Author Topic: Another 6.9 that wont raise (diagnostic tips)  (Read 798 times)

secondslc

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Another 6.9 that wont raise (diagnostic tips)
« on: 17 May 2020, 10:00 AM »
Hi All,

Same old story, new member.

Inherited a 6.9 that came from California which had been parked on blocks since 1999. I won't say there is no rust as the passenger floor had water in it, but it's refreshing working on a car who's fasteners are all easy to turn.

So far I
  • Rebuilt the fuel system, new pump, accumulator, regulator, fuel distributor, injectors, and rubber lines.
  • Rebuilt the ignition system, new coil, new ignition box, new resistors
  • Rebuilt the engine wiring harness with new plugs for the WUR, CSI, and re-wrapped the loom
  • Rebuilt the engine vacuum system with new lines, fittings, and kept the California spec air injection system
  • Flushed the cooling system and got the thermostat to behave, still needs more flushing but it circulates

Thing starts up without any hesitation, and while I still need to fine tune the CIS, it will idle fine, and rev in the driveway. I'd like to get it on the road so I can do more diagnostics...and thats where I am in need of help.

I've replaced the five accumulators which had Mercedes tags and said made in west Germany... The rear leveling valve was replaced at some point as its got paint marker that says 500se on it.

Put the car back down, and of course its fully flat.

Disconnected the outlet of the pump and directed it into the reservoir and started the car up, fluid comes out albeit a little foamy.
Cracked the front bleeder valve with the car off as I have NO IDEA how to do it while running without losing a hand, a little fluid came out.
Next up I'll crack the rear bleeder, and suspect the same.

Now...the questions...

With the car running and the pump line re-connected, my pressure adjuster disk is floppy with no discernible detents.  Do I have fully bypassed leveling valves, do I have a bad pressure regulator, or do I have a bad pump?

Im going to jack the car up next and let the thing run, but im hoping for more diagnostic tips here before I go buy parts. I'm fine doing this right, but I also don't want to replace working parts.

The goal with this car is to get it running, and only drive it when it's dry. It's relatively rust free (yeah...none are clean), and thats the only way I know I can keep it that way. It's got new brake lines waiting to be put on, and I'll be rebuilding the brake system next...but I gotta get it off the ground!

Any and all advice would be appreciated!

Thanks


1979 6.9
1983 urQuattro
1991 200 20V Avant
1993 Corrado SLC
1996 SL600
2009 Super V8

TJ 450

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Re: Another 6.9 that wont raise (diagnostic tips)
« Reply #1 on: 17 May 2020, 10:12 AM »
I’ve had this sort of challenge on a couple of occasions and it can certainly be frustrating.

I’m inclined to think that the pump might be marginal, but the way to be sure it’s not a valve bypassing is to look at the return into the tank... if a considerable volume of fluid is flowing back into the tank then I would then think that the valve(s) are worn and bypassing.

Also, the pump needs to be primed by running the engine and directing flow back into the tank, kill engine then refit high pressure hose and start engine again. Try revving the engine up to say 1,500-2000 rpm and see if that makes any difference.

Tim
« Last Edit: 17 May 2020, 10:17 AM by TJ 450 »
1976 450SEL 6.9 1432
1969 300SEL 6.3 1394
2003 ML500

secondslc

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Re: Another 6.9 that wont raise (diagnostic tips)
« Reply #2 on: 17 May 2020, 10:38 AM »
To me, there is a significant amount of volume coming through the pump when I direct the line into the tank, to the point where there is foam being formed from the splashing, meaning the valves are probably bypassed?

If they're bypassed, would I still not feel detents or do I also have a pump that can't make enough pressure?

I take it there is no way to test the valves?
1979 6.9
1983 urQuattro
1991 200 20V Avant
1993 Corrado SLC
1996 SL600
2009 Super V8

daantjie

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Re: Another 6.9 that wont raise (diagnostic tips)
« Reply #3 on: 17 May 2020, 10:57 AM »
Sounds like a problem in your height control switch and/or pressure regulator. Have you checked the little filter in the pressure regulator?  There is a plug (throttle screw) which houses the little filter.  Remove and clean the channel and filter. Don't lose it as they are NLA.
You can also remove and clean the 3x channels in the height control switch.
Daniel
1977 450 SEL 6.9 - Astralsilber

secondslc

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Re: Another 6.9 that wont raise (diagnostic tips)
« Reply #4 on: 17 May 2020, 11:19 AM »
Filter was clean when I pulled it, and I cleaned it more with brake parts cleaner.

Can I take the height control switch knob apart without things flying?
1979 6.9
1983 urQuattro
1991 200 20V Avant
1993 Corrado SLC
1996 SL600
2009 Super V8

rumb

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Re: Another 6.9 that wont raise (diagnostic tips)
« Reply #5 on: 17 May 2020, 11:37 AM »
I had great difficulty getting my car to rise. The 2 things that I think finally got it to go were 1. taking the height adjusting/[pressure regulator apart and cleaning. Besides the tiny filter there are about 3 small ball bearings. just take apart carefully on a towel to catch errant parts, clean well and re assemble.

The next was to get the 2 height valves to start working.  They only operate in a smaller range and when the car is completely flat it is beyond it's normal range.  Jack the front and back of car up a bit so it is still in the lift/fill range for a while and run car 1000-1500 rpm.  If that doesn't do it then put back on ground and lift up the front of car and bounce it a lot. At some point it will eventually lift the front and then the back will go up.

Not all cars have the bleeder valves.  I would think you would have the car running and then carefully open the valve to let air out.
'68 250S, '77 6.9 euro, '91 300SE, '98 SL500 '14 CLS550

Punker

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Re: Another 6.9 that wont raise (diagnostic tips)
« Reply #6 on: 17 May 2020, 11:53 AM »
I had the same problem. I checked the pump first.  You can easily get the testing equipment at any hydraulic shop. I then measured both my pumps. The measurement setup corresponds to the instructions in the workshop manual: The measurement equipment inserted between the pump and the valve unit.  A throttle valve is attached and then the throttle valve is adjusted so that at 200 bar at 2000 rpm is reached. Then measure the flow rate. The flow rate should be at least 0.2 litres in 30 seconds. After that you will at least know if the pump is ok.

Greetings Björn

1978 Mercedes 6.9 # 4616 - black/black leather
BMW R 25/3

secondslc

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Re: Another 6.9 that wont raise (diagnostic tips)
« Reply #7 on: 17 May 2020, 12:16 PM »
Thanks all, I suppose I can lift the front up too as I was only lifting the back. Lift both ends of the car and rev it... see if pressure builds in the switch and go from there?

I don't have access to a hydraulic shop today, but I can go get two more jack stands so I can support the car on all four corners :)

1979 6.9
1983 urQuattro
1991 200 20V Avant
1993 Corrado SLC
1996 SL600
2009 Super V8

TJ 450

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Re: Another 6.9 that wont raise (diagnostic tips)
« Reply #8 on: 17 May 2020, 12:47 PM »
When operating normally, the pressure should build in the switch first and the car will lift by itself (there shouldn’t be any need to manually lift either end of the car).

See how you go though, hopefully it was just an airlock at the pump as this will cause the same symptoms.

Tim
1976 450SEL 6.9 1432
1969 300SEL 6.3 1394
2003 ML500

secondslc

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Re: Another 6.9 that wont raise (diagnostic tips)
« Reply #9 on: 17 May 2020, 12:54 PM »
oh, so whether or not the valves are bypassed, the switch should show some resistance? At this point I get nothing at the switch even if I know there is good flow from the pump. Sounds like I need a pump
1979 6.9
1983 urQuattro
1991 200 20V Avant
1993 Corrado SLC
1996 SL600
2009 Super V8

daantjie

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Re: Another 6.9 that wont raise (diagnostic tips)
« Reply #10 on: 17 May 2020, 01:03 PM »
What you should do first is clean out the pressure regulator and height control switch in my opinion.  There are a few check/ball valves in the pressure regulator, the main one is under the large plug you can see from the top.  Some folks advocate a smart wack with a soft hit hammer on top of this plug while the car is running, to possibly get the ball valve to reseat itself, though I have never tried it, so cannot say for sure if this is safe and/or works.
You can remove this plug too to clean out any debris that may be present, just don't lose the tiny spring and ball. 

A tired pump may be the cause but checking for a non-seating check valve in both the pressure regulator and heigh control switch is relatively simple and might solve your issue.

I have only even used the bleeder valves to drop pressure with the car off, and you must be careful as the car will drop very quickly at the axle in question and you must be ready to catch the fluid in a catch bottle with hose attached to the nipple.
Daniel
1977 450 SEL 6.9 - Astralsilber

secondslc

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Re: Another 6.9 that wont raise (diagnostic tips)
« Reply #11 on: 17 May 2020, 01:13 PM »
Ok, so the Allen head bolt was removed and I cleaned it. The big bolt next to it (a 22mm) was way too tight to get off easily, so I left it.

Is the check valve under there?
1979 6.9
1983 urQuattro
1991 200 20V Avant
1993 Corrado SLC
1996 SL600
2009 Super V8

daantjie

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Re: Another 6.9 that wont raise (diagnostic tips)
« Reply #12 on: 17 May 2020, 01:56 PM »
Is the check valve under there?

Yes you will have to remove the pressure regulator and clamp it safely in a bench vise, otherwise you run the risk of damaging the mount of the pressure regulator or shearing off the bolts holding the unit to the tank.  Once clamped securely in the bench vise then use a good socket and breaker bar length of choice.
Daniel
1977 450 SEL 6.9 - Astralsilber

secondslc

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Re: Another 6.9 that wont raise (diagnostic tips)
« Reply #13 on: 17 May 2020, 02:05 PM »
all taken apart, going to run to get some new copper washers. All seems fine (not like there is much to check?) I'll then re-assemble and then take the switch apart.

1979 6.9
1983 urQuattro
1991 200 20V Avant
1993 Corrado SLC
1996 SL600
2009 Super V8

daantjie

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Re: Another 6.9 that wont raise (diagnostic tips)
« Reply #14 on: 17 May 2020, 02:16 PM »
OK just a word of caution on the screw which holds the selector disk to the height control switch.  It is glued into the bore at the factory, so usually unless it has been off in the past, this screw tends to shear off if forced.  It is kind of hit and miss to be honest, and I'm sure there's a trick to perhaps heating only the screw to get the Loctite or whatever they used to make it give.  Also note the Teflon washer under the screw head.

Then, make sure not to lose any of the balls or the little "push rods" in the bore and make a note of their orientation in the bore, they are sets of 3, and from memory the top balls are 4.5 mm and the bottom ones are 3mm.  Best to do this on a clean workbench over a towel or similar to keep parts from going AWOL.

Also take note of the small rubber o-ring between the pressure regulator and the height control switch.
Daniel
1977 450 SEL 6.9 - Astralsilber