300SDL Engine/Trans vacuum or Trans sticking? - PeachParts Mercedes-Benz Forum

Johnhef · #1 07-11-2005, 09:26 PM

This is the car in question...

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=4559342337
It's currently sitting in my driveway. What it is doing is not shifting into 4th gear sometimes. If you look at the engine picture in the auction you can see what kind of mess the vacuum system is in. I found one line to the modulator pinches, and corrected that, and found the angles hose out of the IP ripped and the line hanging loose and fixed that and while I was giving the owner a ride back home, it shifted perfectly for those 10 miles. As I was on the interstate coming home, I noticed the RPM's hanging around 3500 and it refused to go into 4th. I slowed down a little and it dropped down another gear for a little while but eventually the car acted like I had it in neutral. I slowed down even more (more like coasting) and put the blinkers on as I made my way to the exit ramp, when it found a gear and came back to life.
After sitting at the stoplight, it shifted fine through all 4 gears again.

I am a little stumped. Could the vacuum system be causing this? Or does this sound like a couple trans valves sticking? I could really use some advice on this one, thanks in advance...

Brian Carlton · #2 07-11-2005, 09:30 PM

AFAIK, the vacuum system cannot cause the problem that you have. The vacuum system affects the firmness of the shifts but won't affect the timing of the shifts.

In hoping for a simple fix, the Bowden cable might be a little too tight. But, this normally delays all shifts, not just 3-4.

It's more likely that the trans has sludge in the valve body and the 3-4 shift is delayed beyond normal. Try a fluid and filter change. Sometimes you get lucky.
The other option, which also requires some luck, is Trans-X, but, I have no personal experience with it.

What did you do..........purchase another one??

Johnhef · #3 07-11-2005, 09:48 PM

I was going to try a trans cleaner and then flush the system.

Its not mine, the guy who sold the Blue 300SDL, the toyota MR2 and the black 560SEL my friends and I just bought, owns it.

MB-Owner-in-ind · #4 07-11-2005, 10:00 PM

Check the down shift switch under the "gas" pedal. It might be stuck.

Johnhef · #5 07-11-2005, 10:25 PM

That was my thought too, I havent checked it yet but the guy took it to a shop when he got it home and they replaced the kickdown switch with no change. The shop then "disconnected the solenoid" which I assume he meant for the kickdown since it doesnt respond when its pressed.

I'd still like to see a vacuum diagram for the vehicle if anyone knows where to find one....

MB-Owner-in-ind · #6 07-11-2005, 11:01 PM

No vacuum will make it shift early and hard. Too much vacuum will make it shift very soft ie slipping. I can't help but think it would be something else. I have ran my car without any vacuum hooked to the tranny and didn't have these sort of problems.

sixto · #7 07-11-2005, 11:05 PM

Pull and cap the line to the modulator on the side of the tranny. See if that changes anything. At the very least it will confirm that it's not a modulator problem.

Sixto
95 S420
87 300SDL

Johnhef · #8 07-12-2005, 07:18 PM

Drove it 25 miles to and 25 miles back home from work today with the modulator line unplugged... it didn't act up once. Modulator holds vacuum fine too. I'm going to leave it unplugged for the time being and see if I can get it to act up. I plugged the kickdown solenoid back in for the ride home too. definitly an improvement there

on the other hand, one of the tie rods is loose as hell so I wont be driving it anymore untill I fix that first...so thats what that knocking noise was everytime I turned the wheel...

twr · #9 07-13-2005, 02:19 AM

John: My “new” 300 SDL had the problem of not shifting into 4Th. And sometimes shifting properly, the problem was gone after replacing the vacuum valve for the transmission (I was told a very common issue with cars with more than 300k miles)
My point is this car high mileage could be the reason for the transmission problems.

sixto · #10 07-13-2005, 02:58 AM

By vacuum valve do you mean that thing on the side of the IP that modulates the vaccum signal to the tranny amp based on throttle position? If so, I was able to revive the one on my SDL. I took it apart and flipped over the foam pad that serves as the airflow regulator. The facing side had a groove worn into it that didn't allow a proper seal.

Taking it apart requires shaving off the rivet flanges. I found that screws used in common household switchplates will thread themselves into the remaining 'rivet shafts' to hold the valve body together.

I attached pics showing the inside of the valve. The control arm is connected to the throttle position input from the IP through a negator spring. I don't know if mine was lazy at 260K miles but there was about 15* of slack between full open and the relaxed position of the the negator spring. In retrospect this may be what makes adjustment possible. The inverted close-up shows the piston that holds the foam pad that presses against the input (or output?) nipple to modulate the amount of vacuum going to the tranny amplifier. As I said, one side of the foam had a permanent groove so I flipped it over to present the fresh side. It seems like I did this a couple of lifetimes ago so I don't remember what resulted. Maybe it's in the archives.

I have more pics of the innards of the valve if anyone's interested but I don't remember what each pic shows :/

Sixto
95 S420
87 300SDL

twr · #11 07-13-2005, 11:01 AM

Quote:

Originally Posted by sixto

By vacuum valve do you mean that thing on the side of the IP that modulates the vaccum signal to the tranny amp based on throttle position? If so, I was able to revive the one on my SDL. I took it apart and flipped over the foam pad that serves as the airflow regulator. The facing side had a groove worn into it that didn't allow a proper seal.

Taking it apart requires shaving off the rivet flanges. I found that screws used in common household switchplates will thread themselves into the remaining 'rivet shafts' to hold the valve body together.

I attached pics showing the inside of the valve. The control arm is connected to the throttle position input from the IP through a negator spring. I don't know if mine was lazy at 260K miles but there was about 15* of slack between full open and the relaxed position of the the negator spring. In retrospect this may be what makes adjustment possible. The inverted close-up shows the piston that holds the foam pad that presses against the input (or output?) nipple to modulate the amount of vacuum going to the tranny amplifier. As I said, one side of the foam had a permanent groove so I flipped it over to present the fresh side. It seems like I did this a couple of lifetimes ago so I don't remember what resulted. Maybe it's in the archives.

I have more pics of the innards of the valve if anyone's interested but I don't remember what each pic shows :/

Sixto
95 S420
87 300SDL

Yes this is the one, mine had to be replaced because it was beyond repair.

Johnhef · #12 07-13-2005, 09:00 PM

sixto, what symptoms did your car have that led you to suspect the valve? This sounds very promising to me, and makes sense why it doesn't act up with the line to the modulator unplugged.

sixto · #13 07-13-2005, 09:29 PM

The transmission would slip during hard acceleration. I tested the vacuum controller per section 07.1-170 in the shop manual and couldn't get a 0 reading.

Sixto
95 S420
87 300SDL

Johnhef · #14 07-15-2005, 09:31 PM

I ordered the valve today. It wasn't too bad, only a $57 hit for it. A drop in the bucket compared to the other $525 in parts alone he ok'd me to get for it today.

It's going to be a very nice car for someone who wants an SDL when I'm done with it.

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