Another Speedo/odometer problem
 

Hi folks,

Trying to fix my Speedometer and Odometer on my 1981 380SL.

I've tried to research as much as I can find and have read lots about the broken gears, etc. I've looked at the multiple threads on this forums (whunter's excellent list:http://www.peachparts.com/shopforum/diy-links-parts-category/188540-instrument-cluster-odometer-speedometer-tachometer-links.html), too.

My case is that the speedo and odo do not work, all other instruments do work. I have the instrument panel out and all the gauges separated. Mine is a VDO type, electric.

I simply don't know if the issue lies with the head unit or the "pulse sending unit" (speed sensor?); or how to figure out.

I wanted to see if I could ascertain if the issue was in the speedo head unit or the transmitter, so I used put the rear of the car on blocks and started it up; first I put the oil gauge wire back onto the oil gauge unit because I didn't want anything spurting out of it, and then started the car and put in drive to idle the rear wheels.

I used my multi-meter to test the wire that has the single-hole plug that goes into the back of the speedo unit; it was reading a voltage on both DC and AC, though I believe this is suppose to be a pulse thing which I know nothing about, but at least there was voltage. Whether it was correct or not I don't know.

Can someone link me information on this "speed sensor" that presumable sends the "pulse" to the back of the speedometer head? All I find when searching for speed sensor are ABS sensors. Perhaps that means there isn't such a thing for this car?

Comments and suggestions appreciated.

Cheers!
Eric

I have pulled apart the w123 mechanical speedometers. these electric spedo are something I haven`t jumped into yet.

some info here that may help.
http://www.peachparts.com/shopforum/diesel-discussion/216780-electric-speedometer-repair.html

or search electric speedometer or insert SL in the search. look in the blue bar above and click on "search". I did the Google search.
hope this helps.


Welcome to the forum. :) Iam mostly in the Diesel section and don`t venture over here much.


Charlie

I actually found a good testing page in the online Mercedes manuals. The "sensor" part is called (according to their doc) the "inductance transmitter". A very good testing approach to determining if problems are with the transmitter or with the speedo head unit can be found here:

http://www.startekinfo.com/StarTek/outside/11883/PROGRAM/Chassis/54-252.pdf

Thank you for posting, I got a 380sl instrument cluster to replace a non working one and was trying to figure out how to make it work.....

Michel