Checking Leads

NOTE:

Many faults in electrical systems are due to leads and connections. These faults are mainly due to interference from other electrical systems, mechanical or chemical effects.

If an intermittent fault occurs, this is often due to a fault in leads or connections. Check as shown below.

1. Procedure
- Check connections visually.
- Check resistance in connectors and leads.
- Check for open-circuits.
- Check for ground shorts.
- Check for shorts across leads.
- For intermittent faults, measure resistance in contacts and connections.

2. Checking connections visually
- With readings or checks which involve undoing connectors, always check connections visually. Look for oxidation which may affect connections. Check that connecting pins are intact and that copper leads are properly connected to pins.

3. Resistance in leads and connections
- In theory, resistance in contacts, leads and connections should always be zero: but there will always be some resistance due to poor connections, oxidation and wear.
- If resistance becomes too great, it will cause operating problems.

Checks
Shake leads and pull on connections gently when taking readings to see if there are any intermittent faults.

Using a loose female pin, check that male pins connect properly and that pins do not come loose if you pull gently on the male pin.

Use an ohmmeter to take readings across the ends of leads: and take readings with switches and sensors in circuit to see if there is any excessive contact resistance in them.

The ohmmeter reading should not be more than a few ohms in a circuit with acceptable levels of resistance in contacts and connections.

4. Intermittent faults
- Intermittant faults are often due to faults in leads and connections. If you suspect there is a transient fault:
- Read off DTCs and erase them
- Wait about 10 seconds
- Read off DTCs again. If the DTC has disappeared, this may be due to a transient fault which has now returned to normal.

5. Open-circuits
- Open-circuits show up when something fails to work. Rubbed or worn leads and connections which have come loose are the usual causes.

Checking

Undo connectors at both ends of lead.

Before testing, check that test equipment reads 0 ohms when the test leads are held together.

Using an ohmmeter, measure across the ends of the lead.

The ohmmeter reading should be approx. 0 ohms if there are no open-circuits in the wire.

In some cases, leads may be difficult to undo. If one end of the lead connects to a ground point, you can put the test lead straight to the ground point.

If one end of a lead is connected to a junction, you can put a test lead with an extra fine point through the lead sheath, then take readings with the test lead inserted on both sides of the junction. If the hole in the sleeve gets too large, cover with insulating tape.

6. Short-circuits to ground
- The usual sign of a short-circuit between a live wire in a circuit and ground is a fuse blowing when power is turned on or something failing to work.

Checking for short-circuits with a voltmeter
Before replacing fuse
- Ignition off.
- Remove the fuse which has blown.
- Disconnect load.
- Connect a voltmeter across the fuse to see if there is any short-circuit to ground.
- Ignition on.
- Shake the lead connected with the load gently and see if the test equipment responds. If it shows a voltage, this means the lead is short-circuited to ground.

Checking for short-circuits with an ohmmeter
- Ignition off.
- Disconnect battery negative lead.
- Remove the fuse which blew.
- Remove the load.
- Connect an ohmmeter to the wires to be checked.

Before testing, check that ohmmeter reads 0 ohms when test leads are held together. Shake the lead gently, while watching the ohmmeter. If the ohmmeter reads infinite resistance with no components in circuit, there is no short-circuit. If the ohmmeter reading is low, approx. 0 ohms this means the lead is short-circuited to ground.

7. Short-circuits between leads
- Short-circuits between leads do not always cause a fuse to blow: the fault usually shows up when something does not work.

Testing
- Disconnect battery negative lead
- Use an ohmmeter between suspect leads to see if there is a short-circuit across them.

Ohmmeter should read infinite resistance when used between leads not connected to one another in circuit.