New A/C Problem 300E
 

Well folks this should be an interesting one.

After pulling the dash and replacing all the vac pods on my 300E, I drove from Texas to Kansas City in August in perfect comfort. Had 39 degree vent temps much of the time until stabilized on the long trip. All was well.

This morning I drove it to town in comfort but without my thermometer in the vent. After the car sat in the 92 degree weather and heat soaked, it would not make cool air at the vent in the stop and go traffic. I put the thermometer in the vent and got 55 degree vent temps on the way home, meaning highway speed for 20 miles.

After getting home I hooked up the gauges and the pressures were normal and sight glass looked good. That was a bit of a relief since my first worry was the evaporator.

I then checked at the diagnostic connector with my homemade impulse counter and there were no error codes present.

The heater valve had a hot hose going in, but no heat on the hose going out.

The temp sensor vent held a square of paper with no problem, so the sampling fan seems good.

I took it down the road and turned off the air to see if I was getting fresh air forcing through with the system off. There was no air coming through the vents at all with the system off at 60MPH.

I had turned the dial down to less than 60 degrees on the way home and it didn't make any difference.

My first thought was that maybe one of the vac pod shafts had come off, but I think that if that were the case it would blow air through the vents with the air turned off.

Any suggestions?

Thanks,

Larry:

If the heater hose is hot up by the firewall, the monovalve is leaking OR the pushbotton control unit is out to get you.

Try clamping off the heater hose and check vent temp -- if OK now it's the monovalve. I suppose you could rig up a power connection direct to the valve to see if the valve itself is bad or the PBCU isn't giving you enough juice.

I suspect it's not leaking enough to do anything more than raise the vent temp a bit -- the heater core is behind the evaporator, so the coolant leaving the core will be fairly cool.

I think I finally got all the leaks fixed on the TE (of the ill fated evaporator change) -- the o-rings on the compressor manifold high side eroded out, dumping yet another charge (and tons of oil this time). I re-sealed the compressor and put new spring plates in since I didn't find the bad o-rings until I put it back together (it eas fun!).

Naturally, the rear muffler fell of on the test drive....

Peter

Peter,

Thanks for the response. What I meant about the heater valve is that it seems that it is preventing flow because the hose going from the valve to the firewall is not hot.

I suppose blocking it off completely would be good to ensure that this is not the problem.

It seems that there are only three basic things that can lower the vent temp; ineffective refrigeration, heat from the heater core or fresh air entering the cabin.

It seems like fresh air is the likely cause, but it would be easier to totally eliminate coolant flow to the heater core before trying to figure out anything with fresh air leaking in.

Thanks again for the reply.

Any comments or thoughts are welcome.

Have a great day,

Larry, just for my own curiosity, what symptoms would a faulty expansion valve exibit? I have long suspected mine as the culprit for my marginal cooling problem on my car.

Sorry Larry, I missunderstood. Yeah, it's working, and yeah, almost anything would be better than taking the dash apart again....

I would pull the glove box and check the vac servos for leaks. I had to replace all the right angle connectors on my switchover valve, none of them would hold vac on the valve after R&R. Cheap fix, eh?

You won't be able to see the flaps, but you will hear them move.

Sounds as if your full recirc is not working -- but this could be due to cooler outside air. I would expect vent temp to stay pretty much the same, though.

A clogged expansion valve will result in very low low side pressure, sometimes (depending on charge and outside temp) a high side pressure too high. If completely plugged, you can blow a high side hose in hot weather (I know, I did, but not on a Benz).

Peter

What role would a heater core play in this?
 

Hi Peter, Larry, all...

I have a problem with getting cold air unless I'm on highway speeds.

As Peter had advised me in the past: I Pulled the glove box, vac lines ok.

However, even with AC functioning optimally I'm NOTICING humid cool or cold air from Vents. I READ somewhere that a sweet smell in the passenger compartment is a sure sign that the HEATER CORE (??) needs to be replaced, but I don't know what "sweet smell" means. Windows are fogging up really bad.

I don't know much about the climate control system, but I'm willing to learn if someone will point me in the right direction.

ANYONE: DOES IT SEEM LIKE I HAVE SYMPTOMS OF A FAILED HEATER CORE? SHOULD I CHECK SOMETHING ELSE????. No obvious loss of coolant.

Thanks if anyone has any feedback.

Preston

Definately start with the easy stuff. I'd first pull the glovebox and check the operation of the vacuum elements. I'll second that the right angle connectors to the seven port valve get tired over time and can fall off. The 55 degree vent temps on a 92 degree day sound just about right for refrigerating fresh air.

- JimY

Cap'n,

Peter described an expansion valve that fails by closing off and his explanation is good and is valid.

I have also seen an expansion valve fail open. In this case your high side pressure will be very low and the the low side pressure will be high. This is because there is no restriction in the system.

LEADFOOT,

The sweet smell is the smell of antifreeze. To know what to smell for, just open the radiator while the engine is cold and take a whiff. That is what you're smelling for. I don't really understand your problem because the a/c SHOULD be putting out cool or cold air from the vents. Please elaborate.

Have a great day,

Leadfoot:

Gurgling in the heater core means all the air isn't bleed out since the last time the coolant was changed OR you have a leak.

Coolant dripping from the evaporator case is also a sign of a leak. In severe cases you will get steam coming out the vents with a warm engine on a cold day! You will also get slime on the windows from the glycol.

Check the system for pressure holding ability, the pressure caps fail with age. This leads to loss of coolant and gurgling heater core. A pressure test will tell you for sure.

Changing the heater core on this car is quite a bit of work.

Larry:

I forgot about on sticking open.

Peter

Peter

I will second Peters statement that changing the heater core is quite a bit of work. Been there, done that.

I hoped to get out and pull the glove box last night but it didn't happen. I sure hope that the problem is simply a vac leak that I can access to fix without pulling the instrument panel again.

Have a great day,

Larry:

Just be careful not to find anything else broken when you do that easy fix....

I got the exhaust done on the TE tonigh (would have been a one hour job except I had to cut the bolts at the front muffler, siezed up like nobodys business).

Naturallyl, I had to look -- needs rear brakes and torque and thrust links.....

Peter

Peter,

Just goes to show that "it's always somethin'."

I pulled the glove box and then got interrupted so I haven't finished checking everything out. I did get to visually inspect the connections at the manifold and they look good.

I won't know until I put the vac source in place, but my worry is that there is something wrong with the linkage pieces for the fresh air door. That means off again with the inst. panel. I'll try to reserve my pessimism until after a better diagnosis.

Have a great day,

I applied vacuum to the recirc connections. They are tight and I can hear actuation. I then looked at the test procedure on the CD so the next thing is to apply vacuum at the firewall to test the one way valve and to see if the manifold is applying vacuum as it should according to the test procedure.

So far there is no indication that the instrument panel will have to come off. The recirc pods seem okay and connected, so it could be the one way valve, or maybe the PCU itself.

Wish me luck,

Does anyone know if the recirc door is normally closed or normally open? I checked vacuum from the firewall to the switchover valve block and all that is okay.

I messed around with a vac gauge at the switchover valve and can't find a problem. If I know if the door is normally closed or open I might be able to put something over the recirc pod to feel of the linkage to ensure that it is actuating, but I need to know if it is closed or open.

This one is getting baffling to the point that I'm afraid that I will have to pull the instrument panel just to see what's happening. Since the a/c pressures are perfect and the hose OUT of the heater valve is staying cold, I'm quite sure that the problem is fresh air instead of recirc.

Do any of the other doors have anything to do with this? If they do I can't figure out how.

Thanks for any help you can offer.

Have a great day,

Maybe you have already checked it. Since you have the glove box off, you can feel or look for the fresh air flap openning while AC is on and in recirculation mode.

Larry,

For what this is worth, my 92 300CE had some problems with the center vent not working. After I tested all the pods and found no leaks the head gasket let go after 150k. Long story short, there was a vacuum hose routed under the battery that had to be replaced. Air works great since then.

Cheers,
Steve

I don't want the recirc open. It must be closed to have recirculation and lower vent temps. I can hear it and I THINK it is opening, but I don't know any way to know that it is physically closing without pulling the instrument panel.

I know the center vent is opening because I can look through the vent and see it open. I wish there were a way to look at the recirc flap.

Thanks for the suggestions. Please keep them coming.

Have a great day,

I seem to remember the doors being open with no vac, but I'm no longer sure.

There is a blend door (the one inside the box) that might cause trouble by letting air through the heater core, but I'm not sure.

Peter

Larry -

You can see the recirc door close with the glovebox removed. Just a bit of the far right-hand side of the door is visible past the HVAC box. Shine your shoplight in there and use the mityvac. Normal (no vacuum) position is for fresh air. You should see the door swing away upward as you apply vacuum to the pods.

- JimY

Jim,

Thanks. I just put it back together because I have to go to Arkansas this week. I will try this when I get back.

Have a great day,

Well, I learned that the problem is intermittent. :(

I stopped for a little bit in Little Rock Tuesday afternoon and the cabin heat soaked. I started it up and it quickly took the vent temp to 38 degrees. Then yesterday afternoon, with it heat soaked, it struggled to get the vent temp to 60 degrees even after several miles.

Anyone have any ideas about what could be operating intermittently? I think I am going to take out the glove box for awhile so I can look at the edge of the recirc door as Jim describes when the problem occurs again.

Any help is greatly appreciated.

Hi Larry

Reading through the thread the only thing I can think of is it may be a switchover valve problem. Once the symptom occurs try monitoring the vacuum output to the indiviual elements. The swithover valves were known for chattering and or intermittent operation. :cool:

I like the suggestion of a sticky/failing switchover valve. With the glovebox removed you could try tapping it while it refuses to activate the recirc flap; that might help isolate the problem. With the glovebox removed a very distinct, single click noise can be heard as the switchover valve activates. Should be able to hear this when activating the recirc dash switch.

The only other idea is a crack in a vacuum hose or rubber elbow. Sometimes it'll hold enough vacuum to pull the element closed, sometimes not. I've seen similar operation from old vacuum elements with the diaphragm on the way out, but that can't be happening with brand new elements.

Mostly I'm clueless. I hate intermittents. My approach to solving intermittent problems is to wait until they become continuous problems...

- JimY

George Murphy, one of the technical directors for the MB club is the AC guru and even akes various ac parts. He would help you. I have heard him speak twice. phone 865-482-9175 george_murphy@compuserve.com

Larry
this is how my SL acted when the mono valve was going bad. I would get a little heat and the vents would get warm. Then go back to full cold. I replace the monovalve and the cars AC worked fine.
Isolate the mono valve hoses and see if the symptom disapears. Or open it up and see the condition of the unit. If it is junked up consider a rebuild kit for it. When mine went bad it was as I said intermitant.

Fresh air flap. Disconnect it from the vacuum element and "tie' it closed. Check the symptoms.

One note.
The 560Sl has a fresh air recirc button that applies vacuum to the fresh air flap for a few minutes. Closes the fresh air flap and then opens it after about 10 minutes.
This circuit may be able to be adapted to to your car.
If you dont' have the 560SL CD let me know, I check mine.

Dave

Thanks for the suggestions. I will pay some attention to the switchover valve.

Dave,

Thanks for the monovalve ideas, but the 124 has a heater valve that is completely different from the repairable monovalve like was used in the 123's and some of the other chassis. Also I have checked the hose coming out of the heater valve several times during this ordeal and it is cool. Also, since it is not cooling upon start up while the coolant is still cold, that seems to eliminate that as a possibility. Thanks for your input and fly safe.

I may run out of hot weather to test it in.



One more possible clue. I drove to town late yesterday afternoon and the weather was raining and damp. I noticed sweat on the windshield caused by air from the defrost vents. I don't think air should be coming from there with the middle button of the climate control pressed. Is this a significant clue?

Have a great day,

Larry -

Condensation on the windshield means the defroster vent is not closing completely. The defroster flap uses a two stage vacuum element. Both sections must be closed to fully shut off airflow to the windshield. On older cars the small (upper) diaphram generally fails first. This is why you see older MBs (and occasionally BMWs) driving around with condensation on the windshield. On older cars like your '88 the foam around the defrost doors which seals the closed door is long gone. So you may get a bit of leakage (and condensation) even if it is working correctly. I can't remember if my wagon had the condensation problem after repairing the CC system or not - definately did before. The defrost element is readily visible with the glovebox removed, so it's easy enough to check whether it is closing partially or fully.

On my '87 124 the large, bottom diaphram would close immediately upon starting the car. This mostly shuts off the airflow. After about 30 seconds the small diaphram would close as well. I'm thinking it was designed to clear any condensation that occurred while the car was shut off.

I'm starting to think your car has a general lack of vacuum to the climate system. Perhaps it's time to check the main vacuum source to the switching valve - I believe this is the solid green line. Perhaps plugging it and then checking from under the hood with a mityvac. Also, disconnect it at the switching valve and check the vacuum reading with the engine running. The problem may actually be under the hood...

- JimY

Jim,

Thanks for the response. I did indeed check the vacuum source last week when I had the glovebox out. I did not measure it at the switchover valve with a gauge, but I did check at the firewall to ensure that the one way valve and the reservoir were okay. I was following the factory info and supplying vac at 400 mBARS as they indicated. I will measure at the input to the switchover valve and see what it is making.

Do you have any idea how much vacuum it should be supplying at the switchover?

I replaced the defrost pod of course, but I did not check the seal around the defrost door. I'm not so much worried about the condensation from the defrost vents. I was just posting this as a possible clue to the problem.

Thanks again,