300SD shifts only at high rpm (3500-4000) - PeachParts Mercedes-Benz Forum

mplafleur · #1 12-29-2005, 11:47 AM

My '85 300SD has started to shift only at high rpms. All forward gears and reverse work well, no slipping or flaring. This is not a B2 problem.

I disconnected the kick-down switch a year ago so it's not that.

I replaced the modulator last Sunday, so It's not that. (it did have a vacuum leak though)

I did notice that if I take it up to ~2500-3000 and back off the throttle quickly, it will upshift without maxing out the rpms.

It acts like the kickdown switch is still hooked up. Where does that input go?

I have not attached a vacuum gauge to the vacuum line at the amplifier output where it goes to the tranny yet.

Any guesses?

TheDon · #2 12-29-2005, 11:51 AM

have you adjusted the bowden cable? mine was shifting high and i adjusted it and it shifted earlier . this is in my 300D-T i belive its tighten to get earlier loosne to get late shifts

#3 12-29-2005, 11:56 AM

I agree with the bowden cable comment. Also, have you verified that the disconnected wire from the kick-down switch has not shorted out somehow?

mplafleur · #4 12-29-2005, 12:16 PM

I did nothing to the bowden cable to cause this to happen. I checked it and it is still attached and I have also adjusted it to either end of it's travel to no effect.

Properly set, it should have a slight slack.

Pete Burton · #5 12-29-2005, 12:56 PM

Quote:

Originally Posted by mplafleur

My '85 300SD has started to shift only at high rpms. All forward gears and reverse work well, no slipping or flaring. This is not a B2 problem.

I disconnected the kick-down switch a year ago so it's not that.

I replaced the modulator last Sunday, so It's not that. (it did have a vacuum leak though)

I did notice that if I take it up to ~2500-3000 and back off the throttle quickly, it will upshift without maxing out the rpms.

It acts like the kickdown switch is still hooked up. Where does that input go?

I have not attached a vacuum gauge to the vacuum line at the amplifier output where it goes to the tranny yet.

Any guesses?

Is this the car that Roy just changed the modulator on? Even new parts fail, and IF they fail, they frequently fail soon. Much more common for something to fail in week#1, rather than, say, week#43.

mplafleur · #6 12-29-2005, 01:37 PM

Quote:

Originally Posted by Pete Burton

Is this the car that Roy just changed the modulator on? Even new parts fail, and IF they fail, they frequently fail soon. Much more common for something to fail in week#1, rather than, say, week#43.

Yeah, I changed it at Roy's. I don't have doors on my garage yet.

It was too easy of a change to mess up. The plastic piece went back in fine. It didn't break. I didn't have to force anything. I don't think the part would be bad right out of the box.

I have a feeling it is something else.

rg2098 · #7 12-29-2005, 02:00 PM

Might be the kick down solonoid inside the trans. On a 722.315 its located at the back passanger side lower corner. It may be at the same place on yours. Try unplugging the wire on it first, maybe the kickdown wire is shorting somewhere else?

mplafleur · #8 12-30-2005, 02:58 AM

I'm thinking it's unlikely it is the cause but I'll check it out. Maybe the wire got pinched by something and shorted together. It seems to be acting as the switch were stuck ON.

whunter · #9 12-30-2005, 04:07 AM

Please review these two threads, and call me tomorrow.

Bowden cable...

Bowden cable, is this the adjustment

I need to drive the car.
We may need to drop the transmission pan.

mplafleur · #10 12-30-2005, 11:12 AM

Already read those.

It's not out of adjustment as I didn't change it. Something broke.

When I pulled out onto the road it shifted through the gears just fine, several times.

When I pulled away from a stop light, it had to go to 3800 rpm to upshift into drive. It actually shifted fine for 2nd and 3rd. The next time I pulled away, it happened for all the upshifts.

I'd like to rule out all other possibilities before I pull the pan.

SD Blue · #11 12-30-2005, 11:29 AM

Quote:

Originally Posted by mplafleur

I did nothing to the bowden cable to cause this to happen. I checked it and it is still attached and I have also adjusted it to either end of it's travel to no effect.

Properly set, it should have a slight slack.

Hmmmm, this might give us a clue. Did you check the transmission end of the bowden cable? On mine, just a couple of turns changes the rpm shift point.

Pete Burton · #12 12-30-2005, 12:01 PM

Quote:

Originally Posted by mplafleur

Already read those.

It's not out of adjustment as I didn't change it. Something broke.

When I pulled out onto the road it shifted through the gears just fine, several times.

When I pulled away from a stop light, it had to go to 3800 rpm to upshift into drive. It actually shifted fine for 2nd and 3rd. The next time I pulled away, it happened for all the upshifts.

I'd like to rule out all other possibilities before I pull the pan.

sounds like the valve body is not applying the appropriate pressure to the bands and clutch packs. By a lot. Could be due to the signalfrom the cable or modulator, shifter partially out of detent, clogged filter, weak front pump or a number of unlikely but possible hangups in the valve body, although the main pressure valve spool (or whatever it's called) would fit the symptoms. Would be nice to get some pressure gages on it.

mplafleur · #13 12-30-2005, 01:49 PM

Does it mean anything that I can get it to upshift by bringing it up to 3000 rpm and then letting off the throttle quickly?

Pete Burton · #14 12-30-2005, 02:29 PM

Quote:

Originally Posted by mplafleur

Does it mean anything that I can get it to upshift by bringing it up to 3000 rpm and then letting off the throttle quickly?

Yeah, it means that you are able to "trick" the valve body into sequencing to the next gear by quickly relieving the cable linkage holding that pressure regulating spool valve on the right side of the valve body. That quick release may help it move when otherwise it gets stuck somehow - I think.

mplafleur · #15 12-30-2005, 11:30 PM

So where should I be focusing my attention?