Tachometer / Rev Counter Stopped Working

Batteries, Alternator, Wiring, Lighting, etc. (Please discuss ignition problems in the Engine category)
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Mike Stevens
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Re: Tachometer / Rev Counter Stopped Working

#11 Post by Mike Stevens » Tue Apr 04, 2017 10:44 am

In my experience, most cars speedos seem to over-read when compared with a satnav. The satnav should be pretty accurate when the speed is constant, but I suspect that they will lag behind a bit when the speed is changing. This is probably due to the update rate that the GPS engine uses.

David - that sounds like a good method to check the calibration. I have found that some tachos have a 'pot' inside to adjust the calibration, while some (maybe later ones?) don't. I use a low frequency signal generator coupled to the tacho input and to a digital frequency meter so I can check the signal generator frequency. A bit of calculation gives the frequencies corresponding to revs and so far it seems to be OK!

I've also been trying to find a suitable way to check the calibration of a speedo. The obvious choice is a 'stepper motor' which you can accurately define the motor RPM. Sadly, I've not found a suitable one yet!

Submitting your article to Kevin is by far the best approach. I shall look forward to seeing it!

Cheers,
Mike.
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David Withers
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Re: Tachometer / Rev Counter Stopped Working

#12 Post by David Withers » Tue Apr 04, 2017 3:22 pm

My tacho is the type fitted to many cars in the late-60s/early-70s and it has a trimmer accessed by screwdriver through a hole in the back of the housing (so the instrument has to be outside the panel for adjusting).

Re: Sat-nav accuracy: Last year my son and I went on a 150 mile journey in his car. As 'navigator' I set my Garmin for the trip, however he'd set his TomTom so we let both run for comparison (with his muted). I didn't see any differences in displayed speeds, though I couldn't tell you about lag as I only did spot checks from time to time.

Mike Stevens
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Re: Tachometer / Rev Counter Stopped Working

#13 Post by Mike Stevens » Sat Jun 10, 2017 10:27 pm

Good article David. I enjoyed that. Of course, being an electronic engineer means I was a bit biased!

Cheers,
Mike.
(South Oxfordshire)
Register Member No 0355
1967 Herald 1200 Convertible rust should be white
1971 2.5PI Saloon Sapphire blue
1973 2.5PI Saloon rust some Honeysuckle
1973 Stag French blue
(1949 LandRover was blue should be light green!)

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David Withers
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Re: Tachometer / Rev Counter Stopped Working

#14 Post by David Withers » Sun Jun 11, 2017 7:31 am

Mike Stevens wrote:Good article David. I enjoyed that. Of course, being an electronic engineer means I was a bit biased!

Cheers,
Mike.
Thank you, Mike.

Not being at all confident with electronics, I was very pleased - and perhaps a little surprised - that the repair went so well. That tacho should now be fine for another 45 years!

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Re: Tachometer / Rev Counter Stopped Working

#15 Post by torque2me » Wed Jun 14, 2017 4:47 pm

Mike Stevens wrote:In my experience, most cars speedos seem to over-read when compared with a satnav. The satnav should be pretty accurate when the speed is constant, but I suspect that they will lag behind a bit when the speed is changing. This is probably due to the update rate that the GPS engine uses.

Mike.
4 seconds.

Kev

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