Speedo misread

[mejermy]

My E250D estate, LHD, WDB124186 type, has a speedo which consistently overreads by 20kph. Interestingly the speed markings on the dial begin at 20kph, and the hand moves as soon as the car does. Question answered, but not problem solved. The cable seems to be the same LHD to RHD, the speedo p/n is correct according to the EPC. I initially suspected the diff/box gearing, but they check out and I'm now sure it's something simple. So simple I can't work it out!!
Before I take the speedo to bits and possibly attempt to reposition the hand does anyone have any suggestions?

 

[millo777]

Surely there is a way to recalibrate the speedo? to account for different rolling diameter wheels for example, so it must be in WIS somewhere?

 

[BTB 500]

AFAIK all the OEM wheel/tyre setups would have the same rolling radius (within a percent or two).

 

[mejermy]

Hope this pic is visible. I am aware that there is a calibration takes place, but I have not found anything in WIS. A US site mentioned that the speedo/binnacle is labelled ex-factory with a number relating to the particular car's gearing. I suspect this is the number 770 on the label in the pic. I took these pics as a matter of course whilst swapping the interior and doing other work. The binnacle was the hardest to remove of 3 I've done, so I doubt it had never been out. Will take the thing to bits if I receive no suggestions, but prefer to make haste slowly.

 

[mejermy]

Wheels/tyres

Yes - although I've changed from steel to 8 hole genuine MB, they're the same diameter and therefore rolling circumference.

Thanks for your thoughts so far.

 

[Ali_H]

If the error is a constant 20KPH it is unlikely to be caused by an incorrect rolling radius or gearing as both of these would produce a percentage error, ie If it was 20 out at 100KPH it would be 10 out at 50KPH and 30 out at 150KPH and so on.

 

[mejermy]

Indeed it would. A uniform 20kph overread occurs throughout the speed range

 

[Dieselman]

It must be the needle position is incorrect on the spindle or the clock spring preload is incorrect.

 

[mejermy]

Thanks DM and everyone for your inputs. I was inclined to this view but didn't want to pull the instrument to bits without several second opinions. I'll rejig the needle position this weekend and post the outcome.

 

[mejermy]

Pulled the binnacle and then the speedo. Altered the needle position, replaced all and tried the car, and it's now perfect to within 1kph.

I didn't take pix, because binnacle out and speedo out have been much covered already and are on the WIS, but a tip I picked up from a US website worked for me. When you have the speedo out, lift the needle gently over (below) the stop and let it find its own stop position, where the spring tension allows it to rest. Mark that with a bit of tape and make your adjustments from there.

Hope this may be of use to others. Thanks to everyone for input.

 

[Dieselman]

a tip I picked up from a US website worked for me. When you have the speedo out, lift the needle gently over (below) the stop and let it find its own stop position, where the spring tension allows it to rest. Mark that with a bit of tape and make your adjustments from there.

A good tip.

I have a car that over-reads by 5mph, so may do this.
Last time I pulled a needle off a speedo the shaft broke off...never been tempted to do it again.

 

[mejermy]

The USA website referred to above advocated using two spoons on opposite side of the needle hub to lever it up, with lashings of patience. Also to protect the speedo face with a cloth. I used two small electrical tester type s/drivers placed under the brass collar of the needle hub where it slides over the shaft. Nevertheless, best left alone where possible. I agree it all looks very delicate.