Transmission adjustments
Proceed at your own risk. You can get into some trouble with this procedure.
If for example you tap into the wrong pressure port with a gauge attached
that is not capable of measure high pressure, the gauge will destruct. In
my case, I just destroyed the Bourdon tube and blew off the hose getting
transmission oil all over myself and the garage floor. A lesser gauge might
have come apart.
Also, DIY can mean two things: Do It Yourself or Damage It Yourself.
Please be careful!
Tools required:
1 meter of hardline vacuum hose
1-vacuum hose "T"
1 Mity-Vac vacuum tester
various vacuum hose adapters to connect the Mity-Vac to the hardline
1 spare transmission plug, threads are 10mm, fine thread, hex head is 12mm.
2 feet of 1/8" copper tubing
4 hose clamps, small (for the rubber tubing)
1 0-60 psi gauge
1 adapter to adapt the 1/4"npt gauge to 1/8th" copper tubing
1 (maybe) a 12mm deep well socket, 1/4" drive
1 tube of BoatLife, a polysulfide rubber caulk
Perfect vacuum: a vacuum of -80kPa held for 3 minutes without dropping 9kPa.
The spec in the book for vacuum testing is different, but this one
will work if your gauge cannot measure 1kPa. I found that new
or good parts will exceed this parameter.
WOT: wide open throttle
1981-85 300D
Before starting, make sure the vacuum system is completely intact and in
perfect operating condition. Doors lock and unlock a minimum of 8 times
without the engine running? Does the engine shut off correctly? Do all
of the ports in your ACC system work properly (defrost, center and outside
vents, floor vents?). Power brakes work properly? Headlight adjusters
working properly? All of this equipment must work as designed or you are
just wasting time proceeding.
The transmissions in the '81 to '85 model years, to the best of my knowledge,
don't have the mechanical switch linkage that is seen on the earlier models.
I don't know about the 240's. As far as I can tell, the transmission is
vacuum controlled through the modulator only. Maybe some shift/speed
information is picked up and evaluated via engine rpm's or drive line rpm's,
that is only a guess (without the presence of vacuum, the transmission will
shift at the red line shift points marked on the speedometer)
The modulator on the '81 version is red and mounted on the right side of the
transmission. It has a cap on it that is O-ring sealed and should hold PERFECT
vacuum when tested with a Mity-Vac.
The modulator on the '85 version is green and mounted on the left side of
the transmission. It has a rubber cap sealing it and it also should hold a
PERFECT vacuum when tested with a Mity-Vac.
I don't know what the model year change was.
There is a vacuum control modulator on top of the IP. It will NOT hold prefect
vacuum, and is designed to bleed off modulator vacuum with respect to the
position of the accelerator pedal. It also will not hold more than -40kPa.
On the transmission, below the modulator valve you will see some plugs. They
look like hex head bolt heads, but they in fact are service ports. On the '81,
there are three in the general vicinity of the modulator. The one closest to
the modulator and the one that is hardest to reach (it lies below the modulator)
is the one you want to use to test and set the modulator pressure. Of the other
two, one will not have any pressure at all behind it and the other will ruin a
0-60 psi gauge as well as blow off the rubber hose clamped pressure gauge. So
be careful and be prepared.
On the '85, the modulator pressure port is directly below the modulator.
Under the carpeting in the W-123 in the driver and passenger footwells you
will find some plastic plugs. If you pry up one of the plugs you will see the
hole that goes through the frame under the car. You can use this hole to pass
through a rubber tube that will connect to a pressure gauge that you will need
to read while driving the car.
At the steering column there is a rubber access boot that the vacuum lines
pass through into the main cabin. Unless you can think of another way, you
will have to use this access point to pass through a hardline vacuum line
from the modulator circuit/ transmission switch over valves and into the
cabin. I cut one of the points off and when finished, resealed it with
polysulfied rubber (a product called 'Boat Life' found at marine stores
works well.)
So to get started:
The first thing to do is to check for vacuum leaks and for flakey vacuum
switches (or transmission switch over valves) that are located on top of
the cam cover. To do this, I T'ed into the modulator vacuum line and ran a
hardline vacuum line into the cabin. I used my Mity-Vac to monitor the vacuum.
I drove the car normally and noted anything unusual. What you should see is
maximum vacuum at idle and a smooth reduction in vacuum as you press on the
accelerator. It drops off fairly fast and should end up at -10kPa at about
half-throttle. THIS HAS TO BE CONSISTENT. This is the vacuum your modulator
sees to govern how much pressure it needs to shift the transmission properly.
I found I had a flakey vacuum switch on the cam cover. Sometimes there
would be a smooth transition and sometimes it would jump around depending
on the position of the accelerator. I drove the car for a week to see how
many problems there were. After replacing the valve, I drove the car for
another week to make sure there were no other underlying problems. That is
how important this stage is.
Also, if a BB should somehow find its way into the feed for the switchover
valves that control the EGR system, some potential vacuum problems will be
eliminated.
In my case, I ended up rebuilding most of the vacuum system as engine oil
had been sucked into the vacuum system at some point in time and had ruined
some of the valves for the ACC system, contaminated the power brake vacuum
assist resevoir, and ruined the key switch. Can you say "what is that black
oil dripping on my worsted wool pant legs?"
After making sure the vacuum system was sound, I proceeded to the modulator.
When adjusting the modulator, you need to set it for the correct operating
pressure. JUST GUESSING ISN'T GOOD ENOUGH, unless of course you like $2000
door stops.
To adjust the modulator, you need access to the modulator pressure port. To
get that, I went to an MB recyclers and picked up a plug from a dead transmission.
I then used a drill press to drill a 1/8" hole down the center of the plug and
soldered a length of 1/8" copper tubing. Make sure to leave plenty of tubing
coming out of the top of the plug because you will need to attach and double
hose clamp a section of rubber hose to it: 2" should be enough. You might
need a deep well socket to install it in the transmission.
Remove the modulator service plug and install this one.
Next, attach the hose and gauge to the plug. The gauge you will need is a
0-60psi pressure gauge. Use 1/8" heavy walled vacuum line if you can't
find the reinforced stuff. I found some reinforced rubber hose at NAPA.
Attach the gauge and run the hose through the hole in the floor. USE
DOUBLE HOSE CLAMPS! You do not want the hose to come off while driving
the car! Set the gauge up so you can read it while driving the car. You
will also want to monitor the vacuum gauge as well.
If you have just replaced the modulator with a new unit, you can simulate
proper vacuum by drawing down the modulator vacuum with your Mity-Vac
to -10kPa (with the car running ) and then set the modulator to 41psi.
THIS IS ONLY A BALL PARK SETTING THAT WILL ALLOW YOU TO DRIVE THE CAR.
In order to set the modulator correctly and with the absence of a dyno,
you need to actually drive the car to make the final setting.
Now drive the car .
With the car at 40mph and in 4th gear, floor the accelerator BUT DON'T
HIT THE SHIFT-DOWN SWITCH. Note the vacuum and the modulator pressure.
You should see they are inversely proportional, less vacuum produces
more pressure which is also why if you were to disconnect the vacuum hose
from the modulator your transmission will not shift until the engine reaches
the redline points on your speedometer. The rates of change for both the
modulator pressure and vacuum should be smooth and linear.
So note the pressure you see at WOT and 40 mph. The spec is 41psi, so set
the modulator accordingly. More pressure will give you harder shifts and
less pressure will give you softer shifts... and flaring. You want good,
crisp shifts. Soft shifts produce wear and shortens the life of your
transmission which should go for 200 to 250 thousand miles if well taken
care of. If your transmission has seen years of abuse, you may be able
to set it for harder shifts to try to compensate. I had to set mine at
about 46 psi to overcome the years of wear that misadjustment had produced.
This was in the form of flaring between shifts. When your transmission
flares, it is in the process of glazing the friction materials inside the
transmission, and contributes to the burning of the oil. Transmission
flares is just prematurely wearing some very expensive parts and should
be investigated ASAP.
Done.
If this procedure does not cure the problem, there is really nothing
more you can do but to tear down the transmission and investigate.
This, I believe is beyond the scope of most DIY'ers. It requires absolute
cleanliness in the working environment, and the documentation and parts
to install all of the latest upgrades for these transmissions (microfische
anyone?). If faced with the dilemma, I would be inclined to purchase a
transmission from MB and install it myself. MB guarantees its parts
regardless of who installs them, as long as the parts are installed
properly. This includes the diligent use of a torque wrench. As for me,
I use a torque wrench when installing the wheels on my car (and a dial
indicator when setting the wheel bearings!) These are great cars and
will last almost forever if well taken car of.
Good Luck and Good Motoring,
Mike Senko
MBCA
Seattle Section
1981 300D
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