1984 300DT A/C problems. - PeachParts Mercedes-Benz Forum

HuskyMan · #1 08-11-2015, 10:33 AM

I thought the system was discharged (wrong). I went to install a new expansion valve this past weekend, as I began to loosen on of the expansion valve bolts, green colored spray blew on me and the floorboard (fortunately I had the carpet out of the car!). So, I quickly retightened the bolt and the spewing stopped.

The A/C blows really hot on all settings. I checked the compressor, it is coming on. I checked #8 fuse, it looks good. Next is the mono valve and climate control. I have a multimeter and need to know how to properly check the monovalve. Also, does this car have a relay that might be causing issues?

leathermang · #2 08-11-2015, 10:45 AM

I know there are really good threads in the archives on your questions...
not so with all questions...

rrgrassi · #3 08-11-2015, 11:34 AM

With the compressor running, does the low side line (with the fitting by the valve cover) get cool or cold? If so, Freon is flowing. Do you have gauges to attach and see what pressures you have?

The monovalve needs 12 volts to close. It's default position is open, allowing hot water into the heater core.

HuskyMan · #4 08-11-2015, 01:11 PM

Quote:

Originally Posted by rrgrassi

With the compressor running, does the low side line (with the fitting by the valve cover) get cool or cold? If so, Freon is flowing. Do you have gauges to attach and see what pressures you have?

The monovalve needs 12 volts to close. It's default position is open, allowing hot water into the heater core.

OK, I just went out and started the car, turned the climate control to blue cold/button second to left pressed (solid up and down arrows). With the car cold, the air blew cool for the first ten minutes then gradually turned to warm. I then took a short drive. When I arrived home the condenser fan was working properly but the low side line was not cool or cold. Vents still blowing warm to hot air.

Question: If I hook up A/C gauges where is the high side fitting located? Also, I have a multimeter. Where do I put the leads to test the monovalve? Thank you for your assistance....

BillGrissom · #5 08-11-2015, 03:05 PM

The H.P. port is on the metal tube running upward behind the alternator. BTW, that is a torturous path which no longer exists in my two 300D's since in adding new hose, I ran it straight to the condenser (as does RollGuy's modded H.P. hose), and I can now easily get at the turbo oil drain gasket and alternator bolts.

Rather than fool w/ testing the mono-valve, you could just clamp off the heater hose to stop any hot water flow which may be over-powering your AC. Find where it is a single hose, either around the electric pump (no longer on my cars) or beside the oil filter housing.

Since your glove-box and passenger knee cover is out, you can feel the AC low-pressure tube right after the expansion valve, before the heater has affected it, and judge how cold the refrigerant is getting. I feel for you. Automotive AC is a life or death issue in the CA Central Valley.

HuskyMan · #6 08-11-2015, 04:41 PM

Good news! I clamped off the auxiliary pump hose to mono valve and the air blows cool (not cold) but a LOT better than full on HOT! Thank you Bill for the idea! Obviously, the monovalve is the problem; fortunately I have a genuine Bosch rebuild kit sitting on my kitchen table ready to go.

Question; I was unscrewing the screws on the top of the monovalve housing and coolant began to slowly leak out. I quickly re-tightened the screws thinking I need to let the engine cool down before proceeding. Do I need to drain any coolant from the system before proceeding with the monovalve replacement?

ah-kay · #7 08-11-2015, 04:53 PM

I don't think so. The mono valve is sitting high enough that draining is not necessary. You can also undo the radiator cap to release the pressure if the engine is hot. It may drip a little but that is it.

HuskyMan · #8 08-11-2015, 09:15 PM

I was able to install the new Bosch monovalve kit; the A/C now blows cold, however I think I may need to add some R12 to the system. I have a 30 pound R12 cylinder and gauges. Is there anything special I need to know to add the R12 freon through the A/C gauges?

leathermang · #9 08-11-2015, 09:39 PM

Wear goggles...
having a great fan in front of condenser is important..
go slow adding small amounts and check for performance...
you can be SEVERAL oz low and it work just fine..
ONE oz over and you are working the system to death ...
Be sure you need it before adding...

HuskyMan · #10 08-12-2015, 12:45 PM

OK, but what pressure readings should the gauges show (high and low side)? How do I know from the readings whether I should add freon or not?

leathermang · #11 08-12-2015, 12:51 PM

Too many variables to state that..... that is why I mentioned the good air into the condenser...
Do you have a legit scale to use ?
Best to do actual road tests...
put a small amount in... go test drive... Stop adding when you have acceptable results...don't shoot for ' max perfect'.. that is how some get overcharged...
but on your temperature during install.. if you use that.. or just want to know... remember that the fan is to be on LOW....
with the other normal install rules...
Road test is the only really good way to get proper air across the condenser...