Thread: More Electricery Q's Speedo.

OOps,
me and my typos, think i found my answer in search

it is said the transducer from the transfer case is the likely culprit ,as on mine when the speedo stops working so does the odometer.
that would explain what happens while the vehicle is moving, but why would the speedo move when it is stationary, that bit i don;t understand.



john

John, I'm no expert in this field although it does fall into the realm of general electronics where I do have some knowledge and I think I can give you a logical answer.

The RAVE CD isn't very helpful in this area - it simply shows a transducer which feeds the multi-function unit. But it's highly likely that the transducer is a Hall effect device which develops a voltage across its faces when a magnetic field is applied to it at right angles to the Hall effect device. This magnetic field is usually generated by a toothed ring on the shaft which you wish to measure the speed of, and the ring is magnetic material. As each magnetised tooth passes close to the Hall effect device, it generates a voltage, as each valley between teeth passes the Hall effect device the voltage goes away. Thus you get a series of voltage pulses from the transducer. If you have, say 4 teeth on the ring, you count the pulses electronically, divide them by 4 and figure out the speed of the shaft. Then you make a further division based on the diff ratio and you know the wheel rotational speed, and then using pi, and a little more maths, you can electronically calculate the vehicle velocity.

Now in your case, it seems that the Hall effect device, or, most likely the connection to it, has become intermittent, and as the pulses from the Hall effect device are probably amplified by a small amplifier in the transducer to give them immunity to general electrical noise in the vehicle, the intermittent connection is probably switching the power to the amplifier on and off randomly, creating additional pulses.

When the vehicle is stationery, it's possible that turning on the air-con fan causes the idle speed to be lifted and the vibration of the revving engine, passed through the gearbox, causes the connection on the transducer to flop around generating a few random pulses at a rate which the processor counts and figures out your apparent velocity is about 100Kph.

Sorry to break the bad news, but your speedo doesn't need an exorcism - just a new transducer. They live in a very unpleasant environment and they're a bit prone to failing, I believe.

It's ironic that series Land Rovers used to have trouble with the worm wheel on the transfer box output housing slipping and causing the mechanical speedo to be erratic, and having moved away from that to an entirely electronic system, you can still get the same symptom!

Many Thanks graham
for the explanation,
the pulses you described make sense on the stationary side, same for when i slow down after a run. now to get to the bugger.
RAVE just raves on, with no help in the 'difficult' dept.
i would have had a crack at it by now, but to get my veh on the stands and have a good fiddle is almost impossible at the moment, its been bucketing since I put the post on, can;t even get it in the shed( too much stuff in the way), and the carport is too low.

bring on the sunshine


john

You're most welcome! I can't believe I'm hearing someone say "Bring on the sunshine" in the middle of a drought! It's all relative really isn't it? Comes a time when enough's enough and you just want to get dry again.

Hope you get a nice day to sort it and when you've finished it rains again (moderately!)

the erratic reading can also be induced by a dying alternator spiking the earth side of the vehicle, induced electrical currents (some very very early traffic light detectors) bad earths and electrical pollution of the harness from other parts of the vehicles electrics. Id suspect that a dogey electical connection might also be able to do it as well.

Thanks dave
when it first happened not long after i bought it, an auto elec suggested that( the alternator) but had it checked out and it was fine.
I have looked at the common denominator , ie what started it, relative to what I have put on it, and that didn;t confirm anything, so with everyones advice and and knowing a bit more about how it works. it all seems to point to the transducer.
never thought about the conductive coils in the road , will keep a closer eye on the speedo when going over them,


john