Bad Vacuum Transducer? (Blue Disc) - PeachParts Mercedes-Benz Forum

::matthew · #1 09-13-2009, 03:07 PM

So at idle I have #12 going to the transmission, and when I open throttle it only drops a little ~#7-9. My understanding is that it should drop more.

I popped the cap off the adjustment port and, using a 4mm socket, turned the screw to the full extent in each direction with no change to the vacuum characteristics.

How can I test my transducer to see if it's operating correctly?

sorry for the title misspelling

Brian Carlton · #2 09-13-2009, 03:57 PM

What's the reading at maximum power in third gear above 3000 rpm?

What's the shift quality at low pedal positions and also at high pedal positions?

::matthew · #3 09-13-2009, 04:44 PM

hard shifting 1-2, OK after that. it's harder than I'd like at both low and high pedal positions.

I've been trying different bowden cable positions to soften this. It does seem a little smoother when the bowden cable is backed way off (shifts lower RPMs) but I'm in 3rd too soon.

So I'll be teeing off the vacuum line going to the transmission for the 3rd gear @3K test?

sixto · #4 09-13-2009, 04:56 PM

At idle, how much vacuum is available to the VCV and how much gets to the transducer?

Sixto
87 300D

Brian Carlton · #5 09-13-2009, 05:25 PM

Quote:

Originally Posted by ::matthew

hard shifting 1-2, OK after that. it's harder than I'd like at both low and high pedal positions.

If you've got 7-9" at idle, it's going to shift a bit firmer than you'd like. Ideally, you want 11"-12" at idle if you want the characteristic soft shifts.

As Sixto mentioned........what's the value of the VCV output to the transducer at idle? The problem may have nothing whatsoever to do with the transducer..........and be upstream of it.

Also, many problems with a hard shift are due to a leaking cap at the modulator. This can drop the vacuum to the modulator and cause the hard shifts.

Finally, any leaks in the EGR or ARV systems will cause a drop in vacuum that cause hard shifts. These circuits should be disconnected and plugged. They are usually on the right side of the engine.

::matthew · #6 09-13-2009, 06:36 PM

EGR and ARV are capped...all associated parts are gone.

Cap on modulator was replaced in the last month. I have a replacement, but was waiting to see...

I'll check vacuum levels at the VCV and report back. Is the transducer getting it's vacuum off the IP or the VCV?

Which port of the VCV are we looking for readings from?

At idle, I have about 12" going to the trans.

79Mercy · #7 09-13-2009, 06:39 PM

Alfie hasn't forgot about your pics, he has been busy, i'll tell him to go take some pics, LOL.

sixto · #8 09-13-2009, 06:54 PM

The VCV is attached to the IP. The IP itself has no vacuum attachments.

After looking at the 123 vacuum line diagram, I realize I shouldn't have gotten involved with this thread

It doesn't look like the 617 VCV works the same way as the 603 VCV.

What I'd do is connect the VCV directly to the transmission modulator bypassing the transducer. Keep one of those dampers (green cylinder) between the VCV and transmission. If shifts are closer to what you expect, the problem is the transducer. If shifts are still off, the problem is the VCV or the modulator.

Sixto
87 300D

::matthew · #9 09-13-2009, 07:01 PM

OK, what I was thinking of as the VCV is the switchover valve.

::matthew · #10 09-13-2009, 07:18 PM

At Idle:
22" going to the VCV (port on top) measured just before the dashpot

12" coming out of transducer to transmission

::matthew · #11 09-13-2009, 07:19 PM

what role does the switchover valve(125) play?

thebenzbar · #12 09-13-2009, 08:36 PM

Matt, PM sent. Let me know if you want more.

::matthew · #13 09-13-2009, 09:17 PM

thanks Alfie

Brian Carlton · #14 09-13-2009, 10:00 PM

Quote:

Originally Posted by ::matthew

12" coming out of transducer to transmission

That's exactly what it should read. If the shifts are too firm with that vacuum level, it's time to back off the modulator. The vacuum system is working fine.

::matthew · #15 09-13-2009, 10:06 PM

thanks Brian

This has probably been covered elsewhere, but do you know which way I need to adjust the modulator to "back it off"?