multiplex wires

I have a few questions regarding muitiplex wires, so I thought I'd start a new thread on the subject. My basic understanding is that a multiplex(MUX) wire is a single wire through which multiple 'signals' can be sent to a computer, usually a body control module(BCM). 

In my case, my 2003 Jeep Liberty, all the mutliplex wires will show different negative resistance values for different functions through the same wire. For example, if I unlock the door the wire attached to the swich will show 426ohms, but if I lock the door the switch will show 1.4ohms.

Here are the first two questions I have.

What happens if I were to apply 2k ohms to the lock/unlock wire for example, would the BCM ignore it, or would it cause problems?

I thought is that if I apply an 'unrecogized" resistance, the BCM will simply ignore the signal. That is, as long as it's the same polarity.

How do you test to see if a wire is multiplexed or not? And if it is, how do you know what resistor you need?

My thought here is that if I test the wire in question, it will show 0 resistance if it's not multiplexed when the accessory is activated. If it is multiplexed, it will show some sort of resistance.

To tell which resistor I would need to operate the circuit with an alarm or remote car starter, I would need to cut the wire and put my DMM(measuring resistance) in series and then activate the accessory.

Is any of this right?

A system like you are talking about is not actually measuring resistance, it is measuring voltage.

If you take two resistors and place them in series and place 5vdc at one end and 0vdc at the other end you can calculate the voltage in between the two resistors as a ratio of one resistance to another - This is called a voltage divider circuit.  The voltage in the middle of the two resistors can be calculated by this equation: Vout = (R2 / (R1+R2) * Vin (more info here: http://en.wikipedia.org/wiki/Voltage_divider)

The monitoring device, in your case the BCM, then reads that voltage through an analog to digital converter and turns the value to a digital number that can be evaluated in software (for example if using a 10 bit ADC the number will be converted in a linear fashion between 0-1023, for an 8bit number it would be 0-255).

Now, because resistors arn't perfect and impedence can be affected by temperature a typical designer will include at least a certain amount of error around the "center" number.  Basically, instead of looking for a 775 for door lock you look for a 750 - 800 or even a 600 - 950.  Generally, the smaller number of multiplexed inputs the larger the error can be around center numbers.  You can take it to the extreme and convert it back to a digital on/off signal by check to see if a signal is 1000 or below (low)  or above 1000 (high) - it all depends on the requirements of the particular system at hand.

Reguarding invalid numbers - if a system is set up to look for 100, 200, 500, or 1000 the firmware may look at a widow between 50 and 150 and if the number isn't in there look between 150 and 250 - if the value isn't between those it would look between 450 and 550 for the 500 value.  If no number there it would check 950 to 1050 and if no value there it would just exit, ignoring whatever the value on the line happens to be.

I hope this gives enough basic information - it is hard to be specific without knowing the performance requirements of the system!

So what you're saying is, by changed the resistance of the resistor in the Z2 position in the diagram below, you're change the Vout. Vout is what is monitored by the BCM.  

Simple enough.

I would like to wire the rear defogger circuit in my Jeep through my remote car starter. I believe the wire is a multiplex wire, but I don't know which resistor to use. What is the best way to determine that? Test accross the switch while pressing the button? Cut the wire and put my DMM in series with both wires?

There are several ways - the "correct" way would be to find the two wires coming from the switch, find the one that rests at ground and then cut the other.  Next, strip and connect one lead to the switch side of the wire you cut and then connect the other lead to ground.  Push the button and measure the resistance.

Another thing you could do is just measure the voltage and work backwards.  The most common "z1" resistor is 1Kohm.  You could calculate Z2 by using the measured voltage and with Z1 = 1kohm.  Then, put Z2 in the circuit, activate it, and measure the voltage at the junction.  If the voltage is correct, you are done, if it isn't correct, use the voltage measurement along with the Z2 resistance to calculate Z1, then use the calculated Z1 with the desired voltage and recalculate Z2!  Sounds like a lot of math, but it isn't bad.

Why do you think the wire is multiplexed?

I'm not sure if the rear defogger is multiplexed or not. I can't really tell from the factory service manual, and the vehicle wiring I got from DEI doesn't reference it at all.

The rear defogger is not multiplexed. I tested everything, then wired it up this morning and it works beautiful.