My Vss Hertz

4 GEARS September 2002

My VSS He

rtz!VSS THE PROPER DIAG...

V ehicle speed sensors come indifferent shapes, sizes anddesigns. But they all have onecommon purpose: To provide the com-

puter with a signal to indicate road speed.In this issue of GEARS, were going tolook at some of the more common speedsensors and learn how they work.

Almost every manufacturer has asensor to measure road speed. The com-puter uses these sensors to determineshift points and overall transmissiontiming. Think of the VSS (VehicleSpeed Sensor) as the transmissionsgovernor, and the TPS (ThrottlePosition Sensor) as the throttle valvecable. These sensors are crucial to prop-er engine and transmission operation.As the throttle signal increases, thevehicle speed will usually increase, too.When the TPS and VSS signals reach apredetermined level, the computercommands the shift. Then the processstarts all over again.

If a speed sensor is acting up, youcan run into a number of different dri-vability conditions: harsh or soft shifts,erratic shifts or shift points you mayeven have a car that stalls the enginewhen you put the trans into gear. Ratiocodes are another common problemthat can be caused by a faulty sensor.

Most vehicle speed sensors gener-ate an AC signal, but there are somethat generate DC. Some vehicles have aremote module that converts the ACsignal into DC; others have that modulebuilt inside the powertrain or transmis-sion control module.

AC-generating speed sensors usu-ally have only two wires going to them(figure 1). You can test these sensorswith a scope or DMM. The AC signalvoltage should rise with road speed andcan reach 20 or more volts. These sen-sors are permanent magnet generators.In the example used here (figure 2a,2b), the vehicle speed sensor is locatedon the differential housing, and it cre-ates a signal based on movement from

by Lance Wiggins

Figure 1

Figure 2A

Vss Lance.qxd 7/31/02 2:50 PM Page 4

4060 Dixon Street Des Moines, IA 50313Phone: 1-800-598-1933 (775) 352-6800 l Fax: (775) 352-6813 e-mail: [email protected]

4060 Dixon Street Des Moines, IA 50313Phone: 1-800-245-2203 (920) 865-4266 Fax: (920) 865-4270 e-mail: [email protected]

Reliable transmission testing takesthe guesswork out of transmissionrebuilding and repair.

The result: lower costs, happiercustomers and a reputation forquality work.

We offer dynamometers, valvebody testers and solenoid testersfor all shop sizes.

TCRS is dedicated to keeping you on the leading edge of technology.

Axi-Line and TCRS are members of ALL NEW: www.helc.net

Over seven million torque converters have been rebuilt withTCRS equipment. Its the choice of quality rebuilders worldwide.

The Auto-Weld-Aligner, the flagship of the line, features air-oper-ated collets, patented auto-tack and auto-weld, the ability to dobowl-buildups and weld on impeller hubs.

Make the same choice as the Big Three: Ford, GM and Chrysler--all use the TCRS Rebuilding System!

Call us for information on special pricing for our NEW start-uppackage. You could be rebuilding next month!

Piston BonderPB-2002

BalancerHB-4000

Air Test StandATS-100

SingleGun

VerticalWeld

AlignerAWA-2002

No Matter How Small Your Shop,a Transmission Dyno Will Pay Back

97000 ECRHManual Controls

BIG

LCDYNO_7-X.QXD 7/31/02 10:31 AM Page 5

the exciter wheel around the differen-tial carrier.

To check this type of sensor, con-nect your positive meter or scope leadto one wire and the negative lead to theother wire (Fig 3a, 3b, 3c). In additionto signal voltage, you should alsocheck the signal frequency; an examplecan be at 55 MPH, sensor frequencywill be about 125 Hz. If the frequencyor voltage appears to fluctuate errati-cally during the test, the sensor or pick-up may be bad, or the gears driving thesensor may be loose or damaged. Ineither case, the erratic signal will causea driveability problem, and the dam-aged component will need to berepaired or replaced.


Figure 2B

Figure 3A

Figure 3BFigure 3C

Some vehicles

have a remote

module that con-

verts the AC sig-

nal into DC; others

have that module

built inside the

powertrain or

transmission

control module.

VSS the proper diag My VSS Hertz!


Youd beamazed at whatsgone into our name.

The Aftermarket BusinessOf Freudenberg-NOK

A lot of companies and brand names, past andpresent, have helped create Corteco. Were theAftermarket Business of Freudenberg-NOK, andwe bring you all the quality and dependabilityyouve come to trust in names like TransTec,IPC and Farnam. And of course, we drawupon the resources of our parent compa-nies Freudenberg and NOK to providethe same OEM quality, design andmaterials to the aftermarket.

While some names may come and go, TransTec ishere to stay. Youll continue to get the same high-quality transmission and power steering kits andcomponents weve provided since 1978. New

logo, same quality. TransTec from Corteco, the Aftermarket Business

of Freudenberg-NOK.

MANUFACTURING CORPORATION

Farnam

Sealing Sy

stems Div

ision

Milan, Ohio 44846 U.S.A. Toll-Free: 800/852-0340 Fax: 419/499-3337 Web: www.TransTec.com

TransTectransmissionand power steering kits.

YOUR NOLOGY SPECIALISTtec

Cortecto Brand ID 2/22/02, 3:14 PM1

Transtec placed eps.qxd 2/27/02 3:57 PM Page 51

DC-generating speed sensors usu-ally have three wires going to them(figure 4a, 4b) (see sidebar on page12). These sensors can also be testedusing a scope or DMM. The DC signalshould rise with road speed, just as theAC sensor did. The difference here isthe DC sensor will only increase in fre-quency. Voltage will always switch onand off, between zero and referencevoltage (figure 5a, 5b). On todays cars,these sensors are typically Hall Effectsensors.

Vehicle computers are digitaldevices: They have no way of readingan AC signal directly. For the computerto use an AC signal, it must be convert-ed to a DC signal. Modules that convertAC to a DC signal can be located insidethe computer itself, or they may beremote.


Figure 4A

Figure 4B

Figure 5A

Figure 5B

These DC-generat-

ing sensors can

also be tested using

a scope or DMM.

The DC signal

should rise with

road speed, just as

the AC sensor did.



Weve added something

extremely valuable to our kits.

(ISO 9002 Certification)

For 26 years, Precision International has manufactured foreign and domestic gasket kits, seal-

up, transfer case, overhaul and master kits to the highest standards. Before we send a kit out

to market, we tear apart the transmission it goes into to make sure every single part fits the way

it should. This assures that you get the best, most complete, kits available. Whats more, every

Precision kit comes complete with installation tips and easy-to-follow instructions. So you can do the job

right the first time, all the time. Plus, our inventory control

procedures have made us a leader in on-time delivery to

distributors all over the world. This kind of attention to detail

has earned us something very few kit manufacturers can boast of registration by Underwriters Laboratories

to ISO 9002 a system of quality standards for manufacturing, products and services for business and

industry. So next time youre looking for OE certified foreign or domestic transmission repair kits, you can

count on Precision to deliver quality, time and time again.The Problem Solvers.

210 Knickerbocker Ave., Bohemia, NY 11716, (631) 567-2000 Fax (631) 567-2640 Florida Office (954) 783-2100 Fax (954) 783-2190, E-mail: [email protected]

prec ad place.qxd 8/1/02 11:20 AM Page 9

Some remote modules are in theinstrument cluster, such as the ones onFord products. Ford uses a PSOM(Programmable Speedometer Odo-meter) (figures 6a, 6b), which converts

the AC signal to DC and distributes it toother modules around the vehicleincluding the PCM.

GM uses a DRAC (Digital RatioAdapter Controller or Speed Buffer)

(figures 7a, 7b). The DRAC module isusually mounted under the dash, but inlater-model vehicles theyre built intothe PCM. Both the PSOM and DRACcan be reprogrammed to adjust the sig-nal for different size tires and variousdifferential ratios.

Some import vehicles use a combi-nation meter (speedometer) to do theconversion. These are similar to theFord PSOM, but they cant be repro-grammed.

To diagnose a VSS correctly, youmust understand the conditions that aretaking place. For example, we recentlyreceived a call on a 1997 Ford truckwith an E4OD and a 5.4L engine. Thetruck would stall whenever you put itinto gear. This is a fairly common callto the Helpline. Invariably the techni-cian tells us, The converter is lockingup when I put the transmission in gear.This is his interpretation of the prob-lem, and it would be valid if therewere a pump problem.

So the first thing we always ask iswhether there were any codes stored inmemory. All too often the answer isNo. Next, we ask: Does the trans-mission stall in reverse? Answer:Yes.

Since theres no fluid going to theTCC solenoid in reverse, there are onlya few things that can cause this specificcondition:

Worn pump Sump screen sucking air Input shaft is too long VSS signal incorrect for conditions

If the pumpclearance is exces-sive, the amount offluid going to therelease side of theconverter is reducedto where it cant holdthe clutch fromapplying. The easiestway to test this is tocheck the cooler flow.Remove the coolerline and start thevehicle. If coolerflow increases whenyou put the transmis-sion into drive or


Figure 6A

Figure 6B

Figure 7A Figure 7B



Before they have topass yours!

At Allomatic, the filters,

friction plates, steels

and bands in our diverse

product line are

put through a

series of

rigorous OE

quality tests.

From the raw

materials

through the finished

parts, we test, inspect

and quality check to

assure Allomatic

products are prepared to

pass the ultimate test

performance in your

customers transmissions.

Allomatic Products Are Ready to Pass Tough OE Tests...

A Raytech Powertrain, Inc. Company

Plant: 609 East Chaney StreetSullivan, IN 47882

Sales: 110 Jericho TurnpikeFloral Park, NY 11001

Toll Free: 1-800-568-0330

Web Site: www.allomatic.com

ALLOMATIC placed.qxd 8/1/02 11:24 AM Page 11

reverse, the pump is causing the prob-lem.

If the screen is sucking air, the rea-son for the lockup is the same: Thereisnt enough pressure to hold the clutchoff.

If the input shaft is too long, itcould force the clutch forward againstthe lid of the converter when you boltthe unit into the vehicle. But before wepull cooler lines or remove the trans-mission, lets check the data screen onthe scan tool, and pay close attention tothe VSS signal (figure 8a, 8b).

On this truck, the VSS signal read123 MPH whenever you put the transinto drive or reverse. Since thisoccurred while the truck was still in theshop, we have to assume this is anincorrect speed signal.

The next question is, can the VSScause the converter clutch to come onand stall the engine? Sure, but at thispoint with the VSS showing 123 MPH,who cares! The bigger question is, willshutting down the fuel injectors causethe vehicle to stall? Okay, I think we allknow that answer to that one!

The point is, there are a number ofdifferent drivability problems that willaffect the transmission operation andthe overall driveability. So the next timeyou receive a vehicle that stalls whenyou put it into gear, make sure the VSSisnt causing that problem and saveyourself from a VSS that Hertz!

D i d Yo u K n o w . . .Most of the DC sensors used on

todays cars are Hall Effect sensors.These sensors consist of a magnet andsolid state pickup to develop a signal. Atrigger wheel with shutters and win-dows rotates between the magnet andthe pickup. When a shutter is alignedbetween the magnet and pickup, thevoltage drops to zero; when a windowis aligned, the voltage jumps to refer-ence voltage.

But not all DC vehicle speed sen-sors are Hall Effect-style. Early GMvehicles used an optical sensor, but theywere never used with computer-con-trolled transmissions.

Early computerized Toyotatransaxles used a reed switch sensor: Asa magnet rotated past the sensor, thecontacts closed, creating a pulsing DCvoltage signal for the computer to useto measure vehicle speed.

And recently, a new type of DCsensor began showing up on the scenecalled a Magneto-Resistive speed sen-sor. These sensors combine an AC, vari-able reluctance-type of sensor with theelectronics to convert the signal to a DCpulse. The sensor uses a signal wheel or

reluctor, just like an AC permanentmagnet sensor does. But instead of anAC signal, the sensor creates a DC, on-and-off signal.

One way to recognize these sensorsis, with the key on, engine off, one ofthe sensor wires will have battery volt-age to it. Depending on the system, thesensor may also have a ground wire, orit may use a case ground. And, ofcourse, itll have a signal wire to provide the DC signal to the computer.


Figure 8A

Figure 8B

Join Lance Thursday, October 10, at6:30 PM for EXPO Kick-Off Seminar:F4E-III/FA-4AEL,Are they really that tough?



sep ad place.qxd 8/26/02 2:12 PM Page 45

My Vss Hertz

Documents

Transcript of My Vss Hertz