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Speedometer calibration

PostPosted: Sun Jul 25, 2021 10:56 pm
by Quart Meg Miles
Thirty years ago I recalibrated my speedo, I can't recall why, and it has indicated about 10% fast ever since and I know now that I put the pointer on too far behind the stop. Having decided to correct it, at last, I have no idea what I adjusted to change the gap between magnets and drag disc (which drives the pointer) and assume I need to increase it. I have a memory of adjusting a lock nut which controlled end float but I don't recognise any of it.

Also, in the last few years, the needle has been reluctant to move off zero in cold weather. This picture shows where the spindle comes out at the top (between the two 5s) and I've sparingly touched sewing machine oil on it where it emerges.

Speedo pointer spindle 204_8957.JPG and

What does the little brass screw do (just this side of the spindle)? It seems to pull up on the similar brass bar below it, but I can't see why that's there unless it supports the spindle. I've tightened it 1/8 of a turn hoping to stop the cold weather jam and perhaps reduce the over-reading.

Any ideas gratefully received, thanks.

Re: Speedometer calibration

PostPosted: Mon Jul 26, 2021 1:49 pm
by lotusfan
(1.19 MiB) Downloaded 55 times

Hi Meg

The attached, from here at some point I think, may help.

Re: Speedometer calibration

PostPosted: Mon Jul 26, 2021 11:53 pm
by Quart Meg Miles
Thanks a lot Mike,

Reading a few pages I appear to have the Intermediate version. His "calibration" is getting one speed correctly indicated whereas I was hoping to change the whole scaling by seperating the drag cup (disk) more from the magnet, which doesn't seem to be an option. I wasn't aware of stressing the spring when I first had it out so can't understand why it started over-reading. At least I know how it all fits together now.

I have the Tacho out as well but that's another story!

Re: Speedometer calibration

PostPosted: Tue Jul 27, 2021 8:58 am
by lotusfan

If you have tacho preblems this a good modification.

Re: Speedometer calibration

PostPosted: Tue Jul 27, 2021 11:53 pm
by Quart Meg Miles
Round two, seconds out! Well, a few hours. So I reassembled, having tweaked that little screw a fraction (haven't seen it mentioned in Mike's pdf) and set the pointer against the little dot. Result: The pointer moved and stuck when I tried to insert the drive cable! Out again, turned the screw back beyond where it had been and now it was free but on the road it read wildly too high, just what it did 30 years ago!

So I took it out again and moved the pointer back to where it had been plus 4 mm (3 mph equivalent) so 30 mph shold read correctly. Result: it reads correctly at 30 mph (against digital Tacho), 50 reads 52 and so 70 will read 74!

I've said before that my tachometer broke its transistor a long time ago and I had to put in a Si type rather than Ge, as a result it reads too high most of the time, especially when warmed up.
I took that out of the car too and got a shock: there were two transistors and they were both Ge types! Also it's not possible to adjust it once it is in its case, there is no hole in the back. So I attached the magnetic loop in the cars wiring, which just stretched far enough, powered it up externally and started the engine. The pointer moved as usual so I revved up to 3,000 rpm (on my digital tachometer} and turned the variable resistor until the dial read the same. Switched off, reassembled and put it in the car. Result: a tachometer which reads the same as my cheap digital one! I now have a proper instrument cluster.

Why is my memory so bad? The first line of the tachometer item has been a mantra, how could I have got it so wrong? And how did I test it to make the adjustment? I'm an electronic engineer so might have pumped a trigger signal into it but it shouldn't have caused a serious misread. Three decades without a proper tachometer, pathetic!

Now I remember a confusing factor. My S130 came with a spare tacho which I didn't pass on, but I did analyse the circuit and devise a way of calibrating it from the mains (line) power (and wrote it up in this forum). Soon after that I posted it to 'nicolacoltro' in Italy for his +2 restoration but it was 'lost in the post', I hadn't traced it. I intended to set my own tacho with that method but never got round to it and dismantled the circuit I'd made. But at that time I was convinced there was a single Si transistor in it.

Re: Speedometer calibration

PostPosted: Thu Jul 29, 2021 11:22 pm
by Quart Meg Miles

Today I had a proper run in the car. The speedometer followed the pattern described above though I couldn't spare the attention to be sure of the high speed error, while the Tachometer is accurate when cold but after a few miles running shows a positive error, perhaps as much as 10%, which is safer than under-reading.