January 02 - Dumb Question Day

[Bengoshi2000]

Yesterday, without warning, my '91 300d starts shifting hard. I've yet to double check vacuum lines or the output of the vacuum pump. In the reading I've done overnight, seems that I have the original vac pump (crimped on cover). Here's the dumb question:

Should I forget trying to diagnose the pump and just replace it as a matter of course? The car has 125k miles. I've got to put a new radiator in it this weekend anyway and it seems like a good time to do it.

[sixto]

Yawn. It takes a far dumber question to top me.

If the brakes work, the vacuum pump works. If the door locks and ACC work, the vacuum pump works. If you still have a vacuum actuated wastegate and the turbo is boosting, you have vacuum. Maybe a vacuum line came off at the IP VCV or transmission modulator. Don’t worry about the bigger black line from the side of the VCV to the cabin. That’s a vent that doesn’t connect to anything. If your car has that blue circular thing behind the ABS pump, there are even more vacuum lines that affect shifting.

Sixto
98 E320s sedan and wagon
02 C320 wagon

[barry12345]

Shifting hard is a better scenario than shifting soft I believe. At least to deal with. If nothing is found by observation. I would want to have a vacuum testing tool though.

[vwnate1]

If you don't know a thing that makes you ignorant, a very different thing than is stupid .

I think this is an excellent question and it should serve as a learning tool for many .

[engatwork]

You ain't priced out a vacuum pump yet have you? Find the leak, fix it and be done for the time being.

[tyl604]

You may be lucky and find that one of your vac lines just came off a nipple.

[Clemson88]

I'd isolate some of the systems and see if I could narrow it down. I don't know how the vac to the transmission is controlled on a 91 but that might be a good starting point.

You should probably look into starting a saving account so you can replace that vac pump too. Fix what's broken or loose and save some money.

[sixto]

A new Pierburg pump is $250-300.

If you don’t mind investing in a gasket, pull the pump and confirm the plastic bearing spacer is intact. If the balls are able to touch each other, keep the car off the road until you get another pump.

If it has a metal gasket, apply some RTV to both sides and reuse it. New Pierburg pumps come with a gasket. I don’t know about Chinese pumps.

Sixto
98 E320s sedan and wagon
02 C320 wagon

[pawoSD]

Originally Posted by sixto

Yawn. It takes a far dumber question to top me.

If the brakes work, the vacuum pump works. If the door locks and ACC work, the vacuum pump works. If you still have a vacuum actuated wastegate and the turbo is boosting, you have vacuum. Maybe a vacuum line came off at the IP VCV or transmission modulator. Don’t worry about the bigger black line from the side of the VCV to the cabin. That’s a vent that doesn’t connect to anything. If your car has that blue circular thing behind the ABS pump, there are even more vacuum lines that affect shifting.

Sixto
98 E320s sedan and wagon
02 C320 wagon

Door locks are not handled by engine vacuum pump on the W124, its an electric vac/pressure pump under the back seat.

[sixto]

Doh! Rear headrest then

Sixto
98 E320s sedan and wagon
02 C320 wagon

[SD Blue]

Rather than going straight to the pump, there are more likely suspects.

The flex fitting at the transmission is constantly exposed to a harsh environment.
Also, a recent oil change, etc.., may have resulted in an accidental disconnection.

[Bengoshi2000]

Thanks for the replies, folks.

Like I said, I haven't had a chance to get out the mityvac and start doing the troubleshooting needed. I'm a bit paranoid about the vac pump knowing that if left unchecked, it can wreak havoc when it fails. Possibly panicked a bit as I haven't been under the hood recently to disturb any vac lines... so the sudden hard shifting was alarming.

As for the vac systems the brakes are fine and the vents respond properly to the ccu... no changes there.

At least my saturday morning is planned now...

[Mxfrank]

You really have two questions here. As has been discussed, there is a vacuum leak somewhere. It's not necessarily in the transmission circuit. If you look at the pump, you will see that the brakes and engine shutoff are fed with a large plastic hose that attaches with a flare nut. Next to that, there is a small vacuum line that fits into a nipple. The small line feeds everything else in the engine compartment. Just start there and test at each branch with your mityvac. You will find a leak. As a matter of best practice, if you have one leaking elbow, Y or connector, then you have twenty more that will fail soon. If you don't want to be doing this often, get a full set of connectors and go to town.

As for the pump, I replaced the one on my 602 at something over 300K miles as a preventative. If you're getting up there, keep your eyes open for a deal and do it for peace of mind. There is a Chinese copy of this pump, but you want to look for the real Pierburg pump.

[Mike D]

If it comes down to it, the pumps are rebuild-able, including replacing the bearings with improved, caged style bearings.

https://www.youtube.com/watch?v=DWvLY82zcFg

Comparison of bearings on posts #14 and #15.

OM606 Vacuum Pump

I recommend using paint thinner/mineral spirits and hot soapy water for the cleaning. I'm not a fan of the brake cleaner/carb cleaner approach.

This job is a lot easier with the radiator pulled

[vwnate1]

I imagine the main thing is to avoid Chinese or third world bearings here .

? Would sealed bearings help any ? (add a P [one side sealed] or PP [both sides sealed] at the end of the P/N if needed .