over adjusting a vacuum modulator? - PeachParts Mercedes-Benz Forum

turbozeke418 · #1 06-09-2017, 12:20 AM

I had the transmission in my 1984 300TD replaced from a donor and the car finally shifts in all gears. I've adjust the vacuum modulator to the proper spec which is 48 psi (3.3 bar) for my car. The shifts are VERY firm in all gears. If I continue to turn it up and it shifts better would there be any issues with keeping it that way? It has a brand new vacuum modulator on it too.
Is there anything else that could help it shift smoother other than the modulator?

Diseasel300 · #2 06-09-2017, 10:18 AM

Make sure the modulator pressure is adjusted to the specific transmission model, not just the car model.

Before getting carried away adjusting things, make sure your vacuum system is operating correctly. This means all the little switches and things on the valve cover as well. Low vacuum is what generally results in hard shifting. High vacuum = soft shift. Low vacuum = firm shift.

tyl604 · #3 06-09-2017, 01:50 PM

I would try a couple of adjustments counterclockwise with the trans modulator to soften up the shifting some.

Have you checked to see that the vac at WOT is dropping to zero between the VCV and the tranny? The vac is supposed to modulate downward as you give it more throttle but basically it upshifts best at speed with no vac. Then you need the vac when it downshifts for a smooth downshift. You should be producing between 10-15 ihg with the VCV doing its thing before it drops off to zero.

turbozeke418 · #4 06-10-2017, 03:27 AM

When I accelerate, the vacuum gauge goes up. When i'm cruising at idle at any speed, it measures 48psi. the same as when stationary.

NZScott · #5 06-10-2017, 04:49 AM

I cannot add anything to this thread so ignore this post if you like, but I'm having a hard time working out how vacuum can be 48psi?

spock505 · #6 06-10-2017, 10:18 AM

Quote:

Originally Posted by NZScott

I cannot add anything to this thread so ignore this post if you like, but I'm having a hard time working out how vacuum can be 48psi?

well spotted

Diseasel300 · #7 06-10-2017, 11:23 AM

Quote:

Originally Posted by turbozeke418

When I accelerate, the vacuum gauge goes up. When i'm cruising at idle at any speed, it measures 48psi. the same as when stationary.

Can you be more specific? Vacuum goes "up"?

Standing at idle or with no accelerator pedal, you should be >12" of vacuum going to the modulator. As you press the accelerator, it should drop to 0" by full pedal travel.

turbozeke418 · #8 06-11-2017, 11:10 AM

https://youtu.be/jFxnJ-6uUao
Here is a link to exactly what I am talking about

The gauge that I got from ************** measures PSI and it says to set my transmission at 48 PSI.

Diseasel300 · #9 06-11-2017, 11:46 AM

When you set the modulator pressure, did you have the vacuum line disconnected? If you didn't, the line is supposed to be disconnected and you are travelling a certain speed to check modulator pressure. See the FSM or do a search on this forum for how to do it.

If you set the pressure with the vacuum line attached, no wonder your shifts are firm. The modulator is supposed to soften the shifting when it has vacuum applied to it and firm it up to the set modulator pressure when the vacuum bleeds off under acceleration.

ROLLGUY · #10 06-11-2017, 12:19 PM

Quote:

Originally Posted by NZScott

I cannot add anything to this thread so ignore this post if you like, but I'm having a hard time working out how vacuum can be 48psi?

Quote:

Can you be more specific? Vacuum goes "up"?

Standing at idle or with no accelerator pedal, you should be >12" of vacuum going to the modulator. As you press the accelerator, it should drop to 0" by full pedal travel.

I too am having a hard time with this. Is the gauge hooked up to hydraulic pressure or vacuum? (rhetorical question?)

tyl604 · #11 06-11-2017, 02:49 PM

Yes, something is wrong. Read my post #3 again.

Mxfrank · #12 06-11-2017, 03:34 PM

He's measuring modulator pressure, not vacuum. The following list is what I have for 722.4's, I'm not sure what you have in your car.

http://www.peachparts.com/shopforum/attachments/tech-help/135627d1459387599-190e-300e-4-speed-trany-vac-modulator-difference-modulators.pdf

Also, check your vacuum valves:

Transmisson Vacuum Valve Lever Replacement

turbozeke418 · #13 06-11-2017, 05:16 PM

I'm lucky enough that I don't have any vacuum lines on the top of my valve cover. Just a couple of 3 ways near the firewall. Then it goes to a bunch of crap on the passenger side behind the light housing. I haven't checked anything under there because that is where the reservoir is for my SLS fluid. Everything but the car shut off going into the cab of the car is disconnected (locks etc.) When I adjusted this I adjusted it WITH the vacuum line connected to the car and with the car running.

Diseasel300 · #14 06-11-2017, 06:45 PM

Quote:

Originally Posted by turbozeke418

When I adjusted this I adjusted it WITH the vacuum line connected to the car and with the car running.

As stated above, modulator pressure is set WITHOUT VACUUM and running at a given speed. Make sure you do the procedure correctly. At idle, your modulator pressure (what you're measuring with the PSI gauge) will be significantly less than the 48PSI you're trying to set it at when you get it adjusted properly.

turbozeke418 · #15 06-12-2017, 12:46 AM

Thank you everyone for bearing with me during my time of idiocy!!! I adjusted it the right way and you can barely feel it shift. Feels like a completely different car now!