I'm trying to diagnose a speedometer problem from 300 miles away. The speedometer in my son's 1984 300D has died completely - no speedometer and no odometer. He's is NC and I'm in GA.

I can see three possible problems: 1) speedometer head 2) speedometer cable 3) plastic speedometer gear in the transmission.

The speedometer head was professionally repaired in 2002 for odometer failure (speedometer OK).

The speedometer cable was replaced also in 2002.

The transmission was rebuilt in 2012.

Any thoughts about what could be the problem? Any tests to try?

Thanks.

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Tom Evans
69 280SE 91K (wish I still had it)
73 220D 98K
78 300D 134K
78 300D 185K
81 300SD 301K (still have)
82 300SD 321K (still have)
84 300D 268K
84 6.2 Diesel GMC Suburban 225K (a whole different story!)

Most likely the cable. Pull the gauge cluster disconnect both ends of the speedo cable. Spin one end and see if the other end spins. If not, you've found the problem.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.

I just went through this; most likely the speedo cable has failed. IF it has broke off up inside the transmission, it will require drilling out the old piece of cable BEFORE a new speedo cable can be installed. You best understand how to properly size the correct drill size to drill it out or you can make the problem worse. I ended up taking mine to a professional mechanic to drill out the old cable and install the new cable.

Step 1: To test if it is the plastic part in transmission; remove the speedocable and attach a drill to the end and let it spin (do forward and reverse to make sure) should the mph or odometer still not work then your problem is either a cable or in the dash.

Step 2: At this point you need to remove the instrument cluster. Once there, remove the speedocable attached to the back of the instrument cluster and attempt the drill test once more. If the end you just removed from the instrument cluster spins then your problem is located inside the instrument cluster.

Step 3: Disassemble your instrument cluster and follow these directions, it is a time consuming repair, however very simple How to Repair Your Broken Odometer

Best of wishes, I currently have to replace my transmission after removing the speedo housing, a short while after replacing the plastic part inside it was chewed up. When we removed the original speed unit, there was jagged metal inside the transmission that shredded the plastic speed sensor. We attempted to smooth it out with a screwdriver to no avail. here are the part numbers should this be your case (take it to a shop and let them deal with it.)
007-997-73-46-02 (Inside seal for the speed sensor, contact Mercedes-Benz Classic Center Irvine, Ca 949-598-4850)
005-997-52-45 (another seal)
115-274-06-28 (the housing)
115-270-03-27 (Speed sensor)
Link to my thread: http://www.peachparts.com/shopforum/diesel-discussion/336951-speedometer-cable-trans.html

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Nosce Te Ipsum "Know thyself"