- Before attempting to diagnose an electrical fault, first study the appropriate wiring diagram, to obtain a complete understanding of the components included in the particular circuit concerned. Noting if other components related to the circuit are operating properly can narrow the possible sources of a fault down. If several components or circuits fail at one time, the problem is likely to be related to a shared fuse or earth connection.
- Electrical problems usually stem from simple causes, such as loose or corroded connections, a faulty earth connection, a blown fuse, a melted fusible link, or a faulty relay. Visually inspect the condition of all fuses, wires and connections in a problem circuit before testing the components. Use the wiring diagrams to determine which terminal connections will need to be checked in order to pinpoint the trouble spot.
- The basic tools required for electrical faultfinding include a circuit tester or voltmeter (a 12-volt bulb with a set of test leads can also be used for certain tests); an ohmmeter (to measure resistance and check for continuity); a battery and set of test leads; and a jumper wire, preferably with a circuit breaker or fuse incorporated, which can be used to bypass suspect wires or electrical components. Before attempting to locate a problem with test instruments, use the wiring diagram to determine where to make the connections.
- To find the source of an intermittent wiring fault (usually due to a poor or dirty connection, or damaged wiring insulation), a “wiggle” test can be performed on the wiring. This involves wiggling the wiring by hand to see if the fault occurs as the wiring is moved. It should be possible to narrow down the source of the fault to a particular section of wiring. This method of testing can be used in conjunction with any of the tests described.
- Apart from problems due to poor connections, two basic types of fault can occur in an electrical circuit - open-circuit, or short-circuit.
- Open-circuit faults are caused by a break somewhere in the circuit, which prevents current from flowing. An open-circuit fault will prevent a component from working.
- Short-circuit faults are caused by a “short” somewhere in the circuit, which allows the current flowing in the circuit to “escape” along an alternative route, usually to earth. Short-circuit faults are normally caused by a breakdown in wiring insulation, which allows a feed wire to touch either another wire, or an earthed component such as the body shell. A short-circuit fault will normally cause the relevant circuit fuse to blow.
FINDING AN OPEN-CIRCUIT
- To check for an open-circuit, connect one lead of a circuit tester or the negative lead of a voltmeter either to the battery negative terminal or to a known good earth.
- Connect the other lead to a connector in the circuit being tested, preferably nearest to the battery or fuse. At this point, battery voltage should be present, unless the lead from the battery or the fuse itself is faulty (bearing in mind that some circuits are live only when the ignition switch is moved to a particular position).
- Switch on the circuit, then connect the tester lead to the connector nearest the circuit switch on the component side. 12 If voltage is present (indicated either by the tester bulb lighting or a voltmeter reading, as applicable), this means that the section of the circuit between the relevant connector and the switch is problem-free.
- Continue to check the remainder of the circuit in the same fashion. 14 When a point is reached at which no voltage is present, the problem must lie between that point and the previous test point with voltage. Most problems can be traced to a broken, corroded or loose connection.
FINDING A SHORT-CIRCUIT
- To check for a short-circuit, first disconnect the load (s) from the circuit (loads are the components which draw current from a circuit, such as bulbs, motors, heating elements, etc).
- Remove the relevant fuse from the circuit, and connect a circuit tester or voltmeter to the fuse connections.
- Switch on the circuit, bearing in mind that some circuits are live only when the ignition switch is moved to a particular position. 18 If voltage is present (indicated either by the tester bulb lighting or a voltmeter reading, as applicable), this means that there is a short-circuit.
- If no voltage is present during this test, but the fuse still blows with the load (s) reconnected, this indicates an internal fault in the load (s).
TO FIND AN EARTH FAULT
- The battery negative terminal is connected to “earth” - the metal of the engine/transmission unit and the vehicle body - and many systems are wired so that they only receive a positive feed, the current returning via the metal of the car body. This means that the component mounting and the body form part of that circuit. Loose or corroded mountings can therefore cause a range of electrical faults, ranging from total failure of a circuit, to a puzzling partial failure. In particular, lights may shine dimly (especially when another circuit sharing the same earth point is in operation), motors (eg wiper motors or the radiator cooling fan motor) may run slowly, and the operation of one circuit may have an apparently-unrelated effect on another. Note that on many vehicles, earth straps are used between certain components, such as the engine/transmission and the body, usually where there is no metal-to metal contact between components, due to flexible rubber mountings, etc.
- To check whether a component is properly earthed, disconnect the battery, and connect one lead of an ohmmeter to a known good earth point. Connect the other lead to the wire or earth connection being tested. The resistance reading should be zero; if not, check the connection as follows.
- If an earth connection is thought to be faulty, dismantle the connection, and clean both the body shell and the wire terminal (or the component earth connection mating surface) back to bare metal. Be careful to remove all traces of dirt and corrosion, and then use a knife to trim away any paint, so that a clean metal-to-metal joint is made. On reassembly, tighten the joint fasteners securely; if a wire terminal is being refitted, use serrated washers between the terminal and the body shell, to ensure a clean and secure connection. When the connection is remade, prevent the onset of corrosion in the future by applying a coat of petroleum jelly or silicone-based grease, or by spraying on (at regular intervals) a proprietary ignition sealer such as Holts Damp Start, or a water dispersant lubricant such as Holts Wet Start.
FUSES, RELAYS & TIMER MODULE TESTING
- Fuses are designed to break a circuit when a predetermined current is reached, in order to protect components and wiring which could be damaged by excessive current flow. Any excessive current flow will be due to a fault in the circuit, usually a short-circuit. The main fuse box, which also carries some relays, is located inside the vehicle below the facial panel on the passenger’s side, and is accessed by a lever behind the glove box.
MAIN FUSE BOX LAYOUT
- Fuse/relay removal tweezers
- Diode
- Multi-plug connections.
AUX: FUSE BOX LAYOUT
- Fuses 1 to 3
- Fuses 4 to 8, 11 to 14
- Relays R2, R5 and R6
- Relays R7 to R11
- Relay sockets for relays R1 and R4
- Diode
- A central timer module is located on the bottom of the main fuse box. This module contains the time control elements for the heated rear window, interior lights and intermittent wiper operation. The module also activates a warning buzzer/chime when the vehicle is left with the lights switched on, or if a vehicle fitted with automatic transmission is not parked in position “P”.
- The auxiliary fuse box is located on the front left-hand side of the engine compartment, and is accessed by unclipping and removing the cover. The auxiliary fuse box also contains some relays (see illustration). Each circuit is identified by numbers on the main fuse box and on the inside of the auxiliary fuse box cover
- Plastic tweezers are attached to the main fuse box and to the inside face of the auxiliary fuse and block cover, to remove and fit the fuses and relays.
- To remove a fuse, use the tweezers provided to pull it out of the holder. Slide the fuse sideways from the tweezers. The wire within the fuse is clearly visible, and it will be broken if the fuse is blown (see illustration).
- Always renew a fuse with one of an identical rating. Never substitute a fuse of a higher rating, or make temporary repairs using wire or metal foil; more serious damage, or even fire, could result. The fuse rating is stamped on top of the fuse. Never renew a fuse more than once without tracing the source of the trouble.
- Spare fuses of various current ratings are provided in the cover of the auxiliary fuse box. Note that if the vehicle is to be laid up for a long period, fuse 34 in the main fuse box should be removed, to prevent the ancillary electrical components from discharging the battery.
- Relays are electrically operated switches, which are used in certain circuits. The various relays can be removed from their respective locations by carefully pulling them from the sockets. Each relay in the fuse boxes has a plastic bar on its upper surface to enable the use of the tweezers. The locations and functions of the various relays are given in the Specifications.
- If a component controlled by a relay becomes inoperative and the relay is suspect, listen to the relay as the circuit is operated. If the relay is functioning, it should be possible to hear it click as it is energized. If the relay proves satisfactory, the fault lies with the components or wiring of the system. If the relay is not being energized, then either the relay is not receiving a switching voltage, or the relay itself is faulty. (Do not overlook the relay socket terminals when tracing faults.) Testing is by the substitution of a known good unit, but be careful; while some relays are identical in appearance and in operation, others look similar, but perform different functions.
- The central timer module located on the bottom of the main fuse box incorporates its own self-diagnosis function. Note that diagnosis cannot take place if the heated rear window is defective.
- To activate the system, press the heated rear window button while the ignition is being switched on, then release the button. Operate the light switch, washer pump switch and all of the door switches one after the other, and check that the buzzer confirms that the input signals are correct.
- Now move the wiper lever to the intermittent wipe position, and check the output signals by operating the same switches.
- Switching the ignition off and on again turns off the self-diagnosis function.