My little pocket size Radio Shack DVM does a pretty good job but if I'm really serious about what's happening I drag out my Tektronix 2213 oscilloscope.

 

yeah, a analog meter on computer outputs is very dangerous, and can get very expensive fast

 

I use a wavtek. It is a Beckman clone. I use mine everyday at work. Love it. Fluke is good as well but you need to be careful about which model. The cheaper Flukes are a disapointment.

 

as long as you dont cheap out on it, a fluke will never do you wrong, as long as there treated well.

 

Had a Fluke that I bought in '84 that performed flawlessly until it failed a shock test in 2004. It got kicked off a 28ft. scaffold onto a concrete floor. Stopped working after that, but I bought another one in spite of that sad performance.

 

well, thatll happen eventually, even to the best of em.

 

For measuring the output of an Oxygen (O2) sensor while connected to the vehicle computer, a good high impedance input analog voltmeter (input impedance in range of 10 to 20 megohms and a full scale range of 1.5 or 2 volts) is needed. The old vacuum tube voltmeters (VTVM) or field effect transistor (FET) voltmeters are ideal. The oxygen sensors are analog voltage devices whose outputs are sampled by the computer and converted to digital for processing. Since the sensor response time is slow, the high impedance meter will track the output very well. But still, a good o'scope or even better-a storage o'scope- is ideal. Or an on-board recorder that collects many engine parameters is even better. The problem with DVM's is not that they can't measure accurately, it's that your brain/eye can't keep up with the changing numbers displayed.

 

You gotta know what you're doin - the problem comes from using the old meters to measure resistance on the higher ranges - anywhere from 3 to 90 volts are used in some antiques. USUALLY the older meters used a 1.5 Volt source on the rX1 range, and increased it for anything higher. That said, a Simpson 360 used for voltage readings will follow a rapidly varying dc voltage, and they are cheap.

 

Simpson 360 or 260 ? :I dont kno about a 360, but I have 2 260's ( 1 + a spare) . My old-timer father-in-law turned me onto the 260, when he saw me struggeling w/a cheap DVM . To me, the analogs are easier to use ; maybe it's all the references on the scale that help me figure out what it's teling me.

Can we use a Simpson 260 to measure volatage on the o2 sensor?

 

Yep, finger slip on my part - meant 260.

 

In my opinion, NO voltmeter is going to work well for an O2 sensor, once the computer gets into closed loop mode and the sensor is actively switching. The switching rate (cross count) is too rapid for the meter to react (or for you to read). Analog meters do a little better -- you can kinda follow the dancing needle with your eyes. But the meter movement will, by nature, average the readings also.

A VOM with some sort of specialized sample and graph function will work. So will an oscilloscope. So will a scanner. But a plain old voltmeter is the wrong tool for the job.

As for the comments about low impedance VOMs and computer damage, I don't quite agree. The main problem with low impedance meters is that the readings are inaccurate, because the meter itself loads the circuit too much.

An automobile presents such a harsh electrical environment that the inputs and outputs of the computer have to be pretty well protected from ESD, opens, and shorts. Hanging an older vacuum tube voltmeter or low-impedance transistorized VOM on a protected input or output isn't going to kill the computer. That said, I wouldn't recommend opening the case on the computer and starting to poke around with your old blunderbuss; the internal circuits are not so well protected.

The "ohms" function is the only one with a possibility of causing damage. In "ohms" mode the meter creates a current-limited test voltage between the probes. Most reasonable meters, even very old ones, are unlikely to put enough current/voltage on their probes to damage a computer. But you aren't going to get accurate ohm readings with the computer plugged in, so you might as well disconnect it before checking that sensor or harness continuity.

 

Allright, why you are all on this subject. A few years ago, I needed a better multimeter, I bought the OTC 3545. Now I have progressed (ahem, getting stuck fixin other peoples problems), to the point where I need graphing capability. What is the differences between a labscope and a graphing multimeter?

I have a few guesses, but I wait,
Thanks

 

If anyone is interested here is a link to one of the best write-ups I've seen on Oxygen Sensor testing: http://auto.howstuffworks.com/framed.htm?parent=question257.htm&url=http://www.wps.com/LPG/o2sensor.html .
There is lots of good information on the "How Stuff Works" website.
P. S.: If you remove an O2 sensor be sure to use the proper anti-seize compound upon replacement or you might not get it out again. UH-OH, link didn't work right-- Go to How Stuff Works website and search for Oxygen Sensor.

 

The how to article is great. It should help me achieve what I set out to do when I started asking about multimeters anyhow.


Thanks,

Matt

 

I myself have a Fluke 77 for work, 87 for home as well two Simpson 260's.
On the wish list is a Fluke 88 to better read relay coils.
I have used the 87 to compare readings from the Autometer Fuel /Air gauge.
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