W123 speedometer interchange?

[ScooterABC]

Hi -

If I put a later turbo diesel speedometer in my non-turbo 300D (that currently has a broken 85 mph speedometer) will it read correctly, or is it dependent on the transmission and axle ratios of the donor cars?

thanks
Scott

[BWhitmore]

The speedometer should match the rear axle ratio of the car.

[Shortsguy1]

The turbodiesel probably had a 3.07 or 2.88 diff ratio, whereas the NA diesel probably had a 3.46 ratio. So the error will be rather large if you just do a swap.

Are both the speedo and the odo broken in the NA car?

[ScooterABC]

Would a 240D and a 300D have different axle ratios? What I'm trying to find out is if I can look at an 85 mph speedo and know whether or not it will match my car. The part number on the face of the gauge is useless. The only other part number I'm finding is on the "sending unit" which is the electrical connection on the back. That part number (on the donor speedo) is 0045426517. EPC shows that as going to a LOT of cars.

So if I have a speedo, is there a way to tell what speedo it is for?

Second question - the back of the speedo has the place where you connect the cable and an electrical connection for something defined as the "sending unit". What is this electrical connection?

thanks
Scott

[Shortsguy1]

I don't know the answer to several of your questions, so hopefully others will weigh in.

According to this website...
Hauptseite/en
... a 240D W123 will have a 3.69 Diff Ratio. I don't know how all this translates into the top speed represented on the speedo. In other words, even though a more powerful car (e.g., 300D turbo) was given a taller diff ratio, I don't know what MB changed regarding the highest speed on the speedometer.

I don't understand why the part number on the speedo is useless. VStech occasionally asks for that number to help people know if they have the correct speedo and mph reading. I would be surprised if the answer to your primary question doesn't lie in the speedo part number.

I assume that electrical connection on the speedo goes to the cruise control, but someone else should confirm.

[ScooterABC]

The part number on the face of the dial gauge is the part number for the face, not the speedo. The part number on the rear sender is not unique to the W123 cars. If the speedo clearly HAD the part number of the speedo on it, things would be easier, but I can't find that.