When A/C is off.. transmission runs smoother? Why?
 

Earlier this year I had my 722.3 transmission on my 92 400se replaced with a rebuilt AT. Since then, after the car reaches operating temperature, I experience "hard" shifts.

The independent mechanic doesn't seem to have the knowledge to diagnose or address the hard shift problem.

It is interesting when the air conditioner is not operating, the shifts are much smoother.

I have checked the vacuum line and connectors from the engine to the modulator valve with a vacuum tester. The line will hold presser.

Any suggestion would be appreciated.

Thanks,
Russell

When the A/c is off the engine develops more vacuum & the modulator on the trans work from engine vacuum. The more vacuum the softer the shift!

A slight counter-clock wise adjustment of the modulator should help.

My 722.4 transmission also shifts harder when A/C is on.
On this diesel car, a separate vacuum pump generates vacuum, and transmission uses that vacuum.

I changed 10 k mile old fluid myself (fluid only, not filter) yesterday, but the shift is the same: harder upshift when A/C is on. I took some of the old fluid in the bottle and will send it for analysis. I did tranny fluid analysis about 2 years ago, and it was fine.

We get this question every summer. The transmission doesn't know or care whether the a/c is on. The more the throttle is open, the lower the vacuum produced by the engine, and the harder the transmission shifts. That's how the car is designed - smooth shifts when you're just strokin' along, firm when hurrying.

With the a/c running, you gotta give the car more throttle to move down the road - the a/c is sucking up a bunch of the power produced by the engine. More throttle equals firmer shifts, as described above. If you want softer shifts, try accelerating more slowly :)

- JimY

i thought hard shifts are good for auto trannies. myth or truth?

First, hard shifts will give you less wear on the clutches, by and large, but more wear on pinion gears, diff mounts, spider carriers, etc. To say nothing of being annoying.

Second, even on the diesel, a leak in the ACC vac system will give harder shifts from low engine vac. Worse on the diesel.

The vac supply on the diesel is split -- main hose goes only to the brake booster, everything else is run off a small line with a restrictor in it. Anything that reduces vac in that subsystem will cause hard shifts.

Finally, check ALL the rubber vacuum line parts, they are likely dead. Replace anthying suspect, and verify that ALL the subsystems excpet tranny control valves on the diesel hold vac. Fix anything that leaks.

"Normal" MB tranny shifts are firm by US standards (the W108/109 and possibly the W116 three speeds are an exception -- can't usually tell when they shift at light throttle). Bangs are not.

Peter

A 722.3 is going to shift more firmly until it is warmed up. It's normal for the shifts to be noticeable during the first 10 minutes of driving. After that it should smooth out a bit. Shifts should be just about seamless and not noticeable after a highway drive, when the tranny is fully warm.

My $.02,

- JimY

More throttle should = firmer shifts ...

but not in my case. When i am light on the pedal and accelerate slow - moderatly, it shifts harder.

When i floor it it shifts through the gears perfectly and i barely notice it. Would this more likely be a modulator problem?

:confused: