My 1984 300D is a little saggy in back, so I'm planning to replace the differential mount to see if that makes any difference. However, I haven't been able to find a good how-to on the procedure. I took a look under there today, and it seems like it would go something like this:

1. support the differential housing with a floor jack
2. remove the four 13mm bolts that attach the mount to the body
3. lower the housing a little bit
4. remove the two large bolts (24mm?) that attach the mount to the differential housing
5. install the new mount in reverse order

Is that all there is to it? Any tricks to making it go smoothly?

Thanks for any insights you can provide.

-Mark

You need to support the body AND the differential so you can separate them.

Agreed.

The body must be on jackstands before you start the procedure. Otherwise, it's correct as written.

Thanks. I was planning to use ramps under the rear wheels. Will that work or must the car be on jack stands?

Ramps are no different from the vehicle sitting on the ground.

You must support the body on jackstands at the jacking points.

When replacing the four 13mm bolts, the FSM reccomends purchasing new bolts with the pre-applied threadlocked...reccomendation here is clean and re apply blue loctite.

I purchased a Febi mount and the bolt holes did not line right up - I put my old mount back on at the conclusion of my rear axle job.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car

Sorry if I'm being dim, but would you mind explaining why stands will work but ramps won't? Won't the springs and tires support the body as I lower the differential?

Thanks,
Mark

Because the hinge point is in front of the wheels and the diff mount is behind the wheels. With the wheels on the ground, they will push up on the diff and it will not come down.

You need to get underneath the vehicle to understand it.

The springs sit between the trailing arm and the body.

The differential is bolted to the subframe.

If you remove the differential mount with the vehicle sitting on it's tires, the diff just sits where it's located...........on the underside of the subframe. It cannot move downward because the springs are under compression and the subframe cannot be lowered in this condition.

When the vehicle is sitting on it's body, the trailing arms are hanging downward and the springs are attempting to drive the subframe to China.

Now, when you loosen the differential mount, the springs will send the entire rear subframe, with the attached diff, downward. This is why you must support the diff.

The procedure cannot be done with the vehicle sitting on it's wheels.

Thanks for explanating, Brian and Lance. I'll try this tomorrow using the stands and report back...

Have you looked at the rear subframe mounts? The suspension will sag if those are shot.

btw, won't a bad diff mount generally produce uneven tire wear? I've been told a bad one throws the camber off.

Anything that causes the back end to sit lower than designed will decrease (more negative) the camber. The camber is a compromise. Too much (going positive) and the vehicle won't handle well in turns and will wear the outside of the tread. Too little (going more than 2 degrees negative) and the the vehicle will wear the inside of the tread.