Electric Speedometer Repair - PeachParts Mercedes-Benz Forum

Jkid · #1 03-17-2008, 08:25 PM

Can anybody tell me how an electronic speedometer works or how to troubleshoot it? I pulled the cluster out and checked the plugs going into the back. I get about 4 volts at the single, threaded connection below the larger rectangle plug (while sitting in the driveway) but have no idea what this means. Both of my odometers work fine, but the speedo doesn't budge. I'm reluctant to start tearing the thing apart and possibly killing my odometer. Any ideas? Thanks...

barry123400 · #2 03-17-2008, 09:51 PM

I know nothing about electronic speedometer repair. It does appear your pulse generator or sender works on the transmission as the odometer is tracking. You did examine the archives? Your problem if not obscure is not all that common.
Those speedometer guys just might have a common site for parts and repair to share with each other as well. Wait awhile first though as a site member may have had some or a lot of experience.

Jkid · #3 03-17-2008, 10:10 PM

I see there are a lot of great repairs and suggestions in the archives for cable operated speedometers, but I couldn't find too much for electric. I figure there must be a way to troubleshoot the wiring going into the speedo to find out where the problem is before I open it up at least. I guess I need to find a wiring diagram somewhere or wait, I know somebody on here must know.

I finally fixed my dash lights and door lock cylinder today so I've been on a role. The speedo is pretty much the last thing that needs to be repaired since I bought the car two years ago.

--That reminds me, I've got to update my sig's mileage by 20k miles, guess it's been a while.

SD Blue · #4 03-17-2008, 10:38 PM

As Barry mentioned, it sounds like you are getting the signal due to the odometer. Which also includes power and ground requirements. The wiring diagram doesn't really show anything else than a 3-wire setup.

Try these folks:http://www.speedometer.com/index.html
A quick call might determine if there is an external problem.

#5 03-17-2008, 10:38 PM

Most likely the gears inside have stripped, melted, worn out. Even though it is an electric speedo it still has gears inside (or at least mine did) Mine did this about 7 years ago, sent it off to be rebuilt and it has worked ever since.
Do a google search for speedo repair on benz. I think I sent it to someone in TX for about $75.

TMAllison · #6 03-17-2008, 10:39 PM

What car is it for?

Is probably just a bad sensor/sender.

vstech · #7 03-17-2008, 11:32 PM

the electronic speedometer from the SD series is pretty complex. there are no gears that drive the speedo, just the odo... if the odo is working, but the speedo is not, my bet is either dirt on the swing arm of the gauge or some electronic component is faulty. it could be a simple solder joint failure, or a resistor/ecm or diode/transistor etc... pull out your speedo and see if the speedo arm swings without friction. if it does, try re soldering all the connections.

deeph2o · #8 01-04-2009, 12:11 AM

-I have a similar situation:

Speedo and Odo do not work. I pulled the instrument cluster, and then pulled the speedo specifically. When I rotate the gauge in my hand, the needle moves freely. Next, on the side, you can move a couple of the small gears on the Odo. I could fairly easily spin up 4 miles on it.

So, it looks like this is in the sensor?

-I will try the screwdriver/pickup test. -But where is the pickup located? (This stuff is NOT in your typical shop manual... they don't even give info on the tach/speedo, and most of the stuff condescendingly tells you that it "can only be done by a mechanic's shop" -To which I say, in the immortal words of Sherman T. Potter:

Horse-hockey...

-Also, as I am an admitted newbie here, can someone point me to any locations that have good info on any mods that are done typically to these cars? (I had a different Daimler product DSM-Talon/Laser/Eclipse-which had a ton of known mods... I suspect that there are at least a few "improvements" that have been figured out...

Thanks-and may your tires have many miles yet on them...

SD Blue · #9 01-04-2009, 10:23 PM

A search brings up: Speedometer Sensor 1981 300sd

benzdude42 · #10 09-19-2013, 03:22 PM

Does anyone know how hard it would be to convert a 300d to read the speed from a 300sd Trany? What I really want is the odometer.

mach4 · #11 09-19-2013, 06:19 PM

The 300SD is an electronic speedometer so you'll need a speedometer movement from an SD (or possibly other electronic speedo MB) to make it work. There is a good chance you can just remove the mechanical movement and swap in an electronic one. Many MB gauges are interchangable - for example I swapped in the tach module from a 300D into a 380SL cluster when I did my swap. It was literally a screw in proposition (now I did have to adapt the faceplate to get the readings to align, but that was just a couple of drops of epoxy.)

I'd go out to the junk yard and pull some electronic clusters and take them apart and see how they are configured. If you can find a 123 and an SD you can try try the swap right there before you buy. It may take a little tinkering, but I'll bet it could work without a whole lot of trouble.

Be aware that, while you can swap the tailcone from an electronic tranny to a manual tranny, you can't go the other way, so you're going to have to get the electronic one to work.

benzdude42 · #12 09-20-2013, 12:48 PM

What else will be needed? Do i just take the rear speed sensor and plug it strate into the cluster? Do you think a GPS based odometer might be be a better choice? I'm kinda mad that someone went through the pain of making this sd Trany fit in a 300d

mach4 · #13 09-20-2013, 04:01 PM

Quote:

Originally Posted by benzdude42

What else will be needed? Do i just take the rear speed sensor and plug it strate into the cluster? Do you think a GPS based odometer might be be a better choice? I'm kinda mad that someone went through the pain of making this sd Trany fit in a 300d

That's basically what you do. Here's a schematic from an '85 380SL which is essentially the same system you have

GPS might work, but this is pretty simple... provided you get the electronic module and it will fit into the cluster properly, which I think it would.

benzdude42 · #14 09-24-2013, 02:42 AM

thank you. the problem i have now is find a cluster that wont cost more than 100 to 150 dollars. my local junk yard has one mercedes, 87 190e, but someone already took the cluster. when i have more money, i'll go remove the sensor from the transmission because mine was cut out :/