To my other GT6 pages

March 3, 2020


Speedometer and Tachometer


GT6 cars of course came with a speedometer, but also with a matching tachometer as an obligatory piece of sports car hardware.  They are simple mechanical instruments, not really designed or built for either longevity or repairability.

These rudimentary instruments both work on the same principle, and have a lot in common internally.  A cable with a spinning inner core is connected to the back of the instrument, and turns a spindle attached to a bar magnet.  The magnet is housed inside, but not touching, an aluminum cup.  The spinning magnetic field induces currents in the cup, which in turn generate their own magnetic field.  The two fields interact such that the aluminum cup wants to spin with the magnet.  But the cup is restrained by a small watch spring on its spindle.  The amount that the cup and its spindle can rotate is determined by the speed of the magnet and the resisting force of the spring.  The result is that the rotation angle of the cup spindle is proportional to the speed of the magnet.  A pointer on the cup spindle indicates the rotation, and indirectly, the speed of the input cable.

The speedometer also has a pair of odometers.  On my speedometer, the input spindle carries a brass worm gear, which drives a pair of spur gears.  Each spur gear has an eccentric shaft that imparts a reciprocating motion to a pawl arm.  Each pawl arm bears against a ratchet wheel, and advances the ratchet one tooth for each cycle.  The ratchet wheels drive the two odometers--the main odometer, and a resettable trip odometer.

My instruments were pretty cruddy.  They were also mismatched, with the speedo having an incorrect chrome bezel.  Since the car has been in storage since 1980 or so, it has to be a 70s replacement.

A simple test is to spin a small screwdriver inserted into the input spindle.  On the speedo, this resulted a little lurch of the pointer, which is a good sign.  On the tach, the input spindle appeared to be seized, which is not a good sign.

I worked on the instruments in parallel, so the pictures that follow might be from either the speedo or the tach.




The front bezel comes off bayonet-style, by twisting it until the bezel fingers reach slots in the housing lip.  It was quite a bit easier to say it than to do it.  The back of the bezels had the remains of some dried-up sealing substance.




Inside, there is an inner bezel that holds the glass.  A thin hollow rubber tube served as a gasket between the glass and inner bezel.




The guts of the instrument are freed up by two screws on the back of the case.




To really see much of the mechanism, the face had to come off, but first the pointer.  It is just a press fit on its spindle.  A couple of prying implements and some card stock to protect the face did the job.




Then the mechanism can be separated into the magnet part and the drag cup part.  The magnet is that long piece on the top of the assembly on the left.  The cup is underneath the assembly on the right.  At this point, the magnet spindle was still seized on the tach.




With some effort, I was able to get the magnet spindle turning, but it was tight and rough.   The spindle had to come out.  The magnet and spindle are held in the body by a keeper and two rivets.




Drilling out the rivets freed the magnet.  The tachometer had the brass worm gear for the odometers, even though it is not used for that.  It just retains the spindle.




It looked to me that the problem was that the magnet spindle just ran in the pot metal housing with no true bushing or bearing.  This surprised me a little, since the TR6 tach I worked on a few years ago had a bronze bushing in that location.

Since the bore was chowdered up pretty well, I decided I could fix the tach and improve the design all at the same time.  I'd just put a bronze bushing in there.

I didn't really have much choice on the size of the bushing.  In drilling out for the bushing, that little nose protruding above the threads would pretty much be gone, but since it looked like it was a necessary feature for the cable seating, I'd have to replace it.  The simplest way I could see was to just let the new bushing be the nose.   This set the OD of the bushing at 3/8".  The ID had to match the spindle, which was 6 mm.




I didn't think I'd have much luck finding a bushing with a metric ID and an Imperial OD, so I just ordered some 6mm x 10mm Oilite bushes, and turned the OD down to 0.376" to be a tight fit in the reamed 0.375" bore.  I couldn't find the length, so I just pushed in two, and trimmed the excess.




Then inserted the magnet spindle with a nice coating of light oil, and riveted it back in place.




Then put everything back together.  The pointer is pressed on so that when at rest, it aligns with a small dot below the zero on the face.  Then the pointer stop is retracted so that the pointer can be rotated past it.  This provides the small amount of preload to the spring so the pointer bears lightly against the stop at rest.




To bring the speedometer to this point was a little different.  The internals of the speedometer work the same as the tach, but the added odometers dominate the construction.




On checking the rotation of the input spindle, it felt very smooth and free, except very occasionally, I could feel a mechanical catch in it.  It seemed to be related to one of the odometer pawl gears, so I took all of those parts out for cleaning and inspection.  They had a lot of grease on them for some reason.




I finally traced the problem to one of the gears that had a split in it.  The gear was still firmly attached to its shaft, but the split spread two of the teeth apart slightly, so the mesh would bind in that area.  Without much hope of finding a good replacement part easily, I just reshaped the teeth slightly with a file so they still meshed, but didn't bind.




There was also the little green jewel for the oil light that I found loose in the case.  I was apparently just glued in place.  I put it back where it belonged, and staked the back with a hot soldering iron.




Then there was the issue of the mileage reading on the main odometer.  Given that the speedometer is not original, plus the fact that it was not even working for most of the time I drove the car, the reading has no relevance to this car.  Also given that the car will be for many practical purposes a new car when finished, I justified my desire to return the main odometer to zero.

One way to zero an odometer is to take it apart and reassemble it with all zeros showing.  I did this for my TR6, and found the process to be harrowing.  I wanted an easier way.  I could spin the ratchet wheel by hand either forward or backward, and change the reading by about 32 miles in a minute.  I did the math, and decided this was maybe doable, but not practical.  I finally fiddled with it and inadvertently stumbled on a way to get individual wheels to turn independently.  I can't explain how I did it because I really don't know.




So, then with both instruments apparently working smoothly, I wanted to have some confidence in their accuracy.  This is a calibration rig I made years ago when I did my motorcycle instruments, and used again for the TR6.  It is basically a universal motor (it is a Craftsman chainsaw sharpener motor, modified to run backwards) on a speed controller, an optical digital tachometer to read the speed of the motor, and a stand to hold the instrument and everything else in the right relationship.  The motor drives the instrument spindle directly.




I did the speedometer first.  Speedos of this type will normally have a number printed somewhere on the dial face that indicates how many turns of the input spindle it will take to rack up one mile on the odometers.  A little thought and arithmetic will show that this is also the input RPM that should indicate 60 MPH on the dial.  On my speedo, this number was 980.

So, by taking readings at various input RPMs, it is easy to graph the results and compare to what the readings should be.

The black line in the graph below shows the perfect relationship between input RPM and dial reading.  The blue line is what my speedometer initially showed.  It was around 18% low across the range.




This is what is known as a "slope error".  The other common kind of error is a so-called "offset error", where the error is a constant number across the range.  The fact that the blue line above is pointed towards zero is an indication that there is little offset error here.

The slope of the line is determined in this kind of instrument by two things:  the strength of the spring, and the strength of the magnet.  Either of these can be manipulated to change the slope.  I chose the magnet.

A stronger magnet will steepen the slope.  Magnets can be strengthened by bringing a stronger magnet in close proximity.  I stacked some rare earth magnets and added some steel extension pieces.  I hoped the extensions would allow me to contact the instrument magnet without taking everything apart again. 

It's also important to know the polarity of both magnets.  I found that by using a magnetic comapass.




It seemed to work OK.  In fact, it worked so well that after the first "strengthening" attempt, the speedometer was reading almost 90 MPH when it should have read 60.  OK, a little too much of a good thing.  Luckily, reversing the polarity will reduce the strength of the weaker magnet.

It took a few iterations, but I was able, partly by luck, to get the red line in the graph above.  The speedo read about 3% low at the high end and slightly high at the low end.  I would have been very happy with 5%.

I repeated the process for the tach, and the results are shown below.  Since I don't really rely on the tach that much, I was satisfied with this.




OK, so I was pretty happy with the way the instruments were working.  All that was left was to button them up.  I blasted the cases...




...powder coated the outside...




...and painted the inside.  I matched the blue color the best I could.  I think it was an attempt to even out the light intensity around the perimeter of the face.




Added the little plastic tubes that isolate the indicator lights from the face lighting, and the rubber isolator for the attachment points.  The original isolator was usable, but barely.  The third picture shows the little white rubber sleeve thingies that were on the ends of the light tubes, apparently to bridge the gap to the jewels.  I found two of the four had buckled over and were blocking light rather than guiding it.  I didn't replace them.




Brought the guts home.  The 28 miles on the speedo was what it took to get the calibration rig set up and the speedo adjusted right.




Then prepped the inner and outer bezels. 




Inner bezels were painted, outer ones were powder coated for durability.




This stuff is O ring cord stock  It is silicone, and pretty soft.  I got it to see if it could replace the seals on either side of the glass.




The smallest size worked well for the underside of the glass, while the largest made a good seal under the outer bezel.  I wiped it with a thin smear of petroleum jelly to make twisting the bezel on a little easier.




After all this, I think they will do.




This one was finicky work.  I enjoy it to a point, but I'm ready to move on to something that doesn't require tweezers.

The only cost was for the bushings, the cord stock, and some consumables.  Certainly under $20.

Comments to Ed at: mailto:elhollin1@yahoo.com


To my other GT6 pages