Someone is going to know the answer to this one:

1983 833/OD. I have an aftermarket electronic speedo, which works with the companion sensor. Sensor bolts to the speedo cable hole on the trans no-problem, and there's a little square pin thingy that goes inside it and that fits just fine. PROBLEM: speedo reads crazy, and the reason is it's reading trans revolutions even when the vehicle is stopped. How can this be? I know because when you go to calibrate the speedo it counts sensor pulses, which are supposed to be 16 pulses per revolution. But it counts them in neutral! It counts them faster in gear and moving, but if I rev the engine in neutral it counts them faster as well. Trans seems alright otherwise. Shouldn't the speedo drive gear not spin when the driveshaft isn't moving?

Do I need a new sensor? I wish I had a mechanical cluster available so I could plug it in and test and see if it's driving the speedometer needle in neutral, but I don't.

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1965 Valiant wagon Turbo slant (work in progress)
2000 Chevy 155" cargo van - The Abductor
1970 Newport convertible
1996 Buick Roadmaster Wagon
1966 CruiseAire motor home
1990 Toyota 1 ton box truck TURBO slant (scraped)

Did you provide a good ground for the sender unit?

Richard

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Part of Tyrde-Browne Racing

I have one of those Autometer sensors in the Hpak Duster...the ground wire reaches to the side of the trany tunnel and is screwed into the pan
for immediate ground...If it lost ground it won't read right...

-D.Idiot

I've not played with any aftermarket speedometers, but I imagine it uses an inductive pickup in the sensor.

I strongly suspect you're getting spurious signals picked up and the speedo is counting those. The cause is most likely a poor connection, crosstalk or a combination of the two.

To isolate the problem, first, disable the charging system and see if the problem goes away - that will pretty much eliminate charging noise as a problem. Then try coasting with ignition on, engine off - if the problem goes away, it's related ignition noise.

If neither helps, then I'd spin the sender with a drill and see what happens. If it gives problems out of then, check for a broken winding in the sensor or a broken speedo.

You can suppress the noise at the source (capacitors, moving wires, twisted pairs, etc.) or you can try putting a 10-100Kohm resistor to suppress it near the speedo. It's also possible the speedo doesn't have proper power regulation and is getting noise that way, but I consider that unlikely.

All good tips KBB. I checked the ground, no problem there. What's funny is that the sensor is unaffected if I disconnect the input power to it. It's got three wires - power in, ground, and signal out. It's just a Hall effect sensor so far as I can tell. Line noise makes the most sense, and in fact I am now chasing a problem in the charging circuit. Basically, my dash mounted amp gauge (which may or may not be accurate) shows that the alternator is putting out 30-45 amps all the time. The battery seems like it might be flakey, and the regulator is a cheap item to replace. We'll see what happens.

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1965 Valiant wagon Turbo slant (work in progress)
2000 Chevy 155" cargo van - The Abductor
1970 Newport convertible
1996 Buick Roadmaster Wagon
1966 CruiseAire motor home
1990 Toyota 1 ton box truck TURBO slant (scraped)

If you still get signals w/o power to the sensor, check that the speedometer and sensor share the *same* ground - a DC ground isn't always the same as an AC ground. You might want to use a twisted pair for signal/ground to run from the sensor to speedometer; usually that technique works very well. Many cables for computer networks, stereo controls, and some telephones have the pairs twisted together inside to avoid noise. I tested flat-34pair twisted cable (SpectraStrip or some similar name) some years ago and was suprised that it did nearly as well as coax for fast signals.

A ~1.5Kohm will help suppress noise at the speedomter end, and you can even use a very small capacitor at the signal end to slow down the risetime if ringing is an issue.

Some very inexpensive multimeters have a 50Hz 5V square-wave output you could use to test the speedometer. I seem to remember (it's fuzzy) that 2000 cable rpm ~ 60mph, so 50Hz ~3000/min ~90mph.

Good luck - please let us know what works.