34 GEARS January-February 2003

In the emergency room, doctorsoften must perform test proceduresvery carefully to provide the properresults, without causing damage to thepatient. Much the same way techniciansmust perform complex diagnostic rou-tines to test systems operation, withoutdamaging sensors or computers thatdepend on these inputs. We’ve alreadylooked at a number of test procedures inthis series:

• Part 1 – We went over the impor-tance of power and grounds andhow to check them correctly(January/February 2002).

• Part 2 – We tested temperature sen-sors and throttle position sensors,starting the tests at the sensor andworking back to the computers(March 2002).

• Part 3 – We looked at park/neutralswitches and accelerator switches.And from park/neutral switches wewent into range selectors, whichare very important for controllingline rise and providing manual gearrange information in electronical-ly-shifted transmissions (October2002).

In this issue of Doctor, Doctor…we’re going to look at pressure switch-es and speed sensors. Once you under-stand these different inputs, you can seehow the computer uses the informationfrom these sensors to make decisionson the outputs controlling shift timingand shift feel on many transmissions onthe road today.

Pressure SwitchesPressure switches aren’t new to

transmissions. On earlier units, theseswitches were used basically for adjust-ing ignition timing and letting enginecontrol units know there would be aload change.

Pressure switches don’t have to behydraulic; they can be vacuum sensitiveinstead.

A pressure switch is just as itsname implies: It’s a switch. That meansit’s either on or off, just like a lightswitch. There are three basic types ofpressure switches:

• Pull-Down — This switch normal-ly receives power, and pulls the cir-cuit down to ground when activat-ed.

• Pull-Up — This switch normallyreceives no power, and pulls up tosystem voltage when activated.

• Toggle — This switch is part of acircuit, and either opens or com-pletes the circuit when activated.

Electrical Testing in theOperating Room; Part 4Electrical Testing in theOperating Room; Part 4

DOCTORDOCTOR…It hurts when I shift!by Randall Schroeder

Figure 1A

Figure 1B

36 GEARS January-February 2003

ARIEL TRANSMISSION PRODUCTS

NEW Standard & Automatic Parts

Quality Checked Reconditioned Parts

NEW OE Transmissions

Rebuilt Pumps

Clutch Kits

Torque Convertor Parts

Automatic Cases

Bearing Kits

ARIEL’S TRADEMARK IS OUR

OUTSTANDING QUALITY

See us online at www.arielgears.com

1-877-943-70401-877-943-7078 Fax

WE HAVE MOVED!

Our New 100,000sf

warehouse is located:

2301 N. Douglas Blvd.

Midwest City, OK 73141

Call T

oday!

You asked for more...,NOW YOU’VE GOT IT!

IF YOU’RE NOT USING ARIEL..., YOU’RE IN THE WRONG GEAR!!!

With that in mind, the tests becomefairly simple. Of course, it’s always agood idea to have a complete wiringdiagram on hand, so you know how farto trace the circuits. On later units, suchas 4L60E, 4L80E, 4T80E, etc., thepressure switches tell the computerwhich gear a driver has selected, similarto the range selectors we talked about inpart 3.

The best way to test a pressureswitch is with a voltmeter. You’ll wantto see that there’s voltage to the switch,and make sure the voltage switchesproperly when activated. Connect thevoltmeter negative lead directly to thenegative battery post and use the posi-tive lead to check the circuit (figures1A,1B & 1C).

Speed Sensors Speed sensors can be kind of picky

when it comes to testing, because somany different things can interfere withthe signal. Basically there are threetypes of sensors: reed sensor, pulse gen-erator, and Hall Effect sensor. We’vepretty much exhausted specific testingwith these sensors in previous issues ofGEARS, but let’s highlight the tests tomake sure you understand how to checkthem.

Speed sensors will have a variety ofnames, such as ISS (Input Speed

Speed sensorscan be kind ofpicky when itcomes to test-

ing, because somany different

things caninterfere with

the signal.

Electrical Testing in the Operating Room; Part 4

GEARS January-February 2003 37

Sensor), OSS (Output Speed Sensor),TSS (Turbine Speed Sensor), and VSS(Vehicle Speed Sensor).

Reed Switch Speed SensorA reed switch sensor (figure 2) is

always a VSS. It’s a set of contacts thatopen and close as a magnetic reactingring or magnet passes by. The computersends voltage to the sensor, and as themagnet closes the contacts, the voltageis sent back to the computer. The com-puter measures speed by measuring thefrequency of the pulses from the sensor.This signal is a DC frequency; that is,it’s either on or off, and the pulses varyin speed based on the speed of the mag-netic ring.

Testing the sensor is fairly simple.Start by making sure you have voltagebeing supplied to the sensor (figure 3):

• Connect your voltmeter negativelead to the negative battery post.

• Disconnect the sensor harness.

• Turn the key on, engine off.

• Probe the wires at the sensor con-nector with the positive lead.

One of the terminals should havevoltage to it (figure 4); this is the sensorfeed wire. The voltage can be as high as12 volts or stepped down to 5 volts,depending on the vehicle.

Now that you know which wireprovides the power to the sensor, checkthe sensor itself:

• Reconnect the harness connector tothe sensor.

• Connect your voltmeter positivelead to the switching wire.

• Rotate the shaft with the magneticring, so the magnet passes theswitch.

The signal should turn on as themagnet approaches the sensor, and thenoff as it passes by. Make sure youreceive a signal back at the computer toverify the harness. You can also drivethe vehicle, and check the sensor signal

Phone Hours: Open 7:00 to 5:00 pm/ PSTCall (800) 421-5580 • Local (310) 547-4016

FAX ( 310) 548-6516Order 24/7 at www.slauson.com

In a Perfect World,Every Transmission Shop

Would Have a Slauson Downthe S t re e t .

Unfortunately that’s not case. Not to worry…!With a single phone call, you can still geteverything you need for the job by tomorrowmorning at a competitive price. And,with the Slauson Catalog & CD byyour side our knowledge andexpertise are right at yourfinger tips.

With The Widest Selection of Automatic Transmission Parts Available.

• Used, New, and Rebuilt Hard Parts• Soft Parts• Electrical Components

Slauson… Help is Just aPhone Call Away.

frequency; it should increase with vehicle speed.

The most accurate way to testspeed sensors is with a scope or graph-ing meter. This allows you to see anyinterference in the waveform that maylead to false speed inputs (figure 5).

Pulse Generator Speed SensorPulse generators (figure 6A & 6B)

are magnetic sensors that create an ACsignal. This type of sensor is used for

measuring both vehicle speed and inputspeed. You’ll see them listed as TSS,ISS, OSS, and VSS. There are twothings you should check on these sen-sors: signal voltage level, also calledthreshold voltage, and signal frequency.

The threshold voltage is the peak-to-peak voltage of the signal; that is, thetotal signal amplitude. In general, youshould see at least 500 millivolts ampli-tude from the sensor, and it shouldincrease with the speed of the sensor or

trigger ring. Low signal voltage couldbe caused by a bad sensor, or it couldindicate the air gap between the sensorand the trigger wheel is too wide (fig-ure 7).

Next, check the AC signal frequen-cy. Once again, the best way to checkthis signal is with a scope or graphingmeter. The signal should increasesmoothly in frequency as the sensor ortrigger wheel speed increases. At thesame time, look for any interference in

38 GEARS January-February 2003

Electrical Testing in the Operating Room; Part 4

Figure 1C Figure 4

Figure 2

Figure 3Figure 6A

Figure 5

40 GEARS January-February 2003

the signal that could cause a false reading in the computer.

Hall Effect Speed SensorHall Effect sensors consist of a

semiconductor switch and a fixed mag-net. A rotating shutter wheel opens andcloses a window between the two.When the window is open, voltagepasses through the switch; when it’s

closed, voltage is closed off. The resultis an on-off, digital DC signal.

Hall Effect sensors have threewires: power, ground and signal. Withthe key on, engine off, the power wirewill have reference voltage; that couldbe either 5, 8 or 12 volts, depending onthe vehicle. The signal wire should beeither high or low — voltage or none —depending on the position of the shutter

wheel (figure 8). As you rotate the component that controls the shutterwheel position, the voltage will drop tonearly zero when the window is closed,and will rise to reference voltage as thewindow opens. When operating nor-mally, the result is an on-off digital sig-nal (figure 9).

As before, start at the sensor itselfand work your way back to the con-

Electrical Testing in the Operating Room; Part 4

Figure 6B Figure 7

Air gap

1-800-776-4433Fax: 1-856-222-4169www.wesellhardparts.com

What Makes H.F.T.So Special?

Experience The Difference!VW Passat and ZF 5HP18 valve bodies:1. Disassembled2. Cleaned3. Valves are removed, inspected and replaced with

new Sonnax valves4. Solenoids are flushed and tested with the

Sol-X 2000 Zoom tester5. Reassembled with the utmost care6. Valve bodies you can use with confidence7. The builder’s only concern is replacing gaskets

if necessary

VW Passat and ZF 5HP18

troller for the input from this sensor. In some cases the sensormay feed more than one circuit so you’ll need to follow it allthe way to each computer.

In the next issue of Doctor, Doctor… we’ll look at thecomputer outputs that affect shift feel and timing as we closeout this series on electrical testing. Until next time, keepthose transmissions in good health!

The Doctor

GEARS January-February 2003 41

Figure 8 Figure 9