Mercedes 450 SLC not changing from 2nd to 3rd gear
 

HI All

My Mercedes 450 slc is not changing from 2nd to 3rd gear. my filter and transmission has been changed and it still does not select 3rd gear.

when I engage Drive ( presume I can call this first gear) good response. when I take off it makes the next change( 2nd gear) at 40km/hr smoothly . then does not change to the next gear ( 3rd).

I tried kick down , but it does not seem to engage. all that happens is the car Increases speed on the same gear( 2nd)

should the valve body be looked at or should i try with the easier option of changing the kick down switch?

your thoughts please.

regards
collin

After the change to what you have indicated is 2nd gear, and if you maintain 40kph in that gear, what is the engine RPM?
What model year is the car? In what country are you located?

HI Frank

see below the info.


1977 Mercedes 450 SLC
South Africa

will check the rpm when I drive it again.

C

HI All,

HI Frank

I took car for drive today to establish rpm.

once in drive, it changes to number 2 at 2500 rpm and drops to around 2000 rpm if I keep it at 40km/hr.

if I accelerate the following is observed , while it stays in number 2, ie does not select 3.

at 60km/hr = 2500rpm
at70km.hr=3000rpm
at80km/hr=3500 rpm.

should I be looking at the K1/K2 piston kit?

your help to diagnose will be appreciated.

thanks
c

There are a couple of things to do before you go digging into the valve body.

I'm assuming you have a 107.024 chassis with a 3 speed 722.004 3 speed transmission.

The first thing to do is to make absolutely sure you have the correct fluid (Dexron II if you can get it) and the correct fluid level. I don't mean to be rude but many people struggle with Mercedes transmission dipsticks.

The next thing to check are your shifter bushings - what you describe is close to how the transmission should behave in "S" and not "D".

The next thing to check is the throttle linkage. I think there's a connection for the modulating pressure on your transmission. This runs from the front right hand side of the transmission up the side of the bell housing onto the throttle linkage.


This data might help =>

If you let the car shift through the gears in "D" at idle going from 1 to 2 should happen at 30 kph. 2 to 3 should happen at 45.

At full throttle in "D" 1 to 2 happens at 68 kph and 2 to 3 at 138

(Data from ATSG manual)


#################


Don't under estimate the fluid level or the importance of the fluid type.

The first and easiest place to check is the gear selector linkage rod. The rod has resilient (plastic and/or phenolic) bushings at both ends. If the bushings are damaged or missing, it quite often happens that the lever on the side of the transmission is not moved fully into the D position, thereby holding the trans in 2nd.
If the bushings are intact and the linkage is properly adjusted, then the next step
is:
to check governor pressures. This involves installing a pressure gauge with enough hose that the gauge can be read inside the car while driving, or on a chassis dynamometer.
The third step involves removal of the valve body to inspect for a stuck command valve.

The operating elements, K1 & K2 clutch pistons, and the B1 & B2 band pistons are not on the list of suspects.

It may interest you to know (if you do not already) that your trans, a 722.0xx (3-speed) is actually a 722.1 (4-speed) with 1st gear permanently disabled, hence, it operates in 2nd, 3rd, and 4th.

I can't find the governor pressures (or any others) for the 3 speed in the ATSG manual I've got Frank - have you got that data?

Stretch:

I do not have regulating (governor) pressures for the 722.0, however if the 722.1 is used as a basis, the 60kph pressure would be ~2.5 bar, and the 120kph pressure ~5.4 bar.
In checking the pressure, a large deviation would be significant; on the order of 50% or more below the design pressure. A deviation of .2-.4 bar would not cause a no-shift.

Hi All

I managed to find a working transmission for the 450 SLC.
Is it easier to remove just the transmission form under the car and replace or better to take the engine and gearbox out and do it outside the car.

your advise will be appreciated.

Any tips to follow when I do his transmission swap. naturally I will change filter and fluid.

thanks and regards
C

Yes!! Pulling the engine just to R&R the trans makes no sense.

I say it depends on the equipment you have.

If you have a driveway a trolley jack and a set of axle stands I would hire / buy / borrow an engine crane with a load leveller and remove transmission and engine in one go. I know it is a lot more work (or seems that way) but struggling with a transmission under a car on axle stands is in my experience an accident waiting to happen. The chances of damaging yourself, the car or the transmission is in my opinion greater.

If, however, you can do the job on a 4 post / 2 post lift with a transmission jack - well it is like you'd be using the right tools for the job!

Thanks for the suggestions.

I have a undercover driveway, engine lift and axel stands.

I think I will give it a try to take engine and gearbox out. I did remove an engine and gearbox previously so should be fun the second time around.

regards
c

Hi All

when it transmission is out of the car, I will change fliter , front and rear seals.

is there any other serviceable items I should consider whilst transmission Iis out of car? eg other seals, pistons etc.

thanks
C

I'd consider checking the B1 and B2 pistons. These are known to get gunked up and cause trouble...

...problem is you're making more work for yourself...

...definitely change the shifter bushing though.

HI All.

I removed the valve body. the fresh new fluid which I filled recently did not look fresh. there was a fair amount of black residue sititng on top of the filter and also on top of the valve body.

Then I looked at the underside of transmission with valve body removed and saw the following. see foto.

on one of the bands the lining is loose and moves. I push it slightly so I could take a foto.

what does this mean?, the band is toast, which band is this?

is this the cause of my problems of not changing into number 3?

your comments is appreciated.

regards
C