Speedometer false reading

Other technical Issues with the Triumph 2000 range
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Grahame
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Speedometer false reading

#1 Post by Grahame » Thu Mar 30, 2017 9:38 pm

Hi
I have a mk1 estate that i have been working on for the past 9 months, when i got it the speedo was a bit rough looking and didnt read correctly it was reading under. I got a low mileage excellent condition replacement and stuck it in and have only now got the opportunity to drive it again and guess what, it still reads about ten miles an hour under the actual speed. Anybody here had the same problem or know of a cause? I know there is only the cable and the drive so it's not that technical but rather than replace parts at random i hoped someone might know where to go to firstly.
any help greatly appreciated
Grahame

johnnydog
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Re: Speedometer false reading

#2 Post by johnnydog » Fri Mar 31, 2017 12:05 am

Does it have the correct diff fitted? You haven't said whether you have a manual or an auto. Manuals were 4.1 whereas autos were 3.7. Given the age of our cars, it wouldn't be unusual for the diff to have been replaced by one of the wrong ratio.
The details are stamped on the flange on the underside of the diff which should identify it - the details can be found in the factory workshop manual.
Incidentally, I've assumed that your car is a 2000 (rather than a 2.5PI - if it was a PI, I'd be rather envious given the very low numbers still in existence!!)
Register Member no. 1596

1967 Mk1 2000 in Gunmetal Grey
1969 Mk1 2000 in Royal Blue
1970 Mk2 2000 in Valencia Blue
1972 Mk2 2.5 PI in Triumph White
1973 Mk2 2.5 PI in Sienna Brown
1976 Mk2 2500S in Carmine Red

tony
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Re: Speedometer false reading

#3 Post by tony » Fri Mar 31, 2017 5:25 am

What tyres are you running?
Tony.
1976 2500 TC. converted to S specs.

Lots of bits

1999 BMW Z3.
2006 BMW 325ti.
Hopefully not needing too many bits.




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Grahame
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Re: Speedometer false reading

#4 Post by Grahame » Fri Mar 31, 2017 8:35 am

Hi
It is a 1967 2000 with manual gearbox and a j type overdrive. The diff had major problems when I got it and this turned out to be two broken teeth on the differential gears. I rebuilt it with good condition used differential gears and reused the crown and pinion gears as these were excellent condition. I am 99 % sure that this was the factory diff assembly, I will check the numbers as I can't recall them off hand.
The wheels are the factory blades! And tyres are what I could get given they don't make the original size anymore. I know I could have got the white walls in the exact size but I don't like them on the car and they are expensive for a daily use vehicle. Again I'll get those numbers as well if it helps.
Were there different gear drives for the speedo take off on the gearbox?
Grahame

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Re: Speedometer false reading

#5 Post by johnnydog » Fri Mar 31, 2017 10:36 am

There are different drive gears for the speedo; the standard drive is 24 teeth, but there are alternative 23, 25 and 26 teeth drive gears described in the parts manual as for competition use (?)
They aren't difficult to remove and check.
You say you have a 'J' type overdrive, instead of the original 'A' type. Is that mated to the original gearbox or is the whole assembly from a later car?
You say that you couldn't get original size tyres (of presumably 175 13). I have seen cars fitted with 175/ 70 13 instead, but they visibly look wrong as they are smaller and as the rolling circumference is less, then the speedo will give an incorrect speedo reading.
A possible alternative is 195/70 13, which will give a more accurate speedo reading.
I am sure a search on here regarding alternative tyre sizes will reveal the experiences of others with different sizes.
Register Member no. 1596

1967 Mk1 2000 in Gunmetal Grey
1969 Mk1 2000 in Royal Blue
1970 Mk2 2000 in Valencia Blue
1972 Mk2 2.5 PI in Triumph White
1973 Mk2 2.5 PI in Sienna Brown
1976 Mk2 2500S in Carmine Red

pete lewis
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Re: Speedometer false reading

#6 Post by pete lewis » Fri Mar 31, 2017 6:33 pm

Chris witor has a usefull download for speedo specs
http://www.chriswitor.com/technical.php


Under the speedo oddometer roll is a number , this is the turns per mile
there are a number of calculations to arrive at this , but a simple way is

mark 52.8 ft down the road
release the speedo cable and stick a card counter on the inner .
push the car accuratley the 52ft ans count the turns of the cable

multiply the turns x100 and that needs to match the number on the speedo dial
around 1000ish

if the speed error is consistent you could remove the bezel and release the innards
hold the alloy disc and turn the needle on its spindle to recalibrate it a little
never remove the needle or the hair coil spring will unwind and then you are in trouble
pete
Pete Lewis
Luton
also area organiser for herts and beds tssc.

johnnydog
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Re: Speedometer false reading

#7 Post by johnnydog » Fri Mar 31, 2017 7:44 pm

That's interesting Pete!
I have always wondered what the 1000 figure meant at the bottom of a Mk1 speedo!
I have seen some speedos with 840 at the bottom; Chris' info has answered a question that I asked myself many years ago!
Register Member no. 1596

1967 Mk1 2000 in Gunmetal Grey
1969 Mk1 2000 in Royal Blue
1970 Mk2 2000 in Valencia Blue
1972 Mk2 2.5 PI in Triumph White
1973 Mk2 2.5 PI in Sienna Brown
1976 Mk2 2500S in Carmine Red

pete lewis
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Re: Speedometer false reading

#8 Post by pete lewis » Sat Apr 01, 2017 7:01 am

We 6sed to have a smiths recalibration machine , with humongous electro magnets
to re flux the drive disc, used to dim the factory lights, it went inthe skip when we closed .

I fitted a different speedo pinion and did a needle tweek on my mk2 with 14" wheels to get good results on the sat nav readings, without having to change the instrument

Its all a bit of a fiddle as the hysterisis of the hair spring and the drive from the magnetic disc are critical to give linear speed readings throughout the range
bear in mind the tolerance used to be 10%+4mph so you could read fast 70 at actual 60
but not read slow.
Pete Lewis
Luton
also area organiser for herts and beds tssc.

Grahame
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Re: Speedometer false reading

#9 Post by Grahame » Sat Apr 01, 2017 3:26 pm

Hi all
I went around the car and got all of the number's that i think are needed.
Starting with the gear box the number on the clutch operating shaft casting is MB146631 and it does have a j type overdrive there are some numbers on the housing between the gearbox and the overdrive but these are probably part numbers. 116561 and 88312305.
The differential number is MPB 13003
The tyre sizes are 165/80/R13 Falken brand
I didnt have time to pop the cable out of the drive at the back of the overdrive but as i had to use the car this morning i also noted as of today that sometimes but not all the time the needle wavers about 5 miles and hour, so i suspect the drive cable is a bit duff.
hopefully some of the information leads to clue as to what is going on!
If everything seems right then what if the speedo magnets were weak, or there was resistance in the needle pin/bushing could this cause under reading?
I was thinking about the tyre sizes and if the tyres were a bit under the original diameter surely this would mean an over reading?
Thanks to all for putting there thoughts in hopefully it wont take much to resolve the issue
Grahame

tony
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Re: Speedometer false reading

#10 Post by tony » Sat Apr 01, 2017 8:16 pm

GPS speedo ? :shock: :shock: :shock: :shock:
Tony.
1976 2500 TC. converted to S specs.

Lots of bits

1999 BMW Z3.
2006 BMW 325ti.
Hopefully not needing too many bits.




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