Modulator Problem, I think.
 

I've just bought two old Mercs, an 81' 300D and an 83' 300D Turbo.
Both have failing Transmissions (Automatics).
They have to be reved very high, or for 10 seconds with a steady rev in order to change into gear. The old one is now parked as it was getting imppossible, and the newer is just starting to go.
IS THIS THE MODULATOR ???
IF SO HOW DO I REPAIR, COST, Should The Part Be New , Will I need any specialized tools to fix this ? I have the kindergarten set, which has severed me so far. :cool:

Thanks for your time and thoughts

-Pat

There are several things that could be responsible for this beyond the modulator or transmission.

On the valve cover there is a contraption with some small vacuum "switches." On the back of the injection pump there is a diaphragm. On the core support, there should be a diagram that shows the vacuum connection. The purpose of all this stuff is to simulate the vacuum characteristics of a gasoline engine. Diesels don't make vacuum, so all this is necesary for the transmission to shift correctly.

Start by teeing a vacuum gauge into the line that goes to the modulator. It should be a brown, stiff plastic line that goes down toward the back near the oil filter housing.

Before you begin, do two things. Make sure that the linkage is connected to the diaphragm mechanism on the back of the IP. Also pull the plastic cover off the contraption on top of the valve cover and see that the cam is actuating all of the switches. You will have to just figure out the vacuum flow and determine where the problem is.

Remember you are simulating the vac of a gas engine. In a gas engine, you have high vacuum at idle and low vacuum when accelerating. The harder you are accelerating, the lower the vac should be. If the vacuum is not coming up, it will not shift into the next gear.

Another common thing to happen is for the nipple from the main supply line breaks and gets patched back together. There is supposed to be an orifice in the line. If there is no orifice, it will not behave correctly.

Start with a vac gauge on the line to the modulator and start the process.

Good luck,