Download pdf - Making the Connection Pt2

14 GEARS May 2002

I n the last issue of GEARS, welooked at how easily you can con-nect a scope or meter to typicalsensors without having a schematic ormenu to guide you. In this issue, welllook at a few sensors that arent 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 flapperdesign, 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

Meeting the demands oftodays market means keeping pace with the industrys latest transmissions.

Allomatic offers a large anddiverse product line that includes filters, friction plates,steels and bands. To keep pacewith market demands,Allomatic adds parts for latemodel transmissions whentheyre introduced. Allomaticdelivers replacement parts thatare manufactured and qualitytested to assure value withconsistent dependable OE performance.

A Raytech Powertrain, Inc. Company

Plant: 609 East Chaney StreetSullivan, IN 47882

Sales: 110 Jericho TurnpikeFloral Park, NY 11001

Toll Free: 1-800-568-0330Web Site: www.allomatic.com

Todays Market Demands the Best. Allomatic Delivers.

allomatic placed 3/20/02 10:53 AM Page 15

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? Heres 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 youreworking 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 youre using a scope, you shouldbe able to tell right away whether one ofthose is a digital signal. If youre 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, thats the MAF sensor signal(figure 2); the other is the air chargetemperature sensor. Start the engine,and the MAF sensors 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 scopes ground lead to a sensorground. It doesnt have to be the groundfrom the MAF sensor: All of the sensorgrounds are pretty much the same. Asyoull 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, were 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 thatsworking properly. The voltage should switch anywhere from once everytwo seconds, to 5 times a second.


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).

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

For now well 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. Heres how those config-urations differ:

One-Wire O2 Sensor: Theres onlyone wire; thats 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, thats 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

Its 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.


On a two-wire sensor, one wire is the signal wire, theother is ground. Once youve 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 youve 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, theres 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 Isnt Just as Good as Another

Think theres no need to switch your meter or scopesground lead to the sensor ground? Think again: Those sensorground circuits are buffered to filter out much of the noisefrom inside the engine compartment. Thats 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 theres 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, youd actually be setting the TPS

18 GEARS May 2002


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.


DELAYED ENGAGEMENT AND CONVERTER BUSHING FAILURE

4TH ACCUMULATOR BORE SLEEVE AND PISTON KIT

MANUAL VALVE

Teflon square cut seal and D-ring seal

Sleeve

Pinless accumulator piston

Valve Body

OEM Spring

Accumulator Body

SCORED OR DAMAGED 4TH ACCUMULATOR BORE

Visit our website dedicated to assist & guide transmission rebuilders

www.sonnax.com

Chrysler rear wheel driveunits do not charge theconverter or cooler in park

CAUSE:

CORRECTION:Install this manual valveto allow the converter tocharge in park. No mod-ifications are necessary.

Debris and piston contactcan score the housing bore

COMPLAINT:

CAUSE:

CORRECTION:

Also: Poor shift quality

Install this sleeve and pinless accumulator piston.

4L80-E

CHRYSLER RWD UNITS '78 ON W/TCC

Manual Valve22771-09Patent Pending

Also Available:Lube-regulated pressure regulator valve22771A-02KPatent Pending

Part No.

22771-09Patent Pending

COMPLAINT:

Sonnax Automatic Drive P.O. Box 440Bellows Falls, Vermont 05101-0440 USA

802/463-9722 800/843-2600 fax: 802/463-4059 [email protected]

and visit our website:

for SonnaxTM reamers & other tools.

See your nearestTransmissionSpecialtiesdistributor & askfor Sonnaxquality engineeredproducts.

Featured product of the month:

THM 400P/N: 34310-01

Accumulator PistonRetaining ring

Also availablefrom SonnaxTM:

ATXP/N:

86940-05KATX BOOST VALVE

& SLEEVE KIT

4L60, 3T40,4T60, 200,

325-4L, 200-4RP/N:

77872-02KUniversal TV Cable

Repair Kit

TM

No valve body modifications requiredwith either valve!

IMPORTANT!Either valve canbe used indepen-dently.

For greatest ben-efit, use as a pair.

Part No.

34805-01K

GRTSAD0502 3/20/02 11:07 AM Page 19

voltage to zero. And since the signalwould read 0.5 volts on your meter,youd 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. Whatsmore, a spike from that noise could dropbelow zero just as it did in this signal causing you to suspect the sensoritself was faulty.

Thats why its important to use theproper sensor ground when measuringthe sensor signal.


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: Thats the only difference between how the two

signals were acquired.

Building Quality Since 1977

1-800-331-6405955 Kiowa Ave. Lake Havasu City, AZ 86403-2838

www.instaclean.com

Out OurSpecials At

20 GEARS May 2002

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 isnt all that diffi-cult. In the next issue of GEARS, youlllearn how to connect your scope ormeter to check an actuator, such as asolenoid or motor.

Figure 10B See description from Figure 10A.

Tel: 507-452-4043 Fax: 507-452-7970

Americas Leading Designer and Manufacturer of Quality Automotive Machine Shop Equipment

Backed byFactoryTrainedSupport TeamsNationwide.Call Barb today forprice and availability

Get more Flywheel Grinder for your moneywith a DCM CASH MACHINEGet more Flywheel Grinder for your moneywith a DCM CASH MACHINEExclusive Features that PAY... 21 Capacity Exclusive piloted 3 point

mounting system: Fastest and most accurate available

Automatic lead screw slack adjuster

Automatic feed option eliminates need to babysit operation ...like getting a free technician

Ergonomic, rugged design for simple operation

Made in USA

1-800-533-5339Tel: 507-452-4043 Fax: 507-452-7970

Keep all of your operations in house and finish jobs faster. Keep the profits in your own pocket rather than sending

them out with your flywheels. Keep your technicians happy by eliminating wait time.

With DCM Tech you get the Best machine for the money, complete standard tooling package, & the best technical support in the industry . . . in short the biggest BANG for the Buck!

DCM Tech, Inc. 4455 Theurer Blvd. P.O. Box 1304 Winona, MN 55987Visit our Website at www.dcm-tech.com

AttentionTransmissionRebuilders

HOT FLUSH, DCI ENTERPRISES, INC.800-852-5713 www.hotflusher.com

Transmission Cooler Flushing

Increase shop efficiency Reduce come-backs 8 models available Automatic backflush Tells you when cooler is

clean 6 to 8 quarts of fluid 10-15 min. heat up time 16 gauge welded steel

cabinet Dual-Filtration system Uses 700 R4 filter & #28

micron catch screen Uses hot oil at 160

degrees + 5 minutes and $12.00 to

change the fluid in the machine

Uses 110 volt electricity

Endorsed byDACCO

Approved Tool
