a search and see what you find. I may get a universal one from AcKits if the scorching summer heat down here gets to be too much.

NC Benz · #10 06-07-2007, 07:09 AM

Quote:

Originally Posted by Lux

I recently rebuilt my system -- full evacuation, flush, new compressor, dryer, expansion valve, and proper charge of 134a.

I'm surprised at how well it cools. It's not freezing cold. On super hot, humid southern days, it takes a while to cool the cabin, but for now, it'll do.

Based on what you have going on, I also suspect the expansion valve . Not that I'm any kind of AC expert here, just based on my research here and my hands-on with my car. The valve has a very small orafice and seems that any foreign debris will clog it. Also, one cannot properly flush they system without dealing with the expansion valve. I removed mine, drilled it out, re-installed it, flushed the system and then replaced the valve with new.

There is also a lot of information here about parallel-flow condensers. Seems to be the missing link in getting our older systems to work well with 134a. Do a search and see what you find. I may get a universal one from AcKits if the scorching summer heat down here gets to be too much.

I am running R134a in my 1983 300D. I get a vent temp of 42 degrees on a 90 degree day. I will take that! The other day I needed to turn down the air when it was 92. Today is supposed to be 95 and this will be a great time to check out the vent temp.

Ken300D · #11 06-07-2007, 12:43 PM

Just speaking on a hunch, I'm thinking that 4 cans of refrigerant is too much charge. That would make your low-side pressure remain too high. Thus you wouldn't get as much of the pressure differential that makes for a cold evaporator.

Ken300D

Kuhlrover · #12 06-07-2007, 02:33 PM

Quote:

Originally Posted by Ken300D

Just speaking on a hunch, I'm thinking that 4 cans of refrigerant is too much charge.
Ken300D

Ken, I agree. 4 - 12oz cans would be 48 ounces. I would expect the correct amount to be about 29 ounces. Although the pressure across the compressor looks good the condensor may be saturated.

TimK

James L · #13 06-07-2007, 04:39 PM

here is a quicky tip I got from a HVAC tec to help check amount of freon; get on the evaporator side of the expansion valve. with a/c running feel the two pipes going from expansion valve to the evaporator. You want about a 5 degree difference between the two.....if the pipe exiting the evaporator is 5 degrees or less than the pipe entering the evaporator you are ok.

Kuhlrover · #14 06-07-2007, 08:03 PM

Quote:

Originally Posted by James L

You want about a 5 degree difference between the two.....if the pipe exiting the evaporator is 5 degrees or less than the pipe entering the evaporator you are ok.

I don't know if I completely by into that. If the expansion valve is partially blogged and the pipe entering evaporator is at 60 degrees and it is at 65 degrees when it leaves the evaporator, then the coolest the air will be coming out of the vents would be 60 degrees. Yes it does indicate a small amount of refridgerant flow but there is not enough evaporation of the refridgerant to drop the evaporator to a level where it going to work the way it is suppose to. If the inlet pipe is closer to 35 or 40 degrees then that is what you would like to see.

TimK

James L · #15 06-08-2007, 12:06 AM

Those 2 temperatures need to be as close as possible within 5 degrees of each other. if the outlet is more than 5 degrees warmer than the inlet you need more refrigerant. if the outlet is more than 5 degrees cooler than the inlet, you have too much freon.

another tip, get on the highway and run a/c with fan on low, this gives the evap time too catch up and triggers that 60 degree stick to move on down to 38-47 vent temps, then run fan on auto and see if it maintains.