TR2/3/3A Speedo WAY off...

[doc50]

On my (71 Revcon) motorhome I installed an adapter in the speedo cable to calibrate the values over a wide range. It works great and they make different ratios. It's an easy way to correct speed readings.
My TR3 reads about 20% low and I wonder if they make such an adapter for our cars? That would be a much easier fix than resoldering whatever springs, IMO (which, according to the articles I've see, only corrects for a chosen, target speed). Anyone seen such an item?

Thom
1959TR3
#TS34909L(O)

 

[Geo Hahn]

I think you will have a hard time finding a ratio adaptor that will fit the threads used on the TR cable -- but what some have done is have a pair of shorter cables made with the TR fittings on each end and the middle fittings in a size commonly available on adapters.

But before you go to that much trouble you might try a single point calibration by simply re-positioning the needle at a mid-range speed, say 50 MPH, and see what that looks like overall.

 

[CJD]

Also...check the odometer for accuracy. If it is also off the same 20%, then an adapter will work. If it is within a couple percent, then the adapter will fix the speed but ruin the mile indication. In the latter case, you are merely treating a symptom instead of fixing the problem (weak magnet).

Spring rates do not change unless the spring is damaged, so you rarely have to mess with the needle spring when calibrating a speedo.

 

[doc50]

Geo, is there anyone who makes these cables? Also, how to 're-position the needle'?
John, odo is off also. The system seems to just be sloooow overall.

Thom
1959 TR3
#TS34909L(O)

 

[CJD]

Sounds like your rear end and/or tires were swapped, then. Adapter would be the easiest, but I don't know of any...

 

[TR3driver]

It's been a few years back, but my local speedo shop said "No Problem". Just bring them the old cable, so they could get the threads right and for about $100 (then), they would build whatever ratio I wanted. Probably not the same threads on the ratio box, but that price included new cables from transmission to speedometer. I'm sure West Valley Instruments https://westvalleyinstruments.com/ or North Hollywood speedo https://www.nhspeedometer.com/ could do the same thing today (though the price might be closer to $150 by now).

But before doing that, I would make sure it is a ratio problem. Are the odometer and trip meter also off by 20%? If the odometer is accurate, then your problem isn't ratio but speedo calibration. The original speedo I got with TS13571L read high by around 20%.

There are various ways to recalibrate, I used an ad-hoc AC electromagnet to weaken the flying magnet in the speedo. As you can see, just a few turns of hookup wire wrapped around a bolt, and my old Weller soldering gun for a low voltage, high current AC source.


As it happened, I weakened the magnets a little too much (who knew it would work so well?), and had to remagnetize. Another ad-hoc electromagnet, DC this time, using a capacitor bank to give a big jolt of current (but too short to burn up the wires).

(The batteries visible in the photo wound up not being used, instead I used the bench power supply to charge the caps.) That took it back to too strong, so I used the degaussing tool again but didn't get as close with it.

The result was darn near perfect. I used a hand-held digital tach from HF to check several speeds on the lathe, and then used the lathe to turn the speedo head so I could easily check at various speeds. The chart I made up matching lathe speed to indicated speed is visible in the first photo above. I worked back and forth between the 24 mph and 65 mph while adjusting magnet strength, then when those indications were good, I double-checked at 40 and 108. IIRC it read just a smidgeon high at 108 (about 110 indicated), but that is plenty good enough to suit me.


Of course, all that is just working with the tools at hand. Lots of other ways are probably better suited to what you have on hand.

Here's an article showing how to get it apart, and move the needle https://app.box.com/shared/embyus13g9

Someone else used a permanent magnet to improve the flying magnet https://bullfire.net/TR6/TR6-40/TR6-40.html

 

[TRopic6]

Thom,

What's the 4 digit number under the odometer? Is it 1180 or something higher? The magnetized bar could have weakened somehow too.

Jeff

 

[karls59tr]

I think you will have a hard time finding a ratio adaptor that will fit the threads used on the TR cable -- but what some have done is have a pair of shorter cables made with the TR fittings on each end and the middle fittings in a size commonly available on adapters.

But before you go to that much trouble you might try a single point calibration by simply re-positioning the needle at a mid-range speed, say 50 MPH, and see what that looks like overall.

How do you re-position the needle at a mid range speed?

 

[Lbp]

I found this on the net: "Repairing Jaeger & Smiths Speedometers". https://obswww.unige.ch/~wildif/cars/docs/Smith-jaeger_speedo_repair.pdf. I've found it to be quite helpful.

Bill

 

[Geo Hahn]

How do you re-position the needle at a mid range speed?

That link (above) to Tony Rhodes' document covers this nicely.

I loosely place the speedo in the dash with just the cable attached & drive while my wife logs indicated v actual with a GPS. We pull over in a handy church parking lot (week day) and I can remove the unit and tweak the needle w/o getting out of the driver's seat. Rinse repeat.

Not a fix for a unit that is way off with a non-linear error but can improve things in many cases.

BTW - I have heard of springs being weakened when an engine lacked a good ground and heavy current was drawn thru (among other sources) the speedo cable. True or not, I cannot say.

 

[TR3driver]

BTW - I have heard of springs being weakened when an engine lacked a good ground and heavy current was drawn thru (among other sources) the speedo cable. True or not, I cannot say.

That's an interesting thought, I hadn't heard that one before. Certainly sounds plausible.

BTW, the link I gave to Tony Rhodes' speedo article is a slightly newer version (2002 vs 2000). Not sure what happened to his web site, it used to have a lot of other good info as well as the speedo article.