Hello. I've been lurking on the site for a few weeks now, ever since I bought my '84 300DT.
I have, indeed, acquired a new 'hobby' since buying this car, since I have been almost obsessed with identifying probs
and fixing it up.

I plan to use to use the car to commute come fall. I'm using biodiesel now (previous owner did, too) and hope to install a grease kit soon ($$).

I've made a list of things to do for it, and so far fuel and oil filters have been changed.
The car runs really well, esp. after warmed up. It's slow as molasses at first. Driving the car really forces you to slow down and chill out overall when driving. You can't be in a big hurry with this car. Even when warm, the car seems to lumber up the big hills. But it's great on the highway.

****I'm wondering about some surging that occurs when cold and when going up hills. (I live in a really hilly town). I bought the car from a friend, who says this prob just came up.
It doesn't seem like a filter issue, more like something to do with the vaccum assisted shifting. On flat ground the car also seems to shift quickly (and hard) into 2nd then quickly into 3rd. It makes me cringe. I've heard a bit about adjustments that can be made to remedy this. NOT sure if surging and shifting are related BUT I think they are! Am I causing damamge by not doing something about it?

I'm looking for more insight/info about shifting/transmission/vaccum on cold vs. warm vehicle. Any comments would be helpful!

Next on my list.....bring back heat blowing from the floor vents!

 

hello wunderbar, i too have experienced shifting problems of a different nature but shifting problems all the same. i would ask you what the full throttle shifts are like. these diesles climb thru the gears rather quickley unlike other cars. diesels are low rpm engines that make tons of torque (what really moves a vehicle in the first place) so rpm is lower than gas engines. sounds to me like a trans modulator problem which controls shifts and helps soften them up. has the fluid and filter been changed? thats easy insurance if it looks dirty. also check the vacuume valves on top of the valve cover on the engine. these crack with age and the slide levers wear out causing hard or nonexistent shifts. hope this helps.

 

Try this web site having to do with adjusting trans vacuum. www.mercedesshop.com. in the shop forum section, very informative! its in the DIY section last article.

 

Thanks for your replies.
We checked out the vaccum valves and were surprised to discover old, deteriorating hoses, one of which was not even in place! Putting it back made things a little better, but not great. I'll keep working on it. Good to know I'm on the right track.

As I've had the car a little longer, I have realized that none of the symptoms I describe happen when the car is good and warmed up, and some days it drives like a dream. This is encouraging, even though I can't exactly figure out why that would be the case.

 

MERCEDES-BENZ TRANSMISSION
Steve Brotherton

February 2002

PROPER VACUUM SYSTEM "TUNING" ACHIEVES OPTIMAL SHIFT CHARACTERISTICS AND AN ACCURATE REPAIR


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.

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).

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

 

I'd do a diesel purge and service the ALDA. Both are simple DI Y'er's.. The Pictorials can be found at dieselgiant.com... Then I'd consider placing bb in the vacuum line that goes into the EGR, which effectively blocks it, thereby giving you greater HP.
You can also by pass the switchover valve by joining the vacuum line from the banjo nut( it'son the manifold next to the firewall. Pics are under AlDA service at dieselgiant.com ) directly to the ALDA. Many reputable German diesel mechanics regularly do this, whilesome peoplesay it's a big no-no. I've done it and it's a vast improvement.The car even shiftssmoother...