New to this forum and to Mercedes Diesel vehicle...

Someone suggested I check the Vacuum Amplifier with a gauge hooked into the rubber fitting position marker TRA. Results were 5" vac when vehilce is at idle. I squeezed the other fitting (dead center one) with fingers and then non teeth pliers and watched the gauge go up. Released grip at 15" just in case of trouble...

Followed the direction of the hose from that location to a 3 way fitting. Squeezed each node, one at a time and found one made a change in pressure.

Followed that hose back towards windshield, through hole in firewall and came to a 4way fitting.

Repeated the squeeze test and found that 2 of the nodes affected vac, so I tried something with my heart in my throat...

I clamped the one node until I increased vac to 10", closed the hood and took a little ride down the road.

Shifts felt fine and when I came to a stop, did not hear the clunk. Did this a few more times and thought I may have fixed the problem and could go eat dinner and remember who my wife was!

Turned the car off, but it wouldn't turn off, Until I released the pressure of the clamp on hose fitting.

My question is, What should the reading be at the TRA point on Vac Amplifier?

Second question is, is this an OK way to test something like this problem?

If so, can someone with a 1985 300SD repeat test if they have time and post results?

Regards,
John Kuhtik

The clunk is caused by low vacuum to the transmission. Measure the vacuum at the line that goes to the transmission modulator as you drive and shift up and down. You can hook up the mity vac, run the line out thru the cowl and leave the mity vac so the gauge is visible thru the windshield. Lock your hood back down.
In most of these cases, there is a vacuum leak somewhere other than the transmission system itself. For instance, I had a leak inthe climate control in my 85 TD. It would clunk on downshifts if I had the heat on but not if I didn't. A simple test for you would be to push in the far right button on the climate control. If the clunk stops, you know a leak in the climate control system is causing your problem.
Plugging the yellow door lock line under the hood would also exclude a leak in that system as the cause of your problem.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Thanks for reply.

I tried turning off climate control by pressing in the far right button but still have problem of clunk sound.

Also tried hooking up a vac gauge like you said. I put it between the TRA point on the Vacuum Amplifier and the green dashpot that leads to the transmission through the Vacuum Modulator.

When I left the driveway it was at 5" or so, and driving along it never went up but slowly dipped down. although it never got to zero and always clunked at 10 mph at the decel...

I will check for the other line that you mentioned.

There are lots of good threads on transmission vacuum here. You should measure the line coming out of the Vacuum Control Valve and going to the transmission modulator. It should be between 12-15 inches and drop down towards 0 the harder you push on the accelerator.
5 inches sounds too low.
I'm guessing you have a vacuum leak somewhere reducing the vacuum in your whole system. Are your door locks working ok? Have you measured the vacuum on the main line coming from the vacuum pump and going to the brake booster? Be careful that main line can get brittle.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Hey Kerry,

I opened the connection to the brake booster and got a reading of 18.5". Is that also too low?

Not sure if I have a Vac Control Valve on the 1985 300SD.? On top of the engine I have no black plastic cover as seen in some instructional archives.

There is an electronic switch that gets pressed in when at full throttle.

However, I did notice that on top of the Vacuum Changeover Valve, there is a small black hose with a screen filter of sorts. it is no bigger than an eraser and the metal band on top of a standard pencil. the side wall of this part was split and debris was getting sucked into the top barb of this three way valve... for how long, who knows. I used a drill bit .090" (SLOWLY TWISTING IT BY HAND) to auger out the dirt but have a feeling that there is more inside the plastic housing... One hose that runs from it, goes to the banjo bolt, it used to be a clear hose i suppose, but it has been filled with dark stuff. looks like it is on the inner wall of the hose but not clogging it. Is there a way to test this valve? If it is clogged, would that affect the vac level mentioned earlier, at the Amplifier? Also, if the hose to the banjo bolt is restricted, would that be adding to the clunk situation?

Regarding the door locks. When I open the car, the drivers side lock opens first and the other door locks pop up one at a time. Probably takes 1.5 seconds for all of them and the trunk. Hear a lot of noise at the trunk when this is all happening. Is that normal time frame? Or are they all supposed to open together?

I did a second driving test, pinching the second node on the 4way connector behind the firewall. It had a clear/blue or white/blue hose running out of it. When I clamped it to the point of seeing the needle go up to 10" at the TRA point on the Vacuum Amplifier, I took the car for a ride and found that the pickup was terrible, shifting felt strange, but when slowing down to a dead stop, I had no clunk sound at 10 mph or anywhere on the decel and I think I was able to turn the vehicle off...

Just came back from errands. Drove the car and put it in 3 rather than keeping it in D when I was slowing down. I dropped the stick to 3 at around 30 mph in the decel and when slowing down, I do not experience the clunk at 10 mph...


Where is the yellow hose you mentioned (for the door locks)? If I plug it, does that mean that the door locks won't work at all?

Lastly, if I remove the Vac Amplifier, can I somehow test its function with a couple of gauges and a hand vac pump? Do they go bad?

Does anyone else have a 1985 300SD? If so, could you do a quick test and attach a vac gauge to the TRA spot on the Vacuum Amplifier... Record reading when vehicle is at Idle... Just wondering how low me 5" reading is.

Sorry, I missed the fact it's an SD. The door lock system is not connected to the engine vacuum pump on your car.
Search for vacuum test with Brian Carleton's user name. I think he was involved in a test of the vacuum amplier a few months ago.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I don't have experience with the '85, but the system is the first year that the vacuum amplifier was utilized and it's identical on the '86, so my recommendations should be valid.

First and foremost, the vacuum level of 5" to the transmission modulator is way too low and it's the cause of your hard downshifts. This figure would typically be about 12", however 10" will also work quite well.

The problem most likely resides in the VCV, the plastic valve that modulates transmission vacuum on the rear of the injection pump. See if you can T the gauge into the outlet line from the VCV to the vacuum amplifier (blue flying saucer). On the VCV, there are two lines coming into the top of the valve. One line is the supply vacuum and the other line goes to the vacuum amplifier. It's the second line to T the gauge into.

See what the vacuum is at idle in this condition and report back with results.

If you have low idle vacuum with this test, then you must test the supply vacuum to the VCV. That's the other line entering the top of the VCV. If this line also shows low vacuum at idle, you've got a leak in the system somewhere else (likely) and that leak must be fixed to bring the vacuum up.

Ok Brian,

I will do those tests later today. Want to (need to) go to the river and fish for trout...

Can I send some photo's with a reply on this forum? If so, how is it done?

Is this the VCV

[IMG]file:///Users/johnkuhtik/Pictures/iPhoto%20Library/2008/04/03/P1010002_2.JPG[/IMG]I hope the attached photo is viewable

Attached Thumbnails

 

yes.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Yes, that is the VCV.

However, it's different to the one I described above. Apparently, the supply vacuum goes into the single top port. The output vacuum to the amplifier must be one of the two lines that extend from the bottom of the unit. One line will go to the amplifier and the second line is the vent...........goes into the cabin via the vent lines. Be sure of the difference.

what is on the far end of the vent line in the cab?

Brian- I don't have an SD, but assuming it is a similar setup, doesn't the single port on top (with green inline dashpot) output to trans modulator?

__________________
'83 300D Turrrbo 295K miles

Here are some other photo's of the VCV. All that oil, does that tell you anything?