The Nuts and Bolts of Electrical Diagnosis by Mike Van Dyke

Diagnosing
GM DRAC
and Speed Sensor Circuits
Understanding and Testing the System

I f you have a 1991 to 1995

General Motors rear wheel drive
truck or van in your shop with no
upshift, no speedometer, DTC 24, or
DTC 72, you’re more than likely going
DRAC and PCM. Most of the testing
and diagnostic procedures will be per-
formed at the DRAC and PCM connec-
tor terminals, so moving the PCM and
DRAC out into the open provides free
1.

2.
3.
Switched battery + (hot with
ignition switch on) at terminal
C9
Ground at terminal C8
A clean AC voltage signal of
to be diagnosing the DRAC/Vehicle access to these components (figure 1). sufficient amplitude from the
Speed Buffer and related circuits. In The DRAC will typically have two speed sensor between termi-
this issue of GEARS, we’re going to connector cavities (figure 2). The larg- nals C7 and C12
cover some DRAC/Vehicle Speed er, 9-pin connector cavity is used for
Buffer basics, as well as some quick power, ground, and the main input and Power and Ground
diagnostic tests that will help you pin- output signals. The smaller, 4-pin con- Battery + and ground are necessary
point the problem. nector may be empty on vehicles with- for the DRAC to function properly.
The DRAC (Digital Ratio Adapter out cruise control, as those terminals With the ignition switch on, you should
Controller), or Vehicle Speed Buffer is are reserved for cruise control module have system voltage at terminal C9.
used in most rear wheel drive General vehicle speed signal circuits. Terminal C8 should provide the DRAC
Motors cars, trucks and vans from 1991 Figure 3 shows a basic DRAC with a good engine ground.
to about 1995, and in some 1996-and- wiring diagram. With an overview of Open the hood and look around.
newer commercial trucks and vans. In what the DRAC does, let’s take a look The DRAC ground is typically con-
this article we’ll refer to this device as at the signals the DRAC needs to oper- nected to the engine, so it’s sometimes
the DRAC. ate: left loose or disconnected during
The function of the DRAC is to
take the AC voltage signal generated by
vehicle speed sensor (or transmission
output sensor in 4L80E applications)
and convert it into separate DC pulse
signals for the PCM and speedometer to
read.
We’re going to look at a 1995
C1500 Chevrolet pickup truck with a
5.7L engine and a 4L60E transmission.
GM DRAC systems are all very simi-
lar; the main difference is component
location. Some PCM pin locations and
wire colors vary, so consult a wiring
diagram and an electrical component DRAC
location chart for the vehicle you’re PCM
working on.
On our truck, removing the glove
compartment provides access to the
Figure 1: Accessing the DRAC and PCM

4 GEARS April 2005

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Diagnosing GM DRAC and Speed Sensor Circuits

Terminal C7 Terminal C15

engine service work. On our
test vehicle, the DRAC
ground (G134) is bolted to
the thermostat housing (fig-
ure 4). This location makes
it susceptible to corrosion,
and to being left loose or
disconnected after engine or
cooling system service. Figure 2: The 9 pin connector on the DRAC is where the power, ground, and main input/output
Quick Test: Eliminate signal connections are made
problems with the power
and ground by jumping bat-
tery + to DRAC terminal C9
with a 2-amp, fused jumper
wire from the positive bat-
tery terminal, and a jumper
between DRAC terminal C8
and the negative battery ter-
minal. By connecting these
jumpers, you’ll bypass any
power or ground problems,
and can road test the vehicle
to see if the condition goes
away.

Vehicle Speed
Sensor or Output
Speed Sensor
Signal
Okay, so we have
power and ground to the
DRAC; what’s next? For the
Vehicle Speed Buffer to
send speed signals to the
speedometer or PCM, it
must first receive a signal Figure 3: DRAC wiring diagram
from the Vehicle Speed
Sensor (4L60E), or Output Speed
Sensor (4L80E). The signal from the
speed sensor is an AC (alternating cur-
rent) signal. There are three main char-
acteristics of the speed sensor signal
that affect how the DRAC receives and
processes the signal:
1. Amplitude
2. Signal quality
3. Frequency

Amplitude
Amplitude is the voltage
“strength” of the signal. You can meas-
ure the AC voltage with a multimeter
for a quick check. To check signal qual-
ity and look for interference or dropout,
you’ll need an oscilloscope.
On an oscilloscope, amplitude is Figure 4: DRAC ground (G134) on thermostat housing bolt
the peak-to-peak height of the

6 GEARS April 2005

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GEARS July 2004 3

Diagnosing GM DRAC and Speed Sensor Circuits
waveform. Figure 5 shows the Vehicle
Speed Sensor signal at the DRAC on
our 1995 pickup, measured between
DRAC 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 on
for the AC voltage, but some factory
test procedures say you should have 7
volts AC (about 10 volts peak) at 10
MPH. The DRAC needs more than 4
volts peak-to-peak to function reliably.
Automotive module circuits typi-
cally use what’s called a “zero crossing
detector” circuit to process the AC sig-
nal from a speed sensor. The zero cross-
ing detector allows the module’s logic
circuits or microprocessor to detect pre-
cisely when the AC signal voltage Figure 5: Vehicle Speed Sensor waveform at DRAC terminals C12 and C7
crosses zero volts. By counting zero
crossings and precisely measuring the line indicates noise or signal dropout. nect the wires at the alternator
time between zero crossings, a micro- You really shouldn’t see more than 300 and tape them up so they can’t
processor can figure out the speed of a millivolts (0.300 volts) of noise or sig- short out, then run the vehicle
shaft in RPM, or the vehicle speed in nal variation on a speed sensor circuit. and see if the interference goes
MPH, etc. With the vehicle stopped, you away.
Amplitude is very important shouldn’t have any signal or waveform 3. A bad connection in any of the
because, to recognize a zero crossing showing on the oscilloscope (the oscil- DRAC circuits, aggravated by
event, voltage must first reach the loscope should display a flat line indi- engine vibration when you rev
“arming” amplitude; in other words, cating 0 volts). You may have a blip or the engine or perform a stall
before it can recognize a negative- brief series of voltage pulses when you test (check grounds!)
going, zero crossing event, the positive shift the transmission in or out of gear
amplitude must first reach the arming at a stop. This is completely normal, Frequency
voltage level. caused by the output shaft moving a The signal must have a regular
In simpler terms, the zero crossing fraction of a revolution because of nor- period (frequency) to provide sufficient
detector first verifies that the voltage mal slack in the driveline and rear end. amplitude and acceptable signal quali-
reached a minimum level before count- This brief movement will cause the ty. Unless there’s a speed calibration
ing a zero crossing. This reduces, if not sensor to send out a few pulses during problem caused by the wrong gear ratio
entirely eliminates, any low voltage AC engagement or disengagement. or damaged reluctor wheel teeth (you
noise from being interpreted as a valid Otherwise, if you rev the engine in would see the latter as an irregular pat-
speed sensor signal, as long as the park or neutral, or stall test the vehicle, tern on an oscilloscope), there isn’t
noise amplitude is below the voltage you shouldn’t see more than 300 milli- much reason to be concerned with fre-
threshold of the zero crossing detector! volts (0.300 volts) of amplitude if the quency, if the amplitude and signal
Basically, we don’t want any low level vehicle isn’t moving. quality are good.
AC noise taking cuts in line and trig- Quick Test: If you see noise in Diagnostic Tip: Just because you
gering our zero crossing detector. park, neutral, or during a stall test, there get a frequency reading from the speed
are 3 main possibilities: sensor circuit on your DMM, it doesn’t
Signal Quality 1. Ignition system interference mean that the VSS signal is okay. A
Signal quality is how clean and — typically this interference DMM can measure the frequency of a
consistent the waveform looks on an will look like short spikes, very weak signal. Most DMMs are
oscilloscope. Referring to figure 5, the increasing in frequency with capable of measuring the frequency of a
waveform should be a clean line, engine speed. This indicates signal that has only a few millivolts of
sweeping up and down evenly from a secondary ignition breakdown amplitude. We need a couple of volts of
positive to a negative peak, centered on interference. amplitude for the DRAC to recognize
0 volts (ground), repeating continuous- 2. Faulty alternator/charging sys- and process the VSS signal.
ly (when the vehicle is moving). Any tem interference — this is fair- Quick test: At this point, if you are
fuzziness or jagged appearance of the ly simple to identify; discon- having a problem with the AC voltage

8 GEARS April 2005

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Diagnosing GM DRAC and Speed Sensor Circuits
signal coming from the vehicle
Speed/Output Speed sensor, you are
going to have a problem with the sensor
or the wiring (as long as there are reluc-
tor teeth inside of the transmission for
the sensor to pick up off of). You can
test the sensor for AC voltage output
right at its connector, or substitute a
good sensor. To eliminate a wiring
problem, you can cut the two wires at
the sensor, cut them at the DRAC (C7
and C12), and run new wires. This will
bypass and eliminate any problems in
the harness.

DRAC Output Circuits

Once you’re sure the DRAC has
the basic signals it needs to do its job —
good power, good ground, and a good Figure 6: DRAC Output Speed Signal Waveform at DRAC terminal C13
signal from the VSS/OSS — you’re
ready to look at what the DRAC needs to 5 volts inside the PCM through a C13 to pull the voltage to zero, then
to transmit its speed signals to the PCM 1.2k-ohm (1200 ohm) pull-up resistor. ungrounding terminal C13 to let it
and instrument cluster. PCM terminal F12 is also connected to return to 5 volts. The key word here is
1. 5-volt pull-up sources from the DRAC terminal C13. ground; if the DRAC itself doesn’t have
the PCM for Output Speed and Figure 6 shows the Output Speed a good ground at terminal C8, it won’t
VSS Output signals (DRAC signal at DRAC terminal C13, with the be able to pull the Vehicle Speed,
terminals C11 and C13) vehicle at about 12 MPH. When the Output Speed, or Speedometer signal
2. 10- to 12-volt pull-up source DRAC sends the transmission output lines all the way to ground. The DRAC
from the instrument cluster speed signal to the PCM, it grounds and doesn’t have another magical ground
(DRAC terminal C15) ungrounds terminal C13 at regular circuit to toggle the Vehicle Speed,
intervals. The result is a square wave Output Speed, or Speedometer signal
Output and VSS Signals to that toggles up and down continuously lines with; DRAC terminal C8 is the
PCM and Speedometer from 5 volts to 0 volts and back to 5 be-all, end-all ground. Make sure
You might ask: “What the heck is a volts. If the DRAC doesn’t have 5 volts DRAC terminal C8 has a good connec-
5- or 10-volt pull-up source?” To supplied to it through a pull-up resistor, tion to engine ground!
answer this question, let’s briefly go the PCM won’t receive the speed signal Quick Test: With the DRAC har-
over the function of the Output and from the DRAC. ness connector unplugged, you should
VSS circuits, and how the signals are The DRAC can’t send out a speed have 5 volts on terminals C11 and C13
transmitted from the DRAC to the signal without a pull-up voltage sup- (figure 7). According to Ohm’s Law,
Speedometer and PCM. plied by the PCM. Failure to under- grounding C11 or C13 should develop
Figure 3 shows a wiring diagram stand this is one of the main reasons for about 4.2 mA of current (figure 8) in the
of a typical DRAC and its related cir- misdiagnosis! circuit, because the 5 volts is supplied
cuits. PCM terminal F12 is connected The DRAC is grounding terminal through a 1.2k resistor inside the PCM.

Figure 7: Check DRAC harness connector terminal C13 Figure 8 : Grounding DRAC harness connector terminal C13
(Output Speed Signal) for 5 Volt Pull-Up to check DC current

10 GEARS April 2005

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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 storage
You should have 10 to 12 volts on if the PCM is losing oscilloscope:
harness connector terminal C15 one or both of the Channel 1- Vehicle Speed Sensor
(Speedometer Signal; figure 9). signals. Channel 2- Vehicle Speed signal
With the DRAC connector still A DSO (digital Channel 3- Output Speed signal
unplugged: storage oscilloscope) Channel 4- Speedometer signal
• Probe harness connector ter- allows you to view
minal C15 with a jumper wire. waveforms, record
• Turn the ignition switch on. them over a period
• Rapidly tap the other end of of time, and store
the jumper wire against a good them for future refer-
ground. ence. The length of
time is limited by the
You should be able to get a 10 to 20 memory or buffer
MPH reading on the speedometer. This size of the DSO.
verifies the speedometer can create a Using a DSO with a
reading if the DRAC is doing its job. good size buffer can
You can perform the same test on let you record wave-
DRAC harness connector terminals forms over a rela-
C11 and C13 while watching the tively long time, so
Vehicle Speed and Output Speed read- you can review them
ings on the scan tool, but this isn’t a to look for signal
conclusive test. Even if everything is dropouts or glitches. Figure 11: Selecting the ‘Zoom Out’ 8x function to view
okay, you may not get a steady reading We’re going to slower signal components.
for Vehicle Speed. The Output Speed use the Snap-on
may only read a low value and will MODIS in lab scope mode. We can A Tip for Successful
come and go. This is because the fre- monitor the VSS input, Speedometer Diagnosis
quency of the Output Speed signal is output, Output speed, and Vehicle Of course, the circuit tests we’ve
pretty fast and difficult to duplicate by Speed signals using the 4 channel DSO discussed here are all fine and dandy if
hand. (figure 10). When viewing a stored there’s a hard fault; one that’s occur-
waveform, you can zoom out (figure ring right now. But if you have an inter-
Using a Digital Storage 11) to view some of the slower signal mittent problem, remember: the only
Oscilloscope components (figure 12). This is helpful valid diagnostic information you’ll get
A DMM will tell you if there’s a if you suspect a faulty PCM or is from tests performed when the prob-
consistent problem most or all the time. EPROM. If the PCM is always getting lem is occurring.
But catching intermittent DRAC sys- a good Vehicle Speed and Output Speed In other words, if the truck comes
tem glitches with a DMM can be a hit- signal at its terminals when a signal loss in with a DTC 24, and the customer
and-miss proposition. problem is occurring, the DRAC is says that once every three weeks the
By connecting a scan tool to the doing its job; which means there’s a truck drops into neutral at highway
vehicle you can check Vehicle Speed good possibility the PCM or EPROM speed and the MIL lights, there’s a good
and Output RPM through scan data. may have lost a couple of marbles. chance that everything’s going to test
You can also record a movie of scan okay. You aren’t going to find the
data when a problem is occurring to see source of the problem unless you can:

12 GEARS April 2005

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Diagnosing GM DRAC and Speed Sensor Circuits

When viewing a stored

waveform, you can zoom
out to view some of the
slower signal components
(figure 12). This is helpful
if you suspect a faulty
PCM or EPROM.
1. Find a marginal problem, such
as a poor ground or intermit-
tent connection.
2. Duplicate the problem while
performing the tests.

This is why it’s important to quiz

the vehicle owner. Ask whether any
work has been performed on the vehi- Figure 12: After zooming out, we can view more cycles of the Vehicle Speed
cle. If so, check the area where the and Speedometer signal waveforms
work was performed for loose grounds,
damaged harness, etc. Ask about the
driving conditions and what’s happen- Watch Out for the Ground Loop!
ing when the problem occurs:
• Is the engine cold or warm?
Whenever you’re using an oscilloscope, DMM, or any other
• What speed? type of test equipment that’s powered by the vehicle’s battery,
• Towing a trailer?
• Does the speedometer drop
cigarette lighter, or OBD-II diagnostic connector, be careful. Some
out? externally-powered test equipment will have its negative or
And so on, so you can try to dupli-
ground test probe connected internally to the tool’s power source
cate the failure. ground circuit. This means that the negative test probe is already
Don’t forget to check the basics:
charging system, powertrain ground,
connected to the vehicle’s ground.
chassis ground (July 2004 GEARS; The GM DRAC VSS terminal C7 is also connected to ground
Check the Grounds!), terminal tension,
battery condition, etc. Before you start
internally (to C8). So if you’re probing the VSS AC signal input to
throwing parts at the car, you need to the DRAC with test equipment that’s powered by the vehicle, and
cover the basics, especially with an
intermittent problem.
you have your negative test probe connected to DRAC terminal
The GM DRAC system has a few C12, you can essentially short out the VSS signal, giving you a false
things to take into consideration before
diagnosis can begin, but once you get
test result.To avoid this type of ground loop problem, always con-
the basics down it’s just a process of nect your negative test probe to DRAC terminal C7 and positive
elimination. Besides, it just gets easier
as time goes on; on most 1996-and-later
test probe to DRAC terminal C12 to check this signal.
GM vehicles, they eliminated the exter- Remember this when testing any other circuits on a vehicle;
nal DRAC; the vehicle speed sensor
signal goes straight to the VCM
you can short out circuits and possibly damage the vehicle or your
(Vehicle Control Module, or what they test equipment.
used to call the PCM). Isn’t it nice
when new technology actually becomes
easier to diagnose?

14 GEARS April 2005

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