I know this is an old thread, but IS there any way to calibrate the speedo on a W123?

Thanks,

PE

Are you talking about the spedo the speed you are traveling? or the odometer, the distance you have traveled?

If the odometer is correct, but the spedo reads a little high or a little low, that can be corrected by removing the needle, and resetting it.

Remove the cluster and then remove the spedometer. On the edge of the face plate between the resting peg and white printing at the bottom, (MPH. and the 123 542 xx xx etc...) there is a very small white mark. Lift the needle up over the resting peg and see where is lines up. It should line up with this mark, so when the spedo is ever totally dissassembled, this is a reference point to reatach the needle for reassembly.

Behind the face plate there is a very fine spring that gives the needle the tention it needs. The other end of the shaft the needle resides on, is an aluminum cup the shaft is attached to The cup/shaft/needle is driven by a magnet that spins, driven by the spedometer cable.

If the spedo is not reading correctly, the needle can be pryied off and reset to the right or left of the white mark.

The other problem is the potmetal gear slipping on the shaft the numbered wheels reside on, then the numbers will not record for a bit, then will when the gear catches on the shaft.

Then it is time to tear the assembly down and go through it. Diesel Giant has a good DIY, and think here on the forum if you search will be some good writeups.

PNP has another 50% sale this weekend, good time to pick up a 123 spedo and use it for your sacaficial lamb. that is what I did to figure out what makes these things tick.
I replaced the 85mph one in the 240D with a 150mph one. went all through it cleaning, painting the needle, fixing the potmetal gear etc... when I was realigning the numbered wheels to get it to the 240~s correct milage, I was stressing over not getting them aligned correctly. had it apart 3 times. :mad:

I found out later the shaft with the grey cogs at the very top can be slid out after filing off the bulge in the shaft where they crimped it. reset the first wheel, added a cog, the next wheel, add a cog etc... until I had them all in.

Then to keep the shaft in place, I stripped off a piece of wire insulation that is smaller than the shaft. slid it on and all is good. the shaft doesn`t spin, just keeps the cogs lined up in their place. the first one only turns at each mile, next one at 10 miles etc...

Ant time changing the spedo, make sure you are using the Differential from the donor car. the spedo is geared to the Diff, not the transmission. the MB transmissions final drive is 1:1. auto`s and the 4-spd manuals. Not sure about the 5 spd with the OD how that works.


Charlie

the problem here is that the jersey shore distorts both reality, and reality tv

I calibrated my speedometers by going to 195/75/14 tires on all three of my W123s, above 55 or 60 it still reads a tad high so its not perfect.

Dear Charlie,

THANK YOU for this useful and detailed information. It is exactly what I was looking for!

I'll give it a try in the next day or two and report back. I do have a couple of spare speedos to tinker with.

Sincerely,

PE

Bump for a ditto to charmalu; I will be doing this myself.

ODO doesn't work yet, haven't replaced the gears yet.

And the Speedo reads 5 mph slow All The Time. I am hoping the idea of checking where the pointer is pointing works on mine. I have heard that the magnets that enable the speedo to work can lose or gain magnetism (gaining more rare), and therefore change the calibration.

We shall see.

My 85 300D speedo has always (20+ yrs) read high When I am driving at 120km/hr, actual speed is about 116.

I used to check it with highway mile markers (actually km markers in Canada!) or by having friend in another car drive at same speed. More recently checked with GPS.

In a way this is a good thing - fewer speeding tickets!

My car is original in terms of cluster and rear end. So, why would this error be there?

Only possible cause I can think of, is a difference in rev/mile on current day 195/70-14 vs the original tires? Most current tires are quite close in size and about 838 rev/mile. As tires wear, diameter could reduce by about 1/2". I guess that could cause speedo to read high.

The other thing, could be soft tires. My tires when inflated to MB specs always seem to have a flat spot at bottom when parked. Perhaps rolling diameter is different from spec numbers?

This doesn't bother me and I am not going to do anything about it. Just curious as to why speedo reads high when nothing has been changed.