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

JamesDean · #1 07-31-2015, 02:29 PM

Quote:

Originally Posted by mikes560

Hi folks just read this chatline and signed up for my 20 cents worth..

Just pulled the cluster from my Australian compliance 1989 560SEC - it was reading 100kph at a real roadspeed of 90kph (satnav check) - no wonder everyone was passing me on the freeway. The cluster has never been changed from new. Replacing the 2 capacitors made only a small difference. I've read a few blogs saying you cannot adjust the speed reading of these Mercedes digital speedometers.

I'm a bit of an electronics nut, so downloaded the data sheet for the drive chip (UAF2115), and found that you CAN adjust the speed reading on these instruments. The 750ohm resistor next to the chip on my PCB (see photo attached) is used to calibrate the meter reading on the dial. I removed it, extending some leads out to find the room to attach a 68ohm resister plus a 20ohm multi-turn precision trimmer. I can now adjust my speed reading by about +/- 15%.
http://www.peachparts.com/shopforum/attachments/tech-help/131236d1438234522-how-w126-odometer-repair-clock-capacitor-replace-560sec-speedo-1.jpg
I'm not convinced this is related to the REAL fault as I still have a linearity problem in this speedo so have more work to do (plus I have to design and 3D print a replacement odometer drive pinion (mine has split).

I'll make another post once I've cleaned up everything and am happy I've got a final solution.

My apologies for a very crude post - maybe I should just stick to electronics!!

I'm going from memory here on a good amount of this so bear with me.

I am fairly certain that resistor is used to configure the galvanometer current. Changing it will cause some linearity problems. I recall that from trying to make a 560 speedo head work for a 300SD.

OdometerGears.com sells replacement gears as does Garagetastic but 3D printing does sound fun so its all in how you wanna spend your time.

Aside from electronics, speed error could be introduced by wheel/tire sizes that are dramatically different from stock sizes. Are you tire/wheels stock or have you changed out?