Climate Control/Vac Leaks - PeachParts Mercedes-Benz Forum

diesel don · #1 11-07-2002, 11:44 AM

I have read hours of posts and have diagnosed my problem but still need some help on my '83. Perhaps PSFred or some of you that understand this problem might assist me.

SYMPTOM: I loose vacuum with the acc dial turned to heat. Door locks cease to function and car won't turn off.

DIAGNOSED: In taking out the center "push-button" control unit I found several leaks.

1. Three of the five buttons (the 3 on the right of the horizontal panel) leak so bad that I can hear air escaping from them.
2. When I push the 4th button to the left (solid air up and down), the leak stops and all vacuum fuctions of car return.
3. If I rotate the dial back to blue, all vacuum leaks stop no matter what button is pushed.

QUESTIONS:
1. Are the dial and three buttons bad?
2. I don't understand servos and what they are? (I'm fluent with door lock actuators and the use of my precious mighty vac. Are the servos similar?)
3. Can these problems be addressed without taking out the dash?

Thanks

Don

Alec · #2 11-07-2002, 12:17 PM

I cant help with your vacuum problems. My 300TDT suffers from some similar issues such as the 'EC' not working enless I hit the defrost first. What a joy.

The reason I wrote is to ask you about your 85 wagon. Mine has 291kmi and I am nervous about the timing chain although I have been told by several mechanics that the stretch is fine and I dont need to change it. Have you changed your timing chain? What are you thoughts?

Thanks,
Alec.

diesel don · #3 11-07-2002, 12:57 PM

I'm not too worried about my wagon. In my 240D, I suspect that a worn chain is affecting the timing. I'd like to practice on that car with chain & IP so that I gain some insight before tackling one of the 300's.

I don't think I'd worry if I were you if you've been told its good and if it functions well.

On the other hand, its not too much $ if you do it yourself. Under $100 if I remember right.

Don

kerry · #4 11-08-2002, 11:37 AM

You've probably got a leak associated with one of the vacuum actuators that operates only on heat. The one that immediately jumps to my mind is the footwell ducts actuator. That one only works when the heat is on. Do you have air coming out in the footwells when the heat is on? My guess is that you don't and that is where the problem lies. That particular actuator is directly to the right of the accelerator behind the ducts. To access it, remove the kick panel under the driver's dash. You will be able to access that actuator and the one for the defrost vents.
If that actuator is not the problem, perhaps it is the one that decides whether to bring in fresh air or recirculate the air. That one is behind the glove box somewhere. Remove the glovebox and you will see at least two actuators back there. I think the one I refer to is to the right.

Steve300 · #5 11-08-2002, 02:26 PM

Kerry, I think I have a similar problem with the footwell duct.

How do you test this actuator to see if it is bad?

Thank you

Steve

_______________
1985 300 D Turbo

kerry · #6 11-08-2002, 06:08 PM

Do search on 'center vents' and you'll see a lot of tips on how to test it. Listen to it to see if you canhear a vacuum when it is supposed to open. Also use a Might Vac to pull a vacuum on the actuator and line to see if and where it leaks.

diesel don · #7 11-08-2002, 06:19 PM

Kerry:

Thanks for the advice. I've watched (with a flashlight) the footwell diaphram open and close with dash adjustments. In addition, I get heat out of there. I'm wondering if its one behind the glovebox. Perhaps I'll try this next unless anything else comes to mind.
Thanks

Don

psfred · #8 11-08-2002, 08:41 PM

Don:

Sounds like you have a bad pushbutton control unit, or bad actuators.

The only way to test is to attach a vac pump to the heat actuator and see if it holds vac. If it does, the PBCU is bad, get another one. If it doesn't, replace the actuator (this one, at least, is easy to get to.

It is possible for the actuator to have a leak and still work, but that shouldn't kill the rest of the vac system.

Do your locks work with the car off? ACC off, or on? If the locks don't work with the car off, or they fail when you select heat, I'd suspect there is more wrong with the vac system.

You should probably blow the $40 on a MitiVac hand pump. I suspect you will find that you have lots of bad vac connections in the engine compartment and have low vac supply along with a problem with the ACC.

When you get the pump, disconnect the green vac line in the engine compartment and pump the system down with the hand pump. Must hold vac. Big leaks will be obvious. If you hear leaks at the pushbutton control unit, replace it, not really a repairable part. Good used one should be around $75.

Peter

diesel don · #9 11-09-2002, 12:16 AM

PSFred:

Thanks for the comments. I'll be digesting them for awhile.

1. I've owned a mighty vac for a year now. Use it almost every day. Hey I own three of these cars and do constant vacuum work.

2. The car holds vacuum perfect. All doors and everything else perfect......EXCEPT when the heat dial is rotated to the red section. On AC all vacuum related functions are perfect.

3. In addition, there is one setting that still causes the car to hold perfect vacuum while on heat. And that is the 2nd button of the five. The one where the up and down arrows are solid.

4. Once again, I say that these cars have personality....even when broken.

5. I have audible leaks from the settings when the 3rd, 4th, or 5th buttons are pushed but ONLY WHEN THE DIAL IS ON RED (HEAT).

6. When I put the mighty vac on the green climate control line from the engine, it leaks down. I know where the leaks are because I can hear them. They are behind the PBCU and are somehow linked to the rightmost 3 buttons. I simply don't know what lies behind it and what the actuators look like. It looks like spagetti in there and I'm only used to the door actuators.

If any of this makes more sense, feel free to steer me clear....

Thanks

Don

MarkM · #10 11-09-2002, 08:09 AM

Based on your descriptions, here are the potential culprits:

When on the second button (solid arrows up and down), there are only two changes when you switch from heating to cooling mode (by the temp. dial):

1) The third vacuum switch (mounted behind the pushbutton unit, in a row of five, each with an electric plug and two vacuum hoses) actuates (opens) allowing vacuum to go to the center vent vacuum element and to the second stage of the two defroster element (the "elements" are the pink and gray plastic things with rubber diaphrams that open and close vent flaps). If when you are in cooling, you get air out of the center vent, then that vacuum element is ok...if the center nozzles dont blow air, you may have a leak in the center vent element, or the defrost element discussed below. This center vent element is toughest to get to, located way up under the middle of the dash, behind the center nozzles. Now, from the same vacuum line, vacuum goes to the second stage of the defroster element....this is located in the driver side foot well, kind of behind the ingnition switch, and has two vacuum attachments, one on the bottom, one on the side. The second stage is actuated by the side attachment....there are two rubber diaphrams in this element upper and lower...if there is a leak in the upper, defrost vents wont open, if leak is in the lower, vacuum will pass down to vacuum switch number one and leak out the bottom of it (this switch is closed, allowing vacuum to pass out the back) you may hear the leak, behind the pushbutton unit. To test the two stage element, pull off both vacuum lines, and pull a vacuum on one nipple, then the other...each should hold a vacuum with the other nipple open to the air.... if not, replace the element. Now to test the center vent element, attach your hand vacuum tester to the line coming off the side attachment of the defroster element, then pinch off the line coming from the third vacuum switch behind the pushbutton unit, and pull a vacuum. Pull and release vacuum a few times....if you hear the center vent flap open and close, and it holds a vacuum, the center vent element is ok...if not, that element is ng...real tough to replace (I think you have to remove the dash!!!).

2) The only other possible difference in the system when you go from heating to cooling is that, you may be in "recirculating" mode (fresh air flap closed) or in "fresh air" mode (fresh air vent open). The fresh air element is located far to the right under the dash behind the glove box. Remove the glove box, remove vacuum line from the element, and test the element to see if it holds vacuum.

That said, the fact that you said that you hear leaks behind the pushbutton unit, the problem may not actually be with those switches...as described above, there are conditions where a leak in a vacumm diaphram element may allow vacuum to escape out of the back of one of these switches. However, it is a good idea to pull each of these switches out, attach a hose to the nipple extending from the front (not the side one), and try suck air through it...you should not be able to get air. Then attach tube to side nipple, and suck air...you should be able to pull air through the back vent of the switch. Then repeat the above while applying 12 volts to terminals, and you should be able to suck air through the front nipple. Then attach tube to side nipple, block the front nipple, and you should not be able to pull air through the vent on back of switch.

Once you are sure these elements are working, then do tests on other system components. I have had to replace two of these vacuum switches on my 1985..with good used ones...Im getting close to the time when I may buy five new ones at about $25 each so I wont have to check them every time I have a climate control vacuum problem.

good luck,

Mark

jerryb · #11 11-09-2002, 12:44 PM

There is another valve behind the pb unit called the deverter? flap, this changes air flow, if the leg flaps open they are ok, the fresh air flap behind the glove box default to recirc if bad.
If you pull the right panel on the trans tunnel you will find a row of vac. switch over valves, they seldem go bad but should be tested, then you can pull vac to each location from one spot to isolate your leak. You should get a vac. diagram they are all color coded.
jerryb

psfred · #12 11-09-2002, 01:59 PM

Don:

I think, upon reflection, that the pushbutton control unit is all electric, the vac is controlled by the switchover valves (a solenoid that opens or closes a vac line). If they are bad, or worse, a hose has been pulled off, you can have a big vac leak.

You will have to get to them, they are buried (hopefully not as badly as the cluster retaining knob on the W108!) and check to see that they hold vac.

Certainly, if you can hear one hissing and it is no the hose, replace it, it's not worth messing about with them.

I'm still curious as to why the door locks die, though -- are the supply lines clogged? You clould also have a weak vac pump, too, if a leak in the ACC causes the door locks to die.

Peter

diesel don · #13 11-09-2002, 02:39 PM

Mark & Peter:

Very thorough analysis and much appreciated. Currently being printed off.
This is alot of food for thought and I'm heading out right now to begin to narrow it down.

Thanks again,

Don

MarkM · #14 11-10-2002, 01:03 AM

Peter had a good point about possible weak vacuum pump....always when diagnosing vacuum problems, first pull off a hose on the main vacuum line from the vacuum pump to the brake booster and check to see how much vacuum you have...ideally you should have a maximum vacuum of about 15 to 18 inches of mercury.

A leak in the ACC system can cause failure of door lock system because you may not be able to build enough pressure to "fill" the door lock vacuum reservoir.

Mark

#15 11-10-2002, 09:53 AM

I went thru some thing much like this on our 78 CD... Drove me crazy.. .. The problem when I found it was the vacuum tubes where they go from the body to the doors.. where it bends all the time.. Inside those rubber covers that have the tubes and wires in them... The tubes had bent time after time.. and cracked.. Problem was some times it leaked and some times it did not.. Under a test with a test pump it would suck it self shut but open the locks and open the door to check some thing and it would leak down... I cut the tubes where they bend and put hose over the bend area where the cracks were... Darn thing almost drove me carzy... Almost replaced the whole AC vacuum system before I found the problem..

Ken