A/C Vacuum Leak in Center Vent? - PeachParts Mercedes-Benz Forum

EricSilver · #1 07-22-2009, 10:24 PM

Is this the result of a vacuum leak?

Car: 1995 e320

Today A/C stopped blowing cold (blows kind of luke-warm now) and there is a persistent hissing from inside the center vent when either of the two A/C buttons on the push button/climate control unit are pushed. When EC or Defogger are pushed, the hissing stops.

I also can hear the hissing inside the engine bay.

Everything else works as it should:

The compressor engages and I see the Freon through the glass on the bottle/receiver.

Hot air blows out the side vents when heater is on (normal)

Cold (or now luke-warm) air is coming out the sides and center (normal, except for lack of cold air)

The defogger blows out the top of the dash (normal).

In the heat of the afternoon, in Auto mode, the blower motor runs at the same speed as in High mode. Switching back and forth does not alter blower speed. In Low mode, the blower runs normally (slow).

Only in the cool of the evenings do the temperature wheel and Auto settings moderate blower speed.

I am correct in guessing the climate control system is attempting to cool the car by running the blower full blast, but the vacuum element, or the line to it, is apparently leaking and needs to be replaced -- a job the requires dismantling the dash?

EricSilver · #2 07-22-2009, 11:59 PM

MORE:
The Recirc function appears only to work in EC mode.

The engine bay hissing is loudest in the area around the brake booster. On inspection, I discovered a loose vacuum line (see attached photos) which I never noticed to be loose before. NOTE: A few years back either a vacuum line, or the switch-over valve was plugged with a BB when the transmission vacuum modulator failed. If the photos show those components, please disregard. However, if one of them is hissing, the BB may have fallen out. (Checked DM and also saw a code 26.)

First Photo:
Loose vacuum line (green) laying atop brake booster. Note component behind it with red/orange wires.

Second Photo
Component with red/orange wires is open at the bottom also appears to be a vacuum element, Is that the switch-over valve?

EDIT: The hissing near the booster was the noise from the expansion valve

#3 07-23-2009, 07:20 AM

Have you reconnected the vacuum line?

EricSilver · #4 07-23-2009, 10:16 AM

Quote:

Originally Posted by LarryBible

Have you reconnected the vacuum line?

No -- I'm not sure where it connects to.

EricSilver · #5 07-23-2009, 09:28 PM

More:

Cabin Temp Sensor
Cleaned the vent. Since blower changes speed after changing temp wheel, I’ll assume this sensor is functioning.

Low Speed Fan
Jumped across pigtail connectors with screwdriver at receiver. Heard fan(s) come on.

Aux Water Pump & Duovalve
Pump & Valve are both hot to the touch. Hoses to heater core are not. Hot coolant flows only when heater/defogger are turned on.

Recirc
Goes on/off in EC mode. Goes On in A/C modes but not Off.

Pipes from Compressor and Receiver/Drier
Relative to the hot metal and plastic components surrounding them, the pipes going from the compressor, and leading to the Receiver, are cool to the touch. But not frosty.

Other
After driving for a while, especially w/blower on low speed, the air gets progressively cooler. Some might call it “cold,” but it is not nearly as icy as it was just 24 hours ago.

Recharge Needed?
I have a hard time believing I need a recharge since the system was charged in summer 2003 – when the evaporator, expansion valve, receiver and other components were all replaced, and fresh R134 w/dye was added. It seems unlikely the system would suddenly take a massive Freon dump when it was working perfectly the day before.

Then again, the weather has been very mild and I have used the A/C only about three times this summer, so perhaps something is dried out, stuck or whatever. What I suppose I’ll do next is examine the vacuum lines under the passenger dash. (And hopefully someone will tell me what that detached vacuum line is in the picture I posted above.

)

Bob Weir · #6 07-23-2009, 09:39 PM

I have 94 e320 and valves are same.

Referencing your first pik. Looking at it left to right
1] Both check valves [bottom ports "Y" connector] get vac from engine manifold
2] Left valve, left port Red line feeds reservoir in right front fender [goes under fan box & battery, susceptible to leaking cracks
3] Left valve, right port Green line goes into firewall feeds ???
4] Right valve, left port Black feeds ???
5] Right valve, right port Black feeds change over valve
6] Change over valve Green feeds Trans Shift Point retard

Re 2] Reservoir out port to ACC controller. You must remove passenger airbag to access it. Easy DIY, but get specific instructions elsewhere in forum.

"Diverter Air Flap" pod located above center vents is probably not working properly allowing warm outside air to mix with cold AC.
Pull the pass air bag and test controller's #6 [2nd from bottom] for Diverter Air Flap.

EricSilver · #7 07-23-2009, 10:00 PM

Quote:

Originally Posted by Bob Weir

I have 94 e320 and valves are same.

Re 2] Reservoir out port to ACC controller. You must remove passenger airbag to access it. Easy DIY, but get specific instructions elsewhere in forum.

"Diverter Air Flap" pod located above center vents is probably not working properly allowing warm outside air to mix with cold AC.
Pull the pass air bag and test controller's #6 [2nd from bottom] for Diverter Air Flap.

Bob, thanks!

The Reservoir out port is open at the bottom. I can see hard plastic vacuum tubing that is not plugged -- is that the normal state?

Would the diverter air flap pod account for the irregularly oscillating hissing I hear?

I will test the controller tomorrow, pulling the air bag per these instructions I found searching threads yesterday:

1) Disconnect the negative battery cable
2) Pull up the front carpet under the passenger's feet
3) Remove the 1 screw holding the plastic or metal shield in place
4) Unplug the big red connector (this is for the airbag system)
5) Unscrew the one long bolt that holds the airbag in place (bottom/center)
6) Pull airbag forward, unplug connector
7) Installation is the reverse of removal.

To do the test, I assume I'll need a MityVac?

EricSilver · #8 07-23-2009, 11:47 PM

After reading more threads about center vent hissing it seems I am most likely low on Freon. The hiss comes from the expansion valve when gas, rather than liquid, Freon passes through it. (There is no hiss at all when liquid Freon passes through the valve.)

For as long as I owned the car I have heard (and ignored) this hissing whenever the compressor engaged/disengaged. A slow, multi-year leak apparently crossed a critical threshold yesterday.

Because it is normal to lose a few ounces of R134 per year I am not going to hunt for leaks.

Bob Weir · #9 07-23-2009, 11:52 PM

The Reservoir out port is open at the bottom. I can see hard plastic vacuum tubing that is not plugged -- is that the normal state?

Bottom of both yellow check valves are connected with "Y" hose that is fed from eng intake manifold. You mentioned Green line disconnected. Someone has modified the vac system. Sorry, I don't have complete info on #3 & #4 vac lines. I'll ask my inde tomorrow.
The only port that isn't used is on bottom of [rectangular] Change Over Valve. A Mighty Vac or similar tool will make testing easiest, but extra hose tapping off eng will do.
Remove battery and check condition of vac lines. Oddly mine cracked under the fan box. Remove right tire and screws top & back of plastic fender well. You'll see reservoir.

Would the diverter air flap pod account for the irregularly oscillating hissing I hear?

Yes, if the pod or feed hose is leaking air. You might be able to see it with the center vents removed. Otherwise, you'd have to remove dash [major major job] to access it. Therefore, I'd check everything else first.

I will test the controller tomorrow, pulling the air bag per these instructions I found searching threads yesterday:

Once you remove 10" bolt through bottom, us a flat head screw driver and gently pry on both sides of dash piece. It slides out very easily.

Your response posted while I was composing this one. Hopefully you're right on the AC juice. As an addendum: I just remembered the switches above the ACC. Center one looks like rotating circle. It's "Air Recirculation" switch that prevents outside air from entering cabin.