14 GEARS May 2002

I n the last issue of GEARS, welooked at how easily you can con-nect a scope or meter to typical

sensors without having a schematic ormenu to guide you. In this issue, we’lllook at a few sensors that aren’t quiteso obvious.

Mass Airflow (MAF) SensorsMass airflow sensors provide the

computer with an indication of engineload. By measuring the amount of airentering the engine, the computer candetermine engine load with muchgreater accuracy than with a typicalMAF sensor (figure 1).

NOTE: Some GM vehicles haveboth a mass airflow sensor and a MAPsensor. On those vehicles, the MAPprovides a backup for the mass airflowsensor, in case the MAF fails.

The problem with testing mass air-flow sensors is there are a number ofdifferent ones in use, with differentwiring configurations. In addition,some of those sensors have an aircharge temperature sensor built in,which adds a couple more wires to theconnector.

But in general, there are only twodifferent types of mass airflow sensors:frequency sensors and analog sensors.

Frequency sensors provide a digi-tal signal that increases in frequency asthe flow of air through the engineincreases. Analog sensors provide avariable voltage signal, much like aTPS does.

There are two types of analog sen-sor: the hot-wire sensor, commonlyused on GM vehicles, and the “flapper”design, which is more common onimports. But in each case, the sensor

by Vince Virgilio, EAST Training, Inc. and

Steve Bodofsky, Steve Bodofsky Productions

Figure 1

Common Mass Airflow (MAF) sensors. Left to right:1990 Mazda B2200 and 1988 Toyota Camry.

Figure 2

A waveform from a digital Mass Airflow (MAF) sensor. The frequen-cy of the pulses will increase as engine RPM increases.

MAKG CONN P2.qxd 4/4/02 4:41 PM Page 14

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develops a DC signal that varies withthe amount of air flowing through theengine.

So how can you use that informa-tion to check the signal from a mass air-flow sensor? Here’s how:

• Connect your meter or scopenegative probe to a goodground.

• Set your meter to read DCVolts.

• Turn the key on, engine off.• Backprobe the wires, one at a

time, and record your resultsfrom each one.

One or two of the wires you back-probe will have a constant voltage of 5or 12 volts, depending on the car you’reworking on. These are the power supplywires for the sensor.

One or two of the wires will have a

constant zero volts. These are the powergrounds or sensor signal grounds forthe sensor (yes, there is a difference!).

The last one or two wires shouldprovide a small voltage signal; any-where from a few tenths to a volt ortwo. If you’re using a scope, you shouldbe able to tell right away whether one ofthose is a digital signal. If you’re usinga meter, switch it to Frequency and seeif either of those signals is a digital fre-quency.

If one of the signals is a digital fre-quency, that’s the MAF sensor signal(figure 2); the other is the air chargetemperature sensor. Start the engine,and the MAF sensor’s frequency shouldincrease with engine RPM.

If neither of the signals show fre-quency, switch your meter back to DCVolts, and start the engine. Then checkboth signals again: The one thatincreased as soon as you started theengine is the MAF sensor signal (figure3). The sensor will create an analogvoltage signal that increases withengine RPM.

Before you attempt to examine thesensor signal further, switch your meteror scope’s ground lead to a sensorground. It doesn’t have to be the groundfrom the MAF sensor: All of the sensorgrounds are pretty much the same. Asyou’ll see, the ground you use can havea big effect on the signal measurement.

If the MAF sensor has an aircharge temperature sensor built in, youmay want to check that as well. The eas-iest way to check the air charge temper-ature sensor is to compare its voltagesignal with the Engine CoolantTemperature sensor voltage signalwhile the engine is cold. They should bealmost identical.

Once you start the engine, theengine coolant temperature sensor sig-nal will decrease fairly quickly; the aircharge temperature sensor willdecrease, but much less, and muchmore slowly.

Oxygen (O2) SensorsYou might be thinking “Wait a

minute, we’re transmission techs: Whywould we care about testing the oxygensensor?” But there are a couple reallygood reasons for a transmission techni-cian to want to check the oxygen sensor

16 GEARS May 2002

Making the Connection, Part 2

Figure 3

A waveform from an analog Mass Airflow (MAF) sensor, such as avane-type sensor or hot-wire sensor. The signal voltage increases asengine RPM increases.

Figure 4

This is how the oxygen sensor signal should look on a system that’sworking properly. The voltage should switch anywhere from once everytwo seconds, to 5 times a second.

MAKG CONN P2.qxd 4/3/02 5:04 PM Page 16

signal.To begin with, the oxygen sensor

signal is one of the most accurate waysof determining whether the computersystem is in closed loop. That regularswitching, from high to low and backagain, tells you the computer is accept-ing the oxygen sensor signal, and isadjusting the fuel delivery based on thatsignal (figure 4).

And the oxygen sensor signal isone of the best tools for examiningengine performance, to help you deter-mine whether a driveability problem isbeing caused by the engine or transmis-sion. Misfires, lean operation and evenvacuum leaks can be easier to identifyusing the oxygen sensor signal (figure5).

We’ll ignore the really unusualones, like the titanium sensor used onsome Jeeps and Toyotas. And we’llcompletely ignore the 7-wire sensorthat appears on the special lean-runningHondas: Those things are an article allby themselves.

For now we’ll just consider thesimple zirconia sensor that appears onvirtually everything else made today.You know them: the sensors that createtheir own voltage that ranges from zeroto one volt. And covering those sensorsalone is enough, because they come infour flavors: one-, two-, three- and four-wire sensors. Here’s how those config-urations differ:

One-Wire O2 Sensor: There’s onlyone wire; that’s the sensor wire.These sensors use the engineblock as the ground.

Two-Wire O2 Sensor: One wire isthe signal wire, the other is thesensor ground.

Three-Wire O2 Sensor: The firstheated oxygen sensor. Onewire is the signal wire, onewire is 12 volts for the heater,and the third wire is a sharedground.

Four-Wire O2 Sensor: A later heat-ed oxygen sensor. One wire issensor signal; one is sensorground. The other two wiresand power and ground for theheater.

So how do you connect your scopeor meter to these sensors, to check the

signal? If the oxygen sensor has onlyone wire, that’s easy: Connect yourmeter or scope positive lead to the sen-sor wire, and the ground lead to aground near the sensor (figure 6).

If the sensor has more than onewire:

• Connect your scope or meterground lead to a good ground.

• Set your meter to read DCVolts.

• Start the engine.• Backprobe the leads, one at a

time.

GEARS May 2002 17

Figure 5

This waveform is from the same vehicle as the one in figure 3, but withone cylinder shorted. This is how a misfire appears on an oxygen sen-sor signal.

Figure 6

It’s easy to connect to a single-wire sensor: Connect the posi-tive probe to the signal wire, andthe negative probe to a goodground, as close to the sensoras possible.

MAKG CONN P2.qxd 4/3/02 5:04 PM Page 17

On a two-wire sensor, one wire is the signal wire, theother is ground. Once you’ve found which is the signal wire,connect your positive lead to that wire and the ground lead to

the other wire (figure 7).On a three-wire sensor, one wire is the signal wire, one

wire provides 12 volts for the heater, and the third wire isground. Once you’ve identified which is which, connect thepositive lead to the signal wire and ground lead to the groundwire (figure 8).

The four-wire sensor is a little different: • One wire is the sensor signal wire. • One wire is the sensor ground.• One wire is 12 volts for the sensor heater.• One wire is ground for the sensor heater.

The problem is, there’s almost no way to tell the differ-ence between the sensor ground and the heater ground. Theyboth offer zero volts. So bypass the problem: Connect the pos-itive lead to the sensor wire, and the negative lead to a sensorground on a different sensor, such as the TPS (figure 9).

One Ground Isn’t Just as Good as Another…

Think there’s no need to switch your meter or scope’sground lead to the sensor ground? Think again: Those sensorground circuits are buffered to filter out much of the noisefrom inside the engine compartment. That’s why many manu-facturers’ specs allow for up to a half-volt difference betweenthe chassis ground and the sensor ground.

Think about it: Suppose the throttle position sensor basesignal is supposed to be half a volt, and there’s a half-volt dif-ference between the chassis ground and sensor ground. If youtried to adjust the TPS minimum voltage with your meter con-nected to the wrong ground, you’d actually be setting the TPS

18 GEARS May 2002

Making the Connection, Part 2

Figure 7 Figure 8

Find the signal wire.Then connect thepositive probe to it,and the negativeprobe to the otherwire.

Figure 9

Four-wire sensors have two grounds: one for thesensor, the other for the heater. Instead of tryingto figure out which is the sensor ground, con-nect the negative probe to the ground at anothersensor, such as the TPS.

Three-wire oxygensensors share aground between thesensor and the heater.Connect your positiveprobe to the sensorwire, and the negativeprobe to the sharedground.

MAKG CONN P2.qxd 4/3/02 5:04 PM Page 18

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voltage to zero. And since the signalwould read 0.5 volts on your meter,you’d have no way of knowing there wasa problem… until the system set a code.

Here are two oxygen sensor signals,taken from the exact same car, using thesame scope and same setup. The onlydifference between the two is theground used: The first shows the oxy-gen sensor signal with the ground leadconnected to a good ground on theengine. The second shows the differencewith the ground switched to the propersensor ground (figure 10).

As you can see, using the properground clears up an awful lot of noise inthe signal. You could easily mistake thatnoise for a misfire in the engine. What’smore, a spike from that noise could dropbelow zero — just as it did in this signal— causing you to suspect the sensoritself was faulty.

That’s why it’s important to use theproper sensor ground when measuringthe sensor signal.

Making the Connection, Part 2

Figure 10A

The signal on the left was taken with the scope connected properly,

using the sensor ground. Then we switched the ground probe to a reg-

ular chassis ground: That’s the only difference between how the two

signals were acquired.

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MAKG CONN P2www.qxd 4/24/02 10:41 AM Page 20

GEARS May 2002 21

So even on the mass airflow andoxygen sensors, finding the properwires to connect to isn’t all that diffi-cult. In the next issue of GEARS, you’lllearn how to connect your scope ormeter to check an actuator, such as asolenoid or motor.

Figure 10B See description from Figure 10A.

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