1968 280SE Trans - PeachParts Mercedes-Benz Forum

ukstevem · #1 05-11-2006, 07:43 PM

I recently got a 280SE as a project, i believe i have a transmission problem with the vehicle which is where i am starting on the project. The dashboard selector shows four speed (2,3,4,N,R,P) however i dont believe it ever gets to fourth as the engine is revving high (too high) at 65mph and i dont feel any shift into the fourth gear. I am aware some vehicles of this vintage were three speed, but the low top end makes me think this is a four speed, is there any way to identify the gearbox, at least that will give me a starting point. Also being rather new to the Benz, does anyone know if this is a common problem? Shifting through the gears that do shift can be described as done with 'certainty' but no excessive jerking or any perceivable slippage.

Thanks for any help!

Steve

Arthur Dalton · #2 05-11-2006, 07:59 PM

Top gear high RPM is due to differential gearing of 3:92.
Unless yopu use the kick-down, the trans starts in 2nd gear.

ukstevem · #3 05-11-2006, 08:35 PM

i thought the car was supposed to have a top end of 100+ mph, there is no way this car will achieve 90mph with the revs it has. Have i got the wrong end of the stick here?

Arthur Dalton · #4 05-11-2006, 08:42 PM

If you suspect it is not in 4th gear , simply downshift to 3rd gear and that will tell you if it was in 4th..
..it is quite commn for new owners of these chassis to think it is not in top gear as the rpms are high due to the gearing..

What does the tach say at 60

ukstevem · #5 05-11-2006, 09:16 PM

no tach, the other dial is the fuel / temp gage, i would say the car wouldnt do more than 85 in top. it doesnt seem to try and shift into 4th, is there any way to determine the gearbox. If it isnt shifting into 4th is there anything obvious i should check before pulling the engine and gearbox.

Tomguy · #6 05-11-2006, 09:59 PM

Like Arthur said... at about 60MPH, pull the gear selector to "3". If the engine RPMs dont change it's not hitting 4th. If they do it is. If your engine is doing 4k RPMs at 70MPH itd be doing 6k at 105.

ukstevem · #7 05-11-2006, 10:20 PM

OK, tried it, it didnt downshift, so i either dont have 4th or dont get 4th, any suggestions on where to start. Im quite interested in pulling the engine and gearbox and pulling it apart, is it worth it or should i just trade the gearbox for reconditioned?

Tomguy · #8 05-11-2006, 10:40 PM

Have you checked the linkage? It's hard to condemn a tranny that works in all but one gear. Check the linkage out, check the fluid and if you need to, check a tranny shop!

We're assuming this is a 6-cyl as you haven't specified. The V8 4.5 had a 3-speed auto.

ukstevem · #9 05-11-2006, 11:41 PM

Thanks guys, i'll give those a whirl to start with, yes it is the 6 cyl.

in your post Tom you said 'if you need to check, a', check a what? leaving me hanging.

Thanks,

Steve

Tristar1959 · #10 05-12-2006, 12:16 AM

Tomguy has a good point on the selector linkage - it would have to be proportionally displaced out of position, but you "enable" 4th in the first position off "N", so if you're shifting into that detent, it should be able to select it when load and rpm permit. The other linkage which is more likely to "hold" your tranny in 3rd would be the "Control Pressure Rod" from the throttle linkage down to the "Control Pressure Lever" on the tranny - it would need to be LENGTHENED some, IF it's lying to your transmission, essentially telling it you're "kicked down" (pulled more forward) when you're not. A general rough "zero" setting would be to have your throttle linkage at idle stop, and adjust the rod length so that, when the linkage is all hooked together, the tranny lever is back against its stop, as well. Easiest to have your upper linkage at idle stop, push the rod all the way back, and adjust the attaching end to fit the throttle linkage with zero "slop" either way.

In the troubleshooting guide for a similar tranny, when it does not shift into 4th, the cause is listed as "1. Governor pressure too low" and "2. Command valve jammed".

By the Book, for the Trial Run, Shift selector into position "4" and slowly accelerate with slight partial throttle. The transmission is in 1st gear.
Shifting up takes place:
at approx 10km/h from 1st to 2nd gear,
at approx 30km/h from 2nd to 3rd gear,
at approx 40km/h from 3rd to 4th gear.

You may be able to "see" the engine shifts if you put some long leads on a simple dwell meter/tach, viewed in the crew compartment whilst driving...plus know what rpm's you're turning at what speeds.

The downshift/upshift test:
Maintaining constant accelerator pedal position, accelerate to 60km/h and then employ kickdown; the transmission changes down into 3rd gear [of course, this assumes it shifted up, to begin with - obviously, if it doesn't/didn't, then you won't get the kickdown.]
Return accelerator pedal to "full throttle" position and accelerate. The transmission changes from 3rd to 4th gear at 125km/h.

There's a whole further test, which I'd rather scan for you if you can read .jpg pics, since I type with two fingers...

Shift On!

Stan

Tomguy · #11 05-12-2006, 12:58 AM

Quote:

Originally Posted by ukstevem

in your post Tom you said 'if you need to check, a', check a what? leaving me hanging.

I do that every now and then. The thought is complete in my head and I'm moving on to the next before I convey the thought verbally or in writing.

Check a tranny shop. Don't be so quick to doom the tranny!

If your shifting is delayed, your kickdown switch may also be shorting, telling your tranny you're constantly floored. This will keep it from shifting into 4th until you're redlining in 3rd.

250 Coupe · #12 05-12-2006, 02:36 AM

I think your transmission has granny low like the W114s. I've not seen an overdrive automatic in the earlier cars unless it's a manual.

If so, you'll not likely notice the 1-2 shift as it happens at 5 to 10 MPH under a light throttle, if at all. Some don't use first unless you floor it or select it before starting out. My 1971 250C uses first all the time so I get a hard shift around 10MPH. My 1976 280C didn't use first unless provoked. If you count the noticeable shifts, you'll only get three.

From a dead stop and with the selector in its lowest position, accelerate mildly and shift manually around what you think sounds right based on revs.

My guess is that you'll feel all four shifts.

Top gear is 1 to 1 just like a normal three speed and is not an overdrive. You should see so tick marks around the outer edge of the speedometer. These indicate the speed to shift out of the indicated gear. As an example, shift out of second into third at the "II" mark. You'll see that the "III" mark is pretty high up the dial but not near top speed. That's because you're not shifting into third but rather into fourth.

Michael

Tristar1959 · #13 05-12-2006, 07:43 AM

Tomguy has a good point about the kickdown switch possibly "lying" to your tranny that you're floored...the same logic I was chasing with the rod/lever adjustment - that perhaps something's telling it to stay in 3rd when it would normally have upshifted.

Also, from the "upshift" test - even with the throttle floored (or seemingly floored), that final "redline" upshift Tom mentions should take place at about 125km/h - if you can get it out on the M to check it out...

Stan

ukstevem · #14 05-12-2006, 11:17 AM

Well, i think that sets my weekend for me, i'll take a look at these items, when driven i feel it doesnt shift into second / third as early as it should, seems to be high revving, so maybe it is a linkage/kickdown problem, i did take it out on the road to 85mph and never got the shift into 4th.

Does anyone have any advice on a workshop manual for this vehicle, i know the haynes/chilton guides can leave a lot to be desired.

Thanks again,

Steve

Tristar1959 · #15 05-12-2006, 12:34 PM

According to my Dec 69 supplement to the automatic transmission part of my Service Manual, the 280SE/8 rear ends changed in November 69, from 3.92 to 3.69, but that could be only the US versions.
If yours is a 3.69, add 5% to the speeds given below. Look for numbers stamped into underside of the differential casing - 47 and 12 for a 3.92, 48 and 13 for a 3.69. If for some reason yours is very long-legged or short-legged, possible other special ratios which could have been installed in your car include a 107/116's 3.07 (46:15) or 3.46 (45:13); 114/115's 4.08 (49:12), and the tree- and Hong Kong Peak-climbing 115's 4.36 (48:11).

And, as pointed out above, the kickdown will hold the lower gears, as well - if, from a stop, you floor my daughter's W123 280CE, it will use and hold 1st, quickly wind it out; then upshift into 2nd, and so on, until it bails out at the redline. Of course, this is only known to me, since Daddy's Little Girl would NEVER stomp on a gas pedal that way (Yeah, Right!!!)...

These are the "book" upshift and downshift spds (all in klicks, not mph):
In selector position 4; accelerator position ( ); shifting; Up/Down
(Idle) 1-2-1: 9.5/5 (Full) 1-2-1: 25/5 (Kickdown) 1-2-1: 25/17.5
2-3-2: 27/20 2-3-2: 53/20 2-3-2: 53/45
3-4-3: 39/29 3-4-3: 121/48 3-4-3: 121/109

In position 3;
(Idle) 1-2-1: 9.5/5 (Full) 1-2-1: 25/5 (Kickdown) 1-2-1: 25/17.5
2-3-2: 33/28 2-3-2: 73.5/34.5 2-3-2: 73.5/63

In position 2;
(Eric) 1-2-1: 37/5 (Monty) 1-2-1: 37/17 (Beckham) 1-2-1: 37/30

Unfortunately, it appears from the table that the UPshift will not change whether at Full or Kickdown - just the Down. Don't even ask me how you get the downshifts at Full or Kickdown throttle settings - unless you're supposed to use the brakes to get down there...ANYONE KNOW?

Feed your navigator/timer/copilot some dramamine or apply scopolomine patches before trying all the above. Also remember to allow some "cool-down" time between hard-working trials [the transmission (and possibly the co-pilot) may get unusually overheated][not to mention the brakes].

Let us know what you eventually find out, please - you're neither the first nor the last to see this gripe...

Stan