Good vac news/Bad vac news - PeachParts Mercedes-Benz Forum

Maroon 300D · #1 07-07-2006, 11:48 AM

The good news: I believe I have figured out the cause of the rough shifting I've been getting on cool mornings.

The bad news: It's a climate control leak! This is bad because I was hoping I wouldn't have to take the dash apart to fix this problem.

This morning I got up a little early and tested the vac to my transmission. It registered at just under 6" hg at idle, down from 20.5" hg at idle when it's warm out (ambient temp.). Now before you freak out about the 20.5 number, realize I have an '85 CA car and the vacuum is a little different on it. In other words, the numbers on Diesel Giant's site and in other places may not be right for my car. I don't know what they should be, though...so if you have any thoughts, please chime in.

Now it was a bit dewy this morning so I used the defrost, which I haven't touched in a while...and the shifting was great! I did some more experimenting and sure enough, smooth shifting with defrost on and hard shifting with it off. Previously I'd tried the climate control in various positions - different temps and settings - and it hadn't seemed to make a difference. Guess maybe I'd neglected defrost. I'd also given some vacuum to the blue/green line that I think goes to the climate control and it held the vacuum, but that was when it was warm out and the problem only surfaces when it's cool.

My purpose for posting this is I did some searching and it seems the vac setup for the climate control in the '85's is, once again, a bit different than the other cars. I noticed on one site I saw that they sold vac diaphragms for climate control, but they were only good through the '84 cars. So in other words, I'm not sure whether the info. I find will apply to my particular car.

If anyone has any ideas about why my shifting would be good with the defrost on but not off...please fill me in! And if you have some theory on how I could maybe fix it without removing the dash, well I'd love to prove you right.

Thanks in advance. Thanks also for reading this long post if you've gotten this far.

Mismost · #2 07-07-2006, 01:36 PM

Keep in mind mine MB is an 83...so take this for what it's worth. But I THINK the defrost flap is controlled by a dual action pod...could be with the defrost in the off position the pod leaks causing bad shifts.

Good news! As I recall that pod is easy to access by removing the drivers side kick panel and it's right there. Just check/replace the vaccum hose connections first...you may get lucky and fix it for a nickle's worth of tubing.

All this vaccum stuff sucks!

Maroon 300D · #3 07-07-2006, 01:45 PM

Quote:

Originally Posted by Mismost

Keep in mind mine MB is an 83...so take this for what it's worth. But I THINK the defrost flap is controlled by a dual action pod...could be with the defrost in the off position the pod leaks causing bad shifts.

Good news! As I recall that pod is easy to access by removing the drivers side kick panel and it's right there. Just check/replace the vaccum hose connections first...you may get lucky and fix it for a nickle's worth of tubing.

All this vaccum stuff sucks!

I wouldn't have guessed the pod was under that panel...but it can't hurt to take a look! It's a good place to start, at least. I guess I'll remove the panel and see if I notice anything moving when I turn defrost on/off.

If you're right that would be wonderful. Thanks a lot!

dmorrison · #4 07-07-2006, 03:50 PM

The manual shows the same AC system in the 85 300D and I can't see Mercedes changing the evap box in the W123 model in 1985. So with that said, your defroster vacuum element is just above your gas pedal. Remove the driver lower panel and look for a black and pink element. this is the defroster unit.
A mityvac will help you determine if it is the element or not. I fthe lelment checks out OK then it could be the vacuum switch behind the AC control panel.
Let us know

Look here

http://www.peachparts.com/Wikka/W123Evaporator

And go to the picture

"080 300TD Unclip the connector base on the drivers side case"

That is your defroster element.

Dave

Mismost · #5 07-07-2006, 08:17 PM

DMORRISON YOU THE MAN! What a piece of work and a great DYI write up! I defere to your greater knowledge.

(but I would slit my wrists before I tackled that evaporator replacement...think I'd rather do brain surgery)

Brian Carlton · #6 07-07-2006, 10:19 PM

Quote:

Originally Posted by Maroon 300D

The good news: I believe I have figured out the cause of the rough shifting I've been getting on cool mornings.

The bad news: It's a climate control leak! This is bad because I was hoping I wouldn't have to take the dash apart to fix this problem.

This morning I got up a little early and tested the vac to my transmission. It registered at just under 6" hg at idle, down from 20.5" hg at idle when it's warm out (ambient temp.). Now before you freak out about the 20.5 number, realize I have an '85 CA car and the vacuum is a little different on it. In other words, the numbers on Diesel Giant's site and in other places may not be right for my car. I don't know what they should be, though...so if you have any thoughts, please chime in.

Now it was a bit dewy this morning so I used the defrost, which I haven't touched in a while...and the shifting was great! I did some more experimenting and sure enough, smooth shifting with defrost on and hard shifting with it off. Previously I'd tried the climate control in various positions - different temps and settings - and it hadn't seemed to make a difference. Guess maybe I'd neglected defrost. I'd also given some vacuum to the blue/green line that I think goes to the climate control and it held the vacuum, but that was when it was warm out and the problem only surfaces when it's cool.

Note the following:

The selection of "defrost" shuts down the vacuum to all actuators. So, if thre are any leaks in the pods, the system won't see the leak because no vacuum is sent to any actuator in this condition.

The climate control system should have a restrictor to prevent the exact scenario that you experience. The SD, like all of the 20 year old vehicles, had two leaking pods for years, resulting in defrost all the time, but the transmission was not affected because the size of the vacuum leak through the restrictor was not sufficient to drop the vacuum to the transmission.

So, if you can't get your pods fixed quickly, get the restrictor back in the system and all will be well.

Maroon 300D · #7 07-07-2006, 11:38 PM

Quote:

Originally Posted by Brian Carlton

Note the following:

The selection of "defrost" shuts down the vacuum to all actuators. So, if thre are any leaks in the pods, the system won't see the leak because no vacuum is sent to any actuator in this condition.

The climate control system should have a restrictor to prevent the exact scenario that you experience. The SD, like all of the 20 year old vehicles, had two leaking pods for years, resulting in defrost all the time, but the transmission was not affected because the size of the vacuum leak through the restrictor was not sufficient to drop the vacuum to the transmission.

So, if you can't get your pods fixed quickly, get the restrictor back in the system and all will be well.

Ok, here's what I've done so far: I found the pink and white (it was white in my case) vacuum element for what I assume is the defrost. It's located exactly where dmorrison said it would be in his outstanding (wow!) pictorial.

I discovered that this particular element holds vacuum, but pumping it up is a slow proicess for some reason. It takes several pumps on my Craftsman vac pump to get up to a level most where one pump would normally take it. Once there it holds it, though.

There seems to be a leak down the line, though, because when I keep that connected to the defrost element it doesn't hold vac. This is a red and blue line that seems to run from the back of the defrost element (as opposed to the vac line that runs straight down) to one of the center vents.

I took everything out that can be taken out other than the dash and there's no way to get to it.

So here's a question: what happpens if I put a BB in the line running from the back of the defroster element to the center vent?

And oh yeah, about the restrictor: what is the restrictor, exactly? I noted a black orifice behind the CCU but this doesn't appear to be directly connected to the defrost element.

Maroon 300D · #8 07-10-2006, 01:26 AM

Well I realized putting a BB in the line would be stupid, especially as that's the line that goes to the vents beneath the windshield and seems fine.

My problem must be in that unit/s behind the CCU somewhere, but I realized I wasn't sure how to test it, exactly, so I will research that a bit and try again another time.