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Speedometer problem

Posted: 22 Nov 2020, 10:02
by TONYT25T25
Speedometer playing up again, I have unclipped it from behind the binnacle and put a few drops of 3 in 1 oil down it, but it is still not right, sometimes it rises in excess of what the speed should be sometimes sticks at a speed even when van has stopped, reversing resets the needle and after a short time when parked up it also drops to where it should sit, MOT soon so concerned it would be picked up as a failure.  Not sure if lack of use is the problem or is it the cable or unit. Wiggling the cable sometimes when driving moves it but still not satisfactory, any advice would be welcomed.

Re: Speedometer problem

Posted: 22 Nov 2020, 10:29
by MidLifeCrisis
I would suggest that if the speedo is sticking at a speed even when the vehicle is stationary then that points to a problem with the speedo unit (rather than the cable that goes to the wheel)

The below website has a nice explanation of how a mechanical speedo works - and suggests that the speedcup/spring/needle is somehow sticking - not sure how easy it is to fix - that unit is quite sealed up to prevent mileage tampering I guess - might be easier to look for a replacement;

https://www.explainthatstuff.com/how-sp ... works.html

I had similar experience which only seemed to occur in cold weather ! Which I assumed was something related to oils getting more sticky or something;

FYI - looks like an ‘impaired operation’ speedo is an MOT minor, and a ‘not working’ speedo is a major - so might depend on garages interpretation?!?

Re: Speedometer problem

Posted: 22 Nov 2020, 10:31
by Mocki
I am pretty sure they do not test ( nor can they ) the speedometer for MOT .

Sounds to me like it’s the actual dash item rather than the cable in this instance from your description
Maybe worth very gently taking it apart and having a look see .

Re: Speedometer problem

Posted: 22 Nov 2020, 10:40
by TONYT25T25
Thanks for replies, mine also plays up in the colder months as it appears ok in the summer, just wondering to narrow down area if it is possible to stick something behind the speedo unit to simulate rotation of the wheel to see if it rises and drops, after reading the wiki which pointed to the nearside wheel as a possible area to look at.  Found this video on you tube which I may also pursue, or leave it until after the MOT and see what happens then to give me more time.  https://www.youtube.com/watch?v=_pDzWH0GebQ

Re: Speedometer problem

Posted: 22 Nov 2020, 13:00
by TONYT25T25
Temporary fix but not solution, I have managed to get it working properly of sorts by placing a piece of high density foam between the cable and where it passes alongside the brake servo drum this allows it to operate again without stressing either part.  Will leave this in place for time being until I can suss out if it is the cable itself having to much of a bend in it or the speedo, not ideal but sufficient for now I think.

Re: Speedometer problem

Posted: 22 Nov 2020, 13:34
by AngeloEvs
With the vehicle stationary, the needle can only stick if the shaft that the needle sits on is somehow interfered with. There is a coil spring that should be returning the shaft and needle to 0.

The needle shaft is not physically connected to the actual speedo drive mechanism as it uses induction to rotate but if the drive unit is not secure then it could cause the shaft to be off centre and cause it to stick. Putting pressure on the cable to be in a particular position could be putting the drive unit into the correct position which is why it now functions as it should.

I would remove the speedo, rotate the needle and check it returns to 0 freely, also checking the securing screws that hold the drive unit are tight.

Re: Speedometer problem

Posted: 22 Nov 2020, 15:12
by TONYT25T25
Thanks for in depth explanation, I wonder if removing the speedo cable from the back of the unit and replacing it has somehow misaligned something, where are the securing screws that hold the drive unit located. Not sure what or where the drive unit is??

Re: Speedometer problem

Posted: 22 Nov 2020, 16:41
by AngeloEvs
Sorry, I should have added that the securing screws that hold the speedo drive machanism can only be accessed by stripping the speedo down (removing the needle, fascia plate, spring and odometer assembly).

Re: Speedometer problem

Posted: 22 Nov 2020, 17:29
by TONYT25T25
Thanks, may have a go at that later on, in meantime hope existing repair lasts.