Is there an easy way to diagnose a non-functioning speedo 300SD? - PeachParts Mercedes-Benz Forum

John Schroader · #1 08-15-2010, 08:54 PM

Is there an easy way to diagnose a non-functioning speedo in a 300SD? I've been meaning to replace the gears on my odometer for some time, but recently the speedo did a couple of quick bounces and then died. If the problem is the sending unit, then I will try to replace that and the odo gears. If the speedo itself is dead, then I hope to just replace the whole unit. I'm sure there is a sure fire method to determine what is dead in the speedo ckt, but I'm in the dark. Any suggestions? Thanks in advance.

Wodnek · #2 08-15-2010, 09:30 PM

If it has a cable then here is a way.

Pull your cluster. Drive the car forward, or jack up the rear and just run the car in drive. Watch and see if the cable is spinning. If it is spinning, squeeze it gently with your fingers and see if it is spinning with at least a little bit of torque. If it stops with the slightest pressure, the sending unit may be the issue. Not spinning at all is likely to be a cable.
If the 126 has an electric speedometer, then this is obviously of no value. I never had a 126, so i don't know.

H-townbenzoboy · #3 08-15-2010, 09:53 PM

Quote:

Originally Posted by Wodnek

If it has a cable then here is a way.

Pull your cluster. Drive the car forward, or jack up the rear and just run the car in drive. Watch and see if the cable is spinning. If it is spinning, squeeze it gently with your fingers and see if it is spinning with at least a little bit of torque. If it stops with the slightest pressure, the sending unit may be the issue. Not spinning at all is likely to be a cable.
If the 126 has an electric speedometer, then this is obviously of no value. I never had a 126, so i don't know.

The W126 has an electric speedometer, so it won't help.

showme · #4 08-15-2010, 10:01 PM

Ours was busted when we bought it. Found out that it was just a busted speedo cable. It busted off right at the transmission. It was an easy fix, and the new cable wasn't too bad either. I actually have replaced it twice now. It seems that the rubber cup around the entrance into the tranny collects water or moisture, then freezes up in the dead of winter, and as soon as you pull off after is has set, it snaps the cable right there at that point. The second time, it only took me about 15 minutes to change. (I also learned how easy it is to remove the instrument panel on these cars when I did that.) Although it might be anything, I would certainly take the 5 minutes to get under the car and check that first. I put it off for 6 months fearing I needed to do the odometer R&R job. I think my cable was about $40. Good luck!

vstech · #5 08-15-2010, 10:10 PM

great suggestions there, too bad none work...
pull your dash panel kick plate, and look near the center, there is a cluster of wires in a connector. pull it off, and clean all 8 connectors. then put it back on. if that's not it, go under the car and pull the wire off the transmission and clean it. last is the speedo itself, pop it out, and look over the wire in the back see if it's on really well, and clean it up.
final lastly or firstly if you like check all the ground connections in the dash area, and clean the ground plate connectors on the cluster with a pencil eraser and reassemble.
report back!

John Schroader · #6 08-16-2010, 07:58 AM

Should have been more specific - it's an electric speedometer (my 300D of the same vintage has a mechanical one). Will check the connections. Just hoping there was (is) some sure fire way to determine which end is bad without shooting in the dark. Thanks for the advice.

TX76513 · #7 08-16-2010, 09:08 AM

John - actually not. if you are looking for the technical information to check the circuits it can be found HERE. But, you still have to pull the cluster and partialy disasseble the unit, which you will probably find the little gear is missing a tooth or is stripped on the shaft.

Helpfull Hint: When taking the cluster apart draw a rectangle on a piece of paper adjacent to you cluster. As you remove screws place them in drawn circles on your paper relative to where they were removed from. This will make re-assembly easier. While most of the screws are the same size, lengths change depending upon location.

Beastie · #8 08-16-2010, 09:12 AM

Even though I know you said it bounced before stopping and you did not mention any other issues. Start at the source and pull the fuse and clean the ends. My speedo would work intermittently until I did so, Also if the fuse has the silver( aluminum) fuseable metal change out to the copper ones.

showme · #9 08-16-2010, 10:14 AM

Sorry, John. I didn't realize you had an electrical speedo. I learn something new every time I get on this forum. But the mechanical one's sure are easy to change! I love our MB...most of the time.

vstech · #10 08-16-2010, 10:23 AM

Quote:

Originally Posted by showme

Sorry, John. I didn't realize you had an electrical speedo. I learn something new every time I get on this forum. But the mechanical one's sure are easy to change! I love our MB...most of the time.

heh, if you think the mechanical ones are easy, try an electronic one.
no oil line to disconnect, no speedo cable to unscrew. just pop off the wires, and it's out.

showme · #11 08-16-2010, 10:34 AM

Aw, man, you just burst my "happy bubble"! But I've never had to disconnect my oil line. I just push the cluster out enough to see the speedo, and reach under the dash to unscrew it. With my luck, I'd get a bad electrical connection, and leave it for another 6 months, fearing the worst case scenario for the job. But, vstech, like I said before--- I just learned something else by your post! Thanks!

John Schroader · #12 08-16-2010, 12:06 PM

Thanks guy (and gals?). Been needing to pull the cluster for a while anyhow. I pulled it a while back and, once again, was impressed with the simplicity of MB engineering.