Yidney

If one is testing the wiring to a sensor - say a cam postion sensor - and the resistance through the ground wire to the connector is supposed to be "at or near zero" what does that mean? "Zero" I can figure out, but how much above zero is acceptable? Is 2 ohms "near" zero? 3?

Also, if I get from 0 to 2 ohms with the ignition off, but about 5 with the ignition on, what causes that change?

Confession: This is my daughter's Audi, but I assume the principles are the same for any car.

Glyn M Ruck

0 Ohms = dead short or total continuity. At or near zero should be well below 1 Ohm in normal conventions. Most cam position sensors are Hall effect sensors - i.e. their output varies with change in magnetic field.

You are testing the wire & not the sensor ~ Yes?

If the wire is isolated i.e unplugged at both ends it should read 0 Ohms or something is wrong such as a dry solder joint, badly crimped connector etc. If values are changing with ignition on then by definition this wire must be in circuit & we need to understand that circuit which means you are not in fact testing the wire.

Hope that helps.

Rudeney

Ohms is a measurement of resistance so "zero ohms" means no resistance. All conductors offer some resistance, so technically, there is no such thing as zero ohms. Some meters are more sensitive than others and there are usually settings for various fractions and multiples of ohms, so that's why they say "at or near" zero. If your meter read something like .008 ohms, that's pretty much a complete circuit and "close enough" to be zero.

Glyn M Ruck

Yep! ~ I've already had amdeutsch remind me the resistance of a piece of wire is also a function of gauge & length which indeed it is. It is also a a function of conductor material e.g. OFC, Silver etc.

However in an Automotive wiring harness even the longest conductor will have a resistance of close to zero of the sort of order you indicate.

Daremo

Keep in mind what your connecting to. For example if your connecting to something that has a coil, such as a relay or a solenoid and your connecting across the coil terminals, you will see a low resistance which can be anywhere from <1 ohm to 4-5 ohms. This is because your measuring the resistance of the coiled wire and that will vary depending on its length, dia. ect. It also matters how clean your probes are and the contacts your touching them to as well.

Generally speaking, if your using your meter to "ohm" something out, your looking for either continuity (up to a couple of ohms) or lack of continuity (on a Fluke meter this is usually displayed as OL). 99 times out of a 100, you only care if you have continuity or no continuity.

The easiest test to demonstrate what I am talking about is to turn the meter on and set it to measure Ohms. You will see an "open" circuit indicated by OL or whatever your brand of meter uses to indicate an infinite resistance. Now short the two test leads together. You will now see somewhere around an ohm depending on how good of a connection your making between the two test probes.

Yidney

What I was measuring was the three prong conector pulled off the CPS. The three prongs are supposedly ground, power, and pulse. From the ground to power prong I measured 5 volts, which is what it is supposed to be. Then it says to measure resistance through the ground wire, which it says is to be at or near zero - makes sense. I cannot actually find the other end of the ground wire - it disappears into bundles of wire behind the engine. So I went from the prong on the plug to any numer of ground points on the engine and frame. If I do that same thing from the negative battery terminal to any number of ground points, I get zero ohms. But from the connector prong I get about 3. I figured ground was ground. No?

Rudeney

3ohms or 3miliohms? 3ohms is too high. Oh, and before we get too deep into this, "close to zero" is a very important mathematical term. Without it, we would not have calculus. And without calculus, we'd still be driving around in wimpy little three-wheel Motorwagens ;-)

Yidney

Ohms. About 2.5 with igntion off, 5 with it on.

Glyn M Ruck

+1^