AC problems 1yr after r134a retrofit - how do I ad 134 correctly? - PeachParts Mercedes-Benz Forum

JHZR2 · #1 05-15-2004, 09:50 PM

Hi,

Last summer, my compressor started siezing up on my 83 300D. Replaced it with a rebuilt one and had a professional retrofit done, where they solvent flushed the system, replaced the o-rings, dryer, etc., and then filled it with ester oil and r134a. Worked great, no issues, cold air.

Last monday, I was driving along and it was particularly hot. Not summer heat, which I know the system CAN provide cold air in after the retrofit, but about 85 and sunny.

All of a sudden, while driving on the highway, the air went warm. I just rolled down the windows and thought maybe the system wasnt keeping up, or it was because I had moved the dial from about 16 to about 24C. I moved it back, and after about 5 minutes, the air went cold.

Later, I was in traffic, and it went warm again. Really warm, more like as if I had turned the heat to max. I drove home with the windows down, and to my dismay, when I popped the hood, the metal fittings on the cold side of the AC system were very cold and had condensation.

This morning I drove the car again. at the start it was cold, but then all of a sudden, as the engine got up to temp, it went hot again.

The aux fan comes on, BTW, signifying that the pressure switch is working. Sometimes I see the refrigerant bubbling in the sight glass, signifying that its working. the metal fittings condense water on them, and are quite cold. So what could it be?

I am thinking maybe the compressor is cycling less than it usually does. Not a good correlation to the fact that the aux fan is going on (low pressure should keep the fan and the compressor off), but its just what I notice.

Out of curiosity, Im thinking of adding some r134a. I need to be careful though, as the way I understand it, you can only put in 85% of the weight of refrigerant, since 134 runs at different pressures. So how would I do it? If I get an r134a gauge (which I have from one of those interdynamics kits), itll tell me the pressure that Im at, and have the ok range at 25-45psi. But how does that correlate to the correct amount of refrigerant, and the correct weight and pressure fill quantities that my system would need, since it should have the 85% of the amount of r12 required?

I drive the car all the time, so I really need it to be blowing cold. It cant be blowing heat that feels like max heat at me.
ACC seems to be working right, as do the vents. No issues with anything on the car at all.

What should I do? Any hints, tips or suggestions? anything would be appreciated.

Thanks for your time reading this long post.

JMH

Palangi · #2 05-15-2004, 09:57 PM

Sounds like you are getting heat and AC at the same time. I would check the monovalve before doing anything to the AC system.

JHZR2 · #3 05-15-2004, 10:00 PM

just replaced the monovalve last winter because of lack of heat. Do they get stuck in an open position? The emergence of heat seems to correlate well with the increase of engine temperature during warmup and/or sitting in traffic.

What exactly should i check for? I dont know how to determine its condition visually.

Thanks,

JMH

Palangi · #4 05-15-2004, 10:18 PM

Well, if it becomes electrically disconnected, it's full heat, so step 1 is to check the connector. Underhood connectors can build up some oxidation, so it can't hurt to clean up the terminals a bit.

Step 2 is to pull the cartridge (4 little screws on top) and check for torn diaphram. If that's it, a monovalve repair kit should fix it.

Step 3 (if necessary) is to remove the monovalve from the car. Carefully rig up a way to put 12 volts across it. With no voltage applied, you should be able to blow air through it. With 12 volts applied, it should seal tight so you cannot blow air through it. I had one that was intermittantly sticking in the bore (brand new repair kit).

(By the way, that plastic has a vile taste)!!

If the monovalve works and you're still getting heat, the problem may with the $$$$ climate control unit. There are some old threads about repairing the solder connections on the unit. I haven't had to do that yet.

My experience with monovalves has been that I only get 1 or 2 years out of them.

JHZR2 · #5 05-15-2004, 11:04 PM

Out of curiosity, if an inline fuse was put on the power line for the aux water pump, sould that cause a problem?

For example, if my inline pump was disconnected (from a bad fuse connection), would that cause trouble? As I understand it, the power line for the aux pump is connected to the ACC (thus the reason for the inline fuse, so that if the pump goes bad, it doesnt fry the acc). If that had something to do with it, and then the acc forces the monovalve open, then the system has water flowing through due to the engine water pump flow, making heat. Feasible?

Ill check out the monovalve ASAP. The only thing that can be checked/replaced is the monovalve insert, sold as the /monovalve repair kit', right (the part that is under the four screws, that pulls out from the housing)? Or do I have to remove the whole assembly with heater hoses attached, etc.?

Thanks,

JMH

Palangi · #6 05-15-2004, 11:19 PM

I wouldn't think aux pump is related. Several guys have them disconnected or bypassed. If anthing, it might block some heater flow with it disconnected.

If you need to go to step 3, that means putting the monovalve kit back in, and removing the complete monovalve unit for the 12 volt open-close test. Just disconnect 2 hoses and remove 2 bolts and it's out.

JHZR2 · #7 05-16-2004, 10:16 AM

thanks for all the help. Ill check the monovalve today if the weather holds, and post results. Hopefully its just the insert, Id hate to have to replace bigger things, or mess with the ACC unit, it had been working 100% until then

JMH

JHZR2 · #8 05-16-2004, 06:45 PM

Update:

pulled the monovalve, and applied dc to it via its wiring harness. The valve popped open and closed as it should. At first it was acting a little funny though.

Maybe it is on the outs, but I pushed a nd pulled on it, and I hope that this exercise will give it a bit more life. Ill take it out for a spin to test now.

Thanks,

JMH

#9 05-17-2004, 09:32 AM

Was the diaphragm on the monovalve leaking? That's typically how they fail.

Good luck,

JHZR2 · #10 05-17-2004, 09:47 AM

no, actually there was no apparent leaking at all, and the rubber was all in good condition with no visible defects, etc. Rather, the monovalve (~1 yr old) was stuck in the open position, but simply mssaging it (manually moving it in and out) unstuck it, and all seems to be well now. The wire coil around the valve was fine with an ohmmeter test, and current was going to the harness.

hopefully itll stay fine now, but I can imagine a sticky monovalve.

To my suprise, there was some orangish sludge on the screen element on the monovalve. I wiped it off, but wasnt happy to see that. I always get the dealer performed citric acid flush, somaybe its some leftovers, its almost due anyway (2 yrs this summer).

Thanks,

JMH

Palangi · #11 05-17-2004, 10:53 PM

If the plunger intermittantly sticks in the bore, that could be what's wrong with it. When it sticks partially open, you have heat.

R Leo · #12 05-17-2004, 11:00 PM

Also check fuse #14 (I think). It's the same one as the horn.

If #14 isn't making a good connection, the monovalve will be fully open all the time. No way can the a/c keep up with that heat load. SWMBO's 300D did this a couple of months ago. I replaced the fuse and that took care of it.

You can also get some of those hose pinching pliers and squeeze off the water line running from the monovalve to the circulating pump. A temp fix but better than sweating your fanny off.

JHZR2 · #13 05-17-2004, 11:03 PM

Nope, all is well now... Just exercising the thing made it work fine.

Thanks,

JMH