16 GEARS September 2004

M odern automotive computersystems use dedicated sen-sor ground circuits (some-

times called signal return). These sen-sor grounds return to the computeritself, rather than to the engine block orthe vehicle’s body or chassis. For ana-log sensor inputs, such as temperaturesensors, this provides better signalaccuracy than you could get with anexternal ground. Digital sensors, suchas Ford MAP sensors and most GMMass Airflow sensors, generally don’trequire much ground circuit precision.But these, too, share ground circuitsthat return to the computer.

The computer measures the incom-ing sensor signal against the internalground level. Since it’s comparing thesignal to its own zero level, the com-puter can be sure that its signal meas-urement is accurate. In addition, the

sensor reference voltage circuit insidethe computer grounds and references tothe sensor ground. This allows the sen-sor inputs to remain accurate, even ifthe computer itself has faulty powergrounds.

For instance, suppose the comput-er’s power ground circuit had an excessvoltage drop of, say, 0.60 volts (0.10volts is generally the limit). Since bothsensor ground and reference voltagecircuits reference to the sensor ground,the sensor signals remain accurate ascompared to the computer’s internalcircuits. This remains true, even thoughthe sensor circuit voltages are 0.50volts higher when compared to theengine ground. Despite this difference,the computer is still able to read thesensor output accurately, so a scan toolreading indicates correct values. Whichmeans automotive computers can con-

tinue to take accurate sensor readings,even if the computer isn’t groundedcorrectly.

Figure 1 shows a typical computersensor input circuit layout. Each ofthese circuits grounds through the sen-sor ground buss:

• Sensor reference voltage cir-cuit

• Analog-to-digital (A/D) con-verter (measures the signal)

• Ground for the sensor itself

The large sensor ground buss isgrounded through a lighter conductor tothe power ground buss. So there willalways be some voltage drop betweenthe two buss circuits. Which meansthere will always be a voltage dropbetween the sensor ground circuit andengine ground.

Of course, there’s a limit to the

Sensor Grounds vs. Chassis Grounds:

There Is a Difference!Sensor Grounds vs. Chassis Grounds:

There Is a Difference!

Figure 1 showsa typical com-puter sensorinput circuit

layout. Each ofthese circuits

groundsthrough the

sensor groundbuss:

ECU

B+

5v

TPS

Sensor Ground

VSS

TPS

Figure 1

by Kevin Carpenter

carpenter.qxd 8/18/04 3:01 PM Page 16

When You Buy From An

ATRA-Member Center…

1. We Handle the Diagnosis

Once upon a time, when a transmissionstopped shifting, it was bad. You replaced it and itworked again.

But today's transmissions depend on amultitude of components and systems to operateproperly. Speed sensors, load sensors,temperature sensors… even the EGR valve canaffect transmission operation.

Our technicians receive specialized training andhave access to tools and equipment designed tohelp them diagnose transmissions accurately. So,when they say the transmission needs to berebuilt, you can bet they're right.

And if the problem isn't the transmission, ourtechnicians will work with you to isolate and repairthe problem… the first time.

2. We Remove the Transmission

Front-wheel drive vehicles are a lot cheaper andeasier for auto manufacturers to build. But whenthe transmission needs to come out, well, that's awhole different story. Today you could easily spend2 or 3 hours just removing the transmission fromthe vehicle… and even more time putting it back!

Our installers specialize in removing andreplacing today's transmissions. They have theright tools and skills necessary to pull thetransmission quickly and easily, so you don't haveto.

3. We Flush the Cooler

When a transmission self-destructs, dirt andmetal particles flow to every corner of the unit.During the rebuild, those particles are carefullywashed away, so the transmission is nice andclean.

With Reman Companies,

The Call Is Just the Beginning…

When you order a remanufactured transmissionfrom a reman company, their job ends when thetransmission reaches your shop. But your job hasbarely begun.

You already had to diagnose the vehicle. Then,once you determined the transmission neededreplacement, you had to identify it, and order thenew one.

After the reman transmission arrives, you stillhave to remove the old one. And you have tocompare that transmission to the one in the crate,to make sure there aren't any differences thatwould keep the new one from working properly.

You have to handle the changeovers. You haveto flush the cooler. You have to install the newtransmission. You have to road test the vehicle, andyou have to take care of any other problems thatshow up along the way.

What's more, you have to handle any comebacksor warranty complaints that come up over the nextyear or two. That's a lot of time and effort you haveto provide. When you add it all up, maybe youdidn't make as much as you expected to.

But there'sa better choice…

But what about the cooler? Those sameparticles and contaminants find their way throughthe entire cooler. If not flushed away, thosecontaminants will wash back into yourtransmission the first time you start the engine.

That's why we power flush the cooler on everytransmission job we do. We flush all the particlesand contaminants out of the cooler, and makesure it's flowing free and clear. So when the newtransmission goes in, the oil remains clean andclear.

4. We Install the Rebuid

Putting the transmission back in can take evenlonger than pulling it out. It's one thing to removea bolt or unplug an electrical connector: the trickis putting all those bolts and connectors back inthe right place, and in the right order.

When you send your transmission work to anATRA-member center, we handle the removal…and we handle the installation. Which leaves yourtechnicians free to work on other cars.

5. We Road Test the Vehicle

Transmission repairs don't end when youinstall the last bolt or top off the fluid… not by along shot. In fact, that's when the most importantparts of the repair are just beginning.

Because that's when the road test begins. Roadtesting means more than just going for a drive,and making sure the unit shifts. In most cases itinvolves performing a full drive cycle, to enablethe transmission to relearn its basic operatingparameters.

Afterward, it means rechecking the installation,to make sure there are no leaks, and thateverything's tight and adjusted just right. It evenmeans making sure that everything's nice andclean, and ready to be delivered.

That's how we return the car to you, so the onlything that's left is for you to deliver it to thecustomer.

6. We Cover the Warranty

No matter how you slice it, the warranty sellsthe transmission job. Customers want to knowthat, no matter where they go, they'll havesomeone to call if their transmission fails.

When You Buy a Transmission

from an ATRA-Member Center,

We Handle All the Work While You Collect All the Money!

This brochure has been provided by:

...your local ATRA-membertransmission repair center.

That's the most important reason to send yourtransmission work to an ATRA-member center.Because, with nearly 2000 members, from coast tocoast, there's always an ATRA-member centernearby. We're the largest association ofindependent transmission repair centers in theworld; nearly three times larger than the largestchain.

So wherever your customer goes - from east towest… north to south… anywhere in the US orCanada - there's probably an ATRA member rightnearby. And you can offer that kind of warrantycoverage… if you send your transmission repairs toan ATRA-member center.

7. You Make the Money

Auto repair is about making money. We want tomake money fixing transmissions, and we believethat you can make money using our wholesaletransmission pricing. In fact, if you add in the extraservices we provide, in most cases you'll discoverour prices are even less than the reman companiescharge.

And sending your work to an ATRA-membercenter provides an additional benefit: Since youdon't have to diagnose, R&R or road test thetransmission, your technicians are free to work onother cars.

How many brake jobs, tune-ups and oil changescan your technicians do during the 5 or 6 hoursyou freed up by sending the transmission job tous? It's like having another technician on board,working to earn you money!

That's why you should be sending all yourtransmission work to your local ATRA-membertransmission center. Because we do more than justdrop off a transmission in a crate. We fix the car,from start to finish, and you collect the money.

Any way you slice it, you can earn more with lesswork and less aggravation by sending yourtransmission diagnoses and repairs to an ATRA-member center. Considering all that, why wouldanyone buy a reman transmission?

ATRA - the Automatic TransmissionRebuilders Association - is the largesttransmission repair association in the world,with almost 2000 members in the UnitedStates and Canada. So wherever you go,there's an ATRA-member repair center nearby.

To learn more about ATRA, or to find an ATRAmember repair center near you, call Toll Free:

1-866-GO-4-ATRA...or go online to:

www.atra.comWe'll provide you with the information youneed about your automatic transmission, andhelp you find an ATRA member repair centeranywhere in the US or Canada.

When you have a transmission problem, thinkATRA: We're here for you.

© ATRA 2004. All Rights Reserved. Printed in USA.

All ATRA-member transmission centers are independently

owned and operated, and are solely responsible for their

business practices and the technical competence of their

employees. ATRA accepts no liability for any disparity between

the information and promises provided in this brochure and the

behavior, business practices or capabilities of any ATRA-

member center.

Making MoneyDoing Transmission Work

Shouldn’t Be This

Hard!

The reman companies are quick to claim how

much you can make by buying their

transmissions. What they often forget to mention

is how much work you're going to have to do

after the reman shows up at your door.

But There's an Easier Way…

ATRA: We’re here for you.

We explore every argument… examineevery detail… on a point-by-point basis. We show them how:

• You handle the diagnosis• You remove the transmission• You flush the cooler• You install the rebuild• You road test the vehicle• You cover the warranty

…and, in the end, they make just asmuch as they would have with a reman.And we make it clear that, when it comesto transmission repair, there’s no placelike ATRA!

These new brochures are just $20 for apack of 50, so they’re a great sales toolat a terrific price.

Call 1-800-428-8489 to order your brochures today, soyou can start selling your wholesale work right away!

Not an ATRA member? Too bad; you’ll have to handle your sales all by yourself. Because these new brochures are exclusively for ATRA members. They’re just one more way that ATRA gives you the CLOUT! you need to go out and take back what’s yours.

When You’re Ready to StartSelling, ATRA’s Here to Help!If you’re one of the hundreds of ATRA members who’s ready to start selling wholesale transmission work, we’ve got just what you need:

A new sales brochure, targeted at general repair shops, that explains why it’s

smarter to let you handle their transmission work than to try to do it themselves.

YOUR

STAMP

HERE

Atra Brochure Ad2.qxd 3/4/04 9:05 AM Page 39

computer’s ability to read sensor valuesaccurately with faulty power grounds.That’s because the reference voltageregulator requires enough of a differ-ence between its power input andground level (the sensor ground buss)to generate a regulated voltage; usuallyabout two volts more than the regulatedvalue. So a 5.0 volt regulator requiresabout a 7.0-volt difference between itsinput voltage and ground.

In addition, DC voltmeters averagethe measurement on the display. Whichmeans the peak value of the voltagedrop will be higher than the measuredreading. But the system can have a sub-stantial voltage drop in the powerground before it affects sensor read-ings.

Why is this level of accuracyimportant? Analog sensors develop sig-nals based on measured values, such asengine or transmission temperature, byproducing a voltage signal that varieswith measured changes.

Consider an Engine CoolantTemperature (ECT) sensor on a FordEEC-IV system: At 194º F, it creates a0.60-volt signal. That’s not much volt-age; it wouldn’t take much inaccuracyin the ground level to cause a hugemeasurement error. If there were a volt-age drop in the power ground circuit of0.60 volts, comparing the sensor signalto the engine ground instead of the sig-nal ground would put the sensor signalout of range. The computer would lightthe MIL and set a diagnostic troublecode, indicating an ECT sensor prob-lem. But with the sensor signal com-pared to the sensor ground, the comput-er can read the signal accurately,despite the excess voltage drop on thepower ground circuit.

To measure the sensor ground cir-cuits for excess voltage drop, you’llneed to access the system’s ground cir-cuits at the computer connector. Thislets you measure between the powerground and sensor ground terminals atthe computer connector. A typical valuefor this voltage drop is 10 to 20 milli-volts, but there’s generally no specifica-tion for this measurement, which willvary from system to system.

Avoid using the oxygen sensorgrounds when checking the ground cir-cuit voltage drop. This is because some

systems connect theoxygen sensor grounddirectly to the engineblock. In that case,measuring the powerground voltage againstthe oxygen sensorground wouldn’t actual-ly measure the voltagedrop through the com-puter’s sensor grounds;it would be like measur-ing the voltage dropbetween the engineblock and somewhereelse on the engine block.

It’s also important to make sure theconnector terminals fit snugly on the

computer pins, and the connections areclean. Otherwise, any excess voltagebetween the terminals and pins wouldadd to the voltage drop measurement.

When confronted with a difficultor unusual problem, you might betempted either to bypass the sensorground circuit directly to the engineblock, or add a redundant ground leadfrom a sensor ground circuit and theengine block. Although either modifi-cation might seem to take care of theproblem, both would violate the integri-ty of the system, and could create newproblems:

• By detaching the original cir-cuit completely, any voltagedrop in the power ground cir-cuit would skew the sensorreadings.

• By adding a redundant

ground, current from the com-puter’s power circuits wouldseek ground through the newwiring. This would cause avoltage drop in the light gaugesensor ground between thecomputer and the sensor, againcausing an error in the sensorreadings.

What’s more, the added circuitcould easily cause the printed circuitinside the computer, which attaches thetwo ground buss circuits, to burn away.This printed circuit is only designed tocarry the low current flow of the sensorcircuits, not the greater current for thepower outputs. This could occur if apower ground circuit developed higher-than-normal resistance, causing thepower output circuits to seek an alter-nate ground path.

Because signal return circuits carryonly small amounts of current, they’refar less likely to develop excess voltagedrop than the power ground circuits.The voltage drop between sensorground circuits and engine groundshould always be a few millivolts morethan the drop between the sensorgrounds and power grounds. So it’simportant to verify the system’s powergrounds and the contact between thecomputer’s connector pins and termi-nals before considering sensor groundcircuit voltages to be out of range.

Voltage may be voltage, but aground is not just a ground. Your suc-cess or failure in testing these is know-ing the difference.

18 GEARS September 2004

It’s also important tomake sure the connec-tor terminals fit snuglyon the computer pins,and the connections

are clean. Otherwise,any excess voltage

between the terminalsand pins would add to

the voltage drop measurement.

It’s also important tomake sure the connec-tor terminals fit snuglyon the computer pins,and the connections

are clean. Otherwise,any excess voltage

between the terminalsand pins would add to

the voltage drop measurement.

carpenter.qxd 8/18/04 3:02 PM Page 18

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Raybestos placed.qxd 8/11/04 1:11 PM Page 19