W123 300D: Harsh gear change & won't shut off - PeachParts Mercedes-Benz Forum

Screwdriva · #1 09-12-2020, 05:59 PM

1984 300D NA Automatic.

Seems to me like a vacuum related issue. Eventually the car did shut off and the problem didn't reappear. Transmission has since shifted smoothly (vs harshly when the issue surfaced earlier today.)

Is there are recommended troubleshooting procedure to isolate the source of the issue? Known part failures that cause these symtoms?

Thanks in advance!

Diesel911 · #2 09-12-2020, 07:20 PM

You pretty much nailed it. When I had accidently disconnected the vacuum line down to the transmission modulator I got near normal upshifts but the down shifts were harsh. So that was defiantly vacuum related.

As you said you disconnect and plug off the vacuum side of the various stuff that uses vacuum one at a time and see if the issue stops. After you disconnect something and the issue stops that is normally the area of the leak.

I had a vacuum loss causing the no shut down and it turned out to be the Check Valve the screws into the vacuum pump was gunked up. Hosed it out with WD-40 and that was back around 2008-2010 with no further issues.
Don't use brake cleaner as it can soften the plastic parts inside of the check valve.
This is an old site don't know if it still works:

Uberwasser vacuum system troubleshooting

http://www.ifixit.com/Guide/Mercedes+W123+Vacuum+Lock+System+Diagnosis+Technique/20201

Screwdriva · #3 09-13-2020, 09:42 AM

I believe a transmission modulator can also do this and that an upgraded green version was launched by Mercedes?

So far the problem hasn't resurfaced yet but I don't expect to be that lucky!

tyl604 · #4 09-13-2020, 09:49 AM

If it fails to shut off, it could be the shut off valve which is located at the back (and inside) the injection pump. There is a vac hose to it from the ignition switch. With the engine running you can attach a Mityvac to the nipple on the shut off valve and pump it up. If it shuts down immediately, it is your shut off valve which has become defective.

Pretty easy diagnosis and pretty easy to re-install a new one.

Screwdriva · #5 09-13-2020, 04:10 PM

Okay will check that one first. Any suggestions on what to check next? vacuum lines and door actuators were tested a couple years ago and faulty components were replaced.

Screwdriva · #6 09-14-2020, 05:28 PM

Took a look to see if I could find the VCV valve (didn't succeed) and saw that my fuel pump was very wet. Any likely causes?

tyl604 · #7 09-14-2020, 06:35 PM

Here is a pic of the vcv on my 300SD. Made of white plastic. I would assume that yours looks the same. It must be removed to get to the stop valve but it is not a big job.

Good luck.