Is the Speedo/Odo on W123 model-specific?
 

Hi there,

The Speedo and Odo on my 240D with 4-speed manual needs replacing.

Are the speedo/odos model-specific? In other words - different cars have different trannys and rear drive ratios. Are the speedo/odos different from model to model, e.g. 300d, 240d auto, 240d manual?

Sorry for being such a noob.

Thanks,

PE

Any 240d speedo for a 123 car should work providing you have the stock diff in the rear.

Ideally, get the same speedo using the part number stamped on the face. An older speedo might only be up to 85, while the other variants might go higher (even if the car didn't really reach those speeds).

I know in '81 there was a weird usage of electric speedometers (no cable). Back to 82 and the cable speedo returned. Our former '81 300TD had this electric speedo.

Are the 240D's immune to this unique one-year feature?

Are the differentials the same in the auto vs the manuals transmissions?

Just about any 123 speedo will fit. But it won't read the miles or the MPH correctly. The only ones that won't are the electric ones mentioned above.

What is wrong with your speedo? Usually the odometer stops, and that can be fixed. There are DIY online for it, but I've never had luck making it work in the long term. I've sent two into a Speedo shop and had them fix them. One of them had 60kmile on it since they fixed it. Junkyard Speedos are going to be a gamble. The speedo is one of the weak points of the 123, they always break. Never trust a 123 speedo.

Here is the DieselGiant how to on fixing the spedo:
How to Repair Your Broken Odometer

All 240D`s use the 3:69 diff. auto or manual trans.

300D/TD Na use the 3:46 77 - 81

300D turbo use the 3:07 82 - 84

300D Turbo use the 2:88 1985

The speedometers are geared to the Differential gear ratio.

Charlie

If you're brave enough ANY w123 cluster can be modified for ANY diff.

I've taken apart at least 20 of these things and I've only found three differences between the different types.

-speedo/odo faceplate
-copper spur gear (seen below)
-plastic worm gear (seen below)
http://dieselgiant.com/mbodo21.JPG

I can't remember the pitch and tooth count on the spur and the worm gears off the top of my head. But if you intend to swap them out COUNT THE TEETH FIRST, they look amazingly similar on the workbench.

that bit about counting teeth is varry interesting. i do have several here from different ratios and i'll look at their components when they're on the workbench.

yes, the speedo's are model specific for the 123 chassis.
the 240 speedo's ALL are geared for a 3.69 rear diff.
the 300 non turbo's are all geared for 3.46 (unless it's a manual trans, then I don't know)
the 300 Turbo's up to 1984 are geared for a 3.07 diff,
in 85 the turbo's got the 2.88 Diff.
the one anomaly is the 81 Wagon. it's got a electronic driven speedometer, and is only useful on that vehicle.

And, don't forget the very ends of the cable which go into the tranny and the end of the speedometer. There appears to be two different types: one is blade type and the other is squared and I suggest you don't jam the blade type into the squared ends and vice versa.

this is true, but it's a transmission issue, not a speedo issue...

Mr. Technical ... LOL :)

THat was due to Mercedes using the 617.951 from the 'SD, right? Is this only the case only for wagons?

Actually there is a difference I have noticed between the speedos for an automatic car and a stick car with the same engine. The maximum rpm hash marks on the face are a little lower on an automatic equipped car due to I presume, slippage. ... one or two mph, IIRC.


Not enough to worry about.

no, it's because the 722.3 was only set up for the SD at the time, the engine is still a 617.952 (the first vehicle to get that variant .952 I think...) the NA vehicles all got the 722.2 trans...

this is a good point, obviously the shift points are going to be different between a slipping power hogging auto and a direct drive manual. but the rear wheel speed to speedo speed should be acurate.