Automotive Troubleshooting With An Oscilloscope.

My Adventures With an Old Church Van
by Jeffrey Bledsoe
4/24/2012
Amended 6/22/2012
Corrected 5/25/2013

Jeffrey Bledsoe, PE, MSEE

WARNING
Some of the images in this presentation are ‘graphic’ and
may cause emotional distress to anyone who’s ever:
• Paid a repair shop $1,000’s for repairs that didn’t fix their vehicle’s
problem
• Attempted to troubleshoot their vehicle after spending those $1,000’s

Apologies to electrical engineers for the “bathtub” metaphor
used in the Excel graph annotations.

VIEWER DISCRETION IS ADVISED
Jeffrey Bledsoe, 4/24/2012
Charts amended 6/22/2012 by adding the Christian Brothers Invoice dated 12/3/2010.
Charts corrected 5/25/2013: (1) Corrected the date for the first visit to CBA in slide # 3. (2) Changed “inverter return” to
“inverter safety ground” in one of the photo’s annotations.


Background: A Church Van’s* Intermittent Ignition Failures

•Random/intermittent engine shutdown
•
•
•

After shutdown, engine wouldn’t restart, usually restarted ~ 10 minutes later
No correlation of failures with weather or driving habits
No diagnostic codes were stored and Check Engine Light was never illuminated

•Christian Brothers Automotive (CBA) Efforts
•

CBA attempted unsuccessfully to find the problem during 2 visits over a period
of 6 months
• 12/3/2010: CBA replaced crankshaft sensor, $392.97 charge
• 5/27/2011: CBA replaced several other parts, ~ $1,230.76 charge*
• $1,623.73 total paid to Christian Brothers Automotive in Weatherford TX
*Included $70 fuel charge

* Emmanuel Baptist Church’s Dodge Passenger Van, VIN = 2B5WB35Z6RK551308


•Christian Brothers Automotive (CBA) Efforts, 12/3/2010 Invoice
•
•
•
•

Diagnostics methods and results were ‘fuzzy’
$392.97 total parts and labor
Replaced Crankshaft Sensor
Result: Van exhibited same failure about five times over the next few months

•Christian Brothers Automotive (CBA) Efforts, 5/27/2011 Invoice
•
•

•
•

No engine failures while in possession of CBA (according to our youth minister)
CBA did the following:
• Ran engine performance diagnostics (no problems reported in receipt)
• Replaced fuel pump
• Replaced IAC Motor (Idle Air Control Motor)
• Replaced ASD Relay (Automatic Shutdown Relay)
• Inspected for damage caused by mice (LOL)
• Refueled the van
• Washed the van
$1,230.76 total parts and labor cost to the church (including $70 for fuel)
Van returned to church on 5/27/2011
• Van failed within a week while fetching children for Vacation Bible School
• Same failure symptoms as usual; and same ones observed before both
visits to CBA

•I volunteered to troubleshoot the problem, starting 6/10/2011


Most likely, engine was occasionally missing due to marginal coil drive

What were the retest results? Did replacing the crank sensor fix it? I don’t think so.


No charge for rodentia
damage inspection.(LOL).

Free wash!

(cont’d)
•Fault Isolation and Repair by Jeffrey Bledsoe
•
•

6/10/11: Rigged timing light to a spark plug and bought alldatadiy.com account
6/23/11: Determined that spark was not present during failure to restart
• From alldatadiy.com descriptions and schematics, 2 probable causes:
• Probable cause #1: ASD dropout after loss of crank and/or cam signals
• Probable cause #2: Intermittent coil driver (in PCM) or intermittent coil

•
•
•
•
•

9/3/2011: Eliminated crank and cam position signals as cause of failures
9/10/2011: Discovered failed coil driver in the Powertrain Control Module (PCM)
11/4/2011: Replaced PCM
1,800 miles later, no failures (miles driven 11/6/2011 thru 04/15/2012)
• Several trips to Fort Worth
• One mission trip to Port Arthur, near Houston
Jeff’s troubleshooting costs =~ $575
• $25 for alldatadiy.com account for the van
• $70 for insulation-piercing probes
• ~$250 for fuel for test drives (~ 700 miles driven over 5 months)
• $230 for new PCM
• ~ 40 hours of research, analysis, and study + ~30 hours of test drives
• The experience: priceless


TROUBLESHOOTING TIMELINE FOR A 1994 DODGE B350 VAN*, JUN-NOV 2011
ACTIVITY
Milestones

Initial Data Gathering
•Is spark present during failure?
•Opened alldatadiy.com account

Instrumentation

JUNE
Installed
Timing
Van Died,
Light
No Spark
(6/10)
(6/23)

06/10

JULY
Set up scope
and collected
baseline data
(7/21)

AUG

Van Died, Cam
and Crk Sensors
Are Good (9/3)
2 WK
VACATION

SEPT

OCT

NOV

Install/Test
Van Died, Bad Coil
Tested Ordered PCM (11/4 Road
Drive Signal
new PCM & 11/5)
Tested
Wiring
(9/10)
PCM Vibe
Good
@ PCM (10/28)
Tests (9/24)
(11/6)
(10/24)

06/23

06/24

07/21

•Schematics
•Parts Procurement
•Installation

Sensor Tests

07/21

09/03

•Crank and Cam Sensors
•8Vdc Pwr/Rtn (power to sensors)
•ASD (Auto Shutdown Command)

PCM Driver Tests

09/04 09/10

•Cam and Crank Sensors
•PCM’s Coil Driver
•PCM’s Fuel Injector #1 Driver

Final Tests / Analyses

09/11

•Vibration and Wiring Tests
•Data Analysis
•Ordered Remanufactured PCM

Resolution

10/28

11/04--11/06

•Installed new PCM
•Test Drove to Fort Worth twice
•Project End
* VIN 2B5WB35Z6RK551308


Schematic With Key Signals
•Key signals: based on research at alldatadiy.com
•Van schematic only, i.e., no instrumentation
•1 slide


SCHEMATIC, KEY PCM AND SENSOR SIGNALS, 1994 DODGE B350 VAN

PCM (Powertrain Control Module)
Cam
Sensor

Cam Sense
Crank Sense

Crank
Sensor

8 Vdc
8 Vdc Rtn
12 V from Ignition Sw

12 V from Battery
ASD Relay Contact

ASD Relay Coil

ASD Driver*
Coil Driver*
Fuel Inj #1 Driver*

Ignition Coil Primary Winding

Fuel Injector #1 Solenoid

12 V from ASD

• ASD == Automatic Shut Down
• * Open/Ground drivers inside PCM


Sensor Test Results
•Instrumentation Schematic (1 slide)
•Annotated Waveforms (3 slides)
•Sensor Test Conclusions (1 slide)


INSTRUMENTATION OF SENSORS, SENSOR POWER, AND ASD DRIVER
Cam
Sensor

Cam Sense
Crank Sense

Crank
Sensor

8 Vdc
8 Vdc Rtn

12 V from Battery
ASD Relay Contact
ASD Relay Coil

Scope**

5 kΩ

Ch 1
5 kΩ

Ch 2

ASD Driver *
Coil Driver *
Fuel Inj #1 Driver *


5 kΩ


5 kΩ

12 V from ASD

Ch 3
5 kΩ
(At TB)

5 kΩ
Ch 4
5 kΩ

• ** Scope floated with an AC cheater plug
Insulation-piercing probe

Resistors
built-in to
harness

Ext Trig
Terminal
Block
(TB)

Van Structure

Here are the baseline waveforms with the van running okay

Note: Because of the voltage divider made up of three 5 kΩ resistors, displayed voltages for 8V power (green scope trace above) are MSEE
Jeffrey Bledsoe, PE, less
than 8 V; voltage can be calculated at ~= 0.7 X the actual voltage.
Q: Why the voltage divider?
A: To detect either a defective 8V power source or a defective 8V power return. If the voltage went to ~8V during a failure, this would
indicate a defective 8V return path in the PCM. If the voltage went much lower than ~6V during a failure, this would indicate a defective 8V
power rail or open-circuited conductor inside the PCM. (The scope was “floated” and referenced to a local structure ground, thus allowing
one scope channel to provide information on the health of two signals (8V power and 8V power return). Of course, in the unlikely event that
both 8 V and 8 V return failed, the voltage at the scope would tend toward 0 V as well.

After many days of test drives, the van died on 9/3/2011

Note: Because of the resistor divider, voltages for 8V power are approximately 0.7 X actual
voltage

ASD driver opens ~ 375 ms after last
crank sensor pulse (maroon trace).
Cam sensor (blue trace) is still at logic
‘1’ when ASD opens.

Trigger at t = 0, PCM says “game
over”, ASD driver opens, voltage
transitions to 13 V as observed via
the ASD relay coil.


Crank and Cam Sensors Are Good During No-Restart
(See blue(CAM) and maroon(CRANK) scope traces below)

Scope trigger at t = 0……..ASD driver
opens, ASD voltage transitions to ~ 11 V
shortly after key  off

Notes:
1. Because of the resistor divider, voltages for 8V power are approximately 0.7
X actual voltage.
2. Had to power the inverter from my truck battery via jumper cables because
the van battery voltage was too low to run the inverter during engine cranking.


Conclusions from Sensor Tests
•The shutdown waveform shows active crank sensor pulses until ~375 milliseconds
before ASD drops out; likely too short a time to have caused the ASD dropout.
•The “No Restart” waveform shows conclusively that the cam and crank position
sensors are functional during a “no restart” scenario; attempts at restarting the engine
lasted 5 to 10 seconds, with cam and crank signals active during the entire period; the
waveform shows only the last ~2 seconds of activity before key-off and ASD dropout,
triggering the scope.
•There was the possibility that, although I could see the cam and crank signals on the
scope, one or both sensor signals were not observable by the PCM. However, if the
PCM lost either of these inputs, it should have activated the Check Engine Light and
flagged a sensor failure. (After failures, the check engine light never turned on and no
failure codes were stored.)
•The next logical step was to reconfigure the instrumentation to look at other high
potential signal failures. The first signal on the list was the coil driver, i.e., monitoring a
wire between the PCM and the ignition coil. {According to a Dodge service manual*,
the PCM has no diagnostics on the output of this driver (most likely, newer vehicles
detect failures in the coil driver(s)).} For information, I decided to also monitor one of
the fuel injector drivers.
*John Hagan’s Service Manual for his 1994 Dodge Dakota, i.e., not the fullsize van.

PCM Coil Driver Test Results
•Instrumentation Schematic (1 slide)
•Annotated Waveforms (6 slides)
•Coil Driver Test Conclusions (1 slide)


INSTRUMENTATION OF SENSORS PLUS COIL AND FUEL INJECTOR DRIVERS
Cam
Sensor

Cam Sense
Crank Sense

Crank
Sensor

8 Vdc
8 Vdc Rtn

12 V from Battery
ASD Relay Contact
ASD Relay Coil

Scope**

5 kΩ

Ch 1
5 kΩ

Ch 2

ASD Driver *
Coil Driver *
Fuel Inj #1 Driver *


5 kΩ


5 kΩ

12 V from ASD

Ch 3
5 kΩ
(At TB)

5 kΩ
Ch 4
5 kΩ

• ** Scope floated with an AC cheater plug
Insulation-piercing probe

Resistors
built-in to
harness

Ext Trig
Terminal
Block
Van Structure
(TB)

BASELINE DRIVER WAVEFORMS, SHOWING ON-TIMES AND OFF-TIMES,
(THINK OF THE ON-TIMES AS LITTLE BATHTUBS STORING ENERGY TO BE RELEASED)

Baseline coil and injector driver waveforms (van running normally), 9/8/2011
CAM (v)
195.00

CRK (v)

COIL (V)

INJ #1 (V)

Note: The coil driver voltage data was corrected (multiplied by 3/2) to account for the voltage divider left
over from the 8V power/return sense scheme.

185.00
175.00
165.00

~ 200 V on coil primary, at beginning of off-time,
produces several thousand volts on the coil secondary,
which is routed to the spark plugs via the distributor.

155.00
145.00
135.00

Typical Injector #1 Driver Off-Time

125.00

105.00

Typical Coil Driver Off-Time

95.00

Typical Injector #1 Driver On-Time

85.00

(during this bathtub/on-time, fuel is delivered to
cylinder #1)

Typical Coil Driver On-Time

75.00

(during this bathtub/on-time, current
saturates the coil primary winding.)

65.00
55.00
45.00
35.00
25.00
15.00
5.00

25

20

15

5

10

0

-5

-10

-15

-20

-25

-30

-35

-40

-45

-50

-55

-60

-65

-70

-75

-80

-85

-90

-95

-100

-105

-110

-115

-120

-125

-130

-135

-140

-145

-150

-155

-160

-165

-170

-175

-180

-185

-190

-195

-200

-205

-210

-215

-220

-5.00
-225

Voltage, V

115.00

Time, milliseconds

Little green bathtubs: coil-driver on-times


CAPTURED DRIVER WAVEFORMS DURING ENGINE SHUTDOWN ON 9/10/11
Engine Shutdown at home, 9/10/2011
Note: Last good coil pulse was at -713 ms; note annotations concerning pre-pulse voltage levels.
CAM (v)

50

CRK (v)

COIL (V)

INJ #1 (V)


45

ABNORMAL COIL DRIVER VOLTAGES IN THIS PERIOD

40

•DRIVER OUPUT IS PRESENT BUT DEGRADED
•DRIVER CAN’T SATURATE THE COIL PRIMARY (NO NICE
SOLID BATHTUBS, NO SPARK TO SPARK PLUGS)
•STRANGE RINGING ON THE SIGNAL
•THE LOW LEVEL SHIFTED TO AS LOW AS -5V
•EVEN BEFORE ENGINE SHUTDOWN, BATHTUBS ARE AT -3V
•NOTICE THE INJECTOR #1 BATHTUBS NEAR 0 V

35

Voltage, V

30

25

ASD DROPS OUT,
12 V IS REMOVED
FROM COIL AND
INJECTOR, SCOPE
TRIGGERS

20

15

COIL DRIVER RINGING

10

5

12 V
0

-5
-850

-750

-650

-550

-450

-350

-250

-150

-50

50

150

250

Time, milliseconds

The last green bathtub before shutdown.


ZOOM-IN ON DRIVER WAVEFORMS DURING ENGINE SHUTDOWN ON 9/10/11
Engine Shutdown at home (zoom-in), 9/10/2011
Note: Last good coil pulse was at -713 ms; note annotations concerning pre-pulse voltage levels.
CAM (v)

CRK (v)

COIL (V)

INJ #1 (V)


20

15

Voltage, V

10

5

12 V

0

-5
-850

-840

-830

-820

-810

-800

-790

-780

-770

-760

-750

-740

-730

-720

Time, milliseconds

-710

-700

-690

-680

-670

-660

-650

-640

-630

-620

-610

-600

The last green bathtub before shutdown.

CAPTURED DRIVER WAVEFORMS WHEN THE ENGINE WOULDN’T RESTART

Notes:
1. The coil driver voltage data was corrected (multiplied by 3/2) to account for the voltage divider left over
from the 8V power/return sense scheme.
2. Had to power the scope via an extension cord from the garage because the van battery voltage was too
low to run the inverter during engine cranking.

•NO COIL BATHTUBS AT ALL
•COIL DC LEVEL AT 6 V, INJ #1 AT 10 V


ZOOM-IN ON DRIVER WAVEFORMS WHEN THE ENGINE WOULDN’T RESTART

Notes:


120-130 ms zoom-in on next page


ZOOM-IN ON DRIVER WAVEFORMS AT 120-130 ms DURING NO-RESTART

Notes:

WAVEFORM TOO SPIKEY TO
SATURATE TRANSFORMER’S
PRIMARY


Conclusion from Driver Tests
•Shutdown waveform shows cessation of good coil driver
pulses at about 713 ms before ASD dropout.
•The “No Restart” waveform indicates a malfunctioning coil
driver circuit; the sensor signals continue to look good.
Several coil waveform abnormalities were noted on the prior
charts; the most important one is lack of coil saturation, i.e.,
no “bathtubs, during restart attempts.”
•I concluded that the PCM’s coil driver or the coil driver pin
at the PCM was failing intermittently.
•The engine failure remained intermittent and I was able to
drive the van around town, risking a shutdown (as usual) in
the middle of heavy main street traffic. I decided to try and
induce the PCM failure with a source of vibration and to
test/inspect the connector pins at the PCM and at the
harness connector that mates with the PCM.

Further Tests at the PCM
•I tried to induce the PCM failure by running the engine with
the scope ready to trigger while vibrating the PCM and
firewall of the van with a large pneumatic chiseling tool*. The
engine didn’t die, the scope didn’t trigger.
•I removed the PCM connector and backshell and tested
continuity of the coil driver wire to the coil driver mating pin
while wiggling the wire in the connector backshell. Also
tested the 3 returns (ground lines would provide a current
path for the coil driver) at the PCM connector in the same
way. No continuity problems at those 4 pins. All connector
pins looked good on both the PCM and the mating
connector: no corrosion, no bent pins.
*Provided by John Hagan onsite at Moore Monuments

Resolution
•Based on the scope data, the PCM’s coil
driver was failing intermittently. I didn’t
understand the failure mechanism and had
hoped the external vibration would induce the
failure.
•I ordered and installed a remanufactured A-1
Cardone PCM from O’Reilly’s Auto Parts
•No “engine shutdown” incidents during the
~1,800 miles since replacing the PCM.

Pictures of Instrumentation


Tektronix TDS2024C Oscilloscope

Bungee cords for scope restraint

My Bent
C-Clamp holding wood block to Ashtray

Signal Wiring

Wood Block

Terminal Block mounted
on wood block
4 Scope Probes

Coax cable connected to Scope’s Ext Trigger

Scope chassis isolated
from inverter safety ground
with cheater plug (orange)

100 W Inverter

Scope chassis connected to the van’s
structure
Scope Power
Cord/Plug

My Bent

Cam
Crank
8V/Rtn
ASD

Thumb Drive for storing
captured data

Signal and Power Wiring
Routed on Dashboard,
secured with duct tape.

My Bent


Power Wiring
Inside Convoluted
Tubing

EZ-HOOK Macro
Probes (insulationpiercing probes)

Signal Wiring
Inside Convoluted
Tubing

Piece of wood trim
supporting probes

Battery Post Clamps:
Power for the inverter.

I “adjusted” the hood structure with
a hammer to minimize pinching of
cables as they exit the engine area.

Signal and power wiring
secured to outside of van with
duct tape.


Black Box: Powertrain Control Module (PCM)

60-pin connector that mates with PCM
(removed by loosening this bolt)

40 wires in the bundle at the PCM connector (using the
schematic and wire color data from alldatadiy.com, I
hunted for the six signals of interest)


Automotive Troubleshooting With An Oscilloscope.

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