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Old 08-09-2013, 11:36 AM
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jay_bob jay_bob is offline
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Join Date: Oct 2011
Location: Columbia, SC
Posts: 3,679
The M-Bs of that era use vacuum to modulate the transmission shifting based on engine load and speed.

On a gas engine the engine vacuum naturally happens based on throttle position.

On a Diesel you do not have a throttle so no engine vacuum. You have artificially produced vacuum from the vacuum pump. So on the fuel control linkage there is a device known as a VCV or vacuum control valve. That varies the constant vacuum coming from the pump into a signal that emulates what a gas engine would be doing at that point.

This goes to the modulator on the transmission to regulate the shift points.

The cap is the adjustment device for the modulator. It is inexpensive and easy to replace once you are under the car.

The hard part is tuning the new one properly. The correct way is to tap in a pressure gauge into the transmission fluid test port and tune it based on fluid pressure.

Lots of posts on here about this.

Go to, on the left is MB workshop resources, click through a couple pages to the 126 service info, you have the entire shop manual for the car available online.
The OM 642/722.9 powered family
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)
2008 ML320 CDI (Older sonís DD)
2008 E320 Bluetec (Younger son's DD)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech
gone but living on
1983 240D (body to greazzer, engine to t walgamuth)
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