Here's what I know...

My model has a super complicated system of shutoffs (EGR?) and stuff, and no bowden cable. I've bypassed everything I could and am trying to isolate the shifting issue...

If I remove all vacuum connections from the modulator (no vacuum to modulator) it shifts late everywhere after 2nd gear. In other words, from 2-3 and 3-4 I have to reach a high speed before it shifts, revving up a lot.

If I attach the brake booster directly to the modulator, bypassing everything else (maximum vacuum to the modulator) it shifts early. This time, it's always slipping and doesn't want to stay in a gear if I floor it... it only wants to go reaaal slow.

If I attach the brake booster to the VCV to the modulator I get this reading:
https://goo.gl/photos/QHS1rU6Bjn1dkNaV9
It looks regular, right? But now it shifts late again.. like having not enough vacuum. Should I mess with the VCV to get it bleeding more/less? Should I play with the modulator? Both the VCV and modulator seem to leak slowly albeit consistently if I plug them into the mityvac..

I know, I've read that modulator shouldn't affect timing, but it does. So
there's something I'm missing... Depending on how much vacuum is going to the modulator, it shifts at different times with respect to speed and/or throttle. Are the vacuum numbers so far off that it's just mayhem, or is the problem elsewhere?


Any tips before I do more harm than good?

You need to state year and model of car you're working on.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]

My bad.. 1981 300D, I just bought it at super cheap, looking to bring it back to life

It is not super complicated and you need to work on one thing at a time.

First get out your Mityvac to see if the tranny is working properly. Unhook the plastic vac line which goes to the trans modulator; unhook it at the VCV and plug the VCV. Hook the Mityvac to the plastic line so you can pump up vac to the trans modulator.

Take it for a drive. With zero vac you should get good upshifts. After reaching fourth gear, pump up some vac and come to a normal stop. You should get smooth downshifts. (You will get clunky downshifts with no vac.)

If it acts this way, your tranny may be OK and the problems you have are likely vac related. (I would not fool with adjusting the VCV yet. Check to see if you get between 10-15 iHG at idle per the Mityvac between the VCV and the trans modulator; if so, you are OK. When you hit the throttle the vac should quickly go down to zero per design.)

I would first block off the EGR. In my 300SD all I needed to do was put a golf tee on one leg of a three legged rubber Y coming off the VCV; the 300D is probably the same. Then begin to look for vac leaks. They will more likely be in the rubber connectors than in the plastic vac lines.

The reason you had no power when running the trans modulator direct from the pump was that you bypassed the switchover valve; I cannot get over 35MPH without the switchover valve.

So work on one thing at a time. You have purchased one of the best and most reliable vehicles ever made and just need to work out some kinks providing the engine itself is in pretty good shape.

Thanks Tyl, that worked, and helped me understand what was going on. After adjusting the VCV it seemed to shift much better, and drove it around fine for a few days.

Today, on the other hand, it started slipping continuously.. i looked under the hood and now, even when I unplug everything, it still slips! What's going on? Perhaps after a long time of the modulator not being used.. it got stuck in the shift position?

Make sure the kickdown switch isn't sticking as this will give you all sorts of strange shifting problems. It will appear to flare and slip.

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )

I unplugged it... still slips / shift early.

Check your fluid level. If OK, then try adjusting the trans modulator bit by bit for crisper shifting.

Then get back to us.

It may be relevant to note that when I took it for a drive this morning it wasn't slipping.. then about 5 minutes into the drive it started slipping. Tested the vacuum hold on the modulator and everything is fine... Just want to think around this before i meddle with the modulator... Anything else come to mind?

If i were to mess with the modulator screw, cw or ccw?

Also... i had changed the engine oil before this started happening.. changed oil and filter... Dont think this would affect anything as damaged nothing..

Clockwise for a sharper shift.

This discussion is interesting for me. My SDL shifts well generally but it goes into 3rd quicker than I would like. Sometimes even does so going up a slight hill at maybe 25 or 30 mph. I was thinking I was largely stuck with it or use the shift lever to get around it.

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Te futueo et caballum tuum

1986 300SDL, 362K
1984 300D, 138K

In post #5 you say you adjusted the VCV and got better shifts. Did you mean the trans modulator instead? The vac and bleed off thereof caused by the VCV does affect your upshifts. But it bleeds down to zero when you hit the throttle so the upshift is smooth. What reading did you get if you adjusted the VCV before and after adjustment - and is it still bleeding off when you hit the throttle? The pump should be putting out about 20 iHG and you should be getting between 10-15 iHG after the VCV.

Rereading the posts, if you are having trouble upshifting with no vac - if it is slipping rather than flaring, it sounds like a transmission problem. Next if you are getting better upshifts with new, thicker transmission fluid - and then when the new fluid warms up and gets thinner, the shifts begin to slip - it again sounds like a transmission problem.

I believe you can rectify a slipping a little and for a short time by adjusting the trans modulator for crisper shifts. Would try this now.

But I am guessing you have bad clutches, etc from wear and that is the real reason the tranny is slipping.

However some forum member who knows the insides of these trannies should opine; about that I know not much.

I'll explain the whole situation clearly in a single post.

My model transmission is pre-bowden cable.

I purchased the car some weeks ago, it had shifting issues due to vacuum leaks. It would shift up from 2-3 only at very high RPMs. After addressing the shifting issue and adjusting the VCV (I have NOT touched the actual modulator yet) it began shifting well and at the correct times with relation to throttle. Also, I bypassed the entire switchover valve part, cutting out the EGR entirely. Now my routing goes directly from vacuum pump to modulator, with the VCV in between, and NOTHING else. The vacuum I get is on spec with what you describe, from high 20s to about 15or10 when I throttle is activated.

It worked fine for a few days. Now, suddenly, it began slipping whenever I push more than 1/4 throttle from 2-3 (it feels like someone is pressing down on the clutch when I accelerate). Then I lay off the pedal as it slips and slowly depress throttle again, and it runs more or less smoothly, albeit slowly. I have to manually alternate between L,S and D controls to really drive safely.

Now, even as I disconnect the vacuum controls entirely, it still slips, so it can't be related to the vacuum pressure. It just seems strange that the actual transmission would crap out on me two days into being fixed... I mean, coincidence? I wanna try anything possible before giving up... is it possible that the modulator is stuck? The behavior feels similar as it did to when I had too much vacuum going to the modulator... Perhaps the pushrod inside the modulator got stuck after years of not being in use? Is it possible to disconnect the modulator entirely (not just the vacuum line, but to physically remove it from the transmission) and drive the car? At least I could test to see if it stops slipping without the modulator?

Really over my head about removing the trans modulator but I do not think so. If you unhook the plastic line from the VAV so that zero vac goes to the trans modulator, you should get smooth upshifts if the tranny itself is OK.

Is this happening? You are not going to hurt anything taking it for a drive like this.

If it is not upshifting pretty well like this, I am guessing again that the problem is with the tranny.

Reread your last post. It is supposed to lose all vac when you hit the throttle so that no vac goes to the trans modulator when the throttle is pressed. If this is not happening - you say it drops to 15 - then it is not bleeding off properly. You are supposed to have 10-15 with the engine at idle and then quickly drop to zero when you hit the pedal. Maybe this what is happening but I cannot tell from your last post.