Oct 28, 2019

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. 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. 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 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. 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). 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.