Hi my SD has a downshift knock from 4th to 3rd gear, I pin pointed it to the climate control system, When only the ec button is pushed, it will clunk on down shift. When I push any other button it goes away ! Dose anyone know
where I should start to look ? I would think it would be after the climate control unit and the ec button but where? also heat is only 104F at max heat
I dont know if both problems are related. Thank you for the help, J

Welcome to the world of M/B vacuum systems.

What you have is commonly known as a vacuum leak. In reality, you've got more than one.

The problem is that the climate control system is leaking vacuum and affecting the available vacuum for the transmission.

The best solution is to fix the climate control system. This requires taking the glove box out and removing the carpeted panel on the right side of the transmission tunnel. You'll need a Mityvac and you'll test the five vacuum pods that run the vacuum doors for the system. You'll probably find that two or all three of the dual pods won't hold vacuum and won't properly operate the doors.

These must be replaced and all will be fixed.

It's a lengthy job and the vehicle must be left in disrepair while you go and get some parts.........but........when completed, the satisfaction of a properly running climate control system cannot be underestimated.

Are the pods on the transmission tunnel or behind the glove box ?? thanks, J

The three most common pods that fail (the dual pods) are all accessed via the glove box. Two control the main air door and are horizontally opposed. The third is the defrost pod and it's tucked over on the very left side of the space.........a bit difficult to twist and remove.

The vacuum switchover valves are behind the carpeting of the transmission tunnel.......on the right side. Getting these out in the open allows easy testing of all the pods with a Mityvac. Otherwise, you'll have to remove the pod to test it......not something that you want to do.........if the pod is good.

One more thing, How hot should the heat come out of the vent or
def ,w/ the dial on max ?? I am getting a max of 104 deg. is that temp a normal max temp? or should I order a mono valve rebuild kit ?? thanks again , J

Unplug the monovalve and repeat the test.

1. I got the same 104 deg temp with the monovalve pluged in or not!

2. When I take the carpet off the side of the tunnel, there is a bank of 6 vacuum relays, do I check the load side, (thats the one with the colored hoses)? Am I Correct

1. Take the monovalve apart and look for a tear in the diaphragm at the end. Be careful that you don't drop the four screws that hold the cover.

2. You are correct. Remove the hose from the horizontal port........connect the Mityvac to the hose..........and see if it will hold vacuum. If it won't, make a note of which relay(s) won't hold (#1 is at the top) and post back on the forum.

Ok here they are !

1. Dose not hold

2. Holds ,but I have to pump it up fast, then you can hear the flap close it holds fine.

3. 3 splits into two lines a red and green, the green holds fine and the red dose not.

4.same as # 2

5 Dose not hold.

6. Holds fine !
thanks

Yep, I believe you've got the problem with two of the dual pods.

1. Remove the defrost pod. It's stuffed into the corner on the left side of the glove box and it's angled downward toward the base of the windshield. They normally twist to remove, but, the older one might be different.

After you remove it, connect the Mityvac directly to the rear port. Pump it up and confirm that it won't hold.

2. Remove one of the dual pods that control the main air door. They're right in front of you as you look into the glove box. They are horizontally opposed and they both work to move the door. One, or both of them are NG. Pull one and connect the Mityvac to the rear port. See if it holds. If it does hold, then reinstall that pod and remove the second pod. It will probably not hold. Due to the common supply line from #5 switchover valve, it's also possible that both of them are bad.

You can obtain replacement pods from this site or from the dealer.

The only one which would not hold vac was the one on the left side the other two on the right were fine ! is that all ? Could you tell me the place where to get them ? Thank you, J

The other two cannot be fine if #5 does not hold. Check #5 again. It must hold if the two pods that operate the main door have no internal diaphragm leaks.

My mistake was I was not letting the pod open fully before reading the gauge,
I guess I am use to electronic work, where every thing happens right away !or when I work on a dump trucks brake chaimber, Its all about FLOW !!
I screwed up , I did not mark the pod hoses ,I think the r/w hose is on the rear and the r/b is in the front ! Who has these Pods ? beside the dealer !

You can get the pods right on this site.

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1XL0VHEI01XL0VXEXP&year=1985&make=MB&model=300-SD-002&category=All&part=Vacuum+Element&appChassis=_any


Best to call Phil on these. One of the two main door pods is a dealer only item and he's got to go and order it. Maybe you'll get lucky and that is the only one that works. Be careful........the two main door pods are different.