Another transmission question... - PeachParts Mercedes-Benz Forum

ngarover · #1 02-09-2012, 02:53 PM

So, yesterday the 300td's transmission worked fine. Worked fine this morning also. Then I took it for a drive and now it doesn't want to shift.

It will hold any gear it's in. I have to shift to neutral, then back into drive for it to goto the next gear. It downshifts automatically just fine. There is no slippage, the fluid is new and fine.

Checked the vac line, seems fine, Bowden seems fine.

Any ideas as to what to look at?

Stretch · #2 02-10-2012, 04:13 AM

Simplest thing first =>

Transmission fluid level

ngarover · #3 02-10-2012, 06:32 AM

fluids good.

It will shift from 1-2 by itself but higher rpm than normal, then to get it to shift to 2-3 you have to shift to neutral, then back into drive, it will then make the change, 3-4 is the same.

If you don't do the neutral thing, you can almost redline the the car, and it will not shift.

shadetreemechan · #4 02-10-2012, 07:48 AM

does it get better when the transmission warms up? Mine has done that for two years now when it is very cold outside. Seals are hard and can't build enough pressure to shift until it warms some.
My fix is to let it idle for a couple of minutes on cold days before heading out.

ngarover · #5 02-10-2012, 08:02 AM

no, in fact it's the opposite.

It has a slight hesitation shifting into gear at first, forward or reverse, then will shift kind of hard at a higher than normal RPM, then will stay there unless I goto neutral and shift it back to drive. I had a transmission shop drive it yesterday, and he said he's never seen anything like it....

Once it in whatever gear you happen to be in, there is no slipping, it pulls strong etc... I really could use the car tonight and tomorrow. Almost tempted to go out and drop the trans, swap the pump from it into my 85 trans and put that back up in there just to have it driving.

Stretch · #6 02-10-2012, 01:50 PM

Which transmission is it? You mention Bowden cable which would indicate 722.3 - but I guess it is worth checking just in case...

If you've got good throttle linkages, vacuum, fluid etc etc I think you need to get yourself a set of gauges and start making some pressure measurements - following procedures in the ATSG manual. Do you have a copy?

My guess (and it is only a guess!) would be the governor... but before you pull it to bits - including just taking the front pump off I'd make some measurements.

ngarover · #7 02-10-2012, 02:49 PM

Dont have the ATSG manual. Is there a link for it or? Somehow I've misplaced my Mightyvac in the last couple days...

... Still looking for it.

Is there a good walk though on replacing the governor? I could pull the one out of my 85's old trans as it was working fine before I blew the pump.

Stretch · #8 02-10-2012, 02:56 PM

Quote:

Originally Posted by ngarover

Dont have the ATSG manual. Is there a link for it or? Somehow I've misplaced my Mightyvac in the last couple days...

... Still looking for it.

Is there a good walk though on replacing the governor? I could pull the one out of my 85's old trans as it was working fine before I blew the pump.

Well there's this thread

Help - stuck on road 722.3 no 3rd/4th gear

Not a happy ending though. Based on the information in the thread I'd be measuring before taking to bits (which is unusual for me).

ngarover · #9 02-10-2012, 03:45 PM

Odd thing with mine is the fact is downshifts fine. It upshifts 1-2 (2-3)? on it's own. Letting up on gas etc does not allow it to upshift, I have to physically goto neutral and back to drive and it upshifts, repeat when I reach the shift point, and it shifts again. I can live with this as is until I hammer out the issue.

The kickdown switch is unplugged. There is a wire coming of the very rear of the transmission thats not hooked up, and has not been since I owned the car.

andrewjtx · #10 02-10-2012, 06:31 PM

What condition is your trans bowden cable in?

tobybul · #11 02-11-2012, 07:02 AM

Most of the time, tranny problems on the W123s are caused by vacuum leaks. Check the vacuum line going to tranny. Check tranny modulator valve. Check all the vacuum connections associated with the vac line going to the tranny.

Does power brake work? If not, possible main vac line from vacuum pump is leaking or vacuum pump is kaput.

ngarover · #12 02-11-2012, 10:29 AM

Quote:

Originally Posted by tobybul

Most of the time, tranny problems on the W123s are caused by vacuum leaks. Check the vacuum line going to tranny. Check tranny modulator valve. Check all the vacuum connections associated with the vac line going to the tranny.

Does power brake work? If not, possible main vac line from vacuum pump is leaking or vacuum pump is kaput.

My understanding is that vacuum does not control actual upshifting, only the harshness or softness of it. Even with vacuum totally disconnected it should still upshift, only it would "pound" into gear.

I'm still leaning toward the governor.

If I'm looking at 1.5 -2k for a rebuild trans then I might as well spend that money towards a 4 speed change over for the car.

Stretch · #13 02-11-2012, 01:06 PM

Quote:

Originally Posted by ngarover

...
If I'm looking at 1.5 -2k for a rebuild trans then I might as well spend that money towards a 4 speed change over for the car.

If you are prepared to "risk" doing it yourself the price of a rebuild is somewhat lower... it depends on your braveness / madness / confidence