transmission vacuum transducer/amplifier - PeachParts Mercedes-Benz Forum

GregZ · #1 11-12-2008, 11:25 AM

Hi All,

My '85 300SD does not shift nicely - I am debugging its vacuum. On the transducer/amplifier I see a good vacuum 18'-20" (on VAC). The TRA line to the transmission is tight (no leaks). What bothers me is that there is no control vacuum (VCV). If I disconnect VCV, then vacuum builds up coming from the vacuum lines. When I connect my "mighty vac" to the transducer's VCV line I cannot establish any vacuum. From what I understand looking at the transducer, I should be able to pull vacuum on the VCV port. Simple question: is my vacuum transducer "kaput"?

Thanks,

Greg

Diesel911 · #2 11-12-2008, 08:52 PM

Click on the "Shop Forum" button, when that page opens instead of going to "Diesel Discussion" continute down the page to DIY by parts. In that section you will find threads on checking/trouble shooting the vacuum that goes into the transmission.

GregZ · #3 11-12-2008, 08:55 PM

Thanks, I think I looked at most of them. My question is specific to the transducer/amplifier and is real simple. Should the VCV connection on the transducer hold vacuum if applied to it by a mityvac (or similar).

GregZ · #4 11-13-2008, 01:42 PM

Well, at the bottom the VCV and PRE lines will switched up. According to the vacuum diagrams the PRE is in the middle and the VCV is on the side - that is wrong. Indeed the VCV is in the middle and the PRE is on the side.

I have uploaded my new corrected vacuum diagram to:
http://zaruserv.uta.edu/300SD/vacuum.tif

Greg

UriahT · #5 11-13-2008, 02:12 PM

The VCV should not hold vacuum if tested from the control side. There's a leak built in to adjust vacuum to the transmission. I think that's what you're asking. If the VCV were airtight, your transmission would be seeing 18-20 of vacuum all the times, which would lead to massive flaring.

GregZ · #6 11-13-2008, 02:16 PM

Nope, that is not what I was asking. I know that the controlled leak is in the line (coming from the leak controller on top of the IP). The question was: if you suck on the VCV on the blue UFO should I be able to pull vacuum. And now I know the answer: yes. The problem was that on the vacuum diagrams the PRE and VCV are mixed up.