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.