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Old January 31st, 2010, 22:56
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Ecomike Ecomike is offline
NAXJA# 2091
Join Date: Dec 2006
Location: MilkyWay Galaxy
Posts: 15,209
Re: Testing Jeep O2, Oxygen sensors

Originally Posted by xjbubba View Post
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.
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.
I disagree with you on the need or desirability of using a digital MM when testing O2 sensors. So does 5-90. But it does need to be high impedance. A high impedance analog VOM will do what a digital will do and what an oscilloscope would do in one instrument when testing the O2 sensor. So I guess the issue is cost, and the minimal impedance needed to get accurate readings, which would be different for the Renix versus the post Renix (post Renix will be much more sensitive to the problem).

But some of what I just read seems to support your position on DVM accuracy over older cheap analog meters. However I will put my FET Analog meter up against your DVM any day of the week! (LOL). I think Rat Shack still has pretty good deals on FET Analog meters for about $50.

I have not heard any complaints over the last 2 years here from others who have used a cheap analog MM to test their Renix O2 sensors (yet). Perhaps we can get one of the EE's here to do some calcs for us and tell us how much error a 20,000V/ohm analog Volt meter would have on each system.

I was not aware of how low the typical cheap analog VOM impedance was, so you have brought up an interesting point.
Quote=8Mud: "Go ahead and put up the best fence you can build, I'll bet on some Mexican with a few dollars of nails and a pile of scrap lumber."
34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091
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