Part ten: Top tips for testing vehicle electrics systems.
PART TEN: Top tips for electrical testing from Jack Moore.
When it comes to electrical testing and fault-finding it can sometimes be difficult knowing where to begin. A decent multimeter doesn’t have to be expensive, but it will be your most valuable tool for electrical diagnostics. Having a good grasp of the basics and following a well-defined test process will yield the best results.
1 Keep it simple
Fuses are normally easily accessible and easy to test. Owners’ manuals will usually give an index indicating fuse location and rating, while fusebox covers will often give a visual or numerical layout of fuses and relays. Fuses can be inspected visually by removing and checking if the fusible link is broken, or else you can use a multimeter to test for continuity across the contacts. Some types of fuse can be tested in place by checking for equal potential (voltage) at the exposed metal contacts on the back of the fuse.
2 Visual checks
If the component you are testing is accessible, visually inspect the wiring to it for burning, chafing or signs of corrosion. Check the block connector for water ingress and confirm the condition of the contacts, which should be free of dirt and provide a good connection between the male and female contacts. Visually inspect any ground points you can access.
3 Electrical tests
In order to function, an electrical component requires a live or positive feed and a ground or negative connection. The ideal way to check these is by taking a voltage drop measurement; this is a measure of the voltage that gets ’dropped’ across the component.
With the circuit connected and switched on, the voltage measured across the back of the pins should be roughly within a volt of battery voltage (this is true for most, but not all, circuits – some circuits work with a different voltage, such as 5-volt sensors/switches). If the voltage drop measured is equal to battery voltage then test the component itself (see Step 4). If the voltage measurement is less than battery voltage, check the live and ground connections individually. These tests should be carried out with the circuit switched on.
Testing the live Place the red lead of the multimeter on the live connection of the component and put the black lead on the battery negative. This should give a reading equal or close to battery voltage. If not, refer to a wiring diagram and trace the live side back to its source.
Testing the ground Place the red lead of the multimeter on battery positive and the black lead on the component ground. This should give a reading close to battery voltage. If not, the component does not have a good ground supply. Trace the ground side of the circuit – this is usually through the vehicle chassis – detaching any ground connections and cleaning them thoroughly. When refitting, ensure a good connection is made.
4 Testing components
If a component has a live and ground side but is still not operating, it’s quite likely that it is faulty. However, if there’s an easy way to test it before replacing it then it’s always worth doing this.
A lot of basic components can be tested with a multimeter for continuity/ resistance. Including bulbs, switches, glow plugs, fuel pumps, motors, relays, sensors, heated windows, solenoids, horns, etc. These components must have continuity through them in order to work. Sometimes establishing the manufacturer specification for resistance can be difficult. If there is more than one of the component you are testing fitted to the vehicle – a glow plug or window motor, for example – do a comparison check.
Take care not to damage electronic components and never check SRS components with a multimeter.
5 Quick four-pin relay tests
Relays sometimes form part of complex circuits that can be confusing. Here is a quick test that’s easily carried out to determine the condition of a relay.
A relay uses a small current to close a set of contacts, allowing a larger current to flow. The smaller current passes through a winding that creates a magnetic field when energised. On a standard relay, the resistance through this winding is normally between 50-120 ohms, which is easily measured with a multimeter. The winding contacts are usually marked 85 and 86 or 1 and 2. To test if the relay contacts are closing, connect a 9-volt battery across contacts 1 and 2. You should hear an audible click. With a second pair of hands, check for continuity between contacts 3 and 5. With the 9-volt battery connected, you should have continuity with little or no resistance.
When troubleshooting, try to arm yourself with as much information as possible. Confirm whether the vehicle has had any repair or servicing work recently. Conduct an all-round function test to see if there are any other issues or components not working. Check footwells, the spare wheel well and boot for signs of water ingress. Anywhere that wires are forced to move or flex – such as the door and tailgate wiring – is a common source of trouble. Always remember the basics and, when the fault has been located, don’t forget to check if there is an underlying cause. Happy troubleshooting.