0.3 Fuses and Relays

Fuses

Fuses are designed to fail when a current exceeding the rating is passed accross the terminals. Some failures can be caused by isolated instances- accidentally shorting out light connectors when changing a bulb. In isolated instances the fuse can just be replaced by a new one. However, frequent failure of a fuse indicates other problems which should be investigated. All the electrical systems and the wiring that is protected by that particular fuse should be examined for a short circuit. Typical short circuit location are:

• Door and tailgate hinges: wiring harness rubs and breaks from motion.
• Improperly installed equipment like radio, CD player, power antenna, alarm system, trailer wiring harness, extra fog lights
• Incorrect replacement for switch or relay: always check that the replacement not only looks similar but is the same pin out.

If the circuit your are checking has more than one item connected, for example fuse #21 below, you can isolate the problem if you have 2 spare fuses. Remove fuse and disconnect all output wires at the fuse box position #21. Replace the blown fuse with a new one, re-connect one by one the circuits on the fuse box. Check all items on the list each time you reconnect a wire. When the fuse blows again the hot wire is found. Let say the hot wire is going the rear lights, check in detail the LH parking light and both license plate lights. In case of emergency, disconnect only the light that short-circuits, you can use the other ones until the fault is corrected.

fuse #21 LH parking lights (front and rear),
License plate light
Lighting for: ash tray, heater, control panel, switch for heated rear window

When buying spare fuses always replace with the same style and amperage specified, do not take for granted that the fuse in the box is the correct amperage, check the fuse label for the specified amperage.

VOLVO 740/760/780 Fuse Box

The early 740/760 series have the fuse box located behind the ashtray. Pull up on the handle below the ashtray, marked electrical fuses - press upward, and lift out the ashtray with housing. The fuse label is on the back of the ashtray. The 700 series came with a fuse removal tool, mounted on the right hand side of the fusebox. This white plastic handle is not really required, simply pull out the fuse using your fingers. To see if a fuse is blown it is necessary to remove it from the fusebox. Push the tool onto the top of the fuse, then pull upwards. Slide it out the fuse from the tool grip. Spare fuses are in position 23, 24, 26 of the fuse box. If you need to get the whole fuse box out of the dash, for example to get to the relays or troubleshoot fuse circuits, follow the instructions in the section "VOLVO 700 Series Relays" below.

VOLVO 700 Series Relays

Relays are electrically operated switches that are used to perform logic functions (wiper delays, bulb out sensors) and enable the control of electronic items without the current passing through the switch. Occasionally relays stop functioning correctly. Some become hot when running and their internal connections can eventually fail.

Fuse Box Removal for relay replacement

The relays are difficult to access in their mounted position. It is best to first remove the storage shelf (or audio equalizer) above the fuse box, then pull the whole fuse box/relay panel into the vehicle.

• Remove the ashtray with housing by pulling up on the lower handle, to get to the fuses.
• Unclip the plastic surround from around the cigarette lighter.
• Undo the two corner screws
• Slide the storage shelf out (or equalizer if equipped) into the vehicle and disconnect the electrics
• Push the 2 white plastic clips next to relays J and N, release the fuse box upward.
• Slide the whole fuse box into the vehicle (may need to shift the gear stick back)
• Installation is the reverse, although the thick wiring unit is sometimes stiff to bend back into position

VOLVO 740/760 Relays
A Bulb Failure Warning Sensor B Seat belt reminder or torque limited (B23FT) C Windshield Wiper Delay D Tailgate Wiper Delay E Fuel Injection System F Front Fog lights (Impulse Relay 1982) G Flasher Unit H Main Lighting Relay (1983) I Overdrive relay	J Window winders/electric fan K Central locking/main lighting relay (1984 on) L Vacant M Auxiliary lights N Rear fog lights O Vacant P Vacant R Vacant/ignition Advance S Oil level sensor

Relay Inspection/Repair

• Once the fuse box is accessible the relay can be pulled out
• Replacement with a known good relay is the best method to verify relay faults
• Some faults may only occur once the relays has 'warmed up' (heat can cause deflection of the metal contacts)
• From personal experience some relays can be repaired. For example, on a 740 Fuel Pump Relay the plastic surround can be unclipped and the little circuit board examined. Points of lost contact may be evident and repaired by soldering together.