Here is going to be 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 vett 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. This is a repository of specifications, if you are looking for tutorials or how to guides I will be working on some of those in the future or check my personal projects section. Thanks.
The 7 step process for general vehicle 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)
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 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
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
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