Jump to content
Sign in to follow this  
kiwi-jim

Speedo reading wrong

Recommended Posts

Hi Everyone 

Happy new year, is there any "home hack" you can do to the speedo to get it more accurate.   currently reads 80 mph when you are doing 60 mph on the gps,    Need it reading more accurately for the new registration compliance.   Or do I just have to get it recalibrated?

cheers jim 

Share this post


Link to post
Share on other sites

That’s a big gap Jim!

is it tyre size related or other reasons due parts or upgrades ?

Share this post


Link to post
Share on other sites

Have a look at Section J7 of the Technicalities CD, which has an article which I wrote in Spring 1977, and make a note of:

- the number written on the speedometer (will lie between 1100-1500)

- rear axle ratio

- tyre size (rear, of course!)

With this information, you can do the sums and determine where the anomaly lies.

Ian Cornish

Share this post


Link to post
Share on other sites

The needle is only an interference fit on the spindle. It may have slipped round. It's not a major engineering operation to correct it, it needs care as the components are delicate. Just lift it off and replace in the right place!

 

james

Share this post


Link to post
Share on other sites
Just now, james christie said:

The needle is only an interference fit on the spindle. It may have slipped round. It's not a major engineering operation to correct it, it needs care as the components are delicate. Just lift it off and replace in the right place!

 

james

If it has slipped it may be possible to simple 'slip' it back rather than removing and refitting.

 

Roger

Share this post


Link to post
Share on other sites
40 minutes ago, RogerH said:

If it has slipped it may be possible to simple 'slip' it back rather than removing and refitting.

 

Roger

Would it not 'slip back' again though Roger?  It could read 90 mph next time!

Maybe as Ian says it is more about ratios?

Kevin

 

Share this post


Link to post
Share on other sites

In my eyes there are two routes to go - depending on HOW the displayed speed is going wrong.
The position of the needle is defined by magnetic forces (rotating disc, driven by the speedo cable) and a little return spring (like a spiral).

If the deviation is a constant value (always showing 20 mph too much), this could be corrected by moving the needle 20 mph "back" - carefully,

If the deviation is a constant factor (always reading 25% too high) then the magnetism is too strong (less probable) or the return spring got weaker by time & age.
In this case the magnetism should be reduced or the spring strengthened (exchanged): Both a specialists job.

 

Regards, Johannes

 

Share this post


Link to post
Share on other sites

As I suspected this is sounding like a specialist technical job. As the 25% is pretty consistant.

   In terms of have I changed anything,  like tyre size , diff ratios etc,  the answer is no, that has all stayed the same during my rebuild.

Ian the technicalities CD's are something we don't have accesss to down here unfortunately.

We do have a specialist rebuild company down here  so  I will contact them , thought I would see if there was a quich option available so to speak.

As ever thanks for the advise and  suggestions.

cheers 

Share this post


Link to post
Share on other sites

Have you checked the odometer against your GPS ?  if you are clocking up the correct mileage, then the speedo is correct for your car, it just needs re-calibrating, this is done as mentioned above by adjusting the strength of the spinning magnet.  Not much you can do to the hair spring, & that is unlikely to have changed.

If the miles covered is less than your odometer reads then you have the wrong speedo for your car.  If it is a standard TR4 then the small number on the speedo dial (under the trip readout) should be close to 1180

Bob.

Edited by Lebro

Share this post


Link to post
Share on other sites

All,


For most of my life of playing around with cars I could never figure out how to calibrate a speedo. I knew they worked by the magnetic coupling of a magnet and an ally disc, and so I (wrongly) assumed that it was all to do with the gap between the magnet and the disk. I bent a few in experiments before I realized that the answer must lie elsewhere.


Only in the past year, with the help of the internet, did I discovered the truth.
The following pages explain how to calibrate a speedo for both over and under reading.
It all seems obvious in principle. But I wonder how many people actually do it themselves in practice ???


http://tinyurl.com/jj6277y


Charlie.

 

Share this post


Link to post
Share on other sites

Well I used the magnet stroking method last year on a low reading rev counter on a friends Morgan (TR engine) a bit hit & miss, but got it right after a few tries.

Bob

Share this post


Link to post
Share on other sites

Just checked the number on the speedo and it is 1184. so it's correct for the TR4

so then I checked my TR3, it has always read wrong as well and it is number 1180.   So next step is I will switch speedos  and see if that reads any better and if so try that to see if that will get me through this speedo test  that's part of registering a new vehicle on our roads.

Ian can you advise me what is the correct number range for a TR3 speedo..

cheers

Share this post


Link to post
Share on other sites

Ian,

Not quite what you want, but a useful reference.

http://britishautosalvage.com/gauges.html

Bob,

You said:

“…a bit hit & miss, but got it right after a few tries. …”

Impressed !!!

I intend to build a “Gausing/Degaussing machine” just for the fun of it. I’m using old XJ6 instruments on my Bitza TR3a. The rev counter is an easy mod. The speedo a bit more difficult.


Charlie.

 

Share this post


Link to post
Share on other sites

Hi Charlie,

         have you dabbled with de-Gaussing before.

A simple de-Gausser can be made with a good length of mains cable (single core ) wound into a big coil with a hollow centre (air core).

When on this will produce quite decent magnetic field inside the core. If you slowly pass a magnetized item through the core it will demagnetize.

To magnetize place item in the core then switch off.

 

Roger

Share this post


Link to post
Share on other sites
3 hours ago, kiwi-jim said:

Just checked the number on the speedo and it is 1184. so it's correct for the TR4

so then I checked my TR3, it has always read wrong as well and it is number 1180.   So next step is I will switch speedos  and see if that reads any better and if so try that to see if that will get me through this speedo test  that's part of registering a new vehicle on our roads.

Ian can you advise me what is the correct number range for a TR3 speedo..

cheers

The difference between 1180 & 1184 is negligable - do the maths  60 MPH * 1184 / 1180 = 60.2 MPH

So maybe you have an odd diff ratio ?

Bob.

Share this post


Link to post
Share on other sites

Roger,

I think we used to degauss things when I worked on the EMI-scanner, but it was a long time ago, and I’ve forgotten most of what I did there.

I expect I’ll spend a week playing around with magnets and bits of wire, get bored with it all and convert the speedo to electronic, with a pick up from the propshaft bolts. (This is why my TR is still a long way from being on the road. Always “Playing around”, not really “Getting on with it”.)

Charlie.

Share this post


Link to post
Share on other sites
On ‎1‎/‎26‎/‎2019 at 4:59 AM, kiwi-jim said:

tuHi Everyone 

Happy new year, is there any "home hack" you can do to the speedo to get it more accurate.   currently reads 80 mph when you are doing 60 mph on the gps,    Need it reading more accurately for the new registration compliance.   Or do I just have to get it recalibrated?

cheers jim 

My 4A had the same problem. Had it serviced/recalibrated by JDO Instruments. Had to take a number of measurements for calibration. Detail is on their website. Speedo is now reading spot on against sat nav.

Alan.

Share this post


Link to post
Share on other sites

Early TRs might well have been sold with cross-ply tyres, and I think these would have been slightly larger than the radials which were fitted to most by the time TR4s were the current offering.  This might account for the 1180/1184 figures.

Ian Cornish

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×

Important Information

By using this site, you agree to our Terms of Use.