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Old 08-09-2013, 10:36 AM
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jay_bob jay_bob is offline
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Join Date: Oct 2011
Location: Columbia, SC
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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 startekinfo.com, 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.
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2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech
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