I'm working on the shift performance on my 1987 300TD. I've gone through the vacuum system, replaced a few hoses where it was leaking and adjusted the modulator on the transmission itself. Overall the transmission is shifting well, however I think the output of the VCV is low.

Vacuum going into the VCV from pump is strong, around 25 inches. The VCV output measured at the line running to the BUFO is around 7 inches (measured at idle). The output from the BUFO to the transmission modulator is also around 7 inches.

I've done some reading on the VCV adjustment procedure (set throttle wide open, twist VCV clockwise until you feel resistance).

My question is regarding the ideal pressure coming off the VCV at idle? Based on my research I *think* it is around 12 inches at idle, but I'm hoping someone can confirm that for me.

Thanks

can't speak for w124 but on a w123 240d it was basically maximum vacuum pump vacuum

and logically speaking your modulator should be getting most vacuum at idle/coasting

Idle vacuum should be roughly 12", measured both at the output of the VCV and at the modulator. The VCV itself may have a clog in it, they're basically a spring-loaded flapper with a rubber disk on the end pressing against a metal protrusion. Over time they can wear away and that metal protrusion can get chunks of rubber in it and clog it up.

Make sure the vacuum hoses are on the VCV correctly. It matters. Vacuum supply should be in the center port, BUFO should connect to the off-center port.

Depending on your setup, there may also be a switchover valve to tie into the boost sense line. If it's leaking by, your vacuum will be low. Measure vacuum output directly at the VCV, that will tell you if you have a problem with the VCV or with something downstream of it.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Completely different system.

Here's a 603 VCV, free of any leaky attachments:

https://www.youtube.com/watch?v=WZzY6t2InrY

When installed, there is a restrictor between the VCV and the BUFO, so lots of opportunity for vacuum to attenuate. Check your hoses.

Yeah, i'm measuring at the VCV and getting the ~7 inches measurement.

Last night after starting this thread I adjusted the valve as per the procedure (held throttle at WOT and turned the VCV clockwise until meeting resistance). The end result definitely landed the VCV further clockwise than prior to the adjustment, however the output still stayed at around 7 inches. It may have changed a bit but the adjustment didn't have a large impact.

Does this indicate that the VCV is probably toast, or are there further adjustments that can be made to increase the output?

What do you have at the feed line where it connects to the VCV? Do you have the same 25" of vacuum? BTW, the vacuum pump can only produce ~22" of vacuum even when brand new.

If you have full system vacuum entering the VCV but only 7" coming out of it, the VCV has an internal leak or the orifice tube is plugged up with rubber debris. There are a few threads on this forum and elsewhere about how to take the VCV apart and rework the innards on it.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I'll double check the actual vacuum at the pump. 25 was an estimate based on memory, but it was definitely over 20 inches and the same going into VCV.