Speedometer MK2 V8 Question
- Thread starter Jorgenson
- Start date
darth sidious
New Member
Jorgenson said:
It depends if the speedometer coupling/drive (i.e. the thing that rotates the inner speedometer cable) on the LT77 has a different ratio to the speedometer coupling/drive on the automatic. That is, assume that the car is travelling at 50 kph for example, does/would the inner speedometer cable turn at a different rotational speed with the LT77 than it does/would with the automatic?
If it is different, they generally recommend re-calibrating the speedometer.
I think there is (or was!) a method acquiring some data/numbers (counting the number of turns the inner speedometer cable makes for a given number of turns of the driving wheels, the diameter (or radius) of the driving wheels/tyres, etc), then plug that into a formula to get some overall figure to give the recalibration people.
However, I'm sure some (many maybe!) on here have done this change before and may know the figure off by heart, or know how to get round it!
There is another way of doing this.
In the UK, test centres for HGV (Lorry) Tachographs generally stock the components needed to drive a mechanical speedo / tachograph from an electronic sender on the prop shaft.
Since lorries are very individual in respect of tyre size, diff ratio, chassis length etc, it means that it generally isn't possible to have a "standard" speedo drive set up. So a common solution is to fit a hall effect magnetic transducer to the prop consisting of a light metal finger to act as the trigger along with a magnetic detector mounted to something stationary - eg the axle or back of the gearbox. This signal is then taken by cabling to somewhere convenient under the dashboard. Here a small "magic box" is mounted that takes the signal from the detector and converts it into power to a small electric motor which drives the "gearbox end" of a short speedo cable. The other end of that then couples to the existing mechanical speedo. The "magic box" can be adjusted so that the right number of prop shaft turns produces the right number of speedo cable revolutions to give a correct speedo reading.
This is a very easy set up to instal and doesn't need precise knowledge of how many turns of the speedo matches what number of turns of the wheels. The adjustment of the "magic box" is done on a set of brake rollers so that you can see visually that road speed and speedo reading are in sync.
I'm sure such centres must exist all across the Europe as the problem for lorries is the same in all countries.
Hope that gives food for thought.
Chris
In the UK, test centres for HGV (Lorry) Tachographs generally stock the components needed to drive a mechanical speedo / tachograph from an electronic sender on the prop shaft.
Since lorries are very individual in respect of tyre size, diff ratio, chassis length etc, it means that it generally isn't possible to have a "standard" speedo drive set up. So a common solution is to fit a hall effect magnetic transducer to the prop consisting of a light metal finger to act as the trigger along with a magnetic detector mounted to something stationary - eg the axle or back of the gearbox. This signal is then taken by cabling to somewhere convenient under the dashboard. Here a small "magic box" is mounted that takes the signal from the detector and converts it into power to a small electric motor which drives the "gearbox end" of a short speedo cable. The other end of that then couples to the existing mechanical speedo. The "magic box" can be adjusted so that the right number of prop shaft turns produces the right number of speedo cable revolutions to give a correct speedo reading.
This is a very easy set up to instal and doesn't need precise knowledge of how many turns of the speedo matches what number of turns of the wheels. The adjustment of the "magic box" is done on a set of brake rollers so that you can see visually that road speed and speedo reading are in sync.
I'm sure such centres must exist all across the Europe as the problem for lorries is the same in all countries.
Hope that gives food for thought.
Chris
darth sidious
New Member
chrisyork said:
Ah, that sounds like one particular solution!