Electronic speedo wiring.....!

Could be: do not know how to test it properly (that pesky “not understanding the theory of operation part”).

I do have a spare, that I can swap in easy.

You can spin the transducer with a drill, Paul - which only will reveal if the transducer fails because it is not driven by the box…:slight_smile:

A transducer is generally something that converts one form of energy to another - in this case 12V DC to AC. Which as Doug says should be readable with a voltmeter - but whether the speedo reacts to signal frequency or to power generated, I don’t know. Ie, if the transducer is a converter or a generator…

With signal out and no speedo response; failed speedo wire connection, wire misconnection at speedo or faulty speedo? Testing, and fitting, a spare transducer is certainly a way forward - or whatever direction…

Frank
xj6 85 Sov Europe (UK/NZ)
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The transducer creates pulses. These are also counted by the trip computer.
The transducer is up in the transmission tunnel so no drill… does it work or not, flying blind?

Aha! That’s an important clue… I’ll check that out.

The “flying blind” part is not knowing precisely the mechanism by which the transducer works: Frank gave me a hint, that Ill measure with my DVOM.

On the Jeep, the transducer is easily accesed.

If you can reach it, good, just spin it with your drill and see if you get any pulses out of it. One of those injector test lights would do well?
I’d bet the speedo works like the tach does internally. The trip counter is separate.
But again, does it not work or do you not know if it works but don’t want to break anything?

All quite clear to me. Signal ignition ground.

Just look at that :cold_face:

Inside there is an ti mic2/c that turns the pulses into a voltage and then a coil that pulls itself (and the needle) around a ring.
The motor is driven with the aid of two blobs. Two diodes and a resistor (I believe) protect the chip so maybe not so easy to fry.

Signal is yellow, motor is leftmost red, leftmost blacks. Needle is rightmost red and the badly soldered black, should anyone ever be that curious.

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That black wire is shockingly badly soldered!!!

Turns out the brown/ black on the transducer is ground.

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The odometer drive, David?

And the drive computer uses the signal from transducer as well, but convetrted differently…?

Frank
xj 85 Sov Europe (UK/NZ)
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The instrument converts the pulses into voltage. That pushes the needle around the dial.
The trip computer counts the pulses and uses them for average speed, distance and consumption.

In the same fashion, the trip computer counts pulses received from the transmission speed sensor (the same signal that goes to the speedometer) to determine distance driven Once again the exact specs on the pulses don’t matter, only the number of them, though the processor can’t handle pulses which are outrageously narrower and more closely spaced than expected. (It really doesn’t like it if the rate of pulses hits 256 kph or 256 MPH. It will still count distance, but the average speed won’t update.)

Tenths of miles are racked up by 812 or 813 pulses (depending on which tenth), while a final 687 pulses brings it to an even mile, with a total of 8001 pulses.

Tenths of kilometers are racked up by 504 or 505 pulses, while a final 428 pulses brings it to an even kilometer, for a total of 4972 pulses.

In the same way that the number of pulses needed depends on which tenth of a mile or kilometer, the number of pulses needed for a whole mile or kilometer depends on which one it is; sometimes it is one pulse more or less than stated. But none of this is random, the scheme is deterministic, e.g. it always takes 812 pulses to go from 0.2 miles to 0.3 miles, while it always takes 813 pulses to go from 0.3 miles to 0.4 miles. It always takes 8001 pulses to go from 1 mile to 2 miles, while it always takes 8000 pulses to go from 2 miles to 3 miles.

Source

The odometer motor is driven by the two blobs. There’s likely a signal every so many pulses, the motor then moves a bit and so on.

Does that mean the last tenth in each mile is shorter than the other nine tenths?

Yes (click the link, it‘s got other tricks up its sleeve as well, it is a bizarre thing)

Ive found the answer…

MOTOR METER RACING W Pro GPS Speedometer Odometer Waterproof for Car Boat Motorcycle White Dial Red LED Included GPS Sensor https://www.amazon.com/dp/B081JN8S8X/ref=cm_sw_r_cp_api_glt_fabc_922XQEFQWBC0DYES3KK2

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But I thought the odo moves steadily, David, not in steps…?

Obviously the speedo uses the transducer data differently from the trip computer? But I guess both will misread if the transducer fails…

Frank
xj6 85 Sov Europe (UK/NZ)
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May well be that the motor runs constantly but it will have to know how far it has turned; I’m sure you agree that simply varying the voltage would not account for manufacturing differences.
8000 pulses per 10 revolutions of the decimal point of the odometer is too accurate. Turning the motor by hand gives perceptible movement.

The speedo turns the pulses into a voltage right in the speedo, the computer counts, calculates and displays. They are the same yellow pulses. Cruise control would use the same.