A/C problems. Help me understand my guages
 

The 87 300d is finally getting the A/C RandR it deserves.
Anyways, I have some gauges and I'm charging the Freon level. I had the compressor off and replaced it and the dryer. I also ordered a new expansion valve but when I saw where it was (Behind the brake booster) it scared me. I pulled a 29"HG vac and it held just fine. Anyways, I started charging it. Here are my numbers:

Car not running:
Low Side-Pegged
High Side-160psi or so

Car Running at idle:
Low Side-110psi
High Side-Pegged until the compressor shuts off. I jumpered the low pressure/level switch on the dryer a moment and the compressor stays on. With out it jumpered it cuts on and off ever 10 seconds or so.


Lastly when the car turns off Low and High should equalize. Am I right??

Any ideas what's the matter here? My guess is maybe the expansion valve.

I do my low pressure reading the the compressor on and running the engine at 1500 rpms. Should be in the mid 30 range with this heat.

if it is higher, you need freon.

Expansion valve is a pain in the neck.... or right hand... there are some hoses that you can move out of the way a bit... but I scraped up my hand a bit..

then i forgot an o-ring and had to do it again.... most will recommend puttign blue nylog on the o-rings. some will say not necessary. i bought the stuff and used it. at worst, it helped keep the o-rings in place.

i took mine to a professional to pull vaccuum... don't have the tools..also then they could tell me for sure if there was a leak.

that's my story... this was on an 89 124.. a 300CE

Is there a way to tell if the expansion valve is good still? I really don't want to change it and there are no leaks based on my vacuum.

HIGH SIDE GUAGE PEGGED WHEN RUNNING?!!!! that's over 500psi! you are overcharged. WAY too much freon in the system. or you have a VERY small condenser.

Sounds way way way way way way overcharged. It might also have too much refrigerant in there. The 10 second cycling is the overpressure protection cutting out the compressor. Evacuate it and start over.

You do know not to open the high side valve on the gauges while the system is running?

- JimY

The low being pegged when not running has me wondering...as when the system is not running, both sides should be equal.

When you are either running the compressor or letting it kick in or out, does the air get cold at the vents?

Did you check to make sure no rubber lines are kinked?

How much freon did you install? 3 12oz cans should be tops, and depending on what freon you are using, it could be over charged with three cans.

So it could be overcharged or it could have too much refrigerant?

If I'm gonna make a guess..... it's overcharged with too much refrigerant.

We don't know that it's not equal. Keep in mind that the high-side and low-side gauges don't use the same scale. 160 psi is off the scale for many low-side gauges. High side gauges typically peg around 500 psi, low side around 120 psi.

Is your auxillary fan working? This could explain your high side pressures. The high pressures you are reading are extremely dangerous. I'm surprised none of your hoses exploded. Make sure you know what you are doing.

Scott

thats scary... good old mercedes parts though, took all the torture.

The other issue is that the original poster doesn't mention whether he flushed the system. If the compressor crapped out and blocked up the expansion valve that could be part of the problem here. Still, I agree the first step is to pull out the refrigerant. I'm guessing we're talking about R134a here?

With low side pressure off the charts?

Naw, you're right. I when I read the guage was pegged, I still mentally was thinking he meant at the bottom.

You might be right; it could be "pegged" on either end of the spectrum.