What would make a 300D decide to stay in 3rd gear... with an auto? - PeachParts Mercedes-Benz Forum

Uriah · #1 11-16-2005, 10:08 PM

Went for another drive today, and noticed two things. First off, the 1984 300D has WAY more power then it should. It seriously runs like it's a gasser... a small gasser, but a gasser nonetheless. At least as much pep as a 2.3 to 2.6 with an automatic. I'm assuming somebodys running a little more boost then normal.

Anyway, it likes to shift late, and on several occasions decided it liked third gear (up to 70mph) and wouldn't come out, unless you slowed down enough for it to shift into second, then back to third again.

The bottom 3 gears work fine, no slipage, but it does shift a little later then it should. Shifts firm (not real hard, but you can feel it, lol... just like a Mercedes should), the 3-4 shift is what concerns me.

Sometimes you pull out, and it shifts proper, sometimes it don't. I spent about an hour with it to really check it out today, since I was thinking about trading my 735iL for it...

When it DOES go into forth, if you give it slight throttle (or use the cruise) it's fine, but much more then very little, and it's back in third. I'm assuming it's a vacuum modulator problem, but can't remember if a 300D even has one.

The fluid don't smell burnt or anything and is a decent shade of red. Not new, (very faint smell, nothing to be alarmed about), and at the proper level... on the stick.

BTW, if you floor it from a dead stop (on dry pavement, even), it'll move 3-4 feet and break the tires loose, lol. First MB diesel I've seen that did THAT. Maybe the last owner did that often? Who knows...

Oh, besides that, the turn signals quit (think it's in the keyswitch, they worked YESTERDAY), it could use shocks (no biggie, but didn't notice until I got on some mountain roads), and it seems to be geared pretty short, it turns 3100 at 60mph, that's when it goes in forth gear.

blueranger · #2 11-16-2005, 10:12 PM

1. there are 4 items apart from your tranny that effect shift.

a. the kick down switch under the gas pedal... it wont down shift if
this is broken... so get a new one...
b. the bowden cable must be adjusted correctly... if its too loose it wont
down shift at all...
c. there are little plastic fingers that need to be replaced inside the box
infront of the bowdine cable....
d. there is a vacum shift control on top of the fuel pump... if this thing is not producing vac it will effect the shift as well...

2. Your fluid level is critical....

Uriah · #3 11-16-2005, 10:20 PM

My guess would be the vacuum shift control, it really reminds me of a modulator problem, like on a old Ford with a C4, C6, FMX or whatever. However, that's just speculation.

I want the car, but not if I have to stick a tranny in it right from the get-go, lol.

blueranger · #4 11-16-2005, 10:38 PM

actually, the vac shift control on the fuel dist
must go to zero vac for 4th gear so it is probably only
decreasing down to say 2 or 3 pounds of vac and not
allowing 4th shiftt.

#5 11-16-2005, 10:42 PM

It sounds to me like either the downshift switch (under the throttle) is stuck or broken, or the bowden cable is in need of an adjustment. See if the switch is stuck down. Try loosening the cable a turn or so. It should up-shift sooner (lower rpm) and be less likely to downshift with partial throttle. Hopefully that will keep it in high gear. If neither of these fixes it, you may want to have it checked out before you decide.

The modulator affects how hard it shifts, but not when it shifts.

With stock gears and tire size, it should do about 70 mph at 3100 rpm in high gear.

Uriah · #6 11-16-2005, 10:42 PM

Quote:

Originally Posted by blueranger

actually, the vac shift control on the fuel dist
must go to zero vac for 4th gear so it is probably only
decreasing down to say 2 or 3 pounds of vac and not
allowing 4th shiftt.

How to I check it out? I've got a vacuum pump/pressure tester, and a bunch of other stuff, so that's not an issue.

blueranger · #7 11-16-2005, 10:46 PM

our friend craig disputes me but
hook your vac guage to the modulator right over
the fuel dist and drive down the road and see
what the vac is.... you will need a little hose
to run it inside the car...

also change the kickdown switch under the gas pedal becasue its
5 bucks...on ebay... i always change that...

also change the little fingers on top of the valve cover... its probably these
things....

#8 11-16-2005, 11:19 PM

Quote:

Originally Posted by blueranger

our friend craig disputes me but
hook your vac guage to the modulator right over
the fuel dist and drive down the road and see
what the vac is.... you will need a little hose
to run it inside the car...

I was referring to the vacuum modulator, on the side of the transmission which controls how hard or soft it shifts. You are talking about the vacuum controls on top of the IP.

blueranger · #9 11-16-2005, 11:29 PM

craig,
what is your experience with the little plastic fingers
on top of the valve cover....???

also where can we buy the k1 kit???

also i have heard you can buy a vavle body?

#10 11-17-2005, 12:03 AM

Quote:

Originally Posted by blueranger

craig,
what is your experience with the little plastic fingers
on top of the valve cover....???

also where can we buy the k1 kit???

also i have heard you can buy a vavle body?

I was under the impression that the plastic fingers were only related to the EGR valve operation, and did not directly affect the transmission. However, a vacuum leak there could affect everything that needs vacuum including the transmission. I have never played with them myself, but it might make sense to block the vacuum line to the "fingers" and see how it affects the shifting.

I have not bought a K1 kit or valve body either. I have seen posts on the subject. I'm sure someone here knows a good source, anyone?

Sorry I'm not being much help.