W140 vacuum problem...at wit's end, help! - PeachParts Mercedes-Benz Forum

plink49 · #1 06-27-2006, 08:43 PM

I am unable to find an answer to this in the archives and it is driving me nuts. 1992 500SEL, the vacuum pump under the rear seat burned out due to almost constant running due to a vacuum leak/problem. I have replaced the pump but the problem that caused it to fail is still there. Everything connected to the pump seems to work ok, but when the transmission is put into any drive position the pump runs constantly at 2-3 second intervals and never stops. Put the transmission back into neutral or park and the pump stops running. I have traced the white (clear) line to the distribution block beneath the passenger footwell, but there is where I am stumped. What part failure is causing this symptom and what does it take to fix it? I'm letting this wonderful car sit in the garage because I don't want to risk burning up this pump again...they haint cheap!

syljua · #2 06-28-2006, 07:38 AM

PSE running when changing gears, should be related
to the reverse parking aid poles on the back fenders.

The line for those are marked RH on the pump.

Do they extend or retrackt?

The one that you have traced is the vacuum supply
to the A/C system, and will probably run until
a certain vacuum is reached. Marking on that
is Pu or P-

br,
syljua

Peter Guenther · #3 06-28-2006, 08:01 AM

The pump does two things, produce vacuum and produce pressure.
1) it produces pressure to unlock, and sends this pressure to all the door, fuel lid, and trunk lock, thru the yellow line ZV. It also shuts down if it does not get a lock signal back in 10 second.
2) It sends pressure and vacuum to the backup rods thru the orange line RH
3) IT sends pressure and vacuum to the trunk lid handle thru the black line SFG
4)It produces constant pressure for the orthopedic back restthru the gray line P+
5) It provides back up vacuum for the HVAC, when the engine does not provide ample vacuum thrut ransparent line P-
you have to figure out what it is doing
If it runs intermittent it is either trying to replenish the pressure reservoir for the lumbars or the vacuum rervoir. I suspect you have a leak somewhere, go to your local auto parts supplier and get a "mighty vac" tool. It is designed to draw a vacuum and has a guage so you can see if you have a leak down. You can plug it into the hoses and see if they hold.
I dont know what would cause it to run while the car is in gear, unless it is trying to retract the back up assist rods?

Brett San Diego · #4 06-28-2006, 11:31 AM

Peter is right. You need to tell us which line or lines are being fed vacuum or pressure when the pump is doing the constant intermittent running thing. Then you can focus on that one system. So which one is it?

Brett

plink49 · #5 06-28-2006, 07:59 PM

The back-up assist rods work just fine, they go up when in reverse and down when leaving reverse (and the pump runs only during the activation of the rods, then stops). It is ONLY when the transmission is put into a forward gear that the pump runs constantly at 2-3 second intervals until taken out of gear. Someone had mentioned a transmission vacuum control valve on an injection pump...does any of that make any sense to anyone. It makes sense in that it has to do with the transmission...where is this control valve? The central locking isn't functioning, but what would that have to do with putting the transmission into forward gear? I'm totally baffled.

plink49 · #6 07-03-2006, 08:37 PM

I bought a MightyVac this weekend and can see where this will be a very handy tool indeed. The tool confirms that the white (clear) tubing is the one that is leaking. After deeper investigation in the passenger footwell, the MightyVac found the culprit to be the Grey/Green tubing connected to port 5 of the distribution block. Fully plugging that port corrects the problem. Now all I need to know is what is on the other end of that grey/green tube! Anyone with some in-depth info?

Peter Guenther · #7 07-04-2006, 10:20 AM

Gray Green looks like it goes from pin #5 on vacuum block to pin #0 on switchover valve which looks like the distribution point for all the vacuum flaps. It is controlled by the the push buttons and seems to be located in the upper passenger footwell 15 hoses coming from it to various flaps.I cant find any more info than that, and still dont know what the shift lever has to do with it, but the 140 constantly provides challenges. You did not indicate if your locking functions came back when you plugged the gray green. You have to remember the mighty vac will show a bleed down if the valve is directing the vacuum somewhere (or the switchover valve is bad)

plink49 · #8 07-04-2006, 09:18 PM

Peter, your efforts are greatly appreciated, thank you! Your reply seems to confound the problem even further...I didn't notice any other distribution block under the passenger footwell plate, especially one with 15 ports! I guess I will need to dig deeper. You are right though, if these are all HVAC flaps, what does the transmission shift lever have to do with anything? Trust me though, the pump runs [I]only[I] when placed into a forward gear. To answer your other question, no, the central locking system did not come back up with the grey/green port plugged. But, after using the MightyVac to test all of the lines I discovered that all but the driver's door tubing is good and holds a vacuum....and I can lock/unlock the doors (except for the driver's) and trunk and gas filler door using the tool. Also, using the key in the trunk locks and unlocks all of the doors but the driver's. So, I obviously have a secondary problem with the driver's door locking/vacuum mechanism. Ah, life with a 140 is indeed an experience! If the car wasn't so damned nice I would sell it.......nah.

Peter Guenther · #9 07-05-2006, 12:32 PM

The work DVD shows it up higher almost up to the glove boc. I would attack one thing at a time, and would suggest getting the DVD it goes into troubleshooting. Have you ever looked at Jims page on running AC diagnostics
http://pages.prodigy.net/jforgione/MB_S500.html