Basic Vacuum Diagnosis Strategy - PeachParts Mercedes-Benz Forum

shertex · #1 07-08-2014, 10:03 AM

Up to this point, any vacuum problems I've had on my 91 and 92 have been easily diagnosed by noticing loose hoses. But now I need to make sure I master some basics.

On the 91, the headrest that took a while to snap back (though it eventually did) and some minor tranny thuds are clear indications of some sort of vacuum problem.

Can anyone point me to the equivalent of "Vacuum Diagnosis for Dummies" or give me some basic advice? I assume I start with the source (the pump) and then work downstream. If the pump is fine (original with 112k miles), then that means that some hose or device is leaking....and that somehow I begin to isolate different sections of the vacuum system to find the culprit.

It looks like the Diesel Giant page is a good starting point for me. Mercedes Vacuum Troubleshooting

Any help much appreciated...thanks.

uberwasser · #2 07-08-2014, 10:22 AM

I wrote this guide on diagnosing the door locks on a W123, but some of the techniques may apply:

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

Cadin · #3 07-08-2014, 12:56 PM

How about using smoke?

https://www.youtube.com/watch?v=mTywIQVOV7k

(Coupled with the vacuum gauge diagnosis mentioned above; you should be able to find most anything.)

shertex · #4 07-08-2014, 01:42 PM

Good excuse to fire up a cigar!

Quote:

Originally Posted by Cadin

How about using smoke?

https://www.youtube.com/watch?v=mTywIQVOV7k

(Coupled with the vacuum gauge diagnosis mentioned above; you should be able to find most anything.)

CarpeDiem51392 · #5 07-08-2014, 02:39 PM

For your headrests, I would check under the plastic left foot rest. On my 87' the footrest rubbed a hole in the rear headrest line at this spot.

shertex · #6 07-08-2014, 03:12 PM

I confirmed that I have a solid 23 Hg of vacuum from the pump, so that's good. However, while driving I also noticed that the braking power seemed to be a bit reduced from what I'm used to.

My understanding is a leak downstream from the accessories port coming out of the pump would not affect the braking. Does it sound likely that my problem is on the brake booster side of things? If there's a leak in the booster, will that also manifest itself in symptoms other than braking?

Dan Stokes · #7 07-08-2014, 03:58 PM

Nope. A leak ANYWHERE in the system lowers the available vacuum in the whole system. Let's picture this as a brake system. If you have a leak at, say, the right front wheel then there's reduced and soon NO pressure in the entire system. Remember that vacuum is technically "negative pressure" and anything true of positive pressure is true of negative pressure as well.

One of the problems with vacuum operated accessories is that they do tend to fatigue and fail hoses, connections, etc. over time so that which worked fine when new ceases to work with age. Ford used to use vacuum door locks and other power assists in the 60s but found that they were problematic in just the way that you're experiencing.

So I'd go hunting for those leaks. The lead on the leak under the left foot rest would be a great place to start.

Dan

Cadin · #8 07-08-2014, 04:44 PM

Quote:

Originally Posted by shertex

Good excuse to fire up a cigar!

Isn't it?! And if you abstain from such... here's even another cheap one...

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

FYI: You can also light a cigarette with the vacuum pump (Hand held!), put it in a metal tube (filter end toward air out side) and apply air pressure.

(How long does it keep the 23" of Hg? Have you vacuum tested the brake booster as well?)

Mxfrank · #9 07-08-2014, 08:12 PM

First off, you should be aware that by '91 there were two, completely independent vacuum systems. There is an electrically powered vacuum pump usually located under the right rear passenger seat. This supports the locks and interior components, and is the likely vacuum source for your headrests.

The engine-driven vacuum pump supports the engine compartment, HVAC, and transmission, in addition to the brakes. The 'brake side" supports only the brakes and the shut-off servo, everything else comes off the accessory port. The two ports are independent.

Before you start diagnosing, you need to invest in the proper tool...you'll find that most MB tinkerers use a Mityvac, which is a vacuum gauge and pump in one unit. You can use a simple vacuum gauge, but eventually you will find yourself compelled to use the right tool.

The way you diagnose is to start at the source point for the component you're trying to check. Say you're trying to check the transmission modulator. The source is the accessory port on the engine pump. You will find that just after the port, there is a multi-way connector. Disconnect one leg, and plug in your Mityvac. Use the Mityvac to pull a vacuum. If it holds, the leak lies along the disconnected leg, while the remaining legs on the vacuum run are fine. Using this technique at every branch, you will find the leak with no more than half a dozen vacuum pulls. It's that simple.

shertex · #10 07-09-2014, 04:21 PM

So let me ask this: since the pump reads a constant 23 Hg at idle, can I conclude that the pump is fine or not necessarily?

Mxfrank · #11 07-09-2014, 04:39 PM

Quote:

Originally Posted by shertex

So let me ask this: since the pump reads a constant 23 Hg at idle, can I conclude that the pump is fine or not necessarily?

The pump is fine, but which pump are we talking about? The engine driven pump is probably not the vacuum source for the headrests.

shertex · #12 07-10-2014, 09:49 AM

Quote:

Originally Posted by Mxfrank

The pump is fine, but which pump are we talking about? The engine driven pump is probably not the vacuum source for the headrests.

I'm referring to the engine driven pump. My (perhaps fuzzy) recollection is that the headrests do feed off the main pump but that the door locks are governed by the electric pump under the rear seat. Happy to be corrected.

shertex · #13 07-25-2014, 08:53 PM

I now know the following:

1. The vacuum pump provides good vacuum.
2. The brake booster side of things is fine.
3. The white line that goes from pump and splits into green and white (headrests) is fine.
4. Door locks are fine.

Since the passenger side of the vacuum system is offline because of the wastegate swap, I guess I know that that problem lies downstream from the remaining vacuum outlet.

That's what I know so far.