Is this the Transmission Vacuum modulator?
 

I have a 1995 E300d and the transmission suddenly has developed extremely hard shifts mostly as lower speeds. I wanted to adjust the vacuum modulator but from what I have read it has a small rubber bot that must be popped off to access the pin to turn. This device as a whole can be turned to counter clockwise 190 degrees but that is it.

How does one get this device open? If I wanted to test the cars vacuum would I just pull off that hose and hook it up to a vacuum gauge? Thanks

Before messing with the modulator look for vacuum leaks. They don't have to be in the line going to the modulator either. What you described sounds like a vacuum leak to me.

I have looked everywhere , took the crossover pipe off and found nothing. Not happy! So what about the modulator? How is it accessed?

What Jim is attempting for you to do is to attach a vacuum gauge to the vacuum system under the hood. You'll probably find that the system will not hold vacuum.

Only if vacuum is high (around 20"), should you proceed to condemn the modulator.

Even before you do that, you should connect a Mityvac directly to the modulator, pump it up, drive the vehicle, and see if your shifting quality returns to normal. If that happens, you absolutely have a vacuum leak and no adjustments to the modulator will cure your problem.

I understand all this but how do I access this modulator? I posted a picture supposedly of this modulator. There is no rubber cap to pop off, before I damage anything I wanted to know how I would access this modulator and test it with a vacuum gauge.

Brian - assume the working is still like my 81 300SD? No vac needed for upshift (in fact it is designed to bleed off vac at WOT). However needs vac for smooth downshift. Most of the posters do not seem to realize that loss of vac will not affect upshift.

Or did something change through the years?

Find the vacuum line to the modulator.

Follow up from the transmission to the engine compartment. When you find the first junction, disconnect the line from the junction and connect a vacuum gauge. Plug the open connector where you removed the line prior to driving the vehicle.

You'll need sufficient line to route the Mityvac into the driver's seat so you can maintain vacuum while driving (most of them leak slightly).

Vacuum affects both upshifts and downshifts by slowing the shift. Instead of shifting instantly, the vacuum delays the shift to...........maybe .75 second............so that the engine and driveline can match speeds without jarring the driveline. It's a delicate balance between clutch wear and driver satisfaction.

At WOT, vacuum is removed completely as the manufacturer realizes that the clutches will wear significantly if the shift is delayed under high power. Additionally, the driver is relatively unconcerned with the shift quality when he wants full power.

Can some just answer this for me? How does the cap come off where I can access the pin if I decided to adjust the modulator and why does this whole device turn to the most 180degrees counter clockwise but wull not turn clockwise? Also does turning this part do anything? or.... must the top come off in some way to access the pin?

This is what I am biting at the bit to know. Thanks

I'm trying to remember is the reason I have not answered. Have you tried turning the cap that is on it? Have you tried pulling it off? It was the earlier 123 model cars that had the removable black cap. I'd have to walk up to the shop to take a look at mine to tell you the specific answer.

yes as you ca tell from the pics and scratches on it. I stopped in case it does not come off and I subsequently damage it. Still would like to know I it turns 180 degrees CCW but not CW from the initial point?

I'll go up to the shop later this afternoon and look at the one on my car to see if I can remember.

Looks like you have found the thread you needed:).

Brian - assume the working is still like my 81 300SD? No vac needed for upshift (in fact it is designed to bleed off vac at WOT). However needs vac for smooth downshift. Most of the posters do not seem to realize that loss of vac will not affect upshift.

Or did something change through the years?

listen to the mechanics that are attempting to FIX your car! adjustment isn't wise until you KNOW what you have!

you could have a damaged vacuum pump, but that would show other symptoms, (like hard braking) so the most likely reason for your symptom is a vacuum line SOMEWHERE else on the car is bad, or the modulator itself is bad. there are a LOT of vacuum connections in the engine compartment, and unless you followed every single one, there's no way to just look at the transmission line and know it's not leaking.
so TESTING the line while you are driving is the only way to know what's wrong.