Adjustment for the mechanical VDO Speedometers.

So for the last couple of days I’ve been trying to keep my original speedometer to use with my new 2.88 ratio differential on my w123 300D. The speedometer is calibrated for a 3.07 ratio differential. I do have a 2.88 speedo, but I am opting to keep my original because it is in such good shape.

Although I am not an expert about this and correct me if I am wrong, but what I know about these mechanical speedometers is that it runs off a circular magnet that spins in a speed cup. As the car’s speed increases, the magnet spins faster and creates a magnetic field that act to spin the speed cup. The speed cup is connected to a hair spring and then to the needle. The hair spring provides enough resistance to the keep the speed cup from completely spinning and keeps it at certain marked speeds. The calibration of the speedometer can be made by either adjusting the hair spring or adjusting the power of the magnet. To get a more precise calibration, adjusting both the magnet and hair spring might be required. Since I have no way of adjusting the magnet (which requires a powerful electro magnet to do so), I am focusing on just the hair spring.

I basically adjusted the hair spring until my speedo closely matches the GPS. I had to do take apart my cluster a few times to accomplish this. I started with about a 15% error across the speed range, than I got a 10% error across the speed range. I ended up getting my speed accurate between 40 mph to 65 mph. Below 40 mph, my speedo error is showing at the most 4% fast and over 65mph at the most 4% slow. Not sure why it wasn’t consistent across the speed range like my other two % errors.

But that is good enough for government work and then that is good enough for me.

This spring adjustment might be particularly useful for someone doing the 2.47 diff swap. I have a thought of using a weaker hairspring from an 85 mph speedo on a 120mph speedo. To make it work, you’ll probably have to adjust the tension by shortening the hair spring to make it have more resistance because the 85 speedo’s needle has less numbers to travel through then the 120 mph speedo. But that is just my thought and will need to be tested out.

Anywhoo, here are some pics of what I did.

I took the speedo out of the cluster.


Removing the needle. To not scratch or dent the speedo face, I placed a thick stack of note paper with slice in it on the face and under the needle. I then used a wire cutter to pop the needle off. It will be tight and might go flying off somewhere.. So be careful.


Hair spring. Because I was showing a speed that was slower then I was actually traveling, I had to loosen up the hairspring by lengthening it. I did so by removing the lock down pin and draw the spring down through the hole. If you run out of adjustment, then you need a weaker spring.


Here is a shot of the original hairspring and another hairspring from the 2.88. They were physically the same size. So I knew it just a matter of adjusting the tension to make it work.


As far as correcting the odometer. I swapped in the 2.88 speedo's gears. Works perfectly.




My adjustment is basically guessing. Not entirely accurate. But I hope this gets us thinking of how to correct a speedo with someone who does not have an option to simply swap out their original unit for another factory unit... like the 2.47 differential guys.

Attached Thumbnails

         

__________________
1983 123.133 California
- GreaseCar Veg System

Richard this is a great writeup and pictorial. was thinking of doing the same.

I have 3 w123 spedo`s on my desk, a 79 a 83 and a 85 plus a 4th from a w126 that came out of a gasser 2:47. was thinking of modifying maybe my 2:88 from parts out of the w126. Not sure if it is doable, since the W126 is electric input, but front half is mechanical.

I used 2 spoons with masking tape on them to remove the needle, I could then rock them back and the needle popped off.

In picture #3, the brass gear at the top right that turns the odometer numbers, also have different # of teeth.
My 79 count is 19
My 83 count is 21
My 85 count is 15

In picture #5, the gear on end of the shaft from my 79, the count is 14 teeth.
also the spacing onthe other end of the two gear shafts you show are different.

on the horizonal gear the spedo cable drives might be a different # of teeth also.

I didn`t think if the little spring being different between the spedos, good to know.

I need to take my other 2 W123`s apart and take some notes.

Charlie

__________________
there were three HP ratings on the OM616...

1) Not much power
2) Even less power
3) Not nearly enough power!! 240D w/auto

Anyone that thinks a 240D is slow drives too fast.

80 240D Naturally Exasperated, 4-Spd 388k DD 150mph spedo 3:58 Diff

We are advised to NOT judge ALL Muslims by the actions of a few lunatics, but we are encouraged to judge ALL gun owners by the actions of a few lunatics. Funny how that works

Please don't quote such a looooong post....

I have a w116 160mph speedo and the hair spring is much stiffer. I'll take some more pics for comparison later.


Take some pictures too!

__________________
1983 123.133 California
- GreaseCar Veg System

sounds promising. my speedo and odometer are off by 40% after my 2.47 swap, I have a 120 speedo which the po installed that was already off by quite a bit

do you think you could drive the speedo cable with a drill motor instead of putting it in the car each time? maybe by setting the speedo to a gps at several speeds, then duplicating the speeds with a drill motor and recording the rpms?

I will be watching this post. thanx for the great work and post

__________________

1982 300CD Turbo (Otis, "ups & downs") parts for sale
2003 TJ with Hemi (to go anywhere, quickly) sold
2001 Excursion Powerstroke (to go dependably)
1970 Mustang 428SCJ (to go fast)
1962 Corvette LS1 (to go in style)
2001 Schwinn Grape Krate 10spd (if all else fails)

I tried it with my drill, but I can't seem to get a consistent results with it. It seems if I angle it a little bit to any random way the speed would increase or decrease. But it does give me an idea (not exact) of what is the drill's top speed on each of the speedometers.

Just curious, what is the part number on the face of your 120 mph speedometer? I like to find out what differential ratio that speedo is set for. I talked to another forum member who when from a 2.88 ratio to a 2.47 and he stated he had a 15% error.

__________________
1983 123.133 California
- GreaseCar Veg System

Remind me again why you couldn't just but the nicer face and needle on the correct ratio speedo?

__________________
Sadly Benz-less

why cant we just change out the drive wheel at the tranny? seems this is what speedo shops do.

__________________
1984 123.193 372,xxx miles, room for Seven.

1999 Dodge Durango Cummins 4BTAA 47RE 5k lb 4x4 getting 25+mpgs, room for Seven.

That would be too easy, what fun would that be? No really, the odometer numbers were also physically shot. No cleaning would of made them look better. Besides, I wanted to test on how these things can be adjusted.

__________________
1983 123.133 California
- GreaseCar Veg System

Because Mercedes doesn't adjust their speedos that way.

For around 150 bucks, there are calibration boxes that you can add on to your speedometer cable.

__________________
1983 123.133 California
- GreaseCar Veg System

For many years, I have run the 'optional' tire sizes on the W116, which was 205/70R14 on the 450 SEL 6.9 variant.

Forum member 'hammerwerfer' used a set of 215/60R14's on his W116 without rubbing or interference of any kind.

Running some basic calculations, I have discovered that going from a 205 to a 215 would change the speedo reading by roughly 1.3 MPH.

Changing to the 2.47:1 diff will find me having to use a 2.88;1 speedo from a 107 SL, so while I will have some speedo errors that will need to be addressed, the readings will not be too far off.

__________________
Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

Interesting, since I have a 2.88 (107) speedo to go into my W116 with the planned 2.47:1 axle swap.

A 15% error at 60 mph should equal about 9-10 mph on the needle...

How best to address this??????

__________________
Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

Your best bet is to keep your current speedometer, figure out the % off and install a "ratio adapter" or Mechanical speedometer cable Calibration Box.







You can contact these guys to see if they can make a custom unit. Usually sells around 100 bucks.

United Speedometer Service in Riverside, CA (951) 742-7117

Repair Frames


or Commercial Speedometer in Sacramento, CA (916)-371-5873



.

__________________
1983 123.133 California
- GreaseCar Veg System

No new cable on that setup????

Next question is, will it clear the floorpan in a W116?????

__________________
Did you just pass my 740 at 200 kmh in a 300SD?????

1978 300SD 'Phil' - 1,315,853 Miles And Counting - 1, 317,885 as of 12/27/2012 - 1,333,000 as of 05/10/2013, 1,337,850 as of July 15, 2013, 1,339,000 as of August 13, 2013



100,000 miles since June 2005 Overhaul - Sold January 25th, 2014 After 1,344,246 Miles & 20 Years of Ownership

A good speedo shop can build a cable with a ratio adapter anywhere along the length of the cable. So if you have clearance issues near the trans, they can put it further up the line.


.

__________________
1983 123.133 California
- GreaseCar Veg System