Diagnosing GM DRAC and Speed Sensor Circuits

4 GEARS April 2005

I f you have a 1991 to 1995General Motors rear wheel drivetruck or van in your shop with no

upshift, no speedometer, DTC 24, orDTC 72, you’re more than likely goingto be diagnosing the DRAC/VehicleSpeed Buffer and related circuits. Inthis issue of GEARS, we’re going tocover some DRAC/Vehicle SpeedBuffer basics, as well as some quickdiagnostic tests that will help you pin-point the problem.

The DRAC (Digital Ratio AdapterController), or Vehicle Speed Buffer isused in most rear wheel drive GeneralMotors cars, trucks and vans from 1991to about 1995, and in some 1996-and-newer commercial trucks and vans. Inthis article we’ll refer to this device asthe DRAC.

The function of the DRAC is totake the AC voltage signal generated byvehicle speed sensor (or transmissionoutput sensor in 4L80E applications)and convert it into separate DC pulsesignals for the PCM and speedometer toread.

We’re going to look at a 1995C1500 Chevrolet pickup truck with a5.7L engine and a 4L60E transmission.GM DRAC systems are all very simi-lar; the main difference is componentlocation. Some PCM pin locations andwire colors vary, so consult a wiringdiagram and an electrical componentlocation chart for the vehicle you’reworking on.

On our truck, removing the glovecompartment provides access to the

DRAC and PCM. Most of the testingand diagnostic procedures will be per-formed at the DRAC and PCM connec-tor terminals, so moving the PCM andDRAC out into the open provides freeaccess to these components (figure 1).

The DRAC will typically have twoconnector cavities (figure 2). The larg-er, 9-pin connector cavity is used forpower, ground, and the main input andoutput signals. The smaller, 4-pin con-nector may be empty on vehicles with-out cruise control, as those terminalsare reserved for cruise control modulevehicle speed signal circuits.

Figure 3 shows a basic DRACwiring diagram. With an overview ofwhat the DRAC does, let’s take a lookat the signals the DRAC needs to oper-ate:

1. Switched battery + (hot withignition switch on) at terminalC9

2. Ground at terminal C83. A clean AC voltage signal of

sufficient amplitude from thespeed sensor between termi-nals C7 and C12

Power and GroundBattery + and ground are necessary

for the DRAC to function properly.With the ignition switch on, you shouldhave system voltage at terminal C9.Terminal C8 should provide the DRACwith a good engine ground.

Open the hood and look around.The DRAC ground is typically con-nected to the engine, so it’s sometimesleft loose or disconnected during

by Mike Van Dyke

Diagnosing GM DRAC and Speed Sensor CircuitsUnderstanding and Testing the System

Figure 1: Accessing the DRAC and PCM

The Nuts and Bolts of Electrical Diagnosis

PCMDRAC

VanDyke-DRAC GM.qxd 3/10/05 8:36 AM Page 4

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engine service work. On ourtest vehicle, the DRACground (G134) is bolted tothe thermostat housing (fig-ure 4). This location makesit susceptible to corrosion,and to being left loose ordisconnected after engine orcooling system service.

Quick Test: Eliminateproblems with the powerand ground by jumping bat-tery + to DRAC terminal C9with a 2-amp, fused jumperwire from the positive bat-tery terminal, and a jumperbetween DRAC terminal C8and the negative battery ter-minal. By connecting thesejumpers, you’ll bypass anypower or ground problems,and can road test the vehicleto see if the condition goesaway.

Vehicle SpeedSensor or OutputSpeed SensorSignal

Okay, so we havepower and ground to theDRAC; what’s next? For theVehicle Speed Buffer tosend speed signals to thespeedometer or PCM, itmust first receive a signalfrom the Vehicle SpeedSensor (4L60E), or Output SpeedSensor (4L80E). The signal from thespeed sensor is an AC (alternating cur-rent) signal. There are three main char-acteristics of the speed sensor signalthat affect how the DRAC receives andprocesses the signal:

1. Amplitude2. Signal quality3. Frequency

AmplitudeAmplitude is the voltage

“strength” of the signal. You can meas-ure the AC voltage with a multimeterfor a quick check. To check signal qual-ity and look for interference or dropout,you’ll need an oscilloscope.

On an oscilloscope, amplitude isthe peak-to-peak height of the

6 GEARS April 2005

Diagnosing GM DRAC and Speed Sensor Circuits

Figure 2: The 9 pin connector on the DRAC is where the power, ground, and main input/output signal connections are made

Figure 3: DRAC wiring diagram

Figure 4: DRAC ground (G134) on thermostat housing bolt

Terminal C7 Terminal C15Terminal C7 Terminal C15

VanDyke-DRAC GM.qxd 3/9/05 1:58 PM Page 6

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waveform. Figure 5 shows the VehicleSpeed Sensor signal at the DRAC onour 1995 pickup, measured betweenDRAC terminals C12 (+) and C7 (–).This signal is taken at about 12 MPH.The signal is about 20 volts peak-to-peak, and is a regular, clean, even,repeating pattern.

There’s no hard specification onfor the AC voltage, but some factorytest procedures say you should have 7volts AC (about 10 volts peak) at 10MPH. The DRAC needs more than 4volts peak-to-peak to function reliably.

Automotive module circuits typi-cally use what’s called a “zero crossingdetector” circuit to process the AC sig-nal from a speed sensor. The zero cross-ing detector allows the module’s logiccircuits or microprocessor to detect pre-cisely when the AC signal voltagecrosses zero volts. By counting zerocrossings and precisely measuring thetime between zero crossings, a micro-processor can figure out the speed of ashaft in RPM, or the vehicle speed inMPH, etc.

Amplitude is very importantbecause, to recognize a zero crossingevent, voltage must first reach the“arming” amplitude; in other words,before it can recognize a negative-going, zero crossing event, the positiveamplitude must first reach the armingvoltage level.

In simpler terms, the zero crossingdetector first verifies that the voltagereached a minimum level before count-ing a zero crossing. This reduces, if notentirely eliminates, any low voltage ACnoise from being interpreted as a validspeed sensor signal, as long as thenoise amplitude is below the voltagethreshold of the zero crossing detector!Basically, we don’t want any low levelAC noise taking cuts in line and trig-gering our zero crossing detector.

Signal QualitySignal quality is how clean and

consistent the waveform looks on anoscilloscope. Referring to figure 5, thewaveform should be a clean line,sweeping up and down evenly from apositive to a negative peak, centered on0 volts (ground), repeating continuous-ly (when the vehicle is moving). Anyfuzziness or jagged appearance of the

line indicates noise or signal dropout.You really shouldn’t see more than 300millivolts (0.300 volts) of noise or sig-nal variation on a speed sensor circuit.

With the vehicle stopped, youshouldn’t have any signal or waveformshowing on the oscilloscope (the oscil-loscope should display a flat line indi-cating 0 volts). You may have a blip orbrief series of voltage pulses when youshift the transmission in or out of gearat a stop. This is completely normal,caused by the output shaft moving afraction of a revolution because of nor-mal slack in the driveline and rear end.This brief movement will cause thesensor to send out a few pulses duringengagement or disengagement.

Otherwise, if you rev the engine inpark or neutral, or stall test the vehicle,you shouldn’t see more than 300 milli-volts (0.300 volts) of amplitude if thevehicle isn’t moving.

Quick Test: If you see noise inpark, neutral, or during a stall test, thereare 3 main possibilities:

1. Ignition system interference— typically this interferencewill look like short spikes,increasing in frequency withengine speed. This indicatessecondary ignition breakdowninterference.

2. Faulty alternator/charging sys-tem interference — this is fair-ly simple to identify; discon-

nect the wires at the alternatorand tape them up so they can’tshort out, then run the vehicleand see if the interference goesaway.

3. A bad connection in any of theDRAC circuits, aggravated byengine vibration when you revthe engine or perform a stalltest (check grounds!)

FrequencyThe signal must have a regular

period (frequency) to provide sufficientamplitude and acceptable signal quali-ty. Unless there’s a speed calibrationproblem caused by the wrong gear ratioor damaged reluctor wheel teeth (youwould see the latter as an irregular pat-tern on an oscilloscope), there isn’tmuch reason to be concerned with fre-quency, if the amplitude and signalquality are good.

Diagnostic Tip: Just because youget a frequency reading from the speedsensor circuit on your DMM, it doesn’tmean that the VSS signal is okay. ADMM can measure the frequency of avery weak signal. Most DMMs arecapable of measuring the frequency of asignal that has only a few millivolts ofamplitude. We need a couple of volts ofamplitude for the DRAC to recognizeand process the VSS signal.

Quick test: At this point, if you arehaving a problem with the AC voltage

8 GEARS April 2005

Figure 5: Vehicle Speed Sensor waveform at DRAC terminals C12 and C7

Diagnosing GM DRAC and Speed Sensor Circuits

VanDyke-DRAC GM.qxd 3/9/05 1:58 PM Page 8

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signal coming from the vehicleSpeed/Output Speed sensor, you aregoing to have a problem with the sensoror the wiring (as long as there are reluc-tor teeth inside of the transmission forthe sensor to pick up off of). You cantest the sensor for AC voltage outputright at its connector, or substitute agood sensor. To eliminate a wiringproblem, you can cut the two wires atthe sensor, cut them at the DRAC (C7and C12), and run new wires. This willbypass and eliminate any problems inthe harness.

DRAC Output CircuitsOnce you’re sure the DRAC has

the basic signals it needs to do its job —good power, good ground, and a goodsignal from the VSS/OSS — you’reready to look at what the DRAC needsto transmit its speed signals to the PCMand instrument cluster.

1. 5-volt pull-up sources fromthe PCM for Output Speed andVSS Output signals (DRACterminals C11 and C13)

2. 10- to 12-volt pull-up sourcefrom the instrument cluster(DRAC terminal C15)

Output and VSS Signals toPCM and Speedometer

You might ask: “What the heck is a5- or 10-volt pull-up source?” Toanswer this question, let’s briefly goover the function of the Output andVSS circuits, and how the signals aretransmitted from the DRAC to theSpeedometer and PCM.

Figure 3 shows a wiring diagramof a typical DRAC and its related cir-cuits. PCM terminal F12 is connected

to 5 volts inside the PCM through a1.2k-ohm (1200 ohm) pull-up resistor.PCM terminal F12 is also connected tothe DRAC terminal C13.

Figure 6 shows the Output Speedsignal at DRAC terminal C13, with thevehicle at about 12 MPH. When theDRAC sends the transmission outputspeed signal to the PCM, it grounds andungrounds terminal C13 at regularintervals. The result is a square wavethat toggles up and down continuouslyfrom 5 volts to 0 volts and back to 5volts. If the DRAC doesn’t have 5 voltssupplied to it through a pull-up resistor,the PCM won’t receive the speed signalfrom the DRAC.

The DRAC can’t send out a speedsignal without a pull-up voltage sup-plied by the PCM. Failure to under-stand this is one of the main reasons formisdiagnosis!

The DRAC is grounding terminal

C13 to pull the voltage to zero, thenungrounding terminal C13 to let itreturn to 5 volts. The key word here isground; if the DRAC itself doesn’t havea good ground at terminal C8, it won’tbe able to pull the Vehicle Speed,Output Speed, or Speedometer signallines all the way to ground. The DRACdoesn’t have another magical groundcircuit to toggle the Vehicle Speed,Output Speed, or Speedometer signallines with; DRAC terminal C8 is thebe-all, end-all ground. Make sureDRAC terminal C8 has a good connec-tion to engine ground!

Quick Test: With the DRAC har-ness connector unplugged, you shouldhave 5 volts on terminals C11 and C13(figure 7). According to Ohm’s Law,grounding C11 or C13 should developabout 4.2 mA of current (figure 8) in thecircuit, because the 5 volts is suppliedthrough a 1.2k resistor inside the PCM.

10 GEARS April 2005

Figure 7: Check DRAC harness connector terminal C13 (Output Speed Signal) for 5 Volt Pull-Up

Figure 8 : Grounding DRAC harness connector terminal C13 to check DC current

Diagnosing GM DRAC and Speed Sensor Circuits

Figure 6: DRAC Output Speed Signal Waveform at DRAC terminal C13

VanDyke-DRAC GM.qxd 3/9/05 1:59 PM Page 10

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You should have 10 to 12 volts onharness connector terminal C15(Speedometer Signal; figure 9).

With the DRAC connector stillunplugged:

• Probe harness connector ter-minal C15 with a jumper wire.

• Turn the ignition switch on.• Rapidly tap the other end of

the jumper wire against a goodground.

You should be able to get a 10 to 20MPH reading on the speedometer. Thisverifies the speedometer can create areading if the DRAC is doing its job.

You can perform the same test onDRAC harness connector terminalsC11 and C13 while watching theVehicle Speed and Output Speed read-ings on the scan tool, but this isn’t aconclusive test. Even if everything isokay, you may not get a steady readingfor Vehicle Speed. The Output Speedmay only read a low value and willcome and go. This is because the fre-quency of the Output Speed signal ispretty fast and difficult to duplicate byhand.

Using a Digital StorageOscilloscope

A DMM will tell you if there’s aconsistent problem most or all the time.But catching intermittent DRAC sys-tem glitches with a DMM can be a hit-and-miss proposition.

By connecting a scan tool to thevehicle you can check Vehicle Speedand Output RPM through scan data.You can also record a movie of scandata when a problem is occurring to see

if the PCM is losingone or both of thesignals.

A DSO (digitalstorage oscilloscope)allows you to viewwaveforms, recordthem over a periodof time, and storethem for future refer-ence. The length oftime is limited by thememory or buffersize of the DSO.Using a DSO with agood size buffer canlet you record wave-forms over a rela-tively long time, soyou can review themto look for signaldropouts or glitches.

We’re going touse the Snap-onMODIS in lab scope mode. We canmonitor the VSS input, Speedometeroutput, Output speed, and VehicleSpeed signals using the 4 channel DSO(figure 10). When viewing a storedwaveform, you can zoom out (figure11) to view some of the slower signalcomponents (figure 12). This is helpfulif you suspect a faulty PCM orEPROM. If the PCM is always gettinga good Vehicle Speed and Output Speedsignal at its terminals when a signal lossproblem is occurring, the DRAC isdoing its job; which means there’s agood possibility the PCM or EPROMmay have lost a couple of marbles.

A Tip for SuccessfulDiagnosis

Of course, the circuit tests we’vediscussed here are all fine and dandy ifthere’s a hard fault; one that’s occur-ring right now. But if you have an inter-mittent problem, remember: the onlyvalid diagnostic information you’ll getis from tests performed when the prob-lem is occurring.

In other words, if the truck comesin with a DTC 24, and the customersays that once every three weeks thetruck drops into neutral at highwayspeed and the MIL lights, there’s a goodchance that everything’s going to testokay. You aren’t going to find thesource of the problem unless you can:

12 GEARS April 2005

Diagnosing GM DRAC and Speed Sensor Circuits

Figure 9: Check DRAC harness connector terminal C15(Speedometer Signal) for 10 -12 volt pull up

Figure 10: Viewing the signals on a 4 channel digital storageoscilloscope:

Channel 1- Vehicle Speed SensorChannel 2- Vehicle Speed signal Channel 3- Output Speed signal Channel 4- Speedometer signal

Figure 11: Selecting the ‘Zoom Out’ 8x function to view slower signal components.

VanDyke-DRAC GM.qxd 3/9/05 1:59 PM Page 12

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1. Find a marginal problem, suchas a poor ground or intermit-tent connection.

2. Duplicate the problem whileperforming the tests.

This is why it’s important to quizthe vehicle owner. Ask whether anywork has been performed on the vehi-cle. If so, check the area where thework was performed for loose grounds,damaged harness, etc. Ask about thedriving conditions and what’s happen-ing when the problem occurs:

• Is the engine cold or warm?• What speed?• Towing a trailer?• Does the speedometer drop

out?

And so on, so you can try to dupli-cate the failure.

Don’t forget to check the basics:charging system, powertrain ground,chassis ground (July 2004 GEARS;Check the Grounds!), terminal tension,battery condition, etc. Before you startthrowing parts at the car, you need tocover the basics, especially with anintermittent problem.

The GM DRAC system has a fewthings to take into consideration beforediagnosis can begin, but once you getthe basics down it’s just a process ofelimination. Besides, it just gets easieras time goes on; on most 1996-and-laterGM vehicles, they eliminated the exter-nal DRAC; the vehicle speed sensorsignal goes straight to the VCM(Vehicle Control Module, or what theyused to call the PCM). Isn’t it nicewhen new technology actually becomeseasier to diagnose?

14 GEARS April 2005

Diagnosing GM DRAC and Speed Sensor Circuits

Figure 12: After zooming out, we can view more cycles of the Vehicle Speed and Speedometer signal waveforms

Watch Out for the Ground Loop!Whenever you’re using an oscilloscope, DMM, or any other

type of test equipment that’s powered by the vehicle’s battery,

cigarette lighter, or OBD-II diagnostic connector, be careful. Some

externally-powered test equipment will have its negative or

ground test probe connected internally to the tool’s power source

ground circuit. This means that the negative test probe is already

connected to the vehicle’s ground.

The GM DRAC VSS terminal C7 is also connected to ground

internally (to C8). So if you’re probing the VSS AC signal input to

the DRAC with test equipment that’s powered by the vehicle, and

you have your negative test probe connected to DRAC terminal

C12, you can essentially short out the VSS signal, giving you a false

test result.To avoid this type of ground loop problem, always con-

nect your negative test probe to DRAC terminal C7 and positive

test probe to DRAC terminal C12 to check this signal.

Remember this when testing any other circuits on a vehicle;

you can short out circuits and possibly damage the vehicle or your

test equipment.

When viewing a storedwaveform, you can zoomout to view some of the

slower signal components(figure 12). This is helpful

if you suspect a faultyPCM or EPROM.

VanDyke-DRAC GM.qxd 3/9/05 1:59 PM Page 14

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