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Every auto-electrician dealing with modern car electrics must understand how vehicle control systems work and why!
A vehicle battery voltage is not always 12 volts!
Static. Ignition on, engine not running 12.8 volts
Cranking. Starter motor drawing 300amps 10 volts
Dynamic. Engine running alternator loading battery upto 14.4 volts
The above suggested voltages can differ depending on other factors as well. Battery condition and if accessories are drawing a load, like radios, heated rear windows and lights.
The vehicle engine control unit (ECU) needs to know voltages from an array of sensors to make electrical powered adjustments to actuators.
Crank sensor: For ignition timing.
Emission sensors: For fuel control.
Throttle settings: For engine speed.
The list in endless...
So with the change in battery voltage from static to dynamic floating from 12-15 volts. Battery open circuit voltages are not stable enough for exact measurements.
To overcome this, every vehicle ECU unit has a regulated 5 volt transformer fitted to feed a stable voltage to all control sensors.
So whatever the battery state. The sensors are supplied with exactly 5 volts all of the time.
Another factor is system intergration.
Most vehicle wiring and control systems are designed to save cost and weight. It is pretty nomal for only one tiny 5 volt regulated transformer fitted inside engine ECU to feed every control system on a car. Aircon, body control systems, ABS, dashboards, smartcharge and CAN LIN P bus systems. All module logic systems and chips work on 5 volt feeds. This list is trully endless...
The problems with fault finding and diagnostics relating to 5 volt problems are as follows:
1. Diagnostic faultcodes from scanning with diagnostic equipment are not really faultcodes. Simply put, faultcodes are the wrong voltages recorded at the wrong time. Your scanner gives them a meaning from diagnostic protocol, software in the diagnostic equipment not the ECU . If you have problems with 5 volts control systems. You will get unreliable diagnostic information (faultcodes and live data).
2. As most 5 volt systems work on volt drop. To explain: A typical coolant temperature sensor is fed with 5 volt, a resistive coil inside of the sensor causes higher resistance when heated and the returned voltage is lower via cabling to the ECU for measurement. As 70% of all 5 volt systems work on the volt drop principle. As no high current or energy is used. All 5 volt systems are made for very low current, all wiring and components are very thin and light weight. Another 15% are hall effect sensors for very low voltage waveforms with very low currents involved. The remaining 15% are for data (CAN) systems transfer node to node, these systems carry very low voltage buad rates with negligible currents involved.
3. All 5 volt systems work on the returned earth principle. Postive voltage is supplied from a shared power source with shared supply cabling and differing voltages are returned thru separate wires to where they will be measured.
Intermittant 5 volt problems can be a nightmare to find.
Loss of function or a recorded faultcode (non permanent type). Maybe nothing connected to the described circuit faultcode or even function loss. But due to a 5 volt suply voltage drop somewhere else in the system. If the logic chips are low on voltage, they will not perform the functions they are designed for.
Mixing low current 5 volt wiring with 12 volt high current.
Breakdowns in cabling insulation causing voltage crossovers from high current 12 volt wiring to low current 5 volt wiring can be catastrophic for the modules connected at each end.
So if you are confronted with a problem you don't understand!
Measure the 5 volt radial circuit both static and dynamic. If you note any low voltages at the supply side of any resistive load sensor. This should ring alarm bells.
Your 5 volt supply should be the same voltage both static and dynamic, this will prove the regulated supply when the alternator ramps the voltage up after the engine goes dynamic.
Checking radial control voltages should be a basic diagnostic starting point like checking for fuel and battery voltage.
Spend less time looking at faultcode meanings and more time checking the voltage to create these faultcodes is correct and stable.
On most vehicles it is very easy to unplug the coolant temp sensor, MAF or MAP and check the voltage to earth with a multimeter.
Page updated 13-03-15