Spredometer reading acting wierd.

[Paul est]

Hello. I bought a 1996 year Transit. The speedometer is acting weird. If I drive 80 km/h at the beginning it works well but after 10 or 20 km it starting showing 60km/h and jumping around. It flickers a bit all the time. Even if my speed is constantly 80 km/h. It's a manual speedometer. I changed the speedometer wire but it is acting the same. The wire is not original, it is an aftermarket spare part and does not fit very well on the speedometer side. (not locking correctly). Can it be the cause of the problem? I also opened the plastic gear (blue) in a gearbox. It seems to be ok. Can it be that inside the gearbox is wormgear, what is thrive the blue gear, is slipping if it gets hot (is it even possible)?

[MinorMatt]

The aftermarket cable is probably a mk3/4 one.

A lot of speedo issues are due the the nut on the output flange of the gearbox. You need a 12 point, 30mm deep socket and it needs to be done up very tight (ideally with an impact gun)

[Paul est]

The aftermarket cable is probably a mk3/4 one.

A lot of speedo issues are due the the nut on the output flange of the gearbox. You need a 12 point, 30mm deep socket and it needs to be done up very tight (ideally with an impact gun)

Thanks for your answer. Not sure what nut you mean. I have a fork there.

[Paul est]

I don't know how to post photo. sorry.https://drive.google.com/file/d/1IxHfk5sRSGXAmwMamN4Qa6fR_wLC5BJn/view?usp=drive_link

[Keef]

imageproxy.jpg

[Paul est]

Ok, thanks. I will check it.

[dumper]

Had a similar problem on a Ford speedo years ago I think the needle is moved by a spinning magnet we had ordered a replacement head before doing the job on striping the dash it turned out to be some debris stuck to the magnet cleaning it out cured it .
This was probably on a mk2 escort or transit so maybe not the same setup.

[cherniy_chack]

Dirt on the speedometer magnet often causes the instrument needle to jerk. I've had this happen twice.

[Paul est]

I did opened the speedometer and it was all clean and nice.
Is it normal that the wormgear is not fixed and you can turn it around with a finger? After tightening that nut it went a little bit tighter but it is still possible to turn it around.

[Paul est]

The aftermarket cable is probably a mk3/4 one.

A lot of speedo issues are due the the nut on the output flange of the gearbox. You need a 12 point, 30mm deep socket and it needs to be done up very tight (ideally with an impact gun)

Thank you. Problem solved. At least after a 10km drive it worked perfectly.