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Old 26-09-2008, 06:46 PM
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Default How does one test the speedo sensor?

The speedo in my ute ain't working so I thought I'd start from the bottom by checking the output of the sensor at the back of the gearbox. Anyone know what it is supposed to put out?

And yes, I did a search on this forum but couldn't find anything on point.

Thanks guys.
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Old 28-09-2008, 09:18 AM
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im only new to the patrol scene, but at work when we have a problem like that on our trucks. this is what we do.

from my understanding the output speed sensor is a variable resistor, so when the car is at a standstill, there should be a reading of 0 volts at the back of the sensor. This voltage goes up with speed.

So say at 60 kmh u might have a reading of 8 volts or something like that. (not the exact figure, just an example)

something u can also try to test your speedometer, get some wire and run it from a 12v source (battery is best) to the speedo wire at the back of the output speed sensor. link it up and if the gauge goes through the roof, you know that it is operating correctly, and your wiring isnt at fault.

so maybe if u could run a multimeter from somewhere on the wire up to the speedo, and take it for a drive, you should get a reading that goes up with speed, if you dont your OSS is probably rooted......

hope that helped :P
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Old 28-09-2008, 10:11 AM
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A word of caution though. Some speedo sensors are as above, being a tach-generator, while others use a similar senser to a Hall-effect and put out a constant voltage measuring speed via a series of pulses.

Putting 12 volts across a speedo designed for 5, or less can result in somewhat less than desirable results...
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Old 28-09-2008, 06:25 PM
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Quote:
Originally Posted by sturmz
im only new to the patrol scene, but at work when we have a problem like that on our trucks. this is what we do.

from my understanding the output speed sensor is a variable resistor, so when the car is at a standstill, there should be a reading of 0 volts at the back of the sensor. This voltage goes up with speed.

So say at 60 kmh u might have a reading of 8 volts or something like that. (not the exact figure, just an example)

something u can also try to test your speedometer, get some wire and run it from a 12v source (battery is best) to the speedo wire at the back of the output speed sensor. link it up and if the gauge goes through the roof, you know that it is operating correctly, and your wiring isnt at fault.

so maybe if u could run a multimeter from somewhere on the wire up to the speedo, and take it for a drive, you should get a reading that goes up with speed, if you dont your OSS is probably rooted......

hope that helped :P
!!!!!!!!!!! WARNING DO THIS ONLY IF YOU HAVE DESIRES OF DESTROYING THE SPEEDO HEAD !!!!!!!!!! this guy, bless 'im doesn't know what he's talking about.

You cannot sensibly test the speedo sensor / pulse generator or the speedo head without electronic test gear. You would need a pulse generator and a 'scope, although some of the better DVMs would give you a good clue as to whats going on. Best test is substitution.
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Old 03-10-2008, 04:23 PM
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Thanks for that guys.

Good job it's a long weekend and I've got 2 GQs to swap parts around on.

Sigh.
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Old 04-10-2008, 01:42 AM
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Well I brought the Patrol maintenance manual today and it covers testing the speedo sensor. From the manual:

To remove and Install
Disconnect the Negative battery terminal
Raise the front of the vehicle and support it on axle stands
Working under the vehicle disconnect the speedo sensor wiring
Place a container under the extension housing to catch any oil which may leak from the transfer case
Remove the speedo sensor assembly retining bolt and withdraw the sensor assembly from the transfer case
Inspect & renew as necessary

To Test
Connect a voltmeter to the speedo sensor wiring as shown in the illustration
With voltmeter set on AC volts, turn the speedo dddrive rapidly clockwise and note the voltage reading
The voltage reading should be aprox 0.5 Volts.

I'm guessing the duration or frequency of the 0.5V pulses = speed...So for that you'll need an oscilloscope...
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