Warning: electronics newbie (software guy) asking hardware question.

I notice that some sensors, like the TPS sensor, have a reference voltage line. I'm guessing that's so that they can compare the sensor voltage (call it Sv) to the reference voltage (call it Rv) and figure out how far the variable resistor in the sensor is (Sv/Rv).

But then most sensors, like the IAT, Oil Temp, MAP and so on, don't use voltage at all but instead use a reference ground and I guess some amount of resistance to give a reading.

So two questions:
A) Does anyone know why most sensors seem to be ground-resistance, but some (MAP, TPS) are voltage reference?
B) Why do the MAP and TPS each need their own separate 5V reference line coming from the PCM? Why not the same 5V reference?

Thanks!
- Dave

 

Just some random thoughts to stimulate discussion....

The sensors that use a 5V reference generally are "displacement" devices, where physical movement alters the position of the "wipe" on the resistance coil. TPS rotation/displacement is obvious. Pressure sensors move a piston. The MAP sensor is just a refined version of this... I think the sensor membrane is a strain gauge in a wheatstone bridge configuration.

The exceptions to the +5V reference voltage are the inlet air and coolant temp sensors. With only 2 wires, a current source is applied directly to the element, and the resulting voltage reflects the resistance of the device. These achieve variable resistance by nature of the "thermistor"... no moving parts - resistance is inversely proportional to temperature.

In all cases, the PCM reads what you call "Sv" and converts that signal to a value.... e.g. kPa, % rotation, etc.... I don't think there is any direct comparison vs. what you call "Rv".

It would be entirely possible to hook several devices to a single reference voltage source. In the 4th Gens that is actually done with the MAP sensor, A/C Evap temp sensor and the A/C refrigerant pressure sensor, all of which draw +5V from pin B29.

Not sure why they use a separate source for the TPS sensor.

 

How do they actually measure the resistance of the oil/water/iat sensors? ie: if you had a voltmeter and a voltage source, how would you simulate measuring it?

I have an Atmel Mega16L that has A-D lines and I was thinking of building something to tinker with by hooking it up to interesting sensors... but not sure how to measure resistance with an A-D line!

 

I think I may really understand this now, but I need some electronics guru to review my logic

To measure the resistance of a ground-referenced sensor, I think the PCM starts with a 5v source internally and it has a resistor on the inside of some known value, and then it connects to the sensor which acts as a resistor of some unknown value.

5v---1kohm-----X----sensor-----Gnd

You could do it without the 1Kohm resistor internally, but then you'd have a dead short if the sensor went to 0 ohms.

So if the PCM seems 3.125mA of current flowing to the sensor, it uses E=IR to figure out that there must be 1600 ohms of total resistance between the 5V and Gnd, so the sensor is at 600 ohms.

If I'm right so far, then you can't "eavesdrop" on a ground-reference sensor since you have no way of knowing the internal resistance. That's why, I'm thinking, with a FAST system or other unit that splices into the factory PCM you wind up robbing the PCM of its view of the sensors entirely and replacing it with a dummy resistor.

With a MAP sensor, refrigerant pressure sensor, or TP sensor, you have a reference voltage, so you can eavesdrop on both the output voltage and the reference voltage, do your comparison, and multiple parties can look at these types of sensors.

So if I'm right in all the above (please let me know), the only question would be why is the factory nice enough to use reference voltages for the TPS and MAP when they -could- get away with just a normal ground referenced sensor?

 

if you want some real fun try figuring out how the computer reads the MAF signal

 

That's easy, its a pulse width frequency signal in the 0-65K range. But feel free to be addititive by answering the question in the thread ;-)

 

As you've figured out, you can't simply "piggyback" an aftermarket ECU on top of the factory computer. It will only work on "3-wire" sensors, where both systems use the same reference voltage (they usually do). But you can't attach two devices to a "2-wire" resistance device like a thermistor. In that case you need to add another sensor if you want to feed two computers. I run primarily off a MoTeC M48Pro engine management system, but retain the stock PCM for idle air control, emissions devices and VSS/speedometer. I have dual IAT and coolant sensors, one for each computer.

Your description of the typical temperature sensor is correct. The sensor resistance (Rt) is connected to a reference voltage (Vr) through a fixed resistance (R). The sensor output voltage (Vt) is given by the equation:

Vt = Vr x (Rt / (R+Rt))

A good reference, that answers most of these questions:

Understanding Automotive Electronics, William Ribbens

 

"resistance is inversely proportional to temperature."

That assumes that you are dealing with a NTC - negitive temperature coefficient thermistor, do you never see PTC in cars anymore? From what I hear they are more expensive to manufacture and you can easily reverse the scale in the computer insted of using one, but I have head some european cars use them.

MAFs use one of 2 ways of measuring airflow.
The Karmon Vortex Maf uses sound frequencys like the other guy said.
The heated wire Maf heats a thin wire to a set temperature and measures the current needed to keep the wire at that temperature.

 

I haven't seen one.

Its "Karman".... and there's some good basic info on this page:

http://www.autoshop101.com/forms/h34.pdf

These are used extensively in Mitsubishis. I don't know if this is the 65Khz reference in the post above comes from... I don't know what the frequency range is. The frequency appears to be generated by oscillations of a mirror that responds to the vortices shed by the vortex generator.... seems sort of complicated and sensitive. I know that some Misubishi "rodders" replace the Karman with a hot-wire, and then convert the hot-wire output signal to a frequency signal that emulates the output of the Karman - not sure if they do this because of the reliability issuie, or to gain a greater flow capability.

The 3rd Gens F-Bodys used a Bosch hot-wire with direct voltage output. The 4th Gens use the more reliable GM hot-wire, with an intenal conversion to a variable frequency output, ranging to max capability of 11,216Hz at 471.5 GPS.

 

What I meant was that the GM MAF internally converts the current measurment into a pulse width modulated frequency signal that is transmitted to the PCM.

Fords, on the other hand, send a 0-5v voltage (and possibly a reference voltage).

To use a Ford MAF in a GM car, you need a circuit that will convert their voltage output to a frequency. That's actually why I bought my Atmel dev kit and what prompted all this fancy learnin' about sensors in the first place!

 

But where does the "0-65K range" come in?