I decided to share a few thoughts on “home made” speedo work. The owners of the speedo repair shop in Warsaw retired and I simply do not know where to send a speedo for repair or overhaul, so decided to do it myself, accepting all the risks (financial and psychological). This followed a successful overhaul of a clock, so I felt strong enough not to write off ca EUR 1000 or more as well as self esteem immediately.
Important: this is about refreshing and adjusting a working speedo. With non-working speedo the issue is spare parts. I think the work is fairly simple, but the spare parts are the issue. Or ability to fabricate them. With a non-working speedo, i would probably open it to see if there is anything obvious that I could fix, but i would be aware of the risk that I may end up with a car I cannot drive as I wait for speedo to be fixed.
I had a wrong speedo in my car, one coming from, I think, W111. It was working perfectly, just was showing the wrong speed (wrong W value). It also had flexible shaft fitting on the wrong side, which resulted in one flexible shaft shredded (by the way: erratic speedo work, needle jumping at low speeds, was caused by partially shredded shaft).
As a result of taking this calculated risk of doing it myself, I have a new looking speedo that works perfectly well for the price of EUR 300 (plus some tools and materials) and lots of satisfaction. I also have a good nice speedo for W111 (I think) for sale.
I had to buy a speedo for repair on ebay (because of the budget) and fix it.
1. The first difficulty I came across was to determine the W value. In our cars there were several differential ratios used and W (or k in the US) value needs to match those ratios. In a lot of cases what you get from seller is that it is “speedo for W113 230 250 280SL”. Sometimes part number from the faceplate, which not always helps to identify the speedo. Often the expensive, overhauled speedos are also not provided with the W value information.
I took a chance and spent EUR 300 on a speedo claimed as “working”, for overhaul, for a Pagoda but with unknown W value.
2. I needed to determine the W (or k) value for the speedo I bought. I needed a speedo for 1:3.69 differential ratio, which would mean W=0.855. It goes like this (also from our Technical Manual):
1:4,08 rear axle diff ratio: 945 revs. W= 0,945 (km) or 1.512 (for miles)
1:3,92 rear axle diff ratio: 908 revs. W= 0,908 or 1.452
1:3,75 rear axle diff ratio: 869 revs. W= 0,869 or 1.390
1:3,69 rear axle diff ratio: 855 revs. W= 0,855 or 1.368
The revs here mean number of revolutions of the flexible shaft to cover 1km. If your speedo is in miles, you need to multiply the W value (or k value) by 1.6.
What I needed for determining the W value was: laser rev counter (ca EUR 20 in any internet store selling tools), power drill, piece of flexible speedo shaft and a table to set it all up (see picture). I made several measurements by observing points marked on the white odometer wheel – to see when speedo counted 1 kilometer of travel, then took the reading of revs (good if rev counter has the "Hold" function). I made several measurements and drew an average. This is not precise, but precise enough for this purpose.
I determined the W to be 0.869. I needed 0.855, but I decided I am close enough – I knew I could adjust it with the speedo needle placement.
3. Dismantling of the speedo. The difficult moment here was removing the needle. I fabricated a puller for this, made of a puller used for repairing watch bracelets and additional bracket. Whatever touches speedo faceplate needs to be cushioned not to scratch it with, e.g. self adhesive 1mm felt.
It is critical to mark position of the needle before removing it!!! You need to put it in the same place or have a reference point for adjustment.
a. Raise the tip of the needle gently and move the needle over and behind the peg so that it bounces freely on its spring
b. Place speedo in level position and mark where the needle was pointing
c. When putting back – drop needle on the shaft while it is pointing to the spot you marked or you want it to point to (if you need to adjust the reading)
4. Mileage adjustment – to match what you had on the car. The shaft with small driving gears needs to be removed, wheels with numbers adjusted and the shaft with driving gears fitted back. The shaft with driving gears needs to be removed to free up the wheels with digits. Of course you can achieve the same by dismantling the shaft with wheels with digits, but I found it more complicated – the simplest was to remove the shaft with driving gears. This was rather a tricky part for me. The key here is that the small driving gears need to come back on the shaft in the same order, orientation and position of smaller and bigger teeth as they were. Once you mark that and put it back together the same way – the odometer will work.
5. Digits refreshment (as they may be “yellow” because of age). I did it with white oil paint marker 0.8mm and black oil paint marker 1mm. Just followed the groves for digits with the white one, made corrections with the black one.
6. Needle refreshment – oil paint marker, white, 1mm.
7. Faceplate replacement. Three points to note:
a. Various suppliers provide faceplates with various shading of the colors of the digits. SLS one was more white than one from another supplier. I chose SLS to have the same shade of white as the digits in the tacho.
b. The faceplate come from suppliers without the center “frame” for odometer. To move this frame to the new faceplate you need to undo the rivet that is holding it to faceplate. You are either skilled enough to re-use the factory rivet or (for me as I was not skilled enough), you need to find 7mm rivets used for finishing eyelets in canvas and use those. You do not see that rivet when the hand is fitted on its shaft. I used a socket and vice to rivet faceplate back in place.
c. If your odometer frame is scratched or discolored, you may need to paint it. I used Tamiya Rubber Black (modelmaking) color – it was closest to what I needed, but for you it may be a different dark grey or black color.
8. Other spare parts. Depends what you need here, but to note:
a. Glass – fairly easily available
b. Chrome ring – easily available
c. Cushion between speedo and dash – available (Mercedes)
d. Odometer knob – available
e. “Foam” housing – available (I could not find the ring holding it, need to re-use this one)
f. Bracket/nut – available
g. Bulb holders – available
h. Peg - not available, but you can widen its hole in the faceplate by a fraction of a mm, buy 1 or 1.2mm rivets used for watch bracelets, cut it, fit it and paint it
9. Cleaning and lubrication. I am not an expert here and I am sure others may provide more correct recommendations. What I did (based on advise of professional on one of the youtube channels):
a. Blew the internals of the speedo with compressed air gently
b. Flush it with some gentle cleaner spray
c. Applied white grease to obvious places (warm gears)
d. Ensured that small drive gears are moving freely on the shaft
10. Speed reading check and adjustment. Note: this will not always work, I think it depends on what causes wrong speed indications of the speedo. It may work ok if, e.g. the needle spring weakened or magnet weakened (as per you tube), but not all the issues can be fixed with this adjustment.
I see two methods here:
a. Just measure time the white odometer wheel goes 1km vs. the speed the needle is showing. If, for example, the time to cover 1km is 70 seconds, you just calculate
1km – 70 seconds
Xkm – 3600 seconds
(3600s x 1km)/70 seconds = 51km/h and see if the needle shows you the truth
b. Fit the speedo in the car, drive and check the speed speedo shows vs. speed you see on any GPS app on the phone (or Waze).
I did the second method and found out my speedo show 10km/h too much speed across the range.
Then I:
a. Marked the position of the needle when needle had been moved over and below the peg
b. Removed the needle
c. Put the needle back, more away from the peg (or to the right) by approx. 10km/h, so that it shows less speed by ca 10km/h
d. Moved the needle gently over and in front of the peg where it lives
e. Fitted speedo in the car and checked with GPS – readings are now perfect! Regardless of the W being slightly wrong to the gear ratio.
11. Pulling it back together. Fitting the chrome ring was the most difficult part for me. We did it in two persons, then it took me 15 minutes. The issue is you cannot rest the speedo against a flat surface to compress it with one hand, hold some punch device in the second hand and a hammer in the third hand. There is the odometer knob that sticks out, so you need to have a hole in the surface you rest the speedo against. Then when you start hitting the punch, speedo moves creating a danger knob is broken off the glass it holds to…. When someone holds the speedo for you and turns it, it goes fine. Start punching in one place, one punch, then 2nd one across it, then 3rd one mid way between the first 2, then 4th one across the 3rd one. Then fill the distances between the 4 places going across speedo every couple of punches.
Cees Klamper suggested better method of fitting back the chrome ring - crimping it with plyers, while protecting chrome from being scratched - more controllable and vibration free method.
12. Admire the results of your work or count losses and determine how to tell your wife/partner.
I was probably more lucky than I realize to have it done, but I prefer to think it was my skills 😉.
