Hard shift on 300SDL. How do you troubleshoot? - PeachParts Mercedes-Benz Forum

hughet · #1 06-06-2006, 01:54 PM

My 300SDL was shifting hard so I checked the line from the (blue) vacuum amplifier to the transmission and found that the vacuum modulator (on the transmission) had a leak.

Figuring that that was my problem, I ordered and installed a new vacuum modulator on the transmission. This one does not leak but now that it is installed the car shifts much harder than before.

I checked the vacuum supply tube on the input of the vacuum amplifier and it has 22" of vacuum. The line from the vacuum amplifier to the vacuum modulator on the transmission has only 8" of vacuum instead of the 16" to 19 that I have read it is supposed to have. My guess is that the transmission shifts harder now because the new modulator is much stiffer than the old one which probably moved more in spite of the leak.

So my problem is that I am not getting enough vacuum from the vacuum amplifier to the modulator for smooth shifts. My question is... how do I troubleshoot it? I can't find any leaks so I either have a bad vacuum amplifier or a bad vacuum control valve or some sort of electrical problem.

Has anybody been through this?

bender · #2 06-06-2006, 02:27 PM

My 300 was shifting so hard that the tires would chirp. I changed the fluid as well as drained the tourque converter, as I was in there anyway, I replaced the filter and pan gasket. I used synthetic fluid and the transmission acted as if it was new. That was year and a half ago. Very happy with the fix. And very easy. Spend the money and use the good stuff.

Hit Man X · #3 06-06-2006, 02:55 PM

I'd check all the vac lines under the hood and start going through them. Chances are, you need new connections there. Looks alone is not a way to go.

Are you certain the correct modulator was installed?

hughet · #4 06-06-2006, 03:10 PM

I bought an original equipment modulator from the dealer (Caliber motors). When I asked the parts guy if he needed the VIN he said that the black modulator which he was shipping was the correct substitute for all of the 300SDL models.

All of the vacuum lines are connected and appear to be tight and leak free but I'll look at that again before I order any parts.

A complete transmission service was done 300 miles ago.

I have not tried to adjust the modulator because it does not make sense to do it until I am sure that it is getting the right vacuum supplied to it.

Hit Man X · #5 06-06-2006, 03:35 PM

My books show the black is correct, but also for many gassers.

Perhaps it only needs an adjustment?

Also, when I'm in there I always seem to accidently knock off this one damned vac line. It doesn't cause the motor to not turn off, but it makes the trans shift very stiff all the time.

Brian Carlton · #6 06-06-2006, 03:58 PM

Quote:

Originally Posted by hughet

I checked the vacuum supply tube on the input of the vacuum amplifier and it has 22" of vacuum. The line from the vacuum amplifier to the vacuum modulator on the transmission has only 8" of vacuum instead of the 16" to 19 that I have read it is supposed to have. My guess is that the transmission shifts harder now because the new modulator is much stiffer than the old one which probably moved more in spite of the leak.

So my problem is that I am not getting enough vacuum from the vacuum amplifier to the modulator for smooth shifts. My question is... how do I troubleshoot it? I can't find any leaks so I either have a bad vacuum amplifier or a bad vacuum control valve or some sort of electrical problem.

Has anybody been through this?

I've monitored the vacuum from the amplifier to the modulator and it typically stays at 10" for most operating conditions. When you get to about 1/2 pedal, it will start dropping toward 5". When you get into heavy boost, it will head down to less than 2".

I believe that the amplifier is adjustable via a screw on the underside of the unit. However, I have not attempted any adjustments to raise or lower it from the 10" baseline. In your case, you are close to the baseline, but the shifts remain too harsh. Therefore, an adjustment would probably be beneficial for you. See if you can make some small adjustments to the screw and drive the vehicle with the gauge attached to determine your results.

Make sure you post them here as many of us are interested in knowing the adjustment parameters for the amplifier.

tstarr · #7 06-06-2006, 05:31 PM

Not that this is necessarily your problem, but right after purchasing my 300SDL I was having the same hard-shifting problem. In fact, it was happening so badly during my checkout of the car that I negotiated the price down to cover a transmission overhaul. The problem turned out to be the small piece of hose going from the vacuum pump up to the vacuum junction right at the front of the engine. There was enough of a leak in the old, spongy rubber that it caused the hard shifting. Put new rubber vacuum line there and it shifted like a champ. It was very, very cool to discount the car $2K and fix it with a 50 cent length of hose. I savored that moment...

- Ted

winmutt · #8 06-06-2006, 05:48 PM

Is it shifting at redline only?

hughet · #9 06-06-2006, 06:58 PM

It shifts at the normal points considering where you have the throttle. It's just real hard. Bam Bam Bam. It was a little hard before I changed the modulator but now it is much worse.
To adjust the vacuum amplifier I need a tiny socket. Maybe a 4 mm so I'll have to get one to see if it does anything to help.

hughet · #10 06-08-2006, 02:23 PM

I got a 4mm socket and tried to adjust the amplifier and found that it was turned up all of the way. I could decrease the vacuum going to the transmission but could get it to go no higher than 8". I was going to check the switchover valve but the plastic was so brittle that the connector broke when I tried to loosen the screw on the clamp. I will have to get another one now anyway so we will see if that was the problem. I'll let everyone know how it turns out.

Brian Carlton · #11 06-08-2006, 02:34 PM

Quote:

Originally Posted by hughet

I got a 4mm socket and tried to adjust the amplifier and found that it was turned up all of the way. I could decrease the vacuum going to the transmission but could get it to go no higher than 8". I was going to check the switchover valve but the plastic was so brittle that the connector broke when I tried to loosen the screw on the clamp. I will have to get another one now anyway so we will see if that was the problem. I'll let everyone know how it turns out.

Put a gauge on the vacuum from the VCV to the amplifier. Make sure this is reading 10" or higher. The amplifier probably won't provide range above the VCV signal........so........if the VCV is not providing sufficient signal, then it's the culprit and not the amplifier.

TX76513 · #12 06-08-2006, 02:57 PM

Quote:

Originally Posted by Brian Carlton

Put a gauge on the vacuum from the VCV to the amplifier. Make sure this is reading 10" or higher. The amplifier probably won't provide range above the VCV signal........so........if the VCV is not providing sufficient signal, then it's the culprit and not the amplifier.

This brings back some memories

My problem eventually became intermittent BUT in the early days two problems I found in the SDL were 1. the little in line orifices (colored green, yellow, etc) had been replaced with in line splices and there is a vac line under, and in a %#$! position under the rear of the EGR - if you have big hands you have to dive under from the engine side, left handed to unhook. I would almost bet you still have a vac leak.

Pete Burton · #13 06-08-2006, 03:06 PM

I'm surprised that nobody here has suggested reducing the modulator pressure by turning the adjusting key CCW on the new modulator. Unless this modulator does not have this adjustment, I'd try that.

hughet · #14 06-08-2006, 03:55 PM

Brian,

There is no vacuum at all going into the VCV port on the amplifier. What is it supposed to read? That line comes off a "y" on top of the switchover valve where the other side of the "y" goes to the vacuum control valve on the injection pump.

I hooked my vacuum gauge Up to the output line for the transmission and tried pinching each of the three lines (bottom, top, and side that go into the switchover valve and the reading stayed at "8". It seems like for there to be any vacuum coming to the VCV port on the amplifier it has to come from the bottom middle port on the amplifier and go through the top and side ports on the switchover valve. In my case the middle port on the bottom of the amplifier measures "0" Wouldn't that indicate that the Amplifier is not working properly?

TX75613,
I don't see any vacuum lines behind the EGR. There are lines going in and out of the vacuum transducer but that is under the air cleaner. I checked them and they are all tight.

Pete,

I'll probably have to adjust the vacuum modulator after i get this problem fixed but I need more vacuum going down there first. It probsably shifted smoother with the old modulator in spite of the weak vacuum because the old modulator was worn and adjusted down all the way.

Brian Carlton · #15 06-08-2006, 04:47 PM

Quote:

Originally Posted by hughet

Brian,

There is no vacuum at all going into the VCV port on the amplifier. What is it supposed to read? That line comes off a "y" on top of the switchover valve where the other side of the "y" goes to the vacuum control valve on the injection pump.

That's your problem.

The lines seem to be routed properly. The VCV must provide a modulated signal to the amplifier. It should start reasonably high.......above 12" and move progressively lower with more pedal.

Just for a trial..........using the Mityvac, put 14" of vacuum into the amplifer in the port for the VCV. Take it for a drive and see what it does.

If the shifts smooth out, then the VCV, or a vacuum leak to the VCV is the culprit.