backnblack

Dose anyone know anything about vacuum and the trans shifting hard?

I have replaced my intake gaskets and had the top half of my engine off and while I was at it I replaced all the vacuum lines I could fine and I'm am now second guessing if I have them all going to the right place.

I also have replace trans lines and found that I may have not had full pressure and
been low on trans fluid.
Now that I have this stuff fixed when I am in bumper to bumper traffic and I am slowly moving along and hit 3rd gear it just slams but if I am accelerating fairly well or hard it shifts nice and firm.
It really concerns me how hard it shifts at low speeds.
Any anyone have any idea's.
thanks

Tails

Your will need to check the modulating valve pressure and adjust to WSM specifications via small "T" handle to ensure your have correct pressure to get correct shifts.

I have my own pressure gauge and fitting and I adjusted the modualting pressure whilst car was on jack stands.

If you don't feel confident to adjust take it to a good Mercedes Benz or a 928 Porsche auto transmission shop and have then adjust your modulating pressure to Porsche's specification for your model gearbox.

There are threads here to adjust it without using a pressure gauge, so suggest you carry out a 'search' or look through my old post for adjustment of modulating valve pressure.

Tails 1990 928 S4 Auto

Bill Ball

There is a vacuum line that should run from the 7-way rubber vacuum connector at the back of the motor down toward the top of the bellhousing and on to the transmission. Find that line and pull it off the 7-way. Hook a vacuum tester (such as a MitiVac) on to it an see if it will hold a vacuum. Or if no Mitivac, you can do the poor man's test of sucking on the line and seeing if it hold a vacuum that way. If it fails to hold a vacuum, check the other end back on the side of the transmission and make sure it is firmly seated on the vacuum modulator which is on the left side of the tranny, shown below just to the left of the "A". If it is seated on the modulator but there is no vacuum, then probably the hose has a break somewhere along the way from the engine to the transmission. There are two rubber sections with a metal section in between. If the line hold vacuum then make sure your vacuum source from the throttle is providing vacuum to the 7-way when the car is running. I'm going to assume the problem is somewhere in the above and not in the adjustment of the modulator as it shifted OK before you worked on the intake area, right? If it shifted hard before the intake service, then Tails' advice applies. Maybe it's as simple as the vacuum line is not hooked up to the 7-way. See vacuum diagram below for more help with that.

backnblack

Thanks guys for the tips, I have the vacuum gage to check it out but my daily driver ( jaguar XJ ) is on my lift dead sucking the life out of me and pissing me off so I hope I can get to this before it is to hard on the car.

Do you guys happen to know off hand how much vacuum the gage should read?

Bill Ball

Vacuum at idle from the throttle body should be about 16" Hg at sea level. A small leak, like a small vacuum line disconnected will drop it 2" or so.

Mrmerlin

if you pop the cap off of the vacuum moduator you will see a small key pull out the key and turn the key 1 revoultion CCW this will reduce pressure to the , Esentially what the mech will have to do if the pressure is up, But first check fluid level and quantity