[W126 Coupe] Hesitation from 60C to 80C

[Kelly Frink]

MERCEDES-BENZ TRANSMISSION

VACUUM CONTROL SYSTEM "TUNING"

by Steve Brotherton

First Published in:  <http://www.import-car.com> ImportCar∞ Feb 2002

 

  _____  

Pneumatics, or air pressure control, is a common item in all cars. However,
nobody uses it as extensively as does Mercedes-Benz. Many have used it only
for off-on control of timing and emissions devices. 

M-B has extended off-on control to the following items: Door locks (brought
out in the '60s), seat back locks (two-door models), A/C mode door controls,
door closing assist (pulls doors to final lock on '92 and up S-Class car
doors and trunk), seat lumbar support controls and other devices, such as
the position indicators that rise from the trunks of S-Class cars. Variable
control also has been used for such things as cruise control, variable EGR
control and intake manifold pressure regulation in turbo diesels. 

Probably the most interesting of these variable controls are the ones that
M-B uses for shift control in M-B diesels. It's interesting because the
engines have no manifold vacuum, actually having positive pressure most of
the time in turbo-diesels (they run with small boost at constant highway
speed). These vacuum-controlled transmission systems appeared in the first
300SD in 1978. That system was pretty simple. All subsequent systems added
more layers of control. Variations of this system are on all M-B diesel
automatic transmissions until electronic control took over in 1996. 

The basic idea was to create a system that presented vacuum to a
transmission modulator. The trick was to simulate the vacuum-to-load
relationship in a gas motor. With a gas motor, under heavy load the vacuum
would be low as the throttle would be fully open. These diesels have no
throttle so there never is any vacuum; the only differences in manifold
pressure occur during boost. 

What makes this system so special is the variety of ways the system can be
adjusted. The basic book adjustment might work for an out-of-the-box,
by-the-book transmission. As it happened, M-B made numerous after-production
changes to these transmissions. The combinations of pieces and the variety
of wear conditions cause these transmissions to exhibit numerous offensive
shift conditions. Among these, the number one condition in diesels is a
harsh 1-2 shift. The next most common is a 3-4 shift flare. Others include
double shifts into either 3rd or 4th, harsh 4-3 downshifts, and various
shift overlap and sequence irregularities. 

There are many ways of approaching the shift conditions of these
transmissions, specifically the 722.3/4 M-B units made from 1981 to 1995.
Many of the conditions should be properly handled with internal repairs and
modified pieces. 

The B1 band was reduced in friction coefficient in the early '80s. This
allowed a softer 1-2 shift. With the original band, the shifts were very
harsh into second gear, unless the modulator (and thusly shift) pressure was
lowered. Lowering the modulator pressure would reduce the later shifts to
such a degree that slipping or flaring would occur. The new band is MB #126
270 18 62 and should always be replaced on units through 1983. 

The variety of 2-3 and 3-4 shifts is increased in older transmissions by
leaks in the clutch pack seals. There are seals within the clutch drums K1
and K2 that require rivets to be drilled and rebuilt. This probably doesn't
happen in many rebuilds. The B2 band servo piston has been redesigned,
reducing the loss of pressure during release as part of the 3-4 shift (new
part M-B #107 270 04 32). The valve bodies were modified numerous times and
M-B offers a reasonable valve body exchange program that both addresses
repair concerns and the improvements they have found to reduce some shifting
sensitivities. 

Normal wear and tear changes the state of the various conditions mentioned
above. All of these conditions present the technician with a number of
decisions to make. The first is whether to repair or replace the unit. In
the case of the vacuum-controlled M-B diesels, many a tech has condemned the
unit when all that was needed was a vacuum system repair/adjustment. The
real problem appears once the unit is changed and the condition still exists
or, as often is the case, it changes (every tranny has a separate set of
adjustments that are most appropriate). 

ADJUSTMENT VS. REPLACEMENT 

  _____  

The ability to adjust the vacuum control system can save many a
transmission. With proper tuning, this system can significantly alter the
performance of these vehicles. Most important is that the conditions change
gradually, and with respect to each other, through the life of the
transmission. As a result, the ability to "tune" this system can be a real
important part of a properly done service. 

An understanding of the system is necessary to adjust or repair it. The
vacuum starts with a vacuum released from the brake booster supply line. 

Note: I will refer to vacuum as a commodity, as thinking of it in quantities
helps the understanding. 

The vacuum source is a mechanical pump run off the injection timer. The
"quantity" of vacuum is most important and is achieved by allowing flow of
vacuum through a specifically sized orifice to the modulator. Control is
done by leaks. A proportioned vacuum leak is attached in parallel. The leak
is achieved by a valve attached to the injection pump (see Fig. 1 and Fig.
2) and regulated by a lever attached to the throttle linkage. The leak
increases with throttle rotation. 
Figure: 1
The system works right when a high vacuum of 10-15 in. diminishes to zero at
full throttle. It is most important to note that achieving the ideal vacuum
depends most certainly on the proper volume of the vacuum source. Since the
leak is of a given variable flow, the size of the source is critical. The
most common problem I see is total lack of vacuum. This gives even, harsh
shifts, slightly delayed in most variations. 

Another common problem occurs after someone breaks the plastic source tee.
During most of the ingenious repairs I have worked behind, the orifice was
omitted. This leaves either a constant high vacuum or a variable vacuum that
is skewed high. Either condition causes slipping or flaring conditions. This
occurs when the vacuum supply is so great that the proportioned leak is
small by comparison, resulting in vacuum that's too high. 

ADJUSTMENT SPECIFICS 

  _____  

Once a proper vacuum curve is created, the real finesse begins. There are a
number of possible adjustments. The first is the relationship of the control
valve linkage to the throttle lever. The later proportioning valve is
mounted to the injection pump and is rotated to achieve this adjustment. The
early adjustment is achieved by adjusting the rod length such that the lever
reaches within 0.5mm of the full throttle stop (see Fig. 1) with full
throttle. The size of the vacuum leak, and thus the range of vacuum, is
adjusted on early versions by turning the adjustment under the plastic cap.
(See Fig. 2). 
Figure: 1
The later version has all of its adjustment in the rotation of the valve
mounting (pointed to with a pen in Fig. 3). All adjustments should be
monitored with a vacuum gauge and should be done in small increments. A
useful tool is created with a standard vacuum gauge, 3 meters of M-B hard
vacuum line (M-B #000 158 14 35 - costs a little over a dollar a meter) and
one rubber vacuum tee (M-B #117 078 01 45, see Fig. 4). This will allow
monitoring while driving (also a necessity to learning the various
overlapping adjustment strategies and boost pressure monitoring). 

The proportioning valve adjustment allows the range of vacuum to be
expanded. For example, a range of 10 in. to 0 in. could be expanded to 12
in. to 0 in. or 15 in. to 0 in. and, in some instances, raised above zero
(such as 12 to 2). Generally speaking, a wider range is better, but older
trannies with a lot of clutch wear often benefit from a reduced range. Worn
clutches are more susceptible to shifts at low pressure. By adjusting to a
smaller range and reducing the basic modulator pressure, a slow shift can be
modified. Moving the vacuum range also has been of use to cover up valve
body problems concerning shift spacing (for example an early shift into 2nd
or 3rd). 

All vacuum adjustments should be done after the proper modulator pressure is
achieved through adjustments at the modulator. The proper method would be to
install a gauge at the case pressure port. Remove the vacuum line and run
the engine at 2,000 rpm. The pressure is adjusted up or down to meet the
figures in the data manual by turning the tee handle (see Fig. 5). We do
this with rebuilt units but, in used units, I always do it by feel as the
necessary vacuum adjustment may never reach zero. I drive the car full
throttle (monitoring the vacuum) and set the modulator such that these
shifts are appropriately firm. All the other shifts are modified from this
point using the above adjustments. The range of vacuum and its relationship
to throttle movement can be varied to achieve relief from the internal
problems described above. 

As one is varying the above adjustments to achieve shift quality, one more
adjustment comes into play on this basic system. That adjustment is the
control pressure cable. Control pressure opposes the action of the governor
and changes the point of the shift. The basic adjustment on all these
transmissions is for the cable to be attached free of play at the point of
initial throttle take up (See Fig. 6). 
Figure: 1
In practice, the shift point can be altered to achieve some measure of
control during certain flaring conditions. The most common flare occurs in
the 3-4 shift, with some occurrences in the 2-3 shift. The flaring condition
exists due to a lack of shift overlap caused by slow filling of a clutch
pack either through greater fluid volume needs of loose clutch packs or
fluid losses through pack seal leaks (described above). Early cars also had
the condition due to low shift pressures, adjusted to keep the 1-2 shift
tolerable. Cars with this condition can have their relative shift pressure
improved by getting the high shifts sooner so they will be made with greater
throttle, giving lower vacuum and greater pressure. If the shifts are made
as the throttle is backed off, then the pressure drop in the modulator is
very high. 

All testing and repairs should be done after viewing the pertinent vacuum
schematic, as the system is intertwined with supply of EGR and intake
pressure control systems. The system has various other controls depending on
installation. On most models, the vacuum is not tied to the proportioning
valve until some throttle is taken up, closing a vacuum microswitch on the
valve cover. This keeps the vacuum high at idle which keeps some models in
second gear until throttle take-up. It also reduces the closed throttle
downshift to an almost undetectable level. 

The most important modification came with the 1985 model and is continued on
all subsequent ones. It involved the addition of a vacuum amplifier to the
system (see Fig. 7). The amplifier takes a large vacuum supply, the
proportioning vacuum signal and a boost signal and creates the final signal
to the transmission. This device did two things. First, it incorporated the
input of boost to further tailor the load control of the modulator vacuum
signal. The vacuum at the modulator now starts high - 12-17 in. - and is
reduced to about five inches through straight throttle movement. As boost
builds, the vacuum is further reduced to zero only at full boost. 

The second thing this system did was reduce the sensitivity of the final
signal to small changes in the proportioned signal. When this signal went
straight to the tranny, a 10-20% fluctuation due to linkage or vacuum
conditions (the orifices are very susceptible to diesel soot restriction in
any of the orifices) caused great changes in shifting. With the amplifier,
the proportioned signal is just one input and the output averages out the
variations. Shifting in diesels was never better at this point. 

Since these systems are designed to fashion the shift characteristic to load
through basically mechanical linkages, it is imperative that throttle
movements give appropriate power output. In other words, if the engine
doesn't run right it will shift wrong. This always has been the case, but is
especially true in these systems. 

For these fuel systems to deliver the extra fuel required with boost (more
air needs more fuel), an aneroid is attached to the governor mechanism. It
alters fuel metering due to both altitude changes and intake boost changes.
The pressure signal from the manifold is regulated for overboost conditions
by a switchover valve (See Fig. 8) in the line. The valve is often sooted
closed causing no signal to reach the aneroid. This severely restricts
power. The line also clogs at the banjo fitting on the intake. If I doubt
the power, I always use my vacuum gauge arrangement (see Fig. 4) to verify
boost before adjusting the transmission. I check to see that smooth boost
occurs and reaches 9-10 psi. It also must drop immediately with released
throttle. I usually check the boost at the aneroid signal line to the vacuum
amplifier (see Fig. 9). This is easier and it also monitors the signal where
it is used. 

I would have liked to give the exact formula to each condition, but I feel
that every transmission is different and the moves are unending. I believe
that it is much like playing music by ear. Driving the car, visualizing the
condition and using the relationships to modify the condition. It is the
best part of being a technician.

- Steve Brotherton

  _____  

 <http://www.mercedesshop.com/Wikka/CategoryDiy> CategoryDiy

 

From: mbcoupes-bounces at mbcoupes.com [mailto:mbcoupes-bounces at mbcoupes.com]
On Behalf Of Day . Ralph
Sent: Monday, February 09, 2009 6:41 PM
To: Mercedes Coupes Mailing Lists
Subject: Re: [W126 Coupe] Hesitation from 60C to 80C

 

Kelly,

 

Can you be more specific?  I got rid of my 1984  because of this, bought a
1991 five years ago, and am getting rid of it because it has now develop the
same problem.  The cars are wonderful, but this borders on safety.  I am
reluctant to sell it to anyone.  My certified Mercedes mechanic cannot
figure it out either.

 

Ralph D. Day

Senior Program Manager/ASQ LSS Certified Black Belt

MDA Master Black Belt

Dynamics Research Corporation

(301) 275-9030 (cell)

 

From: mbcoupes-bounces at mbcoupes.com [mailto:mbcoupes-bounces at mbcoupes.com]
On Behalf Of Kelly Frink
Sent: Sunday, February 08, 2009 12:54 PM
To: 'Mercedes Coupes Mailing Lists'
Subject: Re: [W126 Coupe] Hesitation from 60C to 80C

 

Vacuum 

 

From: mbcoupes-bounces at mbcoupes.com [mailto:mbcoupes-bounces at mbcoupes.com]
On Behalf Of Matt Jurcich
Sent: Sunday, February 08, 2009 12:41 PM
To: mbcoupes at mbcoupes.com
Subject: [W126 Coupe] Hesitation from 60C to 80C

 

Hey all,

My 89 560 SEC (110k or so) runs and shifts fine from cold until about 60C.
It starts to hesitate, tranny shifts way late (it's scary if you're on the
freeway and can't keep speed!) but then when the temp gets up to 80C
(running temp) the power comes back and shifting is normal.  Car is
completely fine until it cools off.

I changed the fuel pressure regulator, had my fuel filters changed at the
pumps about a year ago, tranny service a couple of years ago, changed plugs,
rotor and dist cap last year.

Any suggestions?  It seems like some temperature sensor that's flaking
out......

Thanks.

-m

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