Where can I find a vacuum diagram for a 123? As mentioned before, I have a vac tester but don't really know what to look for or what readings are normal.

I wish I hadn't sold my 300SD, I could have used it as a comparison.

Thanks,

Mr. BILL

90 300SE
85 300D
84 300SD (sold)

What system & year model car? Almost every year & model cars have different diagrams!

Sorry,

It's an 85 300D. I'm looking at everything under the hood. My wife is experiencing weak shifts and clunks upon down shifts. The locking system is intermittent too.

I drove it, and under acceleration, shift points seem normal. The vacuum pump is pulling 23#mm at idle but doesn't seem to increase when I rev the engine.

The vacuum going into the shift modulator is around 12#mm at idle and increases to about 15#mm revved.

Thanks,

Mr. BILL

90 300SE
85 300D
84 300SD (sold)

Mr. Bill,

There is one that will get you by in a 123 Haynes manual.

Good luck,

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Larry Bible
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

Good thing that you don't have that 84 300SD as the vacuum systems on the 85 are completely different. Be real carefull if you use aftermarket info. The systems all changed in 85. One size won't fit all.

I am fairly confused by your vacuum readings and symptoms. Your numbers look like vac in inches not mm. The vacuum shoul be fairly stable once the reservoir is full. The vacuum that you want to monitor for the trans (follow the line to the trans modulator) should have high vacuum at idle and drop with increased throttle to 3-5 inches full throttle. As boost increases this value goes to zero. THIS is one of the reasons the systems are so different in 85. The tranny signal is modified by the big blue amplifier and takes into account not only throttle position but amount of boost.

------------------
Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

OOPS!

I did mean inches not mm. I'll check again.

Thanks Bill

Steve,

I found a pretty good vac diagram in the Performance Products catalog. After playing around, I found inconsistant readings from the top side of the vacuum valve. That blue thing in the corner near the drivers side firewall.

The first time I read the line that goes to the trans, it read 15" at idle and 12" with the throttle open. After checking other lines, I checked the tranny line again, it read 5" and dropped to 2" revved.

I checked other lines and went back to the vacuum valve and checked the tranny line one more time and it read 15" as before.

Before I did my tests, the tranny was making a "clunk" sound when you would come to a stop. After, it shifted and downshifted as it should.

The vacuum pump is reading 23" at idle with little change throttled up.

It seems to me that the pump is O.K. and the Vacuum Valve is suspect. Does it sound like I'm on the right track?

Thanks for your help,

Mr. BILL

90 300SE
85 300D
84 300SD (sold)

I do these tests with a vacuum gauge and a "tee" on a twelve foot section of the hard vacuum line used for vacuum door locks.

I watch the vacuum as I drive (I do this in controlled conditions - BE CAREFUL). One should tee to the line going to the trans for the final overall control signal. This should be high vacuum at idle and drop with throttle rotation linearly to about 3-5 inches of vacuum at full throttle. This can be varified with a quick snap of the throttle. The smoothness of the vacuum transition (from 15+ to 3-5 in. vac)is much easier to see during easy rotation of a medium acceleration on the road or dyno. Once the throttle is full, boost will build (this should be verified at the line to the "blue" amplifier; boost signal should be verified to be complete 0-9/10 psi). As the boost builds the vacuum to the trans should drop from the 3-5 inches to zero as bost reaches maximum.

That is what it should look like. The reason the amplifier has this signal profile is because it amplifies the original trans signal that used to go to the modulator before 1985. This signal comes from the prepared signal at the proportioning valve at the rear of the injection pump. It is something that also should be checked if the end signal isn't reliable.

Thank you Steve!!!

Bill

Hi Steve,

May I interject a question here? Years ago, the plumbers working at the local dealership got my engine so screwed up it would barely run. I could not get kickdown.

Later, after getting the valves and timing properly adjusted, the car downshifted at about 30-35 mph without flooring the accelerator (and still does today). Is the downshift partly a function of the power the engine is developing or just what would cause what I experienced?

Can't help wondering about this. Thanks for your expertise.

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Ted
1979 240D
160,000 miles

Ted,

From your signature I take it that the car in question was a 1979 240D.

That was the last year before the vacuum shift control. It only has throttle valve control (control pressure). There is a link running from the throttle linkage to a lever on the trans. The more throttle the later the shifts.

This tranny also has the electric kickdown. Bushings at the trans lever fall apart and can change the control pressure downshifts over night. Improper assembly of the linkages can affect this also. Its pretty straight forward though; shorten the link = later shifts higer speed kickdowns. Longer link = the opposite.

The modulator is fixed (one adjustment)on these trannies.

Thank you, Steve,
I have always maintained the linkage and check the adjustment periodically to see that it is according to the MB manual specifications.

Your explanation does clarify some of the other things that happen while driving. Not bad things, just things I didn't understand until now.

Appreciate your response. Have a great day!

------------------
Ted
1979 240D
160,000 miles