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Headlamp Circuit Checklist

Headlamp Circuit Checklist

Doug Dwyer

The purpose of this checklist is to describe the XJ6 headlamp circuit and provide a fault finding diagnostic checklist. The XJ6 was built with a variety of lighting configurations dependant on the intended market. This article applies specifically to North American Series III models. However, much of the information can be generally applied to other variants.

Circuit Description

The circuit is comprised of 5 main compenents: the rotary headlight switch, the relay (located at left fenderwell), the headlamp fuse box (also at left fenderwell), the highbeam switch, and the lamps themselves. For low beam operation battery voltage from the headlamp switch goes directly through the relay to fuses #2 and #4 to power the low beam filaments. The high beams are powered from the relay which has a separate source of battery voltage. The high beam/flash switch provides a ground path to the control side of the relay. Fuses #5 and #3 protect the high beam circuit. Fuse #1 in the headlamp fuse box is actually for the electric cooling fan.

Fault diagnosis

This circuit is very simple but before ANY actual diagnosis begins it is important to remember that the vast majority of problems within this circuit are caused by faulty fuses and dirty/loose connectors. If you are having lighting problems you MUST check these items before continuing on with more diagnostics.

First check the fuses. Obviously you'll check to see if any are blown and, if not, at very least you'll want to clean the metal caps with some steel wool. However, since fuses can appear OK but in fact still be faulty it is generally accepted that the best practice is to simply replace them with new ones, period. The terminals which grip the fuses are prone to loosening and corrosion. Using steel wool and/or a good aerosol contact cleaner go to work on these terminals until they look like new again. After cleaning use some dielectric grease or a small swipe with light oil to protect against future corrosion. Additionally, if these terminals don't firmly grip the fuses they must be gently adjusted for a tight fit, using great caution as they can be rather fragile.

At this point your headlights may have come back to life but it is strongly suggested that you continue on with more cleaning......

Under the headlamp fuse box is a veritable nest of bullet connectors for the circuit. This is a notorious problem area as these connectors are prone to loosening and corrosion. Cleaning, lubing, and tightening all of these connectors is one of the best steps to take if you desire reliable headlight operation. Access to this area is made much easier by removing the small thru-bolts which hold the fuse box to the fender'll have to work from behind the front tire to do this but doing so is actually very easy. Disconnecting the battery first would be a good idea. Use steel wool, aerosol cleaner, and a light lubricant as described above. Loose connectors can be gently squeezed with pliers to gain a tight very careful here !

No, you're not done yet. There are more bullet connections closer to the headlights which need the same treatment and, of course, the plug-in spade terminals at the headlamps themselves. Clean 'em all while you're in the mood. Lastly, one-at-a-time remove the wires from the relay and clean the terminals and connectors here as well.

Don't forget to check the headlamp ground wires inside the fender wells and at boths sides of the radiator aperture.

OK, if your lights till don't work we can now dig in with actual circuit testing. We'll assume "known good bulbs" for this section but never forget the possibilty of a simply burned out bulb.You'll need a 12 volt test light. We'll start right at the beginning of the circuit. Reconnect the battery and turn on the headlight switch.

1) At the rear of the headlamp switch the heavy brown wire should have voltage at all times as this is direct battery voltage.

2) With the headlamps switched "on" the blue wire from the switch should have voltage. If voltage goes into the switch OK but does not exit the switch via this wire then the switch has an internal problem. If voltage is good at this wire.......

3) Go to the relay, remove the blue wire, and check for voltage. If no voltage then you have a wiring/connector fault between the switch and the relay. If voltage is good, reconnect the wire and check for voltage at the terminal associated with the blue/red wire. If no voltage here then the relay is faulty. If voltage is present......

4) Check for voltage at Fuses #2 and #4. If no voltage you have a wiring fault between the relay and the fuse box. If voltage is OK....on both sides of the fuses... but the lights don't work then you have a wiring/connector/ground fault from the fuse box to the headlamps.

5) Now for the high beams. First go to the relay and check for 12 volt constant supply voltage at the heavy brown wire. If no voltage then you have a wiring fault from the supply. If voltage is Ok here.....

6) Configure your test light to check for ground. Locate and disconnect the blue/black wire at the relay. Check for ground at this wire whenever the high beam switch is operated. If no ground then you have a wiring/connector fault bewteen the relay and the switch, a faulty high beam switch, or a wiring/connector fault between the high beam switch and the ground point. If ground is good at the blue/black wire, then.....

7) Reconnect the blue/black wire to the relay. Disconnect the blue/white wire. Check for voltage at the terminal associated with the blue/white wire whenever the high beam switch is operated. If no voltage then the relay is faulty. If voltage is Ok, reconnect the blue/white wire and......

8) Check for voltage at fuses #3 and #5. If no voltage then you have a wiring fault between the relay and the fuse box. If the voltage is OK.....on both sides of the fuses.....but the high beams don't work then you have a wiring/connector/ground fault from the fuse box to the lamps.

Well, that's about it. See how easy that was ? Please don't be tempted to skip the cleaning-and-tightening drill as you'll only end up delaying the ineveitable !


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