Advanced Automotive Diagnostics - 7 Step Process and more ...

Introduction

How do you solve a problem?

You need to have some sort of repeatable process for troubleshooting issues. Thats where the 7 step process comes in.

Also on this page is a list of general specifications for different types of sensors, motors, and tests in the automotive space.

This is a repository of knowledge, random facts and specifications. Some of these specifications are not generally available or easily found. These are general rules because there are no original equipment specifications given for most of these types of tests.

Most of these specifications come from experience, aftermarket suppliers videos, school, professional books, professional websites and professional teachers. I try to verify the sources and information posted here but again these are guidelines. This will be updated from time to time.

If you have any suggestions feel free to email me. These methods are not all inclusive but may point you in the right direction.

The 7 step process for general vehicle (or software) diagnosis

1. Verify the complaint / Duplicate the problem
2. Perform a visual inspection
3. Gather data and Understand the system
4. Develop a diagnostic plan
5. Perform tests
6. Determine root cause and Make the repair
7. Verify the repair

(May have to repeat steps 2-5 as more information is gathered after performing tests)

To see an example of this see my article The computer doesn't always know whats right.

Fuel Pump

• Fuel pump amperage draw - most pumps draw around 6-8 amps. - How to test with pico scope https://www.picoauto.com/library/automotive-guided-tests/fuel-pump/
• One rotation should take about 6.5-16.5 ms
• Typically rotate at 5000-6000 rpm, less than 3000 rpm will typically have driveability issues
• The scope pattern should be fairly even and consistent with no dropout signals

Engine Performance

Fuel trim -

• Short term and long term should be close to 0.
• +- 5% is generally acceptable, the computer starts to set codes around +- 10%
• Negative fuel trim means engine is running rich and computer is trying to take away fuel to compensate
• Positive fuel trim means engine is running lean (or thinks it is) and computer is trying to add fuel to compensate
• If you record a data log and take the average of the recorded average fuel trims, this will be the true fuel trim

Vacuum -

• Highest engine vacuum is on deceleration
• Lowest engine vacuum is on heavy acceleration or wide open throttle - around 0-2 inHg
• Atmospheric pressure - manifold vacuum = absolute pressure aka resultant pressure in manifold

5 gas analyzer -

• Low HC, Low CO, High o2 = lean condition
• High HC, High o2 = misfire

Battery

• Battery state of charge = 12.6v = 100% charge, 12.4v = 75% charge, 12.2v = 50% charge
• 9.6v minimum while cranking measured with a DVOM
  8v minimum while cranking measured with a scope
• Battery load test with carbon pile load tester - load battery to 1/2 battery cca rating, watch voltage, should not drop below 9.6
• When looking for parasitic drain, voltage drop the fuses do not pull them out.
• Parasitic drain should be 30mA or less after 30 minutes

Alternator

• Should have 13.5v - 15v while charging with accessories on.
• Charging current after 5 minutes of run time should be less than 5 amps with no accessories on
• AC ripple should be less than 0.5v measured with DVOM
• When measuring ripple with oscilloscope look for consistent patterns or dropouts.

Starter

Engine Cranking RPM - 200-300 RPM

Average voltage starter draw:

• 4cyl starter draw = 150-180A (less than 100A)
• 6cyl starter draw = 160 -200 A (less than 125 A)
• 8cyl starter draw = 185 -250 A (less than 150 A)

Peak Amperage starter draw:

• for 8cyl recorded with scope = about 700-1000A
• typical for 4 or 6 cyl measured with scope = about 500 Amps

Starter Solenoid Amperage draw:

• Solenoid pull- in winding's = 30-60A
• Solenoid hold-in winding's = 5-10A

High current, low speed = mechanical problem
Low current, low speed = excessive resistance

Engine Mechanical Specifications

Crankshaft rotates at 2x the speed of the camshaft. -

• 2 crank rotations = 1 cam rotation
• RPM measures rotations of the crankshaft

Cranking engine compression -

• 100 psi minimum
• No more than 10% variation between cylinders

Sensors

NTC thermostat - Negative temperature coefficient -

• Temp goes down, resistance and voltage goes up
• Temp goes up, resistance and voltage goes down

Pull up circuit -

• monitored on 5v
• unplugged reads 5v

Pull down circuit -

• monitored on ground
• unplugged reads 0v

Pull up or pull down circuit needs 2 resistors to make a series circuit, one known calculated resistor is in the computer, the other is typically the sensor itself.

MAP sensor (Mass Air Pressure) -

• Intake vacuum low = high freq. or high voltage
• Intake vacuum high = lower freq. or Low voltage
• If system only uses a MAP sensor and no MAF it is called a speed density system

MAF sensor (Mass Air Flow) -

• Airflow increases = increased frequency or high voltage
• Airflow decreases = decreased frequency or low voltage
• MAF should read on scan tool approximately 1 gram per second per liter of engine size at idle and should double with every double of RPM

AFR - Air Fuel Ratio sensor (also known as a wide band sensor or wide ratio air fuel sensor) -

• Typical pre-cat sensor on newer cars
• Has a Lambda reading or voltage reading that is viewed with a scan tool
• Lambda reading on scan tool above 1.0 is lean, below 1.0 is rich.
• Voltage and amperage reading interpretation is similar to Lambda - above reference voltage or positive current flow is lean- below reference voltage or negative current flow is rich.
• The computer tries to keep the mixture at 1.0 Lambda or reference voltage which is equivalent to 14.7:1 Air fuel ratio.
• Requires a reference voltage - Typical AFR reference voltage: Toyota-3.3v Bosch and GM - 2.6v
• Operates at about 1292°F - 1472°F

Standard O2 sensors -

• Typical for post cat sensors on newer cars (Pre cat on older cars)
• 450mV average voltage reading, switching between 0.2v and 0.8v at least once per second at a steady 1500 rpm
• Generates its own voltage - no reference voltage required
• When O2 sensor fails computer usually defaults to a richer open loop mode
• Operates at about 600°F

Open loop mode -

• Computer runs off of pre programmed fuel and ignition charts based on MAP or MAF readings till the system is ready to enter closed loop mode

Closed loop mode -

• Computer enters closed loop when the O2 sensor or AFR sensor is up to operating temperature.
• When in closed loop the computer continuously alters standard timing and fuel injection based on feedback from O2 sensor or AFR sensor.

Piezoresistive sensors -

• are for fluid pressure

Piezoelectric sensors -

• are for knock sensors

Thermocouple works on the thermoelectric effect -

• produces a temperature-dependent voltage
  this voltage can be interpreted to measure temperature.

Strain Gauge -

• When a conductor is stretched its resistance is increased
• Typically pizioresistive

Permanent magnet sensors -

• Typically have 2 wires
• AC signal generated from the sensor
• Checking amplitude and frequency
• Amplitude and frequency increase as speed increases

Hall effect sensors -

• Typically have 3 wires, Reference, signal and ground
• Reference voltage provided by vehicle
• Produce a digital signal

Common Conversions and Pressures

• 29.9 inHg atmospheric pressure at sea level - minus 1 inHg for every 1000 ft above sea level
• 1g = 9.8 m/s^2
• 0-200 Hz may be felt as vibration
• 200-20,000 Hz can be heard as well as felt
• 20,000 Hz and above is dolphin and ultrasonic territory

General Electrical Information

• Frequency = cycles per second = measured in Hz
• Cycle = one on and off cycle
• Pulse width = measurement of on or off time per cycle measured in time (seconds or miliseconds typically)
• Duty cycle = percentage of on time per cycle
• According to Bosch- Max 3% voltage loss to circuit resistance (12 x 0.03 = 0.36v) 12v circuit should have at least 11.64v to operate load
• PROM chip - programmable read-only memory - can only be written once EEPROM chip - electrically erasable programmable read-only memory - can be erased and programmed again and again

Lights

Amp draw standard headlight bulbs -

• Headlamps low = about 4A
• Headlamps high = about 6A

Communication networks

Data Link Connector Pins:

• Pin 16 is 12v power
• Pin 4 is chassis ground

Communication protocol is basically the "language rules" of the protocol -

• Voltage thresholds - how are the 1's and 0's defined?
• How message is structured - What is the priority? - How is arbitration handled?
• The number of wires for the system
• Speed - sampling rate
• Topology = Physical layout of system (bus, ring, star common)
• Gateway = “bilingual” ECU - links multiple networks together

GM class 2 voltages -

• rest = 0v
• Dominant = 7v

CAN voltages -

• High rest = 2.5v
• High Dominant = 3.5v
• Low rest = 2.5v
• Low Dominant = 1.5v

Volumetric Efficiency

Sub Title -

• At idle typical volumetric efficiency below 40%
• Higher vacuum is, the less volumetrically efficient the engine is.
• Opening the EGR valve can change manifold vacuum and volumetric efficiency.
• Volumetric efficiency is typically higher when the throttle is open than when it is closed.
• Volumetric efficiency drops 2-3% for each 1000 ft above sea level