Vacuum Modulator cause a no-shift? - PeachParts Mercedes-Benz Forum

wilburtual · #1 07-22-2014, 12:03 AM

Trying to figure out if my transmission is toast or not... it won't shift over 2nd gear. i've replaced all the vacuum lines and connections so they all hold a vac. since doing this, I have gotten 1st gear back (which i never had before) but it still won't leave 2nd. kick down switch is disabled/unplugged. (but when applying voltage to the wire, I can hear the solenoid clicking)
the vacuum modulator holds vacuum when applied with a mityvac, but when I replaced the o-ring on the modulator the inside of the modulator was full of transmission fluid.. which I've read is a bad thing..
I am wondering, if the modulator is bad, would it cause a no-shift situation? or is the modulator just to regulate the softness of the shifts?
Basically, I'm wondering if trying to swap the modulator even makes sense to try, or if that is obviously not the problem..?? (BTW, the modulator is a red body type with cream cap, which is a special order part for $85, so hate to just throw $85 at it if it couldn't possibly be the problem)

I also tried draining and changing the transmission fluid and filter, but no change when that was done.

Strand · #2 07-22-2014, 09:17 AM

From what I've read, a jammed/malfunctioning kickdown switch could cause your symptoms. Probably worth ruling out the simple things first.

SD Blue · #3 07-22-2014, 09:58 AM

This sounds like B2 piston failure (from what I've read) but hopefully someone with more transmission experience will chime in here.
b2piston

Here is a link to the DIY section on transmissions that may help narrow it down:
Transmission:

Since your modulator holds vacuum, I'm not sure about it being bad. Are you saying it was full of ATF in the vacuum chamber?

wilburtual · #4 07-22-2014, 03:59 PM

when the problem first started, the modulator did not hold vacuum. i took off the cap and replaced the o-ring, after which it does now hold a vacuum. However, when i took the cap off the modulator, it was all wet inside with ATF. not sure if this is coming from within, or if there just happened to be some leaking/ spraying on there externally or something... I thought I remember reading somewhere that finding fluid inside is a bad sign for the modulator. But, I have no idea if that would actually cause a no shift scenario.

I haven't actually pulled out the kick down solenoid , but it clicks when I apply power to it, otherwise, kick down switch is disabled at this point to take it out of the equation.

but, it seems more likely it is the B2 piston or something more serious...

Biodiesel300TD · #5 07-23-2014, 02:19 PM

Vacuum controls shift quality. No vac causes harsh or hard shifts, and too much causes soft shifts and flaring. I'd guess your problems is inside the trans. A B2 and K1 swap are cheaper and easier than a rebuild or replacement. So I'd go those routes first.

Zacharias · #6 07-23-2014, 08:55 PM

You haven't indicated which car you are talking about, or the model number of your transmission.

wilburtual · #7 07-23-2014, 09:58 PM

the car in question is a 1981 240D, I believe it is the 722.118 transmission.

Once in the past, I also had this same issue where it would not shift from 2nd, and then I found that the vacuum line going to the modulator had gotten disconnected. When I connected it again, shifting resumed as normal. This is why I am wondering if the modulator can really cause a no-shift scenario, since it seems it kinda did in the past. The modulator is for sure getting vacuum this time.

I'll look in to the B2 and K1 swaps to see if that is something I could handle on my own...

Junkman · #8 07-23-2014, 10:07 PM

I found transmission manuals available when looking. Search Mitchell and transtech. They have so.e diagnostics.

dude99 · #9 07-23-2014, 10:59 PM

I can never remember, is 81 the year that had the vacuum only transmission? or was that 1980?

wilburtual · #10 07-23-2014, 11:46 PM

I am not sure about the year(s) they were used, but the particular transmission in my 81 240D is the vacuum only type. there is no mechanical cable connection to it. the only thing attached is a vacuum line to the modulator on the transmission.

dude99 · #11 07-24-2014, 01:08 AM

Ahhh, ok. That makes all the difference. You can ignore the posts above saying that the modulator only effects shift quality, that doesn't apply to you. Yours is a one year only exception to the rule.

As your transmission is shifted only via vacuum there are two common problem areas. The modulator (you said yours was leaking ATF so I would replace it) and the 3\2 valves. These are located on the top of the valve cover under the black plastic cover, although the cover is often missing. These are plastic and are always worn out.

Without the modulator and the valves working your transmission will not shift right.

wilburtual · #12 07-24-2014, 10:34 PM

I have recently replaced the little white 3/2 valve (mine only has 1) and *assuming* the new one I installed is good that hopefully isn't the problem. I have also replaced all the vacuum lines and the vacuum rubber connectors & splitters under the hood.
I tried running the car while applying vacuum directly to the line connected to the modulator with my mityvac and still no up shifting, whether there is no vacuum or if its holding vacuum with the mityvac.
Is there any way to diagnose a modulator being bad, such as if I pull it out is there something to check and verify to tell me for sure it's toast?

Zacharias · #13 07-25-2014, 12:29 PM

On the .117 and .118 transmission, vacuum issues will not cause the transmission to not shift up.

I am not a transmission expert but I know a bit about these transmission models, as I had a similar issue with my 1980 300td (which has the trans from my old 240d in it).

1. Shifts are controlled by an internal governor.
2. However, if the operating pressure is too high, this will impede upshifts. I am not sure if a faulty modulator would cause that or not.

My issue was fixed by freeing up the seized governor.

I would check the operating pressure before paying for a new modulator. I think they go bad pretty infrequently.

Zacharias · #14 07-25-2014, 09:18 PM

... that you can try.

True my problem was the stuck governor, but when I got the car back it was shifting oddly and didn't want to shift up past third except on special occasions.

I did this adjustment/fix and now I have better shifts and it goes into fourth with no issue.

http://www.benzworld.org/forums/w123-e-ce-d-cd-td/1261744-transmission-flaring-clunking-vcv-diy.html#post1889497

It might be a good idea to at least eliminate this, before you go further with other stuff, as it's a no-cost** fix.

** unless you are unlucky enough to have one of the VCVs that cannot be adjusted (uses some sort of rivet instead of a nut to retain the spring). In that case you will need to get hold of a later one, which I did, from an '83. When I did, I found the springs in both were nearly slack.

Also I strongly recommend that when you take the VCV off the car, take the associated linkage and its plastic clip along with it. That clip will be fragile from years living under the hood, and it's a dealer item ($17). Comes off easy, not so much the other direction, unless you have slim fingers and the dexterity of a watchsmith. The linkage is a PITA to reinstall, but nothing like fighting that little clip.