W116 300SD speedo calibration
 

My brother is wanting to put a 2.88 diff from a W126 300SD into his W116 300SD.

This is to take advantage of the better designed W126 axles. The W116 replacement axles are pricey and more difficult to deal with. Also, potentially to add ABS to the car for added safety/saving the tires. I have two full systems from a W201 I could use, just need W126 sensors from the yards. Idiotic Atlanta drivers warrant the need for ABS unfortunately.


What would have to be done to calibrate the speedo/odometer? The original ratio is 3.07. There is no 2.88 instrument cluster for the W116 AFAIK

Hello W123fanman,

One possible solution, assuming your budget extends to an additional $85, is to put an electronic adapter in between the transmission output signal and the speedometer. I used one from Dakota Digital (see URL below), to improve the accuracy of the speedo on my '81 300SD due to the change from the original 14" Bundt to 16" AMG wheels. What is nice about the Dakota Digital is that you use the +/- to fine tune the accuracy. Use a good Sat Nav that displays speed as your reference device. Works amazingly well.

Universal Speedometer Signal Interface

Hope it helps.

W116 speedometer is mechanical vs electronic

I'm not familiar with the application, but I can give you general guidelines.

First of all, the speedometer is actually two instruments in one case...the speedometer and the odometer. The odometer calibration is changed by changing a gear. Odometer gears may be available from:

Odometer Gears - WELCOME

The speedometer is a bit more complicated. The way it works is the cable turns a bar which has a magnet on either end. The magnets ride inside a steel cup which is attached to the needle, and works against a spring. The faster the magnets turn, the more drag is exerted on the cup. This system can be calibrated by adjusting the force of the magnets or by replacing the spring, but I know of no source for these components. You can fudge it if the ratio difference isn't too great, by repositioning the needle. Use a drill to spin up the speedometer and see where it reads. Then calculate where it should read based on the speed of the drill and the axle ratio. Remove the needle, and place it at the correct reading while spinning the speedometer.

I have a writeup on my site about calibrating Smiths speedometers. This isn't directly applicable, but the concepts are identical:

Calibrating Smith's Speedometers for Borg Warner T5, Tremec and Toyota

Once upon a time, there were local speedometer shops that could do recalibration, either by rebuilding the instrument or by installing a conversion gearbox. If there's a nearby shop that can do this for you, then that's the easy thing to do...it will be worth the cost. Otherwise, Palo Alto Speedometer gets good reviews:

http://www.paspeedo.com/

Other question would be: Can you swap the drive gears between the W116 unit and the W126 ABS unit?

I'm looking at the possibility of one of these:
Universal GPS Speedometer Interface Module

After recently updating my differential to the 2.47 vs 3.07, I've wondered about how to calibrate the speedometer as well. However, the ~$200 price tag leaves me to using my phone app.

If that's within your price range, it might be an alternative by using the W126 speedometer.

If you have the donor car's speedometer, you may get lucky and be able to swap the entire works of the speedometer into the target car's speedometer case. It may also be possible to disassemble and exchange gears. The interchange may or may not work, so unless someone can report they've done it, you're on your own. This would be the best solution, however, the size of the wheels and tires affects calibration, so you may have to switch tires to compensate for any differences.

Will thee tires calibrate it? :rolleyes: :D


http://2.bp.blogspot.com/_1qOTad3JCl...+Abu+Dhabi.JPG


if you are able to swap in a 123 spedo assy and use the 116 face plate, then
the speeds would probably be off where the needle lines up.

It seems like the W116 face plate is smaller than a W123.

Charlie


sent from my pos computer

i realize that these are for american iron but a clever M.E. might make it work
Speedometer Gear, Speedometer Gears, Ford, GM, Dodge 700R4

Let me rephrase that. Could I swap the gears from the 116 rear diff to the 126 rear diff? I'd lose the benefit of the batter ratio but gain ABS. W116 ABS differentials are hard to come by.

He isn't set on doing this, but it is an idea.

I would suspect that you can swap the entire diff. between the 116 and 126.