So, who can tell me how the speedo drive works...

[northwind]

Well, I know how it works... Instrumentation sends 12v down one cable, 5v down another, and there's an earth in the third. The sensor spins, hall-effect sensor does it's thing, and something happens that tells the clocks how fast you're going. But what is it that actually goes back to the instruments, and on which line/lines? Pulses, presumably... But what's pulsing I don't know. Or maybe it shorts to earth and the fluctuations in draw work the clock?

[Sid Squid]

Originally Posted by northwind

how the speedo drive works...

Magic.

That and pulses, but I've never measured what or how much.

[Stu]

And if you go over 70mph it gets on it's mobile to plod

[northwind]

Sufficiently advanced magic's indistinguishable from technology, I hear

From memory there is a how to test the sensor in the factory manual. If the voltage is applied to the same cores as the loom then the signal will be on the same core as the measured output (this makes more sense if you read the manual). Perhaps then you could rig up a peak reading voltmeter to give an idea of the RMS value of the pulses.

A little google-fu returns this:

Originally Posted by t-inter'web

Modern speedometers are electronic. A rotation sensor, usually mounted on the rear of the transmission, delivers a series of electronic pulses whose frequency corresponds to the rotational speed of the driveshaft. A computer converts the pulses to a speed and displays this speed on an electronically-controlled, analog-style needle or a digital display, the latter of which is more common nowadays. Pulse counts may also be used to increment the odometer.

But that doesn't answer the specifics, I know, merely confirms what was already said.

A little more google-fu says this:

Originally Posted by t-inter'web

The mechanical analog speedometer is driven by a cable housed in a casing and connected to a gear at the transmission. This gear is designed for the particular vehicle model, considering the vehicle's tire size and rear axle ratio. In most cases, the speedometer is designed to convert 1001 revolutions of the drive cable into registering 1 mi on the odometer, which records distance traveled by the vehicle. The speed-indicating portion of the speedometer operates on the magnetic principle. In the speedometer head, the drive cable attaches to a revolving permanent magnet that rotates at the same speed as the cable. Floating on bearings between the upper frame and the revolving permanent magnet is a nonmagnetic movable speed cup. The magnet revolves within the speed cup, producing a rotating magnetic field. The magnetic field is constant, and the amount of speed cup movement is at all times in proportion to the speed of the magnet rotation. A pointer, attached to the speed cup spindle, indicates the speed on the speedometer dial.

The quartz speedometer utilizes an accurate clock signal supplied by a quartz crystal, along with integrated electronic circuitry to process an electrical speed signal. This signal is generated by a permanent-magnet generator mounted in the transmission. This permanent-magnet generator, designed to be used with both quartz and digital speedometers, provides a sinusoidal speed signal that is proportional to vehicle speed at the rate of 4004 pulses per mile (2503 per kilometer).

In the digital microprocessor speedometer, the vehicle speed is monitored by the permanent speed sensor mounted in the transmission. The signal is transmitted to the microprocessor where the counter converts the speed signal to a digital signal and stores it in memory. The timing circuit has the capacity to handle the counter and memory storage in less than 0.25 s. Memory circuit signals are sent to the electronic display circuit, which selects the display numerals representing the vehicle's speed, according to the number of pulses received from the speed sensor.

[Luckypants]

Quiet in work today?

Originally Posted by Luckypants

Quiet in work today?

In the style of a certain dog on TV:

Oh yes...

[northwind]

Thanks folks. Basically what I'm trying to do here is to get one speedo drive to run two sets of instruments, both of which use the same pickup method. It's a set of standard clocks and a Veypor datalogger toy. Either one will run off the stock speedo drive individually, but not together. Which is quite logical really, since it's not a simple case ofhte drive sending a signal, the drive takes its input from the instruments so both are sending 12V down one cable, and 6V down the other.

I'm not that good at electrical stuff to know what happens when you have 2 DC voltages connected into a speedo drive that's designed for 1, other than that it doesn't work

So, I'll fix tails into the cables and find out what they do when they're doing what they do next, and hopefully find some clever way to make it go.