Digital Multimeter

[jpian0923]

My digital multimeter goes crazy (readings make no sense at all and are wacky and random) when I try to use it on the running engine.

It works fine when engine is not running.

I'm going to change the multimeter battery tomorrow but, I don't see how that can help.

Anybody see this before?

[jhwelch]

What are you trying to measure (voltage, resistance) and what two points are you using to make the measurement from?

[Kelly]

Jim,

Don't change your batteries quite yet. I'm pretty sure that fluctuations, depending on what you are measuring, are quite common when the engine is running. I'll let someone else explain the why...

[smosher]

The meter is most likely picking up the output from the coil.

Steve

[ArtJ]

The old fashioned analog meter works best for most general engine work if you
can find one ( the ones with a needle).

[jpian0923]

I have an analog meter as well. Have not tried it though.

I'm trying to measure voltage.

I use the engine ground mounting bolt as ground and the red lead to measure any and all points that I can measure...battery charge, coil positive etc...

[sastanley]

Jim, I had a digital meter that I think was reading a couple volts high. I thought I was seeing 16 volts at the battery posts!

I have a couple digital and an cheapy analog as well. I like the simplicity and level of detail I can get with the digital for measuring resistance and volts in the ignition circuit, provided it works!

[joe_db]

Mine does not do that, but I suspect the leads are picking up RF from the ignition system.

[edwardc]

If you were to use an oscilloscope look at the +12v buss while the engine was running, you would see that the actual voltage is quite noisy, varying a lot with time. There's spikes induced from the coil, noise insertred from the alternator, etc. But the average hovers around +13.x volts.

An analog meter, with a needle, can't respond that fast, due to the inertia of the needle, and it does the averaging for you. A digital meter, on the other hand, can be capable of responding almost instantaneously. They typically have a "sample and hold" type of logic, where the voltage is measured, then displayed for a short period of time while the next sample is being measured. As a result, they can be much more sensitive to the noise on the +12v line, as each quick sample could measure at a different point, sometimes at the peak of a spike, sometimes not. This causes the digits of the display to jump all over the place, making it difficult to read.

Some digital meters have an averaging capability built in, others don't.

[jpian0923]

I'm going to try the analog tonight.

I guess my main concern is safety...I can deal with meter flaws.

I don't want to electrocute myself, or any of my guests.