Computer sensors

can basically be

divided further

into three

categories,

depending on the

number of wires

they use

20 GEARS April 2002

W henever we tell techniciansthat they can get more outof a scope that doesnt pro-vide menus, invariably their response isthe menus make it easier to set up andconnect the scope.

While easier setup may be a some-what valid argument, connection reallyisnt. Thats because connection is sim-ply a matter of determining what youwant to examine, and then choosing the

right wire to pro-vide the signalyoure lookingfor.

One way todo that is byusing a schemat-ic. But mostshops only have alimited numberof schematicsavailable. And acorollary toMurphys Lawindicates theprobability of

your having a specific schematic isinversely proportional to the likelihoodof that vehicle appearing in your shop(you didnt know Murphy fixed cars,did you?).

So if you dont have the rightschematic, how can you determinewhich wire to connect your scope ormeter to?

The answer isnt all that difficult; itjust depends on understanding the

nature of the different sensors and actu-ators on a vehicle. In this article welllook at common sensors and actuators,and see how easily you can identifywhich wire is the signal wire.

To do this, were going to separatethem into sensors and actuators.

Sensors and Their SignalsComputer sensors can basically be

divided further into three categories,depending on the number of wires theyuse. With only a few exceptions, theylleither be one-, two-, or three-wire sen-sors.

And as youll see, that difference,along with its location, tells you justabout all you need to know to identifythe sensor, and determine which wire isthe signal wire.

One important rule to remember:when checking the voltage signal fromany sensor, the harness must be con-nected to the sensor and the ignition on.To check sensor resistance, the sensormust be disconnected and the ignitionoff.

by Vince Virgilio, EAST Training, Inc. and

Steve Bodofsky, Steve Bodofsky Productions

Figure 1: Except for some oxygen sensors, all one-wire sen-sors are temperature sensors. To check the signal from these

sensors, connect your positive lead to the signal wire, andyour negative lead to a ground location near the sensor.

Figure 2: Since most temperature sensors are NegativeTemperature Coefficient (NTC) sensors, the voltage signal will be

high when the engine is cold, and will drop off as the enginewarms up.

One-Wire SensorsVirtually every one-wire sensor in

use is some type of temperature sen-sor and even those are pretty rarethese days. The only exception is thatsome oxygen sensors also have onlyone wire. Well talk more about oxygensensors in the next issue.

If you do run into one of these sin-gle-wire temperature sensors, identify-ing the signal wire is about as easy as itcan be, since theres only one choice.

These sensors provide a varyingresistance, which changes the resist-ance to ground, to pull the signal volt-age down as the sensor warms up. Sothe signal will be a voltage thats highwhen cold, and drops off as the sensorwarms up most of the time.

Connect your scope or meter posi-tive lead to the wire coming into thesensor, and the negative lead to a goodground, preferably near the sensor body(figure 1).

The signal you get will depend onthe specific vehicle youre working on.Since nearly every temperature sensorin use is a NTC (negative temperaturecoefficient) sensor, the resistance willdecrease as the temperature increases.So the voltage signal will also decreaseas the engine or transmission tempera-ture increases (figure 2).

But some systems most notablyChryslers and some GMs have a lit-tle twist to them. At a certain tempera-ture usually about 120 F (49 C) the computer switches a second resistorinto the circuit (figure 3). This causesthe voltage to jump, and start droppingall over again (figure 4). This is nor-mal; the computer does this to improvethe sensor resolution at high tempera-tures.

Two-Wire SensorsIf threaded into the water jacket or

intake manifold, two-wire sensors willalmost always be temperature sensors.If the sensor sits down near the cam orcrankshaft, or is mounted into the sideof the transmission or transfer case, itsprobably a permanent magnet (PM) ACgenerator.

22 GEARS April 2002

Making the Connection Part 1

Figure 3: Some manufacturers switch in a secondresistor in the coolant temperature circuit as the

engine warms up. This is to improve sensor resolutionat higher temperatures.

Figure 4: Whenthe computer

switches the sec-ond resistor intothe circuit, the

voltage signal willjump back up,and then swingdown again as

the engine contin-ues to warm up.

Figure 5: The second wire on a two-wiretemperature sensor is simply the sensor

ground. To test the sensor signal, connectyour positive lead to the sensor signal wire,

and your negative lead to the sensor ground.

Figure 6: While some schematics might indicate otherwise, Permanent Magnet (PM) AC generators

don't really have a positive or negative side. To checkthe signal, connect one lead to one sensor wire, the

other lead to the other sensor wire.

To identify the signal wire on atwo-wire temperature sensor:

Turn the key on, engine off. Connect your meters negative lead

to a good ground. Probe the wires, one at a time, with

the positive lead.

One will be ground, the other willhave voltage to it: the one with voltageis the signal wire (figure 5). Onceyouve found the signal wire, connectyour scopes positive lead to it, andmove the negative lead to the sensorground wire.

Permanent magnet AC generatorsdont really have a signal and groundwire: both wires provide the signal. Tocheck the signal, connect your scopespositive lead to one wire, and the nega-tive lead to the other wire (figure 6).

The signal will be an AC waveformthat varies in both amplitude (peak topeak voltage) and frequency (cycles persecond), with the speed of the devicebeing measured (figure 7). So if its acrankshaft sensor, the signal willincrease with engine RPM. If its avehicle speed sensor, itll start at zero,and increase directly with vehiclespeed.

Three-Wire SensorsCAUTION: Some two-wire AC

generators have a third terminal; this isa ground for a shielded housing, to pre-vent signal errors caused by induction.Dont confuse these sensors with three-wire sensors.

Depending upon its location, athree-wire sensor will either be sometype of potentiometer (such as aThrottle Position Sensor) or a Hall

Effect sensor, such as a camshaft,crankshaft or vehicle speed sensor. Themajor exception to this is the Ford MAPsensor. But the procedure for identify-ing the signal wire on Ford MAP sen-sors is the same as for a potentiometer;heres how:

Turn the key on, engine off. Connect your meters negative lead

to a good ground. Probe the wires, one at a time, with

the positive lead.

One wire will supply power (usual-ly 5 volts), one will be ground, and thethird is the signal wire (figure 8). Asbefore, once youve found the signalwire, connect your scopes positive leadto it, and move the negative lead to thesensor ground wire.

GEARS April 2002 23

Figure 7: The signal from a Permanent Magnet (PM) AC generatorwill be an AC signal that varies in frequency and amplitude withthe speed of the sensor wheel, or reluctor. As the speed increas-

es, the frequency and peak-to-peak voltage increase with it.

Figure 8: Whether you're connecting a potentiometer or a Ford MAP sensor, the connection procedure is still the same:

positive lead to the sensor signal wire, negative lead to the sensor ground.

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The signal from a potentiometershould be a smooth voltage that variesbased on the action of the sensor (figure9). For example, a throttle position sen-sor should provide about a half a voltwith the throttle closed, and shouldincrease smoothly as you open the throt-tle. At full throttle the voltage should beslightly below reference voltage.

Once again, the exception is theFord MAP sensor, which creates a digi-tal voltage signal that varies in frequen-cy as the vacuum level changes (figure10). But the procedure for finding thecorrect signal wire is the same as for apotentiometer: the only difference is thesignal you get.

For connecting to a Hall Effect sen-sor, theres one extra step in the procedure:

Turn the key on, engine off. Connect your meters negative lead

to a good ground. Probe the wires, one at a time, with

the positive lead. Actuate the sensor slowly as you

probe each wire.

For example, if youre checking thesignal on a crankshaft or camshaft sen-sor, youll have to tap the engine aroundwith the starter as you check each wire;if its an output shaft speed sensor,youll have to turn the wheels slowly.

The object is to rotate the wheelwindows past the sensor, to actuate thesensor, and turn the voltage on and off.

What youll find when you checkthe wires to a Hall Effect sensor is one

wire will have referencevoltage usually 5 or 12volts, depending on thesensor. Another wire willalways be ground. Butfrom the third wire thesignal wire you shouldget a voltage signal thatswitches on and off as youactuate the sensor.

Connect your scopeor meter positive lead tothe sensor wire, and thenegative lead to the sensorground wire (figure 11).Then operate the sensor.This may involve startingthe engine, driving thevehicle, or turning the wheels. The sig-nal will be a digital, on-off signal (fig-ure 12).

Of course, there are some oddballsensors, such as Mass Airflow sensors,which have more than three wires. And

then there are the oxygen sensors thathave three or four wires. Well take alook at how to connect and check thosesensors, along with solenoids and actu-ators, in the next issue of GEARS.

24 GEARS April 2002

Figure 9: A potentiometer, such as a Throttle Position Sensor,will develop a continuous sweep signal as you work the sensorthrough its range. Any sudden dropouts or glitches indicate a

problem with the sensor.

Figure 10: Ford's MAP sensor produces a digital signal thatincreases in frequency as manifold pressure increases. At idle,

the frequency should be about 110 Hz at sea level; key on,engine off, it should be about 159 Hz.

Figure 11: To check the signal from a Hall Effect sensor, connect your positive lead tothe sensor wire, and your negative lead to the sensor ground.

Figure 12: A Hall Effect sensor creates a digital, on-offsignal that varies in frequency with the speed of the

component.

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