Author Topic: 6.9 distributor fixes  (Read 1684 times)

raueda1

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6.9 distributor fixes
« on: 06 September 2018, 01:46 PM »
Background:
I've long suspected that my 6.9 wasn't performing quite as well as it should be.  Last week I had the good fortune to attend the Fall "Lodefest" in Oregon.  Lodefest is an event organized by the M100 group which covers the 300sel 6.3, the massive 600 and the 6.9.  There I had the chance to swap 6.9's with somebody so we could compare notes.  This confirmed my suspicions - my euro 6.9 was certainly no quicker than the other guy's USA version, and possibly even a bit slower (subjective judgement based on seat-of-the-pants driving on various mountain roads). 

Fortunately Mike Morris of Morris Motors https://www.morrismotorsusa.com/, regarded by many as a demi-god (or more!) in the world of old Benzes, was in attendance so we were able to discuss the issue.  Mike was adamant that something was going on with the distributor/timing and to do nothing else until that was sorted out 10000%.  I looks like he was 10000% correct.  My timing was kind of right, in a ballpark kind of way, but still funny.  Sometimes the idle would spontaneously change a little.  Sometimes idle speed would hold steady when AC was switched on, other times it would drop.  Sometimes the vacuum advance at high rpms would move around.  So, I decided to take the step of taking the whole damn distributor apart and looking for problems.  And I found some!  Going through the process would have been a lot easier if I had "how to rebuild your distributor" guide, so I figured it might be helpful to document it all.  What follows is a guide to dismantling the distributor and what I found when I did.  Here goes.

The Process:
Step 1:  Remove the distributor, duh.  When you take the cap off be certain to mark the position of the rotor so you're in the right place when you put it back together.  Otherwise you may go nuts, either from trial and error or trying to crank the engine to find the TDC reference.  Plate 1:  distributor as removed from car.  These distributors have 2 compartments.  The upper contains the ignition mechanism communicating with the coil (stator magnet and rotor) and the vacuum advance/retard mechanism. The lower compartment contains the centrifugal advance mechanism. 

Step 2:  Remove vacuum advance pod and cable connector.  For the vacuum pod remove the screws and carefully disengage the arm from its pin inside the distributor.  The pin faces down, so gently wiggle the arm off in a downwards direction.  Note that the vacuum pod has 2 small plastic parts on either side of the shaft (Plate 2).  These are fragile but can be wiggled out.  Don't try to force!  The cable pulls straight of the distributor body.

Step 3:  Now let’s examine what we’ve got and how it’s assembled.  The components are labeled in Plate 3.  The base plate is fixed to the distributor body by 3 hex screws (3mm heads).  The coil is fixed to the base plate from underneath.  The stator (magnet) rotates about 30deg on the central shaft. In my case the stator rotation was impossibly stiff and didn't rotate freely at all.  On the bottom side of the stator is the pin connecting to the vacuum advance pod.  this pin limits the rotation of the stator magnet.  The rotor is held on the shaft with a c-clip and index pin (see plate 4).

Step 4:  To disassemble first remove the c-clip and spring washer on the shaft.  You can then remove the rotor, though it may take some effort.  Note that there's a miniscule alignment pin that locates the rotor on the shaft.  Don't lose it!  Next remove the entire base/coil/stator assembly.  First remove the c-clip and washer so the assembly id free to slide off the shaft.  Then remove the hex screws fixing the base plate to the distributor body.  It’s fiddley but can be done (see plate 5).  Set the assembly aside and have a look at the now exposed centrifugal advance components (see plate 7).

Step 5:  Yikes!  All the centrifugal  elements were gunked up!  The brown is gum and varnish and maybe a tiny bit of rust.  The action was sticky and sluggish.  The parts are shown in Plate 7 as they were found, not a pretty sight.  Removal of the weights, springs etc is straightforward.  They are shown in Plate 8.  I blasted everything clean using ground coconut shell as the blast medium followed by air blasting and solvent wash.  Plate 9 shows everything cleaned up.  Pretty nice, eh?  The action was then perfectly smooth and immediate.  It's worth noting that when you lubricate the wick under the distributor rotor, this is where that oil ends up.  It's not surprising that it turns to gum over the decades and literally gums up the works.  I'd hazard a guess that many, or most, of these older cars have something similar lurking in the same place.

Step 6:  Now let’s return to the upper section.  The coil needs to be freed from the bottom plate to remove the stator magnet.  This is accomplished by removing the 3 flat head screws on the underside of the bottom plate (see Plate 6).  These screws are extremely tight!  I had to use very strong downward pressure to not strip the screw heads (which were perfect beforehand and are now a bit scarred).  The stator coil and magnet can then be removed.  Plate 10 shows the bottom plate separated from the stator magnet. Note that the magnet plate needs to be able to move freely on the bearing on the bottom plate.  Mine did not, it too was gunked up.  After cleaning it up the stator magnet rotated freely.  For the vacuum advance to work right the stator magnet must be able to rotate freely.

Step 7:  Reassemble.  A couple tips:
   1.  Keep the distributor in an upright position (like in a vise) and do not pull up on the central shaft
        until everything is put back together.  Doing so may unseat the springs.
   2.  The stator/bottom plate/coil assembly can be reassembled and installed as a unit.  Dropping the
        hex screws into the hole in the bottom plate before installation makes this easier.

Step 8: check and set timing.  This has been discussed in depth elsewhere so I won't repeat it.

The results:
  • Idle speed stays rock-solid whether AC is on or off.
  • Timing could be set exactly per the specs and it seems stable.
  • Performance was significantly improved!!  I had previously recorded 0-60 mph times around 10.5 seconds.  Results are a bit inconsistent cause I'm using a GPS phone app and it just never gets exactly the same number twice, but still gives an idea.   New 0-60 time was 8.5 seconds, and the improvement was very seat-of-the-pants noticeable.  There was no knocking or pinging.  It's worth noting that the 8.5 sec number was at 5000 ft elevation and on a very mild upgrade (closest place I could test near my house).  At sea level the improvement would doubtless be more.  I could clearly feel the difference between driving in Oregon (~1200 ft) vs locally. 

Next?
My timing setting above was really a first approximation - somewhere within spec, but the specs are a range and I didn't pay too much attention to exactly where.  Since there's no knocking I'll see if I can squeeze out a little more by a hair more advance.  What needs to be stressed here, I think, is that there really is such a thing as "close," and just close isn't good enough.  In addition, the problems were totally invisible until the distributor was taken apart. 

In closing, this project brought major performance gains at no cost and with no parts.  If you've got any symptoms that this might explain then go for it!  There's nothing to lose, much to gain.  Happy motoring and 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 distributor fixes
« Reply #1 on: 06 September 2018, 02:30 PM »
Wow Dave what a monster post, nice work!
A bit off topic and not sure if it has any bearing, but are you running the rev - limiting rotor on your 6.9?  I think it has the rpm cut - out setting printed on the rotor somewhere, might be like 5 300 rpm or thereabouts from memory.  or maybe 6 300.. ::)
« Last Edit: 06 September 2018, 02:49 PM by daantjie »
Daniel
1977 450 SEL 6.9 - Astralsilber

rumb

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Re: 6.9 distributor fixes
« Reply #2 on: 06 September 2018, 02:32 PM »
Wow!..  I see my disti being pulled in the future and cleaned up.

Thanks for the how to.
'68 250S, '77 6.9 euro, '91 300SE, '98 SL500 '14 CLS550

raueda1

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Re: 6.9 distributor fixes
« Reply #3 on: 07 September 2018, 09:34 AM »
Happy distributor cleaning, gentlemen!  As for rev-limiting, I didn't notice any indications on any of the parts but I wasn't looking either.  One thing I didn't mention is that the WOT shifts are noticeably later than previously and significantly closer to the speedo dots. 
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

TJ 450

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Re: 6.9 distributor fixes
« Reply #4 on: 07 September 2018, 09:10 PM »
Fantastic thread! This is something that’s been on my to-do list for years now. My 6.9’s never been quite right and the only thing that hasn’t been touched is the distributor, although I did swap it with another, probably equally as tired.

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

irvine

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Re: 6.9 distributor fixes
« Reply #5 on: 08 September 2018, 10:07 PM »
Great idea for a proper distributor clean up. Giving the prices for new points etc I’m liking the idea of a big clean and refresh before I spent a few $100 on points.
What sort of time scale did it take to carry out I’d love to get stuck in on my next weekend off.

Neil.

raueda1

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Re: 6.9 distributor fixes
« Reply #6 on: 09 September 2018, 08:10 AM »
Great idea for a proper distributor clean up. Giving the prices for new points etc I’m liking the idea of a big clean and refresh before I spent a few $100 on points.
What sort of time scale did it take to carry out I’d love to get stuck in on my next weekend off.

Neil.
It was maybe a couple hours going very slowly and some interruptions.  I stupidly forgot to mark the rotor position when I took it out so I had to rotate the crank to get back to TDC and then all was well.  But rotating the crank was damned hard! 
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

john erbe

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Re: 6.9 distributor fixes
« Reply #7 on: 03 January 2019, 06:10 PM »
Raueda1. After not checking the 116 website in some time, saw your post and thought I mention that you might want to check the part number on your distributor. Your photo caught my attention showing that black wire, as it's consistent with the domestic model. On the Euro distributor it typically is green.

slfan

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Re: 6.9 distributor fixes
« Reply #8 on: 30 January 2019, 08:19 PM »

1. Great post!

2. Curiosity: Is the 6.9 distributor unique to 6.9 or will a "regular" 450 SEL distributor also work on the 6.9?

Thank you and regards
« Last Edit: 30 January 2019, 08:24 PM by slfan »
1978 - 450SEL 6.9 - 3170
1978 - 450SEL 6.9 "Parts Car" - 2973

raueda1

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Re: 6.9 distributor fixes
« Reply #9 on: 31 January 2019, 04:44 PM »

1. Great post!

2. Curiosity: Is the 6.9 distributor unique to 6.9 or will a "regular" 450 SEL distributor also work on the 6.9?

Thank you and regards
1.  Thanks!  I'll add more in due course as I get it tweaked better.
2.  I don't know, but I doubt it.  Per post above even euro and USA 6.9 distributors aren't the same.  My guess would be that it will physically fit but that timing curve is different.  But like i said - just a guess.  Someone else may know better.
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

raueda1

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Re: 6.9 distributor fixes
« Reply #10 on: 31 January 2019, 04:47 PM »
Raueda1. After not checking the 116 website in some time, saw your post and thought I mention that you might want to check the part number on your distributor. Your photo caught my attention showing that black wire, as it's consistent with the domestic model. On the Euro distributor it typically is green.
thanks for that John.  Actually mine is green.  It didn't photograph well and the insulation was discolored blacker closer to the distributor itself.  As PVC ages is almost always discolors towards black or brown.
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

slfan

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Re: 6.9 distributor fixes
« Reply #11 on: 03 February 2019, 03:30 PM »
Dave,

Thanks for the information.  After seeing your post, I now have a new project.

Regards
1978 - 450SEL 6.9 - 3170
1978 - 450SEL 6.9 "Parts Car" - 2973

irvine

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Re: 6.9 distributor fixes
« Reply #12 on: 11 February 2019, 01:21 AM »
Any updates to how the dizzy is running after getting some miles on it?
Thinking about doing this in the next few weeks.

Thanks,
Neil.

raueda1

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Re: 6.9 distributor fixes
« Reply #13 on: 13 February 2019, 01:15 PM »
Any updates to how the dizzy is running after getting some miles on it?
Thinking about doing this in the next few weeks.

Thanks,
Neil.
Not yet.  Car has been in winter hibernation.  But do the project anyway.  You can't go wrong.
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

irvine

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Re: 6.9 distributor fixes
« Reply #14 on: 10 May 2019, 02:16 AM »
guys, I'm trying to do this clean up but I'm stuck trying to remove the first section under the dizzy cap, the one that has the injector contact and condenser screwed to it.

it looks to be heled in by 3 little indents in the distributor body, it looks like its simply twists round to align the indents with 3 gaps and off the plate should come. I've tried this but nothing budges, am I missing something??????

neil