722.118 Modulator Pressure is way high - PeachParts Mercedes-Benz Forum

StaggerLee · #1 03-13-2012, 10:12 PM

This is a 1983 Euro spec 300d. I put in a "Vacuum Only" transmission from a 1980/81 wagon and I'm getting a quick 1-2 shift, a 2-3 flare and a ultra smooth 3-4 shift. New Fluid at the right level.

I've been doing lots of reading and got all set up to do the tests.

First thing I did was measure vacuum between the T in the main vacuum line and the green dash pot; It bounced around between 13-15

Then I measured it between the green dash pot and the IP: It was a steady 15 and went up with acceleration.

Then I measured it between the IP and the transmission: It's a steady 13 and drops to 3-5 upon initial throttle, however, it climbs right back up to where it was at 13. If I let off the throttle and hit it again, the same thing happens. Is that normal?

QUESTION #1 Do I have too much vacuum going to VCV so it can't "spill" enough fast enough? long enough?

The next test I did was from DieselDan where you take a banjo bolt, some hose and a pressure guage and attach it to the test port.

The specs in his thread showed that my transmission should have 2.8 bar of pressure at 30km/h. Mine idled at 10 bar

and went up from there!! I'm thinking OK, I'll just back out the T handle on the modulator and watch the guage drop (engine running) until it's right. The pressure never dropped when I backed the T handle out. It did drop to 8 after numerous turns in (clockwise) but then stopped.

I think I can deal with the vacuum issue but the high pressure in the transmission has me stumped. Army? Govert? Any suggestions? Could the filter cause that? PO said "recent change" but that's first on the list tomorrow unless its snowing again.

Also, I changed out the 240D radiator that had been rigged in there and when I unscrewed the trans cooler lines, not much fluid came out; not what I was expecting anyway. I checked the fluid before this with the car at operating temp and in neutral so it seems like there should have been more fluid in there.

whunter · #2 03-14-2012, 12:11 AM

Quote:

Originally Posted by StaggerLee

Then I measured it between the IP and the transmission: It's a steady 13 and drops to 3-5 upon initial throttle, however, it climbs right back up to where it was at 13. If I let off the throttle and hit it again, the same thing happens. Is that normal?

Sounds like the vacuum control valve is failing.

Here is one I found on Ebay.

Mercedes VACUUM CONTROL VALVE 300d 300sd WVO diesel | eBay

.

Stretch · #3 03-14-2012, 03:48 AM

Best of luck StaggerLee - I'm not really the right one to ask about vacuum adjustments just yet... but let me know if you need some documentation - I can point you in the right direction for that.

StaggerLee · #4 03-14-2012, 09:42 AM

Thanks Roy but I have a M pump. Could the linkage have anything to do with it?

Any thoughts on the high fluid pressure at the modulator port?

Stretch · #5 03-14-2012, 02:00 PM

Quote:

Originally Posted by StaggerLee

Thanks Roy but I have a M pump. Could the linkage have anything to do with it?

Any thoughts on the high fluid pressure at the modulator port?

Sorry I missed the bit about the pressure at the modulator port.

(EDIT - oh crap you even wrote it in the title and I got all wrapped up in the vacuum part of your story)

10 bar?

10 bar?

Are you sure?

Modulating pressure should be between 0.65 and 3.8 bar from what I can make out.

Are you by any chance measuring working pressure?

In first gear you should be in the range of 6 bar to about 14 bar but there should be a drastic reduction by the time you slide on into second. That's meant to be in the range of 3 bar to 7 bar.

Please note I'm going from data in the ATSG 722.1 manual which I have found to be confusing / unreliable at times. I've got a copy of the original FSM in German with the transmission chapter - but I have to crank up Windows to view it...

...it will take some time - I'll be back...

StaggerLee · #6 03-14-2012, 02:33 PM

No problem Army!
I should note that I tried two different ports to get a reading.

The bolt I removed to get the reading was dead center on the bottom just in front of the mount and the other one (which I thought was the right port-closer to the modulator and tucked up a little tighter) didn't register at all.

Perhaps the port I used is for working pressure like you say?

The non-registering port leaked out "some" fluid but stopped. The port where I got the reading kept leaking until I got the banjo bolt in place. DieselDan's DIY didn't day duch about deakage.

Couldn't resist..DieselDan's DIY didn't say much about leakage.

I'll try to work on a photo of the two ports I used so we can clarify that. Back soon!

StaggerLee · #7 03-14-2012, 02:43 PM

10 bar?

10 bar?

Are you sure?

Yep, I'm sure. Pegged out and Destroyed the gauge. It now zero's out at 40 psi, which coincidentally, is about 2.8 bar; where it's supposed to be when running.

Stretch · #8 03-14-2012, 03:57 PM

This is a screenshot of the German FSM - how's your German?

Here are the measurement points

Point

1 => Modulating pressure

2 => Working pressure

3 => Regulating pressure

StaggerLee · #9 03-14-2012, 04:06 PM

Army - Here is a picture of the two spots I tried to get a reading. Am I even in the right place?

Edit: I hadn't seen your whole post. It looks like I was measuring working pressure. Doh! Let me revisit the test and I'll report back.

Stretch · #10 03-14-2012, 04:09 PM

I've just edited my post putting in the measurement point info...

If I've just unscrewed my head and got it the right way round for your picture it looks like you've been measuring regulating pressure...

I think the one you need is to the left of the dashpot in your picture - is that right?

StaggerLee · #11 03-14-2012, 04:35 PM

It looks like I was measuring regulating pressure and I think I have the right port identified now. I flipped the picture around to make it easier to see.

Army - I think you're right - the greasy one correct?

Govert · #12 03-14-2012, 04:50 PM

First the vacuum has to be a-OK before adjusting the modulator.

The vacuum control has to main components: the switchover valve on the valve cover and the vacuum control valve on the IP. From what I read you are not measuring the vacuum correctly.

Measure the vacuum at the red spot:

The switchover valve switches between full pump vacuum (blue dot) at idle and controlled vacuum from the VCV (green dot) when the accelerator pedal is pressed.

StaggerLee · #13 03-14-2012, 05:06 PM

GoVert - I have no switchover valve...it goes straight from the VCV to the modulator. It seems to start in 1st without issue.

I made a restrictor valve last night so I'll be able to adjust the volume without hunting around for the right sized orifice.

Govert · #14 03-14-2012, 05:16 PM

Quote:

Originally Posted by StaggerLee

Then I measured it between the IP and the transmission: It's a steady 13 and drops to 3-5 upon initial throttle, however, it climbs right back up to where it was at 13. If I let off the throttle and hit it again, the same thing happens. Is that normal?

That is not normal. The leakage isn't enough. You can adjust the VCV:

1. Engine off.

2. Loosen the two bolts of the VCV, just loosen, so that the VCV can be moved.

3. Put linkage in full load and keep it there

4. Rotate the VCV until you feel resistance.

5. Tighten the VCV in that position.

It could be that the VCV is dirty inside. Unfortunately the VCV cannot be opened easily, but you can use a spray can of brake cleaner to clean it. Remove the VCV and douse the VCV through the openings with brake cleaner. Let the VCV dry before installing. Be careful with brake fluid, it is very flammable.

Check if the green dashpot really is a dashpot and not a valve.

If you don't feel resistance (step #4), the VCV does need adjustment or replacement. Both not very easy. I don't know if adjustment is possible (as it is with the VCV of the MW pump), but the VCV had to be drilled open. Replacement is not easy, as they are not very popular in the States.

StaggerLee · #15 03-15-2012, 11:25 AM

I was finally able to change the filter and fluid yesterday and everything looked fine in the pan. No change to the shift characteristics.

I checked the vacuum again and it was the same. I then followed Govert's instructions for adjusting the VCV. Now the readings are...a little better:
1. Idles at 13
2. Drops to 0 upon hard throttle, and
3. maintains around 5 (instead of climbing back up to 13) This is measured between the VCV and the Modulator so it's dumping a lot more vacuum.

From what I understand, the 13 is still a little high. Shouldn't that be around 10?

I tried the adjustable valve I made and turned it down until there was hardly any air getting through, but, it produced the same vacuum reading. Is the purpose of the restriction to slow down how fast the vacuum equalizes on either side of the restriction or does it actually change and maintain the amount of vacuum? I assumed that the valve would perform the same task as a restrictive dashpot.

I'll try to find some different dashpots if I can.