Author Topic: 6.9 Aux Trans Pump and Harsh Shifts  (Read 10086 times)

p. rex

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6.9 Aux Trans Pump and Harsh Shifts
« on: 29 October 2019, 07:52 AM »
Hello all,

Had an interesting episode with the 6.9 Sunday night.  I was driving back from Houston when the A/C compressor clutch burned itself up.  I needed to get back home and my research indicated that the transmission fluid circuit operates just like the Euro model when the aux transmission pump is out of operation.  Since I needed to get the AC compressor disconnected, I just cut the belt and headed off on my way, paying close attention for signs of transmission overheating.

So, the car did just fine.  It did better than fine, actually.  Before I'd cut the belt, upshifts under heavy throttle were quite harsh -- nearly enough to break traction on the 1-2 upshift, even with my 8-inch-wide rear tires.  With the aux transmission pump out of commission, the transmission shifting behavior was distinctly different.  The shifts weren't soft or mushy at all.  They were still pretty positive and distinct, but the harshness was gone.  I'd describe the shift behavior as perfect, actually. 

So what gives?  I don't understand automatic transmissions that well, but I turned up a couple of diagrams of the aux pump, transmission cooler, and related valves in a forum thread here (see link below).  I guess my question is this: if the volume control valve in the aux trans pump got stuck in the open position, could it cause excessive working pressures in the transmission, and thus the harsh shifts?

I am thinking the correct move might actually be to leave the aux trans pump disconnected for now.  We're finally getting cool weather in Austin, so the transmission should be okay functioning like a Euro model without the aux pump to increase flow at idle.  Come springtime, I can explore my options with the aux pump.  I was unsurprised to read that new replacements from MB are stupid expensive, so maybe I'd pull it and see if Sun Valley or another MB trans rebuilder would be willing to test it and overhaul it if necessary.

See koan's post in the thread linked here for useful diagrams from the AUS 6.9 dealer instruction materials:
https://forum.w116.org/mechanicals/6-9-tranny-oil-cooler-hose-routing/
1979 450SEL 6.9 #7184 “Die Marschallin”

daantjie

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Re: 6.9 Aux Trans Pump and Harsh Shifts
« Reply #1 on: 29 October 2019, 09:01 AM »
Ah yes, the lesser spotted auxiliary transmission pump ;D  This little part confounds most as it is so very rare, and frankly IMHO just another example of German over - engineering...

Granted, if you are idling in 50C+ in Death Valley or somewhere then fine, I guess the extra cooling will be dandy, but I really fail to see the cooling benefits of this add - on in anything other than extreme operating conditions.

The bypass circuit is there for this very reason, belt failure, and it is also stated as such in the service manual.  The problem can come in if the trans has been removed, and the check valve has not been installed properly.  There is a check valve where the bypass circuit connects close to the bellhousing.  Also very important to not to mix up the 2 connections as they are pretty close to one another.

Overall though, I would advocate to remove the pump and check valve, and then to just have the fluid circulating via the "normal" route.  Thus you will have to block off the line where it sucks from the pan and send fluid to the pump.  Unless you are a stickler for originality, in which case I would take a hard look at the pump, clean and strip it on the bench to make sure all is well.  They have a reputation to have been temperamental over the years.  There is a rear seal which always leaks over time.  Plus an o - ring on the back cover which can also leak.
Daniel
1977 450 SEL 6.9 - Astralsilber

p. rex

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Re: 6.9 Aux Trans Pump and Harsh Shifts
« Reply #2 on: 29 October 2019, 09:54 AM »
Daniel,

Glad to hear that I was right that the system circulates normally with the pump inoperative.  You recommend removing the pump?  I would need to fit lines directly from the trans to the radiator then, right?  Also, would I need to fabricate a pulley to keep the AC compressor connected, or would it be as simple as connecting the compressor to the engine prop shaft with a smaller belt?

Perhaps more importantly — does it sound plausible that if the aux pump’s flow regulation valve sticks in the open position, increased pressure through the return line from the pump to the transmission could cause the harsh shifting?  The check valve on the bypass line by the bellhousing keeps the bypass line closed when the pump is running, so that flow doesn’t reverse through the bypass line and skip the transmission cooler, correct?

Thanks!
1979 450SEL 6.9 #7184 “Die Marschallin”

daantjie

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Re: 6.9 Aux Trans Pump and Harsh Shifts
« Reply #3 on: 29 October 2019, 10:26 AM »
I have my pump removed and no ill effects, but "your mileage may vary", as they say...However I really do not think that the pump has much effect in other than idling in very hot ambient for extended periods of time.  Maybe some Aussies can weigh in as this was also spec'ed for the Oz market, I am guessing temps in the Outback can get pretty crazy.  That is if they have recovered from their dismal Rugby World Cup performance ;D...

Basically what I did was to remove the ball/check valve unit from the connection at the bellhousing, but of course keeping the housing intact to keep the spacing for the connection.  Then this line gets connected directly to the cooler on the LHS (in direction of driving).  Then the RHS outlet flow stays as is, but you have to remove the "suck from pan" line and block off the hole in the pan.

For the pulley, yes you will need to fit a new pulley and bracket.  I used a generic one from a 107 I think, you can find them on ebay pretty cheap, and then just fit a new pulley so you have a fresh bearing in there.

I am not 100% sure to be honest how the flow is throttled through the pump, however it has a "flow control" check valve built in to ensure a constant flow of fluid as per the manual.  So perhaps a sticking flow control valve "could" cause higher control pressure in the trans and thus firmer shifts ????
Daniel
1977 450 SEL 6.9 - Astralsilber

p. rex

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Re: 6.9 Aux Trans Pump and Harsh Shifts
« Reply #4 on: 29 October 2019, 11:00 AM »
OK, thanks.  I may have to put that on the to-do list.

Do you know where the cooled fluid reenters the transmission?  Does the return line dump it in the fluid pan, or somewhere else?
1979 450SEL 6.9 #7184 “Die Marschallin”

rumb

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Re: 6.9 Aux Trans Pump and Harsh Shifts
« Reply #5 on: 29 October 2019, 11:48 AM »
I think raueda1's spare car has a trans pump on it.  He may be willing to part with it.
'68 250S, '77 6.9 euro, '91 300SE, '98 SL500 '14 CLS550

p. rex

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Re: 6.9 Aux Trans Pump and Harsh Shifts
« Reply #6 on: 30 October 2019, 07:09 AM »
Before moving further, I'd like to gain a better grasp on how the trans cooler and pump connect to the transmission.  Does anyone here have a copy of the mythical TRANSMISSION MANUAL?  I have all the other manuals (116 chassis manual, M100.985 engine microfiche, assorted other materials), but no dice on the transmission manual, which seems fairly nonexistent.  Perhaps someone would be willing to send me scans of the relevant pages?

Thanks,

Jordan
1979 450SEL 6.9 #7184 “Die Marschallin”

daantjie

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Re: 6.9 Aux Trans Pump and Harsh Shifts
« Reply #7 on: 30 October 2019, 08:14 AM »
All you get in the trans manual is how to strip down the aux trans pump, it does not show the routing of the lines and such.

The post you linked from Koan in your fist post is the only reference to the routing, and this is actually not in the trans manual, go figure.  I have seen those 2 pages contained in the "Model year" manuals, and specifically the Australian one which Koan took his from. 
Daniel
1977 450 SEL 6.9 - Astralsilber

raueda1

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Re: 6.9 Aux Trans Pump and Harsh Shifts
« Reply #8 on: 30 October 2019, 09:21 AM »
I think raueda1's spare car has a trans pump on it.  He may be willing to part with it.
It does and I am.  I could also try to get some pix of how it's plumbed if that would help (though radiator is removed and now in Rumb's custody).  The parts car is up on pedestals and pretty easy to get underneath.  PM me if interested.
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

p. rex

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Re: 6.9 Aux Trans Pump and Harsh Shifts
« Reply #9 on: 30 October 2019, 10:06 AM »
Daniel, surely DB published some kind of official documentation *somewhere* on how to, say, disassemble or rebuild the transmission.  A hydraulic schematic of the transmission's internal hydraulics, with external line connections labeled, would be a tremendous help.  I cannot imagine that this doesn't exist somewhere.

Dave, I really appreciate the offer.  I am not ready to do the pump surgery yet, though.  I have just spent a fortune on the car, and since the trans seems to be functioning well as currently plumbed, I will probably hold off on further major work until the spring.  What I might appreciate is pictures of the line connections to the transmission.  I may not be able to interpret what the placement of the lines on the bell housing means on my own, but I'll bet we can figure it out if we put our heads together.

It's also possible I am imagining the change in shift behavior.  I have been driving the car gently the last couple of days because of wet weather here (accidentally verified that my limited-slip diff is still working yesterday when I stepped on it a little to clear a yellow light!).  But if tomorrow is nice and sunny like the forecast suggests, I will take the car out in the evening and do a little stoplight derby to observe the shifts.
1979 450SEL 6.9 #7184 “Die Marschallin”

raueda1

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Re: 6.9 Aux Trans Pump and Harsh Shifts
« Reply #10 on: 30 October 2019, 05:11 PM »
It's worth noting that the throttle linkage rod has a profound effect on shifting.  I posted about it last year (I think).  Not sure how that would impact your situation but still....  Cheers,
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

daantjie

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Re: 6.9 Aux Trans Pump and Harsh Shifts
« Reply #11 on: 30 October 2019, 07:29 PM »
Indeed those linkages can be like a bloody Rubik's cube to get just right :o  Of course your vacuum modulator and the little rubber hose connecting the hard line to the modulator itself must also be in top shape.
The firm shift is by design, and should never be like the mushy POS American transmissions of the time.  But also should not snap your neck off, that's a bit too hard, or at least that what she said... ;D
Daniel
1977 450 SEL 6.9 - Astralsilber

p. rex

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Re: 6.9 Aux Trans Pump and Harsh Shifts
« Reply #12 on: 02 November 2019, 07:45 PM »
Daniel, you are Canadian, aren’t you?  I am going to have to speak in defense of American automatic transmissions.  We did invent the damn things!  You are right, though, that the old ones are sort of mushy by design.

I’m out on a long run with the 6.9 now.  Shift quality has definitely improved.  “Firm” sounds about right — it’s positive and direct, with a little perceptible jerk.  Makes me think the very hard upshifts I was getting before aren’t right.  I checked the ATF and it’s not burned at all after a couple hundred miles of driving without the aux pump.  Red and ever-so-slightly viscous with no real perceptible odor (well, maybe an odd chemical odor if you really put your nose to it).  I am going to conclude that the bypass circuit is working as designed and leave the pump and AC compressor alone until the spring.  Dave, I may take up your offer of a pump then.

Thanks, everybody!
1979 450SEL 6.9 #7184 “Die Marschallin”