Is my transmission modulator okay???
 

While performing a routine 20,000 mile transmission fluid/filter change last week, I opted to adjust the shift modulator counter-clockwise a few clicks hoping to smooth out increasingly rough shifting. (While I know how to make this adjustment, I've never before done so.)

From the exterior, the modulator appeared to be in great shape (no vacuum leaks, dried out rubber, or any other obvious problems.) So, I was surprised when I removed the modulator cap (black on my '89 420SEL) because there was dark fluid trapped inside. (I assume this was tranny fluid, but I don't know that for sure.) Anyway, I cleaned and dried the modulator and carefully eliminated this dark fluid residue. I then adjusted the T-bar as described, replaced the cap, and lowered the car.

Transmission shifted better after the adjustment and fluid change, but not smoothly enough to satisfy me. So, I decided to go back under again tonight and further adjust the modulator. SURPRISE... It was wet under the cap again (this time with clean fluid). I dried and cleaned things again, made the modulator adjustment, tested it out, and it feels better still.

My questions are these: Is that fluid under the shift modulator cap supposed to be there? Or is the modulator bad? Am I a fool to be making these adjustments to this original tranny with 215,000 miles on it? (i.e. Should I be happy it's still functioning normally--rough or not, and leave well-enough alone?)

Thanks for any experienced guidance here...

Eric

The cap is supposed to be dry. It sounds to me like it is leaking and needs to be replaced.

No, you are not crazy trying to adjust the shift quality by backing out the modulator. I just did this on the 91 300E (211k miles) AND adjusted the bowden cable to where it shifts very nicely. Actually I am amazed I have driven it almost 100k miles with a too firm shift.

The only thing you want to watch out for is the flaring (slipping). As long as it works properly enjoy.

I would replace the vacuum modulator.

Haasman

Thanks for the diagnosis and the vote of confidence, Haasman!

Can anyone provide insights into replacing the transmission modulator? (I did a cursory ShopForum search and didn't see any specific instructions.)

Specifically, do I need to drain the torque converter and pan before this R&R? Is it a "plug-and-play" swap, or will special tools be required?

(It looks rather straightforward and simple; but I'd appreciate confirmation of this before I dig-in!)

would you please let me know where is this vacuum modulator is located. I guess I need some adjustment too.

Can you post a pic ???
Thanks much...

It is on the left side of the transmission. It is about 2-3" in diamenter. It looks like a small plate sitting on the side with a black cap in the center.

Haasman

For the record, the modulator is on the Driver's side (Haasman's reference to the "left" side might be confusing to some.

Also, the caps vary in color depending on which tranny is installed. Haasman and I have black caps on our respective modulators, but there are also versions with other color caps (i.e. green.)

Thanks Eric, good points.

Also it has a vacuum line going into it.

Haasman

I doctored-up an old picture to highlight the modulator. Note that I was in error in my previous post, it's not the modulator cap that varies in color, rather it's the modulator body itself...

Hope this helps!

goldstone, ...haasman...thanks much. I greatly appreciate it.

Thanks.
 

Eric,

Thanks for posting the picture.

As I only have 2 small wheel ramps I was unable to find the modulator, but with your pic, I knew exactly where to look!

I have finally managed to solve my gearbox juddering at 1,500 rpm just before it changed up a gear with a load thud.

All I had to do was adjust the modulator by one 'segment/click' and it changes gear as smooth as silk.

Thanks again for the picture.

Hello friends,

My 1984 500 SEL is also in need of tranny adjustment. I´ve spent the last week trying to adjust the modulator to find the right setup.

I adjust the transmission in the evening and drive it a day or two to asses the change before doing additional adjustments. It seems as if the shift-quality does not change very much when I turn the T-handle (about 5 clicks per adjustment), which makes me belive that something is wrong with the vacum lines or the modulator itself.

Now to my question - How do I check the modulator and what vacum should the modulator see (I assume that You check it at idle)?

Please help me with help for troubleshooting.

Thanks,

Mikael Westerberg
Vanersborg Sweden

mikael,
What is it doing? Have you tried testing for vacuum leakage on the line going to the modulator?

Sorry Jim,

I was to focused on my questions but missed all the background. Ok here goes.

1) Shifts above about 60 % throttle shifts are fine. This tells me that there should be enough pressure in the system to handle all other shifts.
2) Light throttle shifts below about 25 % throttle are also fine.
3) Medium throttle shifts between about 25 - 60 % throttle flares. This is more pronounced when the car is cold and has been standing for a couple of hours.
4) I have 18 inches of vacum at idle when I measure at the green modulator the vacum holds steadily and drops to almost 0 when a I step on the throttle.

I have done several adjustments lately.

-I put a little bit of slack into the bowden cable to get the shift points at lower speed (the shift pattern was a bit to sporty for my taste). The shift points are now fine. This adjustment did nothing to the flaring shifts (both 2-3 and 3-4).
- I replaced the short rubber hose between the metallic vacum line and the modulator.
- I have in total, and in steps of 4 -5 clicks, adjusted the T-handle CW about 20 clicks but this has only reduced the flaring somewhat. As I wrote before I drove the car for a day or two after each adjustment.

I have also read many many postings regarding the modulator and the general concensus seems to be that if the WOT shift don´t flare adjusting the modulator should do the trick. On the other hand many posts describes how sensitive the adjustment is and now that I have adjusted about 20 clicks or about 3 full turns I´m getting nervous that I might screw something up.

At the moment I plan to take the car out and drive until warm then I will disconnect the vacum hose to the modulator by which i expect that most shifts will be rough. This I assumes will show if the modulator works or not.

Any ideas what I´m up against?

Many thanks/ Mikael

:(

Mikael - sounds like you are on the right path. Have you changed the fluid/filter yet? Make sure you drain the torque converter too during the change.

I can't really speak on how much you should have to adjust the modulator.

Thanks Jim,

Thanks for the input. I will step by the shop tomorrow and get a filter and oil. I have no idea when the oil was changed. I´ll get back with more info as soon as I´ve fiddled around some more and let You all know how things are going.

This list is great.

Thanks again,

Mikael