Correcting a GTV6 speedo with a Jaycar kit

[Al Campbell]

I had the warped needle issue on speedo and tacho. Ended up replacing them with plastic right angle sections. Tried to recalibrate with a GPS but found that it read 85km/h when the car was doing 100.

I mad a Silicon Chip speedo corrector kit from Jaycar, but it wouldn't drive the Veglia speedo at all until I removed a zener diode that limits the output to a maximum of 8.2v. The ALFA sensor (2 wire type) has one side connected to 12v supply.

Still couldn't cancel out the 15% error. I ended up putting a small lead weight on the back of the needle boss (the black disc), positioned so that it is balanced when the needle is horizontal (50) and has the most effect when vertical.

Speedo now reads 1km/h high at 50, spot on ay 90 and 1/2 a km low at 100km/h.

AL.

[Al Campbell]

Some pictures. Note the damned acetone splash marks on the face of the speedo when I knocked the damned acetone bottle flying. Third one shows marks I used to help align the needle every time I removed it.

[Duk]

That's brilliant!
Surprised/disappointed the Jaycar corrector couldn't work, they are normally very good and versatile kits.

[Cool Jesus]

Just want to sound this out... Are you running stock rims and tyres? Not that I can imagine a change from stock would throw out the reading by 15%, bit still this may have accounted for some of the discrepency. Theres plenty of these applications on the net, Miata.net have a tyre calculator where you can enter the cars stock set and then enter the non stock size to see what difference it makes on speed readings. Just a thought?
Nice work on the speedo too by the way. Damn you acetone, you go to hell, you go to hell and die

[Anthony Miller]

The speedo on my 3.0l 75 was out by 15% with standard 14"rims and 195/65 tyres, when I went to 205/60 tyrse it cured about 3% of that error and now it's only 12% i.e 100 on the gps reads 112 onthe speedo. My wife's '09 Forrester is out by 10% as are most new cars that I sit behind in the right hand lane, this would explain the "what's your problem" looks you get from those who think they are doing the speed limit, but in fact are 10 kph under

[McAnnik]

  My 2 cents worth!   Speedo calibration corrections are better off carried out on a test bench. OK! I know you havn't got one,But any changes that are made to any speedo component, no matter how insignificant it may seem will alter your reading.The fact that a change was made to a speedo needle,a slight weight difference here is significent,and that is enough to alter the pressure on the needle return spring which will respond to any such mods as defined already in this thread.The set up of modern electronic/digital speedos and tachs is a delicate operation and does require a little skill and without proper equipment can mean many frustrating hours trying to calibrate an altered or recaltitrant instrument. The use of a GPS and continual roadtesting is a waste of time. It matters not that your instrument is the latest hi-tech digital wotsit you can buy, its not much good if its gives false readings (especially in hot conditions) by even worse aftermarket electronic gadgetary that promises to calibrate to the nth degree. Older Cable driven Instruments  are a little easier to contend with but not much because the problems are the same. the instrument looks simple enough but it isn't.Dont assume your almost as new ex 105 speedo that you just bought is accurate,any mishandling in its past life could well have knocked out even a small portion of internal magnetism meaning it will need to be re- magged and calibrated again after you have found out that it reads wrong.Dont always point first off to your latest choice of fancy rubber as being the culprit,your car may have had a diff change or even a speedo gear drive change although tyres will  run a very close second.  There are very few people left operating in the Auto Instrument Repair game these days, far fewer than in my time so I'm not sure where you would go to have your work done. If you take your instrument to a repair place they will only repair what you give them and to original specs,to recal to suit other tyre sizes requires some caculating to get it right and you may have to revisit his shop more than once.Is it worth it? The cop radar doesn't care if your speedo is WRONG! (Long before digital I used to calibrate copcars, and they had to be accurate!!).....Cheers

[Darryl]

There are very few people left operating in the Auto Instrument Repair game these days, far fewer than in my time so I'm not sure where you would go to have your work done.

Well - these guys are still around and have always had a good "rep": http://www.ottoinstruments.com.au/

[colcol]

My 33 needles, tacho and speedo were warped so much i used to get paralax error, that is, looking at it from anywhere other than directly in front, the needle won't line up with zero, [when stopped], so i had the needles replaced with Commodore needles by a reputable speedo repairer, and it seemed ok, until i drove through one of those local councils, 'what speed are you doing now?' speed indicators, and my speedo was about 5 kph out, this was before the days of GPS's so how else do you know?, their are about 3 of these devises on freeways in Victoria, but they are about an hour from home, and 50% of the time, they are not working, so i went home pulled the speedo apart and repositioned the speedo needle on the speedo shaft and drove back to the place where the council had set up the speed indicator to check the accuracy of my work, after about 2 hours of back and forth, i had the speedo reading within 1kph of the spped limit in that street of 50kph, years later when i got a GPS, and checked the speedo against it, my 33 speedo is 1 kph slow at 50kph, 2kph slow at 60kph, 3kph slow at 80kph, 4kph slow at 100kph and 5 kph slow at 110 kph, the gearbox and diff are original, the rims are for that car, the tyres are for that car, but you would find that the speedo when new was within the tolerance of accuracy required for the Australian Design Rules, its most likely over the years they loose their accuracy, long as you know, and check now and then your speedo versus GPS, Colin.

[Cool Jesus]

Bout time someone chimed in with some solid info. I'll keep that website on my favourites list.
Yeah the ADRs do account for a surprising amount of tolerance on accuracy, even in current vehicles. Can't be bothered looking at the figure, but I do recall it was surprising. I think the accuracy was more aimed at being closer to the mark near the more regularly travelled 60kph zone and as Col noted, then the tolerance wanes some what as the speed increases, although I believe the ADRs require that tolerance to show a faster speed, rather than slower. Just to make sure people aren't going harder than what the instrument is telling them.

McAnnik, I guess the question begs to be asked. What might be the consequences of Al's fix, if any?

[Al Campbell]

Car has been fitted with 16" Telephone dial wheels of a 156 and currently has 205/50 R16 tyres fitted. Theory/formulae says the circumference should be within 0.6% of the original 195/60 R15.

McAnnik, You make many valid points and yes it is dodgy trying to adjust and instrument with uncalibrated devices and especially "Free Apps"on a smart phone, however needs must.

This was a long term occasional project and in the interim I fitted a bicycle speed computer as a speedo. Hunted around for one that reads over 100km/h and takes wheel circumference in mm (Bloke in the Bike shop says he sells a lot for Motorcycles) - no guarantee of accuracy though. Pumped the tyres to a known pressure and rolled the car down along a flat surface for as many wheel rotations as I could get to come out with an average tyre circumference of 1,868 mm, which is about a 3% error over original. Tyres have only done about 1,000 km. Checked tyres on my Subaru too and I suspect that tyre size is not that close to theory. Why would it be? Cheap tape measure from Bunnings. But Bike computer agrees with 2 GPS (both Smart phones) - all within the first significant figure.

I first changed the needles to Aluminum pointers and tried to check the balance by hanging them from a sewing needle inserted into the speedo needles shaft and hung from a piece of cotton, to find they hung horizontal as near as I could tell compared a builders spirit level. Proves nothing much I suppose, but I suspect the originals were much heavier in the direction of the long side of the needles.

Remembering back to the dim dark ages of the first year of my Engineering degree, errors compound and reduce accuracy, so I cannot under any circumstance claim the accuracy I've stated. However 3 independent methods of measurement agree within 1 km/h. Doesn't mean it's right though.

I don't know if the diff has been changed, but I have receipts for various work carried out on the car (burnt valve repairs &c..) and no mention of diff swap. Does anyone know how many "teeth" there are on a 1983 GTV6 speedo transducer "wheel", for want of a better description? I thought it was 6. I have a digital multimeter with a frequency meter function and tried to measure the speedo sensor output for 6 teeth and a 41:10 diff, but I was off by miles so I'm wrong somewhere there.

As for the Jaycar kit not working at first. They have tried to make a universal device that caters for a variety of inputs and outputs. It has a default automatic input detection function and picked up the signal no problems as indicated by a flashing LED. The issue (I think) was that all the output circuits are based on a negative earth, i.e. sensor signal is referred to 0 volts, whereas the ALFA sensor is referred to 12 or 13.8 volts. The kit is limited to maximum rise of 8.2 volts which wasn't enough and either needed a either a higher voltage or a wider swing. I haven't measured either since I pulled limiting Zener diode. So if you want to build one of these all you need to do is not put in the Zener and it works as advertised.

For what it's worth I have the kit set for 110%, which has the odometer reading about 1% less distance than actual distance covered.

AL.

[Al Campbell]

Photos of Bicycle speedo botch.

First image actually shows another plastic needle attempt which looked too fat for my liking (changed the pointers three times now). Bike computer is inactive so is showing time in this picture. If you look above the 12 noon on the clock you can just make out a black lump which is a green LED I put in to shine on the bike speedo at night so I could read the thing. One of the reasons I tried to re-set the ALFA speedo was because this looks so awful, now I can remove this dodgy business.

Second photo shows the sensor cable-tied to the trans-axle tube and magnet glued to the axle hub. I actually drive the Jaycar kit and a "Command" brand cruise control off the ALFA sensor, so this can come off now. Cruise was necessary to hold the speed steady to set the speedo.

My god am I sick of undoing the 7 screws to remove the speedo and open it up.

Incidentally the Veglia tacho has a three spoked wheel at the back with what looks like lead each end. One piece is over twice the size of the others and they look like they've had bits added and removed with a soldering iron so I presume I've sort of copied how they calibrate it on a bench in the factory.

At the stage where I was fine tuning, I did think about doing it on the bench as my multimeter does measure frequency, but only down to 0.01 kHz, so I was thinking of an oscillator using a 555 timer followed by three 4017 decade dividers driving another 555 as a one-shot to make a short pulse. That way I could simulate a wide range of linear speeds to balance the needle then check on the road with the GPS. Only thought of this a late stage,  would have saved heaps of time.


AL.

[McAnnik]

 The repositioning of speedo/tacho original or replacement needles was a normal part of instrument calibration as was the slight relocation of the hairspring on the needle stem to compensate for any weight bias of components not forgetting it also has to return the innards to zero. On the bench this an easy task because you are making adjustments while looking at a test meter or tacho showing the actual operating revolutions of the instrument you are testing. Here is the basic layout of a testbench.Every one I ever used was a homebuilt unit because they weren't a thing you could buy readily.The drive for the bench was 240V small capacity rotory motor coupled to a variable drive two pulley belt system with ratios to run up to 3000 rpm or more.Electronic switching was used to double or treble the test readouts for high reading tachos and such,  It was usual to have two or three calibrated (in RPM) test instruments on the faceboard,one mechanical, one electronic at least one rotating output spindle to attach speedo drive cables and various wiring outlets for output signals from the also rotating speedo generators fitted to the whole shebang. Gearing of most speedo drives is set at 1000 Revs Per Mile,this equating to 60MPH or these days as we now use metric the drive is 1600 Revs Per Kilometre = 100 KPH.Some cars are different again but they can all be  catered for.The numbers are often found stamped on the case or printed very small on the dial These are the numbers used to calibrate at the speedo head.Odometers were driven direct on older mechanical speedos and geared to match those input numbers. on later electronic speedos the odo is driven by impulses given to stepper motors but again reliant on correct information coming up the wires from the sensors.If your cars gearing is not correct because of tyre sizes etc. (try a Jaycar kit). Sorry I wasn't much help!!!!

[Al Campbell]

It looks like the Veglia replicates a mechanical speedo in that there is a rotating disk inside a cup, which is connected to the needle and hair spring.

I assume that the motor is a basic DC, variable speed motor not a stepper as it only has two wires. I'm therefore also guessing that the drive electronics is a frequency to voltage converter and given that the Jaycar kit drove the Veglia with 0% adjustment (i.e. 1:1) then the Veglia is edge triggered.

I'm also guessing that the major component of the error is needle weight as my first replacement aluminum needles were "balanced". I think the latest plastic ones must be lighter at the pointer and (original counter weights are still there) as the tacho has a built in stop on the spindle - not a pin that the needle rest on like the speedo, With the latest plastic needle fitted to the tacho I calibrated it (roughly) using a very cheap analogue Dwell & RPM meter and when the engine is stopped the needle actually floats above the stop.

The Tacho could be entirely calibrated on the bench but because I couldn't work out the theoretical speedo pulses (measured tyre circumference, 41:10 diff but unknown number of teeth on sensor wheel) I needed to road test it to get a baseline frequency.

Did that bore everyone?

AL.

[Cool Jesus]

Don't short change yourselves guys, this is a good thread. Its certainly providing me some insight on speedos.
Al, you certainly seem to enjoy punishing yourself with this fix.
McAnnik, which speedos did you find to be the most reliable?
Would it be possible to retro fit into any of these old Alfas?