so what does the a/c expansion valve do? - PeachParts Mercedes-Benz Forum

#1 08-15-2007, 01:57 AM

according to ol' rip off diesel giant it's very necessary to change that unit when working on your ac system. ( the thing, even though there's oil going through it all the time, can rust internally??) I wonder why, and most of all, what does that thing do???
I'm certainly no specialist here, in europe we have a rather mild climate so nobody cares that their MB's AC-system is complete junk
input greatly appreciated.
also, I wonder, why not just drain the accumulator when working on the ac. I mean, there's moisture in it, so get it out and re-use it, right?

dannym · #2 08-15-2007, 08:02 AM

Google works great!
http://home.howstuffworks.com/ac1.htm

It seperates the High side from the low side and supplies low temperature low pressure liquid refrigerant to the evaporator.

I don't know what Mercedes recommends. When I worked on my system I reused the old one.
It should not rust because there shouldn't be any moisture in the system. Moisture will freeze at the valve and clog it. That doesn't mean the valve is bad.

That is why it is so important to pull a vacuum for a long time. IMHO 1 hour isn't long enough. I let the vacuum sit overnight and hit it again in the next day. Provided the vacuum holds. If it doesn't you have a leak that needs to be fixed.

Danny

#3 08-15-2007, 08:26 AM

No offense, but it does sound like you should have your AC system professionally serviced.

When you ask about the "accumulator," I assume you are referring to the dryer. The dryer is not just a tank to collect water, it contains desiccant to remove water from the coolant, it does need to be replaced and should not be exposed to the atmosphere or an uncharged system.

The expansion valve provides the pressure drop between the high and low pressure portions of the system. Rust is not an issue, but these valves can get clogged if there is any debris in the system (i.e., from a failed compressor). After having a clogged valve take out a new compressor, I now insist on having them replaced when I replace the compressor (they are cheap, maybe $50).

vstech · #4 08-15-2007, 08:40 AM

the moisture that the receiver separates from the system, is not collected, it's absorbed by a desiccant chemical in the receiver. you have to replace the desiccant, so you have to replace the receiver. the expansion valve is a mechanical device with a pressurized temp adjustment... it can fail, and it can wear out over time... like 25 years of service... I don't know about rust, but corrosion is possible from the acid that moisture from the flexible lines contaminating the refrigerant.

Zwoobah · #5 08-15-2007, 09:48 AM

Any time I open an AC system, I replace the expansion valve and receiver/dryer. It's like doing a tune-up for your AC. Both parts are relatively cheap, and can/will cause destruction if they fail.

rrgrassi · #6 08-15-2007, 10:19 AM

Usually you do not have to replace the expansion valve, but you do if the compressor failed and sent shaving throughout the system. The valve is cheap, and not a bad idea to replace.

I had a 70's Pontiac that had the accumulator/reciever with a replacable dessicant bag. IT's part was called a VIR (Valve in Recovery) unit. The unit was on the suction side, not the discharge.

#7 08-15-2007, 10:36 AM

Quote:

Originally Posted by Craig

No offense, but it does sound like you should have your AC system professionally serviced.

When you ask about the "accumulator," I assume you are referring to the dryer. The dryer is not just a tank to collect water, it contains desiccant to remove water from the coolant, it does need to be replaced and should not be exposed to the atmosphere or an uncharged system.

The expansion valve provides the pressure drop between the high and low pressure portions of the system. Rust is not an issue, but these valves can get clogged if there is any debris in the system (i.e., from a failed compressor). After having a clogged valve take out a new compressor, I now insist on having them replaced when I replace the compressor (they are "cheap, maybe $50).

no 'professional' will ever touch my car. This is the item I was talking about:
https://www.***************/search/product.aspx?sid=3au50l55ymgbjo55w2vrrv55&makeid=800016@Mercedes&modelid=1194027@300SD&year=1983&cid=accumulator@accumulator&gid=6972@Receiver%20Drier/Accumulator
"accumulator/ receiver drier"
an expansion valve should not run more than $20 at http://www.hirschinds.com. I'll change it.

#8 08-15-2007, 10:48 AM

Quote:

Originally Posted by henrydupont

no 'professional' will ever touch my car. This is the item I was talking about:
https://www.***************/search/product.aspx?sid=3au50l55ymgbjo55w2vrrv55&makeid=800016@Mercedes&modelid=1194027@300SD&year=1983&cid=accumulator@accumulator&gid=6972@Receiver%20Drier/Accumulator
"accumulator/ receiver drier"
an expansion valve should not run more than $20

OK, you asked. It just didn't sound like you were very familiar with AC systems. Do you have the equipment to evacuate the system?

It was obvious from your question that you didn't understand how the "accumulator/ receiver drier" works, I've never hear it referred to as just an "accumulator." In any case, replace it.

If you can source the expansion valve for $20, even more reason to replace it too.

tangofox007 · #9 08-15-2007, 11:02 AM

Quote:

Originally Posted by henrydupont

an expansion valve should not run more than $20

Sometimes you get what you pay for. The cheap ones tend to leak, at least in my experience.

vstech · #10 08-15-2007, 11:12 AM

accumulator is on the suction side of the compressor and it accumulates any residual liquid to prevent compressor damage. the receiver is on the discharge side of the condenser and it works as a buffer for high pressure liquid reserve depending on conditions. rarely will you see an accumulator on an import. mostly domestic cars use accumulators. both have desiccant in them for moisture removal.

deerefanatic · #11 08-15-2007, 08:43 PM

Yep... But...... Heres a thought... Most desicannts (like silica gel) can be heated and the water will bake out of em... What about receiver/dryer's?

nah....... Not worth the risk on second thought.........

#12 08-15-2007, 08:46 PM

Quote:

Originally Posted by deerefanatic

Yep... But...... Heres a thought... Most desicannts (like silica gel) can be heated and the water will bake out of em... What about receiver/dryer's?

nah....... Not worth the risk on second thought.........

Not worth the trouble to save the cost of a dryer anyway.

#13 08-16-2007, 03:42 PM

Quote:

Originally Posted by Craig

OK, you asked. It just didn't sound like you were very familiar with AC systems. Do you have the equipment to evacuate the system?

It was obvious from your question that you didn't understand how the "accumulator/ receiver drier" works, I've never hear it referred to as just an "accumulator." In any case, replace it.

If you can source the expansion valve for $20, even more reason to replace it too.

I'm also not familiar with the language... and all I know about ac systems is that whenever you buy a mercedes, there will be something wrong with it.
I have a vacuum pump, that's about it. the wild mix of R12, R134, Freeze 12, and Enviro-safe I'll happily send into the atmosphere

not sure how to get all the oil out. maybe I'll just add a little ester oil when I recharge it with freeze 12. the system was never opened, maybe I should not mess with the expansion valve?

E150GT · #14 08-16-2007, 03:51 PM

Quote:

Originally Posted by henrydupont

I'm also not familiar with the language... and all I know about ac systems is that whenever you buy a mercedes, there will be something wrong with it.
I have a vacuum pump, that's about it. the wild mix of R12, R134, Freeze 12, and Enviro-safe I'll happily send into the atmosphere

not sure how to get all the oil out. maybe I'll just add a little ester oil when I recharge it with freeze 12. the system was never opened, maybe I should not mess with the expansion valve?

If you had all those refrigerants in there I would definitly flush and replace expansion valve and drier. since no professional will ever touch your car, I suggest you do a lot of reading, and buy an A/C manual as I did.

#15 08-17-2007, 12:59 AM

Quote:

Originally Posted by E150GT

If you had all those refrigerants in there I would definitly flush and replace expansion valve and drier. since no professional will ever touch your car, I suggest you do a lot of reading, and buy an A/C manual as I did.

I think I'll do it like the professionals, suck it vacuum for 2 minutes and fill it with whatever gas you have

and the funny part is that that even works!

in my case I'll change the drier, put a vacuum on it and fill it with freeze 12 unless I go to mexico before I start working on it