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1987 190E 2.6 Midnight Blue
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Discussion Starter #1
When driving I hear a hiss coming from inside my dashboard, the sound comes and goes throughout my commute but it stops when I turn on the center vent or defroster. My radio is currently removed and it seems to be coming from directly inside that area.

The car has no A/C and only blows heat, not ambient air. I know I have a vacuum leak. What parts would you guys look at first?
 

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89 190E 2.6 x2
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I'd start with testing all of the vacuum pods for the climate control system. The most common failure is the defroster pod behind the instrument cluster. It's a 2 stage pod and the rubber belows tend to rip. Get yourself a handheld vacuum pump to test the pods. Easiest access point is to pull the glovebox out, then the right vent. This will give you access to the switchover valves and from there you can test each pod to see if they hold vacuum.
 

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1987 190E 2.6 Midnight Blue
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Discussion Starter #3
Okay sounds like a fun project, I’ll see if I can get one of those from harbor freight or something. It seems that air is constantly seeping out from somewhere in the dash. Could this possibly be where my ac leaked out?

im not too familiar with how climate control systems work
 

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A/C repairs typically are not do it yourself repairs. They should be done by an experienced a/c technician.
 

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1987 190E 2.6 Midnight Blue
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127 Posts
Discussion Starter #5
A/C repairs typically are not do it yourself repairs. They should be done by an experienced a/c technician.
I agree and wouldn’t dare try to repair it myself. It’s just really expensive to have a shop do it and it’s not absolutely necessary as we head into the cooler months.

I was curious as to whether the leak I’m hearing could potentially be the source of my refrigerant leak?

If so, I’d like to fix the leak so I can refill my ac system and go from there before paying to replace the compressor (which might actually work idk) or converting the entire system to r134

i also just don’t like my car making weird noises while I drive it in general🙃
 

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W201 Moderator
89 190E2.6- 5-speed Manual, 95 E320 Sportsline-sold, 2001 E320 4matic Wagon-sold
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3,495 Posts
This is one diagnosis/repair you either DIY or don't repair it at all. 95% of mechanics will not be able to help you or it will cost you an arm and a leg if they claim they can fix it.

355 has given you good advice, you would do well following it. He has done these repairs himself, multiple times on his cars.

Run the car without AC and actuate different flaps using the center console buttons. See if you can make the hiss come and go. If you can, it is not related to your AC but just your vacuum controlled flaps/switchover box.

The flap actuators are starting to disappear from the shelves but the following is a list of the flaps one could buy 3 years ago when I replaced 2 of my actuators for the hiss/leak.

- Cheers!

2651868
 

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1987 190E 2.6 Midnight Blue
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127 Posts
Discussion Starter #7
Thanks for chiming in Dolucasi! I guess I will try and repair it on my own.

I don’t daily drive the car and don’t have the convenience of storing it at home so it takes me a while to actually carry out some of these projects but I will update per usual. Just needed to know where to start
 

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89 190E 2.6 x2
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Yep, with these cars don't be afraid to dive headfirst in and tackle the issue! Dolucasi and I have both done a lot of troubleshooting on these vacuum controlled HVAC issues over the years.
And honestly, I believe that AC systems can be a DIY project as well. Over the summer I successfully replaced the AC condenser and recharged my system. I just asked my neighbor (retired mechanic) and borrowed all his AC equipment. I've also got other friends with 2.6's and we've successfully repaired their ac systems.
 

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1987 190E 2.6 Midnight Blue
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Discussion Starter #9
Wish you guys were close by to help me out haha. So I can confirm that all my buttons work on the climate control box and the hiss goes away when I turn on the vents.

The center vent sometimes doesn’t turn on but it works more often than not. It seems like heat is constantly seeping out even with every button turned off as well.
 

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89 190E 2.6 x2
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Ok, you've definitely confirmed that you have a vacuum leak somewhere since theese systems default to heat on whenever there's a leak. Once you've got that vacuum tester you'll really be able to isolate the issue.
 

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1987 190E 2.6 Midnight Blue
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Discussion Starter #11 (Edited)
So today when i was leaving work the low level coolant light was on. Very weird because the car was sitting all day. Second or third time this has randomly happened with no signs of any leaks around the radiator, reservoir nor coolant on the ground.

Got on YouTube to watch Mercedessource and apparently leaking heater control valves are a common issue on our cars and I believe the hissing sounds, the fact that the car only blows heat in addition to the coolant slowly leaking out makes this the most likely culprit. The leak tends to be very slight and is difficult to detect since it’s deep in the dash.

question is how does the heater control valve work in relation to the a/c? If I replace this part could my a/c possibly start working again? Wishful thinking , I know haha
 

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89 190E 2.6 x2
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Sounds like you've pinpointed your issue. Heater control valve controls the flow of coolant into the heater core and they usually have to be replaced every 15 yrs roughly. Replacing it won't fix the AC issue though because they're not connected. I've had a bad heater control valve and working AC before.
 

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W201 Moderator
89 190E2.6- 5-speed Manual, 95 E320 Sportsline-sold, 2001 E320 4matic Wagon-sold
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To add to 355's comments the control valve is actually accessed from under the hood. I think you may have to remove the black leaf cover to get to it.
It however does not normally make a hissing sound when it starts leaking coolant, your must also be leaking vacuum.
 

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1987 190E 2.6 Midnight Blue
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Discussion Starter #14
You’re correct dolucasi it’s inside the leaf cover. I’ll pull it to see how bad the leak actually is. And thanks for letting me know I may not be off the hook with the vacuum leak quite yet.

It seems I will need to repair my climate control system entirely. Just discovered my fan speed switch is now jammed and won’t go past 2.

radio is already out and i know to pull the glove box to get to the vacuum pods but what about the fan speed lever?
 
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