Tach Sensor Test Rig

[BDCarrillo]

So I got bored...

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IP gear is spinning at 620 RPM, which is the same as 1240 engine RPM. The gauge is powered by a 12v wall wart.

Up next, I'll be testing a Duralast SU1181 or Dorman 917-784. They can be cut shorter than the stock sensor and have a metal end cap which I'll try threading. If it works out, it'll be a $13 sensor for trucks with the vac pump up high. (This sensor didn't work... Case diameter is too large to be cut threaded. If it could be roll threaded, then maybe, but that's beyond my capability)

Other trucks can use the 1989 2.0 cavalier sensor just fine (detailed in the tech section).

The sensor in the pic was known to be bad, so I ran it through the ultrasonic cleaner and sealed up the wires. Worked fine after that.

A little tech on our sensors: they're variable reluctance magnetic sensors, and generate AC sine waves as gear teeth pass by. The tach picks up on the frequency, not amplitude, and drives the needle. No multiplication is necessary, since the sensor outputs a direct hertz reading. Each high/low contributes to this, essentially doubling the output (compared to a hall effect sensor) to match crank speed.

Ours sit right at 2500 ohms, plus or minus about 50.

Voltage output is .248 vac (NOT dc) at 1240 hz... I would have expected more... But the airgap on my rig isn't perfect.

 

[BDCarrillo]

Yup, those sensors are a Halls effect. Try a CPS from a PSD, you'll get the same thing. Get one that can pickup about 50 BB's, it has a real strong magnet.

Our sensors are NOT hall effect... Hall effect sensors are sensitive to magnets passing by, whereas variable reluctance sensors require only a ferrous material passing by.

The main purpose I built the test rig was to test Ford sensors of unknown status, and verify outputs of other aftermarket sensors that are much cheaper and can be modified to work.

 

[tenlug]

That's good work. With your set up a man can test both tachs and sensors. Thanks.

 

[BDCarrillo]

That's good work. With your set up a man can test both tachs and sensors. Thanks.

Oh I forgot to mention that the tach circuit board has some potentiometers that I'll have to fiddle with...

 

[OLDBULL8]

Correction accepted. Was thinking the wrong way.

How about testing what differences in clearances make? There has been a lot of discusion on the different CPS sensors of different Brands. How much does a couple of thousandts make?

 

[BDCarrillo]

Correction accepted. Was thinking the wrong way.

How about testing what differences in clearances make? There has been a lot of discusion on the different CPS sensors of different Brands. How much does a couple of thousandts make?

That's the beauty of VRS... Clearances effect amplitude, not frequency, so distance doesn't affect the RPM output numerically. The worst case is amplitude output below tach sensing threshold (sensor too far from gear), which gives a zero readout, not a false high or low. Already verified this on my rig.

I have yet to be able to duplicate a bad tach sensor, which I've seen two symptoms of: overly high reading (internal short or resistance?), and zero at idle to about ~1000 (too large of a gap?? Low amplitude output?). The only failure points are the copper windings getting corroded or demagnetization of the permanent magnet.

Anyone have bad sensors that can be tested?

 

[fx4wannabe]

I have 3 sensors. I know the one in my truck isn't working and haven't tried the other 2.