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TR4A tacho and speedo under reading


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My slightly basket case 4A back on the road after a 15 year slumber and things are settling down nicely the more it's driven. However, the tacho and speedo are not really catching up with the rest of the car - both under reading. Guessing that the magnets have become a little demagnetised after years of stuck in the same position. I've managed to get some oil down the tacho cable but no difference. Any other possible causes?

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Hello Roger

Tyres - the tacho/speedo were originally fitted to the car when crossplys were the norm. Now people run radials which have a different rolling radius. If both are under-reading then not the magnets. You can adjust the needle position of the speedo. The speedo can be checked against a gps or satnav. Personally I would try to do some research to try to work out what effect the tyres size are having on the readings before taking things to bits.

The speedo will have the number 1185 on the face if it is the cross ply original version. At 60mph in top gear (no overdrive) the rpm is 3000. If that is the case then the instruments are reading in agreement. My speedo reads high. Not a problem because if I believe the readings then I am always below the speed limits.

Also putting oil down the cables not a good idea. It usually ends up in the instrument and causes more problems. Different people will have different ideas but I pull out the cable, wipe it clean and then wipe with a lint free cloth or paper towel with thin oil on. Just enough to lubricate the surface with a thin (microscopic) layer. This is sufficient. Also make sure that the cables don't have any sharp bends.

Keith

Edited by keith1948
correcting a mistake
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The attached table is helpful. 

The Revs/Mile column shows the figure which appears on the TRs speedometer.

I can't recall how I acquired this table - I seem to have collected rather a lot of stuff in 56 years of TR ownership!

Ian Cornish

TR Speedometers cropped.jpg

Edited by ianc
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12 hours ago, keith1948 said:

Hello Roger

Tyres - the tacho/speedo were originally fitted to the car when crossplys were the norm. Now people run radials which have a different rolling radius. If both are under-reading then not the magnets. You can adjust the needle position of the speedo. The speedo can be checked against a gps or satnav. Personally I would try to do some research to try to work out what effect the tyres size are having on the readings before taking things to bits.

The speedo will have the number 1185 on the face if it is the cross ply original version. At 60mph in top gear (no overdrive) the rpm is 3000. If that is the case then the instruments are reading in agreement. My speedo reads high. Not a problem because if I believe the readings then I am always below the speed limits.

Also putting oil down the cables not a good idea. It usually ends up in the instrument and causes more problems. Different people will have different ideas but I pull out the cable, wipe it clean and then wipe with a lint free cloth or paper towel with thin oil on. Just enough to lubricate the surface with a thin (microscopic) layer. This is sufficient. Also make sure that the cables don't have any sharp bends.

Keith

Thanks Keith, that's interesting I have non standard wheels (American Racing 'Silverstone',  to which I have fitted 'Continental' tyres), but I feel that the under-read and erratic needle swings might be due to something mechanical in the units. The speedo maxes out at 40 mph, even though i'm going faster and the tach won't go past around 2k RPM. Take you point about cable oiling -will drain it back out.

Cheers,

Roger. 

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49 minutes ago, ianc said:

The attached table is helpful. 

The Revs/Mile column shows the figure which appears on the TRs speedometer.

I can't recall how I acquired this table - I seem to have collected rather a lot of stuff in 56 years of TR ownership!

Ian Cornish

TR Speedometers cropped.jpg

Thanks Ian, this is useful. Not sure what axle ratio I have, the car is a solid axle 4A re-import from USA, non overdrive.

Cheers,

Roger

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Ok that's rather different info from your first post Roger and it does sound as though the mechanisms are not working properly rather than being just a bit slow. It could be that internal lubrication is needed. This link covers overhauling them:

http://bullfire.net/TR6/TR6-40/TR6-40.html

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1 hour ago, RobH said:

Ok that's rather different info from your first post Roger and it does sound as though the mechanisms are not working properly rather than being just a bit slow. It could be that internal lubrication is needed. This link covers overhauling them:

http://bullfire.net/TR6/TR6-40/TR6-40.html

Thanks Rob, that's a very concise set of instructions

Cheers,

Roger..

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For method to determine rear axle ratio, see my article in section E2 of Technicalities CD (original written written some 40 plus years ago!) - it includes the table which I pasted above.

Ian Cornish

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  • 3 weeks later...

I have a problem with my speedo under-reading by about 10 mph but when you reach 60/70 it becomes a bit more accurate. Once I have installed new tyres, 165/80/15, I am sending it off to Speedograph who will service it, re-calibrate it and return it for about £75 all in. To give them the correct information you have to roll the car over a set distance and count the number of revolutions of the tyre and fit a cardboard pointer to the speedo drive cable and count the number of revoutions over that distance. Full details are on their website. I thought the price was reasonable compared to the possibility of a speeding fine!

Andy

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20 hours ago, Ocheye said:

I have a problem with my speedo under-reading by about 10 mph but when you reach 60/70 it becomes a bit more accurate. Once I have installed new tyres, 165/80/15, I am sending it off to Speedograph who will service it, re-calibrate it and return it for about £75 all in. To give them the correct information you have to roll the car over a set distance and count the number of revolutions of the tyre and fit a cardboard pointer to the speedo drive cable and count the number of revoutions over that distance. Full details are on their website. I thought the price was reasonable compared to the possibility of a speeding fine!

Andy

I got a quote of £85 + VAT + £12.95 P&P from Speedy Cables but not asked for the information you mention, maybe they just recalibrate to original specifications.

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