RPM's during shifts.. plateau at 1800 always? Kind of weak feeling. - PeachParts Mercedes-Benz Forum

300Drestoration · #1 10-25-2017, 12:04 AM

So i've got a restoration thread going for my 1985 300D but i am hoping to hear some thoughts on one specific performance issue i may have...

Shouldn't the RPM's dip a good bit during all upshifts? Regardless of how hard or soft shifting may be set?

If i am at a stop light and ahead of me is a moderate incline and i accelerate around 50% throttle, once i arrive at about 1800 RPM's it will shift to 2nd and then eventually to 3rd and then finally to 4th. The spacing of shifts seems ok. But throughout all of this, the RPM needle just stays about 1800 the entire time, including during the actual upshifts. I would expect the RPM's to drop momentarily then gradually increase until following shift...and also would expect the sound of the engine to correlate to these RPM changes.

As of now i'm just getting a plateau of RPM's, both on the meter and also in the sound of the engine, throughout all of it and it's all at about 1800. Once i'm cruising upwards into higher speeds then i will gradually go into high rev's and all will seem good. But between the time i'm hit second and the time i hit 4th, it's just a constant 1800 RPM and the engine seems weak during this when going uphill.

The power/torque that i have in the lower speeds definitely leaves me wanting more. There are other factors to work on still, such as valve adjustment.

Any ideas?

300Drestoration · #2 10-25-2017, 08:24 AM

Also want to add this... shouldn’t I be reaching higher rpm’s per gear before shifting? Seems the only time I reach higher rpm’s is when I’m finally in 4th and go up to higher speeds.

97 SL320 · #3 10-25-2017, 09:52 AM

If you are against the torque converters stall speed and shifting early, you are using the torque converter as a variable transmission.

The engine speed will remain constant but TC output speed will increase, trans shift, TC output speed drop but motor stays at same RPM.

RE; the TC output never catches up with engine RPM.

Diseasel300 · #4 10-25-2017, 09:52 AM

Keep in mind that your transmission lacks a lockup torque converter. At light loads or cruising, it is always slipping by design. Light acceleration in a high gear sounds about right at 1500-1800 RPM.

Floor it from a stop and see where your shift points are. They should be >4000 RPM. If they're less than that, you need to tighten up the Bowden cable. At low load, you'll shift up into 4th early and the car will be gutless and reluctant to downshift, even if you're slamming your foot on the kick down button.

300Drestoration · #5 10-25-2017, 10:20 AM

Ok i checked the bowden cable and it seems to be not in a bad place as far as i know. Some immediate tension when pulled. But i am willing to tighten it and see what happens.

But first, i wanted to share this.. I finally went through the VAC connections and found two discrepancies... They are notated on this image below.
The red lines are a non-connection. The blue line is a new connection to cabin.

Any thoughts about how this could relate to my issue?

What if PO or PM had bypassed the "65-Vacuum Control Valve" because it is faulty?

The broken connection from turbo to "81-Switchover Valve, Vacuum Converter" seems to be just that the tube popped out.

As far as i've found thus far, everything else is in tact.

300Drestoration · #6 10-25-2017, 11:34 AM

Reconnected turbo line after blowing it out and putting on new end connectors. Not sure what to look for in difference in performance so wasn’t able to conclude anything after test drive.
Then I adjusted the bowden. First 4 full turns tighter, test drive, then another 4 so 8 total, test drive. Yep, this was the ticket.
Just did another 2. 10 total. I want to see where this brings me.
So much better already.

300Drestoration · #7 10-25-2017, 12:29 PM

It’s a huge improvement. Basically my overall concern is gone. I will continue to overhaul the vacuum connections and test everything, and then will want to fine tune the quality of shifts because that seems just a bit too hard.

BUT: Is it possible for shifting to be too hard?

Right now it’s like hitting a solid bump in the road.

Shifing is so great overall now though. Perfectly clear shifting activity across the board. I may need to go back down a turn or two, since sometimes it can take till 3500 rpm’s to shift which is a bit high for my taste. But as it is now it’s enjoyable having completely confidence when approaching a hill and being certain I’ll not be in too high of a gear and have lots more “boost” in action.

Maxbumpo · #8 10-26-2017, 03:15 PM

Quote:

Originally Posted by 300Drestoration

I may need to go back down a turn or two, since sometimes it can take till 3500 rpm’s to shift which is a bit high for my taste.

On your speedometer there are Roman numerals (little tick marks) on the outer edge that show where the upshift should occur at full throttle. IIRC, there should be a "I" for the upshift from first to second, a "II" for second to third, etc. When your engine is producing full power and the transmission is shifting correctly, you should hit all these marks at about 5000 rpm before the upshift occurs. If you're not getting to 5000 rpm, bowden cable adjustment may help. The cable should be just slack at accelerator idle, and then you can tighten or loosen a bit from there to get the shifting point behavior to your liking.

Diseasel300 · #9 10-25-2017, 03:02 PM

3500 RPM as a shift point shouldn't be any concern. That's still the middle area of the rev range. You're driving a car, not a big tractor trailer. If you keep the revs low all the time you'll tear the engine up, it's counterintuitive, but it's one of the WORST things you can do to an engine next to frequent idling.

You have the bowden cable adjusted properly when the transmission is shifting ~4300-4500 RPM at 2-3 and 3-4 with your foot welded to the floor.

When shifting properly, the shifts should be firm, but not jarring. If it feels like a cushioned slush box, you have it too soft. You mentioned earlier that your VCV is disconnected. Being an '85, you need to make sure the vacuum lines match the original vacuum line diagram for your car. That includes the one-way valves and restriction orifices. The VCV and blue-UFO together control your shift firmness. At light throttle, it softens the shift, at WOT it makes it firmer.

300Drestoration · #10 10-25-2017, 03:49 PM

In other words, I need that VCV!
I’m assuming it is broken and I will be pulling it from my trusty parts car.
Any pointers on directly testing the VCV ?

Diseasel300 · #11 10-25-2017, 08:56 PM

The VCV is good or bad. In one position it passes vacuum, in the other it bleeds off to atmosphere. To adjust it, loosen the 2 bolts that hold it in place and have someone else hold the accelerator pedal to the floor. Turn the VCV clockwise until you feel it "stop" and tighten the bolts that hold it in place.

Without the VCV active, you have no vacuum management going to the transmission modulator. The result is VERY firm shifts since there is nothing to soften them. The good news is that a shift hard enough to feel like you got rear-ended generally means the clutches in the transmission are in decent shape!

300Drestoration · #12 10-26-2017, 06:32 AM

Quote:

Originally Posted by Diseasel300

The VCV is good or bad. In one position it passes vacuum, in the other it bleeds off to atmosphere. To adjust it, loosen the 2 bolts that hold it in place and have someone else hold the accelerator pedal to the floor. Turn the VCV clockwise until you feel it "stop" and tighten the bolts that hold it in place.

This adjustment is done with the engine off, correct? The pedal/linkage to VCV relationship is physical?

Quote:

Originally Posted by Diseasel300

Without the VCV active, you have no vacuum management going to the transmission modulator.

Pardon the complete ignorance here..

Is there a way for me to vacuum test the VCV by myself to determine if it's bad? With a mighty vac or other gauges? And what is the process of removing and replacing it?

If my VCV is broken, i'm weary of just plain hooking it back up and running/driving the vehicle. There has to be a reason it's disconnected and i don't know consequences there might be to driving with a faulty VCV.

When i dig around for info on the VCV, it's all pretty much just about adjusting it, assuming it's a working VCV.

Diseasel300 · #13 10-26-2017, 10:06 AM

Yes, adjust the VCV with the engine off.

There are tons of 80s MB's driving around with the original VCV working fine. Hook it up and see if it works. If it's stuck closed, you'll still have excessively firm shifts. If it's stuck open, you'll turn it into a slush box. That's your test really.

300Drestoration · #14 10-26-2017, 10:11 AM

Quote:

Originally Posted by Diseasel300

Yes, adjust the VCV with the engine off.

There are tons of 80s MB's driving around with the original VCV working fine. Hook it up and see if it works. If it's stuck closed, you'll still have excessively firm shifts. If it's stuck open, you'll turn it into a slush box. That's your test really.

Terrific.

And is it a terrible idea to consider swapping VCV's between vehicles? Am i opening up a big can of worms in terms of physical alignment of it or is a pretty straight forward process...?

The one on this good car has had both of its connections wide open to engine air for what looks to be a long time. They weren't plugged.

Diseasel300 · #15 10-26-2017, 11:49 AM

You can swap VCV's. Just do the adjustment when you install it in the "new" vehicle. There's nothing more to swapping other than the vacuum lines and the 2 bolts that hold it in place.