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Old January 31st, 2010, 21:59
xjbubba xjbubba is offline
NAXJA Member # 1524
Join Date: Jan 2004
Location: santa barbara, ca
Posts: 1,270
Re: Testing Jeep O2, Oxygen sensors

You're correct that the issue is the impedance of the volt meter. The common analog volt meter most people have will be low impedance; I have a number of them. high impedance analog meters are expensive, and impractical for most home "techs". The majority, if not all, digital VM's are high impedance, and should be used when measuring the voltage of an O2 sensor--even pre-'90 Cherokees. The digital meter will give an adequate indication of voltage values above and below the .450 nominal reading of most common O2 circuits. A reading that varies between 150 and 850MV indicates a good sensor. The other parameter is the ability to rapidly swing above and below the .450v center voltage. This is very hard to test with a voltmeter. As long as the reading doesn't "linger" above, or below the .450 level indicates adequate switching speed. To really test an O2 sensor, you need an oscilloscope, or a good engine scanner that gives you O2 cross counts.
Here are just a few sites from the "web" regarding the use of a digital VM, versus an analog meter; there are too many to post here:
Cheap voltmeters will not give accurate results because they load down the circuit and absorb the voltage that they are attempting to measure. A acceptable value is 1,000,000 ohms/volt or more on the DC voltage. Most (if not all) digital voltmeters meet this need. Few (if any) non-powered analog (needle style) voltmeters do
Things You'll Need:
High-impedance Digital Voltmeter
With the inspections completed, test the output of the sensor with your Digital Multi-Meter. Don't make the mistake of using an older generation analog meter to test the sensor, as possible damage to the electronics of the DME computer and the O2 sensor can result.

You should use an analog meter to test a Throttle Position Sensor (TPS), or any other mechanical-style variable resistor (such as the fuel tank level indicator) for discontinuity, and use a digital meter to accurately measure voltage levels.

Last edited by xjbubba; January 31st, 2010 at 22:06.
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