HOW-TO: W126 Odometer Repair, Clock Capacitor Replace - PeachParts Mercedes-Benz Forum

JamesDean · #1 05-05-2012, 02:14 PM

Hey everyone,

My green 300SD's odometer failed yesterday so I decided to investigate. From what I've learned here the drive gear and one of the other gears fail. Sure enough, this was the case for my 300SD as well. What I didnt see on here was too many pictures so I decided I'd take some and post up..

Step 1) Remove Instrument Cluster from Car.

Step 2) Remove Speedometer section from cluster.

Step 3) Remove plastic backing plate via small screws.

Step 4) Inspect your gear train for damage.

As you can see in my second photo my drive gear is missing several teeth as well as the gear that it meshes with.

Step 5) Source replacement gears. You can either buy them for ~$65 from Odometer Gears - WELCOME or if you have a spare speedometer see if the gears are compatible. Luckily I happened to have a spare 560SEL cluster with the same tooth count gears so I simply swapped them over.

Installed replacement drive gear:

Finally:

Step 6) Replacing Capacitors.

Since you have the speedometer out, you might as well replace the capacitors. For my 300SD, there were three. The replacement were easily obtained from Radioshack for about $4.50 or so.

To gain access to the capacitors you must desolder the speedo from the circuit board. There are a few connections to desolder, these are indicated below in the picture:

After that you are free to replace the capacitors, the values are as follows: 2 x 220uF @ 6 V, 1 x 47uF @ 16V. RadioShack carried ones that were rated at 35V so we were set.

Here are my replacements installed:

Step 7) Reassembly

After you've finished soldering, just re-assembled everything back as it was. Be sure to re-solder the contacts for the speedo unit to the circuit board and you're all set.

JamesDean · #2 05-05-2012, 02:32 PM

My buddy (source of the spare 560SEL cluster) was having clock troubles so I told him I'd replace the capacitors in this one. I did the same for the 300SD as the cluster was on my desk. The procedure is pretty straight forward, just be careful with the units.

Step 1) Remove clock/tach pod from cluster

Step 2) Remove circled screws and desolder as indicated by arrow:

Step 3) Remove face screws.

Step 4) Replace the capacitors

Flip unit over carefully, giving yourself access to the back of the clock and its capacitors.

Now, my 300SD had two capacitors whereas the 560SEL only had one. Both were 100uF/16 volt capacitors. I had previously ordered a batch of 100uF/25 volt capacitors from Mouser so I was set. Radioshack should carry 100uF/35 volt capacitors as well.

560SEL Old:

300SD Old:

560SEL New:

300SD New:

Step 5) Carefully reassemble the unit.

Be sure to fully seat everything and solder that one point that we de-soldered in order to gain access to the capacitors. After that, put your screws back in and put the pod back into the cluster, and you're done!

Hope this helps some of you out there.

If you've any questions let me know.

-Kris

TnBob · #3 05-05-2012, 03:24 PM

Nice clear writeup JamesDean. Thats pretty well what we have come to expect from you though.

For those unfamiliar, a capacitor of higher voltage can almost always be used if there is space to do so. If a capacitor is polarity sensitive the plastic surrounding it will be very plainly marked with either a + or - symbol.

Hit Man X · #4 05-07-2012, 11:21 PM

Good stuff Kris.

JamesDean · #5 05-20-2012, 04:57 AM

Well this was short lived. My dad said his odo/trip failed yesterday and sure enough the "replacement" 12-tooth drive gear I sourced from a 560SEL had broken in half.

The main 48x12 gear was healthy though. So I think my buddy has another cluster I'm gonna swipe for yet another drive gear.

JamesDean · #6 05-20-2012, 11:45 AM

I ended up just buying a single 12-tooth drive gear replacement from odometergears.com. It was like $30 all said and done.

Will_w202 · #7 06-11-2012, 08:32 AM

Anybody know if the r129 has a 12 or 16 tooth gear? That website advises pulling the cluster to find out, but I'd rather know so I can order something pronto. Is there no way to know via prod date or VIN?

Walkenvol · #8 02-04-2013, 02:05 PM

concerning the clock repair, my Radio Shack only had 1 F35V100uf35. TNBob said the voltage didn't matter so I also purchased a F50V100uf50. Should this work or do I need to find a second F35V100uf35? Physically they are the same size.

thanks!

JamesDean · #9 02-04-2013, 02:06 PM

As long as the voltage exceeds that of the original you are good to go!

Walkenvol · #10 02-04-2013, 02:17 PM

Thanks Kris, I'll see what happens.

Walkenvol · #11 02-05-2013, 08:12 AM

Althouth my tach is working, it would only make sense to replace the capacitor while things are apart. Accessing the capacitors on the clock has been well explained and is fairly straight forward once things are apart. The tach capacitor however doesn't appear to be accessable without removing the needle and faceplate. Am I missing something? I'm very apprehensive about removing the needles.

JamesDean · #12 02-05-2013, 08:17 AM

Hmmm it looks like, MAYBE with some careful work you could get it out without having to remove the needles. Its hard to say, you might be able to get your soldering iron in there at an angle, and a pair of pliers to pull the cap once you've heated up the joints. The new one is probably smaller so it might be easier to put in...

You might even be able to loosen up the gauge face and move it around for some more room.. I'm not sure.

Walkenvol · #13 02-05-2013, 08:29 AM

The cluster removal was prompted by the recent odometer failure. Speedometer still works. What I think I found was the green gear on the end of the shaft that runs through the odometer was slipped out and not engaging the white screw type plastic shaft. After playing with it a bit the green gear appeared to still be on the metal shaft but the whole shaft assembly had shifted to the left when viewing from the front of the meter. It appears the dark grey gear on the right side of the odometer is no longer firmly attached to the metal shaft allowing the shaft to shift to the left. After carefully manipulating the green gear, was able to get the gear and shaft to slide back in to place. I did not attempt to spin the meter to confirm it would work when in the correct place, but I would be surprised if it would as I thing the grey gear needs to be attached firmly to the shaft also? I don't think any of the gears are missing teeth but I can't really see it well as the faceplate blocks the view.

If indeed this is accurate, does the faceplate have to be removed to "operate" on the odometer? If so, how does the faceplate come off without removing the speedometer needle? If the needle must be removed, what's the best technique to accomplish it? If the "operation" is successful, how is the needle returned in its proper position?

As stated before, I'm really apprehensive about removing the needles.

Walkenvol · #14 02-05-2013, 08:31 AM

Also, is my Weller soldering gun capable of r/ring the capacitors or do I need a smaller soldering iron? Never messed with circuit boards before.

BDBENZ · #15 02-05-2013, 01:53 PM

If the speedo works but the odometer and trip do not, what can be the problem besides the gears? I have taken apart twice, inspected each gear and spun the stator motor wheel by hand and the odometer works fine. Can a capacitor or board cause the problem? Not sure where to look or how to test.. Appreciate any ideas.

Thanks,

Bill