A/C problem - 1993 300D 2.5 - PeachParts Mercedes-Benz Forum

James L · #1 08-17-2007, 03:27 PM

I recently changed my serpentine belt/pulley, while I was at it, cleaned both sides of the condenser, radiator and cleaned and lubed aux fans. Now my a/c is acting funny.

It has a hard time getting vent temps below 60 F. If I get going and put the blower on low it goes down to 42-55 F. as soon as I put blower on auto or high it climbs to 60-65 F. Belt is tight and a/c compressor is engaging (no kilma shut off problems), freon in sight glass as before.

Low side 30 psi, high side 190 psi, 90 F ambient, aux fans work as they should, would not take any more 134a. 110 psi low and high with engine off.

Read the led fault codes and it flashed 30 times (circulation pump-aux water pump) The mono valve is the only thing I see on my car and it checks out on ohms and ccu sends 13 volts to it with a/c on and less than 9 volts a/c off as stated in fsm. changed out a spare ccu unit and still same results.

Put kleenex on temp sensor vent at dome light and it stays, so sampler blower is working.

I am thinking evaporator temperature sensor, cabin temp sensor at dome light or monovalve/circulation pump?

Thanks for advice!

jcyuhn · #2 08-17-2007, 04:34 PM

Sounds as if you are in fresh air mode; not getting any recirculated air. A ~30 degree drop is typical when passing outside air over the evaporator. Suggest you pull the passenger airbag and check the vacuum lines off the 7 port vacuum valve. I suspect you'll find at least one of the two lines for the fresh/reciculated air flap do not hold vacuum. A small hand vacuum pump (AKA Mityvac) is a useful tool for this exercise.

- JimY

James L · #3 08-17-2007, 04:47 PM

Would the recirculated air switch on the dash do the same thing? I clicked on that switch and same results. I used to hear a flap closing, now I do not. I am getting strong air flow through the center vents. Could be on to something Jim. Thanks

James L · #4 08-17-2007, 11:04 PM

Not to trying to argue, but why didnt' the impulse counter pick up the fault code for switchover valves 50 - 57 (fault code for fresh air/recirculation is 56 for large lift and 57 for short lift)? The fault code reading is heater valve/circulation pump related 30. Could hot water be entering the heater core mixing with the evaporator? Where is the aux water pump on my car? Is it under the monovalve? It is not under the coolant tank as in the gas engines.

I will remove passenger airbag and pull vacs on the switchover valve block. If it is the vac element for diverter flap or fresh air/rec flap this requires complete dash removal. if this turns out to be the problem, then I do not feel like doing the 2 day dash removal job and will not fix it.

I am reading up on the control of air flaps and air flow 83.0742-600/8. it says the control of the air flaps is influenced by the temp sensor on the heat exchanger, temp wheel on ccu and temp sensor at dome light . when transitioning from heat to cooling or vice versa.

help! and thanks!

James L · #5 08-20-2007, 11:10 AM

Pulled the passenger air bag and checked the Y7 block connections with the mity vac. The #2 (fresh air/recirculation-large lift) holds a vacuum and flap works. The # 1 (fresh air/recirculation-small lift) does not hold a vacuum-does it need to or is it boost? How can the other line hold a vacuum to the same pod? Diaphram must be ok?

Noticed some type of black-paperish film in the center vent, pulled out with tweezers and took photo. Could this be a tell-tell sign?

Clamped off the hose to the monovalve, no change. Did notice a pump underneath the heated winshield washer tank-is this the aux water valve recirculation pump?

The line going from the expansion valve to evaporator feels alot cooler than the one coming from the evaporator to the expansion valve. On my other car the two lines are about the same cool temp. This could mean an airflow problem or needing more 134A? Can the fresh air/recirc flap affect this?

Things left to try:

An ohm test on the temp sensor, heat box and evap sensors and the aux water valve if i can ever find it.

Evacuate, pull a long vacuum and recharge.

help-advice-opinions! thanks!

chomamma · #6 08-20-2007, 12:12 PM

I am having same issues. Where is the temp sensor in the dome light in the roof? I am also looking for the sensor in the evap. I assume my 91 is the same as yours.

James L · #7 08-20-2007, 12:24 PM

Yes in dome light assembly, hold off on evap sensor! check aux water pump, I think mine is under the washer tank. Do you have an impulse counter?

chomamma · #8 08-20-2007, 03:26 PM

I think one of the pumps are near the firewall in front of the drivers side. That may be the aux pump. The in front of the battery I think is the monovalve. I thought about clamping both off on mine and see if it cools. My compressor runs, have have 38 psi low side 170 high side and 90 off. Yours may be a bit low. Temp is around 95 here. I cant get any cold air through, so I am surfing for fixes. I'll look for the temp sensor. Don't have an impulse counter, but may can find one. Wonder if there is another way to check. You have a pic of the temp sensor?

James L · #9 08-20-2007, 04:06 PM

Wait on the sensors for now. I have been studying the fsm 83-604; checking electrical components of the climate control ground activation, step 10 S24 recirc switch on dash, step 11 monovalve, step 12 circulation pump; with defrost on check voltage on jacks and additionally test circulation pump operation by feeling with the hand. If above is no, circulation pump short circuit or ccu. step 13 is checking switchover valves Y7 fresh/air recirc.

The wiring to the dome sensor/aspirator blower is ground and then the other to the monovalve, then monovalve to ccu, then monovalve gated to 16a fuse that is gated to the circulation pump that goes to ccu.

Also did a search here for aux water circulation pump and read alot about worn/bad circulation pump amperage draws and ccu problems.

What I am trying to say is that the fresh air/ recirc, water circulation pump, monovalve, dome temp sensor and ccu are all related to each other somehow and my fault code 30 (water circulation pump) that I read seems to be dead on. I am going to try to see if I can feel/hear it working and check volatge without removing it. On my 1993 it is under the heated washer tank. If not, may remove and try to clean and test.

Chomoma, does your car have an X11/4, 8-pin diagnostic test coupling? (above and to the right of the monovalve-L shaped box with lid). If so, search here for JimF and go to his website for a diy $10-15 radio shack led code reader. You can read alot of fault codes with it like ccu, engine boost, eds, emission problems, ect.

jcyuhn · #10 08-20-2007, 05:25 PM

The fresh/recirc actuators (there are two hidden behind the box) are dual stage - each has two diaphrams. The large lift line switches from fresh air to 80% recirculated. Adding in the small lift is the additional 20% to 100% recirculated. Obviously, you are limited to 80% recirc.

Activating the dash mounted switch engages 100% recirculation. So the reason you no longer hear the flap thunking closed is because it no longer does. While this will noticably degrade the a/c performance, I don't expect it explains everything. You've not specified where you live - how hot & humid is it?

The aux hot water pump is indeed below the washer tank. It is only powered up when the CCU is calling for some hot water in the heater core. Not likely to be a factor mid-summer.

The high-side line (small one) should be quite hot to the touch. The low-side line should be cold enough to sweat condensation.

I didn't think much about the high/low pressures you posted initially. But check the FSM - it will have graphs for the expected pressures across various temperatures & humidity. I'm thinking your high side is a bit low. Perhaps you are a bit down on refrigerant. Combined with 80% recirculation that might account for the not-quite-sufficient cooling.

- JimY

James L · #11 08-21-2007, 11:05 AM

JimY, I live in eastern NC, very hot and humid with heat indexes pushing around 97-104 F. I do have the fsm with the graphs you mention.

Could a worn fan clutch be a cause, keeping hot air up near the condenser? if so, my high side would be higher?

I think I may need to evacuate, weigh refrigerant, pull a long vacuum, check for leaks and recharge, should I change the receiver dryer? Maybe the descident in it is all used up, since I have owned the car for 7 years and never done anything to the a/c.

Thanks

jcyuhn · #12 08-21-2007, 04:12 PM

I don't think I'd worry about the fan clutch if the car runs at reasonable coolant temperatures and the a/c high side is within bounds. Does your '93 have the dual electric fans in front of the condensor? These seem to be much more effective than the large single fan used in earlier model years.

If you're just recharging I don't think the dryer need be changed. Only if the system is physically dismantled does the dryer get replaced. At this point I'd be looking at a refrigerant recharge. If you can weigh the extracted charge that's great - you'll know how low it was. I recommend adding some flourescent dye to the new charge, makes it very easy to locate leaks in the future.

With only 80% recirc you will notice some degradation of performance with that level of heat/humidity. I modified an earlier generation (123) Benz to convert it from 80% to 100% recirculated air and it dropped the discharge temperature about 5 degrees in humid 100 degree weather.

- JimY

James L · #13 08-21-2007, 04:20 PM

Thanks JimY, mine does have the 2 aux fans. Will I lose any refigerant oil during evacuation? If so, should I add an oil charge? What was oe for 1993, pag or ester oil?

Good point on the dye, will try to find a 12oz can with 134a and dye. My fsm says 1000 grams of 134a for a W124, 1000 grams=2.2lbs=35oz (3-12 oz cans).

Thanks

jcyuhn · #14 08-21-2007, 04:54 PM

No, you won't lose any oil during evacuation. PAG was OE for these cars. I forget which viscosity PAG is used - I think it's the lowest.

Three cans of refrigerant is just about exactly the correct amount. It's rather convenient.

- JimY