rrich, a few posts ago, you said o2 sensors are like little batteries, now you are saying they are like resistors. I don't really understand where the similarity is...

This post here explains what was hooked up, and might answer some other questions for you Rrich .https://www.cherokeeforum.com/f2/ren...ce-rms-133153/

Hey, something that has bugged me since I was knee-high to a Hodaka. Can you tell us, in laymen terms the difference between impedance and resistance? They are both measured in ohms? Is one "dynamic", as opposed to static? That has always bugged me. We're just talking apples and oranges on the sensors. I haven't even looked much at the newer OBD type. I'm mainly interested in Joe blow seeing if his RENIX sensor is even working, and fishing for some clew...is the high impedance voltage reading, (from a cheap meter), some predictable amount different than a lower one, (from an expensive meter). Since the new sensor provided the goal, "desired" reading with the high impeadance meter, I'm thinking it's not to far off.

Thanks Hank. So I guess part of the deal is there IS sort of a "dynamic" side to it.

"Impedance, however,(as opposed to resistance) is dependant on a property called reactance, and frequency. Impedance only exists where there is AC, or fluctuating DC (AC with a DC bias). In a coil or capacitor, the reactance changes due to the way the component works with electricity. A capacitor, will increase its resistance as the frequency gets lower. A coil on the other hand, will increase it's resistance as the frequency gets higher."


Again thanks, but still it's a sqrirmy one for me. I think I'll need to settle for just seeing what the meter says! Although I could guess something fluctuating every second or so might have a pretty high "DC Bias". Maybe that's why it seems to read about right. I saw somewhere, I think it was in regard to true sign wave, as opposed to modified sign wave inverters, that if you had a high impedance meter you could add or subtract some % to end up with a more accurate figure. (for the voltage from the modified sign wave inverter).

Hey! OK rrich. When I saw you noted the incorrect voltage from the ECU, and again didn't seem to read that I had measured just that, I missed the importance of the rest of that post.

This makes sense for me. Just scrap the term impedance, and insert resistance, and it's plain and simple.

So I put my digital on the analogue and got .48 M ohm resistance. (on the 2M scale)

Most generally the term resistance is used in DC circuits.
Impedance is more for non pure DC circuits. It also includes reactance and resistance.
Reactance is the dynamic resistance term used more for inductors and capacitors. Xl and Xc.
Impedance includes it all.

All are forms of resistance, measured in Ohms.

LOL - Picture traffic flow - Resistance is the limiting factor to traffic flow (bumps, turns etc,)
Impedance is what's inhibiting (impeding) it - that naked girl waving to everybody.
Reactance is how you react to it. All frustrating, - measured in swear words, strokes, and heart attacks!

I said it's a very weak battery before I realized the sensor used is stone age Vauxhall parts. But still it's a chemical reaction that can easily be destroyed by too much load on it.

Like your car battery - seems rugged but it too is a chemical reaction. It has internal resistance that limits it's output. Normally it can supply the starter just fine - with up to 300 amps or so.

Place a big screwdriver across the terminals (a big load) to draw far more current. The 700-800 amps it puts out destroys it internally. Why does it only put out 7-800 amps instead of near infinite amps? Internal resistance (impedance.)
A low impedance meter does the same thing to a sensor.

The voltage that you see at the sensor (or the other end of the wire) is not what's really there - use a scanner to see what it really is.

Notice on yours - when you connect/disconnect the voltmeter the engine changes - load. Sensor connected or not.

Edit.. NM. Can't read, lol.

The little GB meter is rated at 2,000 ohms per volt input impedance. Thus if you are measuring a 5 volt signal, it is presenting 10,000 ohms to the circuit under test... in parallel with the circuit you are measuring. That can be pretty significant with modern electronics.
2K/per volt is pretty low input impedance by today's standards. 20,000 ohms/volt was pretty much standard for analog meters.
Digital (DVM)'s typically run 10 Megohms or better and generally cause an insignificant loading and/or distortion to most circuits.

Certain sensors can be damaged by using a meter with a low input impedance. Compared to the extremely high input impedance of the PCM, the meter looks like a short circuit.
It is possible that shorting an operating sensor could draw too much current through the sensor and damage it.
Whether that's the case with O2 sensors or not, I don't know. No point in risking it...just use the high impedance DVM.

Resistance is opposition to flow of current in a circuit. It is generally a static value. A carbon resistor is an example of a pure resistance, it's value is the same for AC or DC and does not change with AC frequency.

Impedance is also opposition to flow of current in a circuit, but in addition to pure fixed resistance it also includes capacitive and inductive reactances (basically more resistance), which change value based on frequency. Impedance therefore applies to AC circuits.

Capacitors present less opposition to current flow as the applied AC frequency increases and more opposition as the frequency is lowered, down to DC where they pass no current at all.
Inductors (coil, choke) do the opposite, offering extremely high opposition at high AC frequencies down to just the minimal resistance of the wire itself at DC.
These values, referred to as reactance, change as the frequency of the AC voltage in the circuit changes, and are added to the fixed resistances in a circuit to come up with the total impedance.

Thus in an AC circuit, the fixed resistances + the capacitive and inductive reactances = impedance.

Using a wirewound resistor as an example:
A 10-ohm wirewound presents 10 ohms resistance to a DC current. That is it's resistive value. 10 ohms. Period.
But...things change when AC is applied. Such a resistor is constructed of a coil of wire, and a coil presents additional opposition to AC, increasing as the frequency of the AC rises. Let's assume 5 ohms reactance at 10 Khz.
So adding the base resistance of 10 ohms and the inductive reactance of 5 ohms, the impedance of the resistor at 10Khz is 15 ohms.

Very well put!

Fantastic! I can even see the inductance from AC/, (or a dropping field), affecting the resistance in a coil. You guys moved a ball down the field that has been stuck on the 20 yd line for well over 30 years!
Impedance does seem to be "dynamic resistance", requiring AC, or at least interrupted DC, to come into play affecting resistance.

I'm still not seeing that grounding the wire, or lessening the resistance to ground on the sensor wire is gonna wreck the sensor. (a sensor which it's self sometimes grounds that 5 Volts to only 1). I do now see how the meter will affect it's own reading. Schrodinger's cat comes to mind!

I didn't even hook up the DVM, but should have. I understood that the meter's "sample rate" or some such would cause it to read total gibberish. Now that I have a better grasp on whats going on I'll rig it up again and take a closer look. Might even pick up one of those $10 WallMart analogues, (purely in the name of science!) That, and rereading your prior posts over a few more times will take a while, but in the mean time, much thanks to you for taking the time to explain. Don

Just so I can follow along - this thread is fascinating - you guys are saying that that the PCM normally presents a very high impedance, correct? Limiting the flow through the O2 so it doesn't get damaged? I think what rrich and Radi are getting at is that the cheapo analog meter has such a low impedance compared to the digital unit (and PCM) that using it to measure output voltage from the sensor is like shorting anything else ground due to the O2's sensitivity.

I guess the 91 and later might have an output from the 02, to your pcm. The Renix ECU has a 5 volt output that is grounded by the 02. Sort of the same principal as your temp gauge. That is if I have that straight. I really didn't look at the OBD types? much myself. I should have put Renix in the thread title.

Har har :P

So the Renix computer measures just the one end of the circuit instead of both?

The electrons in the wire are like billiard ***** in a tube. You push one in one end, one comes out the other. Both ends are the same. They will go freely in a good conductor, or run into resistance. No doubt the ECU limits the current available, so a small "shorting" by the sensor will lower the voltage.

The cigaret lighter on the other hand gets plenty of current and not so much resistance.