Norfolkluegojnr
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| posted on 23/2/15 at 03:15 PM |
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Electronic Speedo - how do they work?
As above really. All other electrics are now working no the car expect the speedo.
its not a mechanical drive that i'm used too, so where to start with for fault finding? Is there some kind of standard diagram? the instruments
appear to be labelled 'CA' - can post up pics if it helps.
Thanks in advance!
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Sam_68
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| posted on 23/2/15 at 03:39 PM |
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They can work in one of three ways, usually: by taking signals directly from the car's ECU, by means of a Hall Effect sensor attached to the
gearbox cable connection, which 'counts' the rotations of what would be the speedo cable connection, or by counting magnetic pulses from
magnets attached to the propshaft or a hub.
The latter is most common, and least reliable... it is quite sensitive to the position of the magnets and their clearance to the pickup, and can often
become faulty due to build up of dirt on the sensor, the sensor position being disturbed, corrosion of the wiring connections to the sensor, or simple
loss of a magnet (they're often epoxied in place, and simply drop off!).
'CA' sound like the CAi logo Caerbont Automotive Industries, who now also do the traditional 'Smiths' instruments. The
'i' of the logo is just a dot over the italicized serif of the 'A', so it's easy to misread!
Information here may be useful.
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britishtrident
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| posted on 23/2/15 at 03:50 PM |
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If you have a digital multimeter set it to a low range and you should be able to read the pulses from the sensor.
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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Norfolkluegojnr
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| posted on 23/2/15 at 04:01 PM |
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quote:
Originally posted by Sam_68
They can work in one of three ways, usually: by taking signals directly from the car's ECU, by means of a Hall Effect sensor attached to the
gearbox cable connection, which 'counts' the rotations of what would be the speedo cable connection, or by counting magnetic pulses from
magnets attached to the propshaft or a hub.
The latter is most common, and least reliable... it is quite sensitive to the position of the magnets and their clearance to the pickup, and can often
become faulty due to build up of dirt on the sensor, the sensor position being disturbed, corrosion of the wiring connections to the sensor, or simple
loss of a magnet (they're often epoxied in place, and simply drop off!).
'CA' sound like the CAi logo Caerbont Automotive Industries, who now also do the traditional 'Smiths' instruments. The
'i' of the logo is just a dot over the italicized serif of the 'A', so it's easy to misread!
Information here may be useful.
Thanks Sam,
Nail on the head - checked that website and the details match up, so I'm one step closer!
No ECU to speak of, so must be a magnet/pick up arrangement. I'll see what i can see with the car up on ramps - how big is the pick up?
Presume you can replace the magnets?
Thanks again.
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Sam_68
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| posted on 23/2/15 at 04:06 PM |
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quote:
Originally posted by NorfolkluegojnrI'll see what i can see with the car up on ramps - how big is the pick up?
Presume you can replace the magnets?
Not very big - think in terms of something that looks a bit like an in-ear headphone. And yes, you can replace the magnets - they're usually
just 'button' sized circular discs - not much different in size to a calculator/watch battery.
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Norfolkluegojnr
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| posted on 23/2/15 at 04:32 PM |
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Great thanks. Will have a look over the weekend.
LCB to the rescue as always! 
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