Transmission Vacuum - PeachParts Mercedes-Benz Forum

tobybul · #1 11-11-2006, 09:41 PM

Troubleshooting the vacuum of my 84 300D tranny... tested the vac lines with mityvac.... fixed central lock but vac line to tranny does not hold vacuum .... and line to the central plug is also not holding vac. This is with engine not running.

I assume its not normal that vac will not hold.

Therefore, what then could be wrong? Assuming that the vac line has no leak, is there a device at the tranny that may be leaking? Modulator Valve? My tranny does shift funny, like 1st to 2nd too hard, 2nd to 3rd to quick, downshift 3rd to 2nd hard.....

What about the line to central plug? Vac control valve? EGR valve?...

Brian Carlton · #2 11-11-2006, 10:19 PM

If the line to the transmission won't hold vac, the modulator or the connection at the modulator is leaking. There is a small rubber hose that connects the Tecalan line to the modulator at the transmission and it's in a harsh environment, so, I'd take a look at that first. The trans will shift exceptionally harsh if that line has any significant leak.

I'm not understanding where the "central plug" is located? Is this the plug to the black box on the top of the valve cover?

tobybul · #3 11-12-2006, 01:49 AM

Quote:

Originally Posted by Brian Carlton

If the line to the transmission won't hold vac, the modulator or the connection at the modulator is leaking. There is a small rubber hose that connects the Tecalan line to the modulator at the transmission and it's in a harsh environment, so, I'd take a look at that first. The trans will shift exceptionally harsh if that line has any significant leak.

I'm not understanding where the "central plug" is located? Is this the plug to the black box on the top of the valve cover?

Yes, Brian, its the black box on top of the valve cover.... I will chk the modulator and the hose tomorrow... can the modulator be repaired in case its leaking? And where exactly is it? Thanks

Brian Carlton · #4 11-12-2006, 10:15 AM

Quote:

Originally Posted by tobybul

Yes, Brian, its the black box on top of the valve cover.... I will chk the modulator and the hose tomorrow... can the modulator be repaired in case its leaking? And where exactly is it? Thanks

Ok, with regard to the black box on the valve cover. Please follow this carefully:

There are three Tecalan lines that go into the black box:

1) Supply vacuum. This comes from a T over near the brake booster. Remove this line and toss it away. Now, you've got a T with an open port. Remove the T completely and connect the two remaining Tecalan lines with a short piece of 4mm vacuum hose.

2) Vent. This also comes from a T over near the brake booster. Remove this line and toss it away. Now, you've got a T with an open port. Remove the T completely and connect the two remaining Tecalan lines with a short piece of 4mm vacuum hose.

3) Output to EGR. This line goes to a mechanical temperature switch on the thermostat housing. Remove this line and toss it away.

If you accomplish the above, all of the leaks that are currently happening in your 3-2 valves will be eliminated and the trans may magically shift better.

The modulator can be replaced. But, let's hope that the hose connection is leaking. Just follow the Tecalan line down to the transmission. It's on the driver's side. You can't miss it.

larry perkins · #5 11-12-2006, 10:32 AM

one of the most common problems i find on these old cars is a leaking rubber cap on the modulator,this is the one you remove to adjust the modulator pressure.the cap has to hold vaccum.
thanks for the line removal instructions brian,got it printed.
larry perkins

tobybul · #6 11-12-2006, 10:51 AM

Quote:

Originally Posted by Brian Carlton

Ok, with regard to the black box on the valve cover. Please follow this carefully:

There are three Tecalan lines that go into the black box:

1) Supply vacuum. This comes from a T over near the brake booster. Remove this line and toss it away. Now, you've got a T with an open port. Remove the T completely and connect the two remaining Tecalan lines with a short piece of 4mm vacuum hose.

2) Vent. This also comes from a T over near the brake booster. Remove this line and toss it away. Now, you've got a T with an open port. Remove the T completely and connect the two remaining Tecalan lines with a short piece of 4mm vacuum hose.

3) Output to EGR. This line goes to a mechanical temperature switch on the thermostat housing. Remove this line and toss it away.

If you accomplish the above, all of the leaks that are currently happening in your 3-2 valves will be eliminated and the trans may magically shift better.

The modulator can be replaced. But, let's hope that the hose connection is leaking. Just follow the Tecalan line down to the transmission. It's on the driver's side. You can't miss it.

Brian, thanks for the suggestions.... I will do as you say... but the analytical mind in me needs an explanation to the above as to why 1,2,& 3 above do not need to be connected anymore. Also, why the vent? What's its function?
I am sure you've explained this to idiots like me several times already. If you don't mind going thru it one more time or point me to a thread that covers it, I would appreciate it.

Thanks, George

Brian Carlton · #7 11-12-2006, 11:35 AM

Quote:

Originally Posted by tobybul

Brian, thanks for the suggestions.... I will do as you say... but the analytical mind in me needs an explanation to the above as to why 1,2,& 3 above do not need to be connected anymore. Also, why the vent? What's its function?
I am sure you've explained this to idiots like me several times already. If you don't mind going thru it one more time or point me to a thread that covers it, I would appreciate it.

Thanks, George

The three lines are strictly for EGR on your vehicle.

Line #1 is supply vacuum to the 3-2 switches that operate the EGR.

Since "vacuum" is the absence of air, a vacuum system needs to have an air inlet at the other end of the line from the vacuum pump. On most M/B's, the air inlet is underneath the dash so that relatively clean air is provided to the system. Apparently, M/B believes the air under the hood is filthy........wonder why??

So, most vacuum consumers that are sensitive to filth will have a vent line into the cabin.

Line #2 is this vent line. If you follow line #2 back to the T, you'll see that one end of the T goes into the cabin and the third line goes to the VCV on the top of the injection pump. The VCV also needs "clean" air from the cabin.

Line #3 is the output line to the EGR valve. It stops at the temperature switch in the thermostat housing to prevent EGR when the engine is cold. The second line from this switch goes directly to the valve. You can remove this line and toss it as well.

tobybul · #8 11-12-2006, 07:11 PM

Ok, did a few things this afternoon....

looked at the tranny modulator valve. Looks like Larry was right about the rubber cap. It looked pretty cracked all around. Since I don't have a spare cap (not sure yet if it can be purchased separately), I relied on my the 'ol black silicone. Dabbed it all around the cap and a bit inside of it carefull not to let it touch the copper tee that would be under the cap. Put it back on and let it cure. Fast forward - Not sure if I was imagining it but the upshift from 1st to 2nd was less noticeable when I tested after the above fix. Downshift from 3rd to 2nd still a bit harsh but not as much (I did not yet plug the 1, 2 and 3 items Brian suggested). And I don't really know if this is making the line to the mod hold vac nor if the mod valve is really functioning (didn't test it with the mityvac yet either).

Also, replaced the stat and noticed my temp climbed to just above 80C. This is good b/c it was staying at 40C b4 and heat was not very good. Now I'm getting good heat.

Just make sure next time you get the right gasket with the stat. NAPA's did not allow for the the bypass. It was just round.

I plan to do Brian's suggestions but I'd like to replace the rubber joints first and see if this will make the vac hold with the components in the engine such as the temp sensor, egr, etc. I guess call me old fashioned.

Open to tips and suggestions anytime. Thanks so much.

running-snail · #9 11-12-2006, 07:27 PM

You and I have the same situation.

I tried to solve EGR problems, purchased the little levers for the 3-2 valve under the black cap on top of the valve cover to fix vacuum problems. This is explained on DieselGiant's web site - good write up. If you decide to go this route, let me know and I'll send you the new levers - no charge - however...

Several months ago I bypassed the EGR - I think based upon a previous Brian Carlton post (very knowlegable & helpful member). I encourage you to do this. There are many benefits, few detriments - none from an operating standpoint.

Keep checking the modulator line vacuum with your vac. guage. THIS IS A KEY TO RESOLVING SHIFTING PROBLEMS.

I have purchased, but not replaced, a modulator valve o-ring in the hopes that this might resolve a slow vacuum leak. When I started, the line to the modulator valve wasn't even attached - it took me some time to even find it (caught on top of the trans). I connected the line to some vacuum hose, then to the modulator and viola, smooth shifting.

Anyway - my vote is that you 'get rid of' the EGR lines...

tobybul · #10 11-12-2006, 09:23 PM

No question it will be my fallback plan, to disconnect or toss as Brian suggested. I know I am reinventing the wheel by trying to revive the system as many have probably tried... But it'll be a good project ....

I ordered some vac lines from ********.com. They seem a tad smaller in diameter as the orig. I think I ordered 3mm and 3.5mm ID. Does this mean the origs are 4mm? How about the rubber connectors? Can I just go to an AZ and get like a 7/64" rubber hose and cut it to length?

I also noticed tranny oil wetness just infront of the tranny pan. I checked the pan bolts and noticed the the 2 front ones where not as tight as the others. The middle 2 where somewhat tight and the back 2 where tight. So I tried to even them all out. Hopefully, the leak was coming from there.

These are times when I wish I had a hoist.. I'm tired of crawling under the car and working looking up. I had the front of the car on ramps this time but working looking up really gets to me sometimes.

running-snail · #11 11-13-2006, 09:21 PM

I picked up 3mm - this seems fine for connecting to the hard tecalan line Brian referred to, at least on my 300. A tight fit is preferred, I would think, for vacuum lines.

Let me know if you need the 3/2 valve levers (or another 3/2 valve, for that matter).

I hope your leak is from a loose pan. I ended up replacing my pan - extremely warped, as it turned out. I'm still leaking from the front seal or pump.

tobybul · #12 11-13-2006, 10:48 PM

Quote:

Originally Posted by running-snail

I picked up 3mm - this seems fine for connecting to the hard tecalan line Brian referred to, at least on my 300. A tight fit is preferred, I would think, for vacuum lines.

Let me know if you need the 3/2 valve levers (or another 3/2 valve, for that matter).

I hope your leak is from a loose pan. I ended up replacing my pan - extremely warped, as it turned out. I'm still leaking from the front seal or pump.

Sure I'll the knockers... oops I mean the levers... its hard to concentrate with your aviatar staring at me.... How would you send them? I did open up the central plug housing today and noticed the 2 levers were worn....

Brian Carlton · #13 11-13-2006, 11:03 PM

Quote:

Originally Posted by tobybul

Sure I'll the knockers... oops I mean the levers... its hard to concentrate with your aviatar staring at me.... How would you send them? I did open up the central plug housing today and noticed the 2 levers were worn....

Why bother with those levers?

Take the old one's out and toss 'em.

Then follow my instructions as noted above.

This is the EGR system. If you aren't running EGR, you're wasting your time and money.

aceofspd · #14 11-14-2006, 12:28 AM

I spent a month thinking my modulator valve was loosing vacuum too fast.
It turns out the 3/2 valve was opening at first movement of the throttle (as it should) and bleeding off to a small leak. When trying to find the small leak, it became a big leak. I just kept tracing back and clamping off until I had done as Brian has advised you to do. I then had to readjust the modulator valve that I had been fiddling with, and WOW perfect shifts.

Now advise anyone with vacuum/shift issues to block those lines atlest temporaryly before they do anything else. They just control the EGR valve wich does nothing anyway.

I use a set of hemostats (roach clip) when tracing vacuum leaks. I always clamp the line to the heating system right after I clamp the EGR circiut.

GREG

PanzerSD · #15 11-14-2006, 12:58 AM

Yeah, I agree that Brian's name should sctually be spelt BRAIN.

a great asset to this forum.

but what about us guys that aren't running the EGR setups? what are the likely leaks on these engines?