300SDL. Trying to fix hard shifting - PeachParts Mercedes-Benz Forum

TurboTim · #1 07-16-2014, 07:55 AM

So I have been doing some research on hard shifting and vacuum modulator. I only have 8in of vac @ line to trans at idle. I got good vacuum to VAC. What I dont have is good vac coming from dohicky on side of injector pump. I have good vacuum going to center tube but it doesn't flow through the system. I did take the cover off the top and found 1 of the tiny lines plugged but all that gives me is about 1 inch of vac at idle and 0 when reved. Is this gizmo broke? Should I see more. I had read somewhere that this is just a contolled bleed but to me it seems it should be feeding more vac to bottom of pressure transducer.

vstech · #2 07-16-2014, 08:07 AM

The vac quantity is reduced through an orifice, if you are teed into the trans line the amount will be low if the modulator leaks. Test the modulator itself to see if it holds vacuum. Also test the main vacuum source to see what main vacuum quantity is right out of the pump.

TurboTim · #3 07-16-2014, 09:37 AM

When I was checking vacuum of the valve on the injector pump I did not T into it. I measured vacuum directly off of it. Though I will check to see if that transducer hold vac later. What is the correct name for the thing on the injector pump?

TurboTim · #4 07-16-2014, 09:39 AM

Oh and the main vacuum source was around 20 inches of vac going to thing on IP. Just nothing came out of it on other side.

ROLLGUY · #5 07-16-2014, 09:47 AM

Quote:

Originally Posted by TurboTim

What is the correct name for the thing on the injector pump?

VCV- Vacuum Control Valve

TurboTim · #6 07-16-2014, 10:57 AM

So the center port should be full vac and the outer port should be? So does the VCV send a vacuum signal to the transducer? Im guessing so because you said the transducer should hold vacuum.

ROLLGUY · #7 07-16-2014, 07:06 PM

Quote:

Originally Posted by TurboTim

So the center port should be full vac and the outer port should be? So does the VCV send a vacuum signal to the transducer? Im guessing so because you said the transducer should hold vacuum.

The modulator has to hold vacuum (black hose going down to the trans).

loneranger · #8 07-16-2014, 08:47 PM

the output port varies to simulate the vaccum on a gas engine, varying with engine speed going to the tranny. If you have nothing coming out you have to replace the modulator.

TurboTim · #9 07-17-2014, 08:44 AM

Quote:

Originally Posted by ROLLGUY

The modulator has to hold vacuum (black hose going down to the trans).

It does hold vacuum. Do you think you could answer my actual question now?

TurboTim · #10 07-17-2014, 08:46 AM

Quote:

Originally Posted by loneranger

the output port varies to simulate the vaccum on a gas engine, varying with engine speed going to the tranny. If you have nothing coming out you have to replace the modulator.

I am understanding this but do you know any actual specs? I am looking for specs for the VCV or how to test it. Not the TRA port on the tranducer.

TurboTim · #11 07-17-2014, 08:59 AM

I am really thinking its the VCV. Varying the throttle I get a max of 9 inches and a min of 8. I dont seem to see the VCV listed anywhere for sale though though.

ROLLGUY · #12 07-17-2014, 10:00 AM

Quote:

Originally Posted by TurboTim

It does hold vacuum. Do you think you could answer my actual question now?

Your question in post #6- " So does the VCV send a vacuum signal to the transducer? Im guessing so because you said the transducer should hold vacuum."

No one else but you has said anything about a "transducer". I will answer your question- NO. The VCV sends a vacuum signal to the modulator, and it should hold vacuum.

TurboTim · #13 07-17-2014, 11:19 AM

Not really sure what you are saying or what you mean? So I will just ask again, what I am wondering is what reading is should be seeing from the VCV?

Junkman · #14 07-17-2014, 01:13 PM

Quote:

Originally Posted by TurboTim

Not really sure what you are saying or what you mean? So I will just ask again, what I am wondering is what reading is should be seeing from the VCV?

Begin with confirming that the modulator holds vacuum and is adjusted to correct pressure. I found modulator pressure online by Googling "Mercedes 722". You want to know what trans you have. The number is stamped on the passenger side directly above the pan gasket. 722.xxx.

There are specs in the FSM regarding the VCV. You begin with getting full vacuum to the line going to the VCV. I think there is supposed to be an orifice in the line that reduces the vac.

VCV vacuum is adjusted to between 8 & 12" at idle. Vacuum is supposed to drop to near 0" when the pedal is dropped. Again, there is a measurement that is difficult to describe. I made a gauge by drilling a 1/8 in hole as close to the center of a nickle that I could.

Hard shifts frequently indicate too little vacuum. The only way to get things right is to go through the process starting with the modulator.

I "think" i ended up with 42 on the green modulator for a 722.303 and 12" of vacuum at the VCV.

Transtec has a pdf manual and Mitchell has another that I found online.

Alec300SD · #15 07-18-2014, 06:09 PM

Quote:

Originally Posted by TurboTim

I am understanding this but do you know any actual specs? I am looking for specs for the VCV or how to test it. Not the TRA port on the tranducer.

Forum member phantoms has posted very helpful vacuum diagrams here:
Vacuum Diagrams

The link for adjusting the VCV is here: MZF-Bayer Transmissions

I found them searching the forum when I was troubleshooting a flaring shift on "Rena 2.0"

("Rena 2.0" is still for sale - http://www.peachparts.com/shopforum/mercedes-cars-sale/352524-fs-%91rena-2-0%94-1983-300sd-1982-617om-turbodiesel.html PM me if interested)

I hope this info is helpful in resolving your problems.