Uncommanded Downshifting after warm-up

[NoMoreSIDS]

Uncommanded Downshifting after warm-up:

A 1994 SL 500 with around 130k usually goes through its shifts beautifully until all is fully warmed, then will stay stuck in 2nd.



If at speed, will be a sudden uncommanded downshift to 3rd, or to 2nd. During this first misbehavior of the day, some games with shift to neutral for a couple of seconds at-speed, then back to 4th might re-achieve a 4th, or a 3rd, but only at best for a minute or two.


Same symptoms on surface streets; play the shifter games when it begins to misbehave to re-achieve a 3rd / 4th, but this only works briefly before that’s it for the day: it will then stay in 2nd.



Fluid level perfect. Indicated water temps are completely normal.
Saw no reason to mess with cable.



Since getting this at auction, I’ve done nothing but been running Auto-Rx to see if this helped. It seemed the symptoms disappeared until much longer well-after full warm up, but ultimately the same symptoms onset returned. So, the only change was of fluid character.


Without changing fluid/filter, I was biased about there being a broken or unstable part to deal with before dropping the pan, but could fluid/filter alone really have such a profound effect?


Before I do the fluid/filter change...any ideas consistent with great running while cool?


Sincerely, Mark



Here are some related posts:

300e transmission downshift woes

350SD - Transmission Downshifts

'94 E500 Automatic Transmission Wierdness?

[mbdoc]

Of course since the trans is 99% mechanical with only electrical kickdown function,
most likely there is a fluid pressure issue.

Start with new filter & make sure there isn't any restriction in the cooler OR cooler lines.

If you still have downshifting issues, most likely internal issues.

[97 SL320]

For hydraulic shifted auto transmissions in general and not make specific, it is rare to have unrequested downshifts. If a clutch pack / band starts to slip when hot, the engine will flare and feel like a down shift but it really isn't.

Does anyone have a apply diagram for this trans? ( This is a grid that shows what is applied in what range )

There are two things that make the trans shift at the proper time, road speed and engine load. Road speed is measured by the governor and converted to variable hydraulic pressure. As road speed increases, the pressure goes up.

Engine load is measured by a cable or linkage attached to the throttle or a vacuum modulator referenced to the intake manifold. This also converts engine load into a variable pressure. ( This is sometimes called TV pressure, Throttle Valve Pressure ) Some transmissions have both using the linkage for kickdown.

Each "speed , gear" has a shift valve ( spool valve ). Governor pressure is on one side and TV pressure on the other. When governor pressure exceeds throttle pressure, the valve moves and the shift occurs.

There are a few valves in the mix for fine tuning, but the above is enough for a basic description.

So , to down shift, governor pressure must drop off ( car slows down ) or TV pressure to rise ( push harder on the gas ) . Even if the electric kickdown is stuck, the trans should up shift given enough RPM.

Any sort of "trans fix in a can" is more like "trans destruction in a can" . Put this in a trans that does not have friction material or piston rubber trouble, the frictions and rubber seals will be damaged.

A valve body leak / governor pressure loss of can cause a downshift but that isn't common. Installing presure gauges can give you some information.

The last question is did you buy this car to flip or keep?