The MGA With An Attitude

ET-103 covers various lighting problems and diagnostics of the unfused live lighting system.

At 09:00 PM 9/9/03 -0400, Steve Demko wrote:
".... having a heck of a time getting electricity to both the rear stop/tail and rear turn signal lamps and ditto for the front parking and turn indicator lamps."

familiar If you are not entirely familiar with the general characteristics of Lucas electrical systems, you should start by reading the "Lucas Fault Diagnosis Manual". Moss part number 990-035, $7.95 list price Sept 03.

This sounds like more than one problem, but it might all be related. In this article I will cover the lights which can be turned on without switching on the ignition, including front parking lights, tail lights, headlights, fog lights, dash lights and map light. These are all powered live from the battery and are NOT fused in the MGA. Those lights which are powered through a switched fuse (brake lights and turn signals) will be covered in another article to follow.

For all live wired unfused lights, start with this diagram and print for reference:
MGA live and non-fused lighting circuits.
Also read my article on Wire color codes and trouble-shooting.

".... I get good readings at both the main lighting switch (3 red, blue and brown/blue wire) and the turn indicator switch (green, green/yellow, green/blue wires) but no juice flows to the tail and side lamps. The panel rheostat and map lights work when those switches are turned on."

First note: You can do all of your headlight and tail light testing without ever turning on the ignition switch, because these lamps are powered live from the battery and will work without the ignition being on. All parking light power wires are red wires starting from the headlight switch. This includes a power supply wire for the fog lamp switch, the dash lamp dimmer switch, and the map light switch, so those lamps will go off when the parking lights are switched off.

The front parking lights (bulb low intensity filament) and rear tail lights (bulb low intensity filament) and the license plate lamp are not related in any way to the turn signals, except that they share a common ground path at the lamp housings. Bad ground connections are a fairly common cause of bulbs that don't light up. Start by using an ohm meter or a battery powered test light or continuity tester to check resistance from each lamp housing to a bumper to see if each one is actually grounded. They should all show close to zero resistance or light the battery powered test light.

You can also use a simple test light to check for ground. Use a long jumper wire if necessary. Connect the test light to the hot terminal on the battery, or at the main battery cable at the starter switch. Touch the probe to a bumper and it should light up. Touch it to the lamp housing and it should light up just as bright. No light means no ground. Dim light means you have a bad connection to ground, a high resistance joint in a bullet connector, or rusted connection from black wire to the body or frame of the car. The ground connections and any related bullet connectors all appear on my wiring diagrams. Every outside lamp fixture has (or should have) a black wire connection to chassis ground. If any of the front or rear lamp housings are not grounded, take care of that problem first. This is usually a matter of cleaning up the bullet connectors and the points where the black harness wires are screwed to the body or frame.

All you need for a light is power and ground and good connector contacts. Looking at the diagram (you did print it), you say you have power on the red wires at the light switch. And then you say:

"Also where the main harness connects to the mid section harness I can NOT get a good reading where the 2 red wires from the main connects to the tail lamp section of the harness hence no juice flowing from that point to the tail lights."

There is a connector here (near the starter switch) 4-way with 3 red wires. One of those red wires comes directly from light switch. If it's hot at the switch it has to be hot at this connector. Unless the wire has been shorted and completely burned up, the wires within the harness will never fail. It is always a problem with the end connectors. Check again to be sure you have all FOUR red wires connected to the light switch. Dash lamps and map lamp are obviously working. The other two reds in question are parking lamps and fog lamp. You can check fog lamp red wire power at the fog lamp switch. Then the only place you can check tail light red wire power is at the 3-way connector in the engine bay.

With the switch on, one of the reds from the main harness has to be hot. You might have a dirty connection where that wire goes into the junction connector tube. You may have to pull the wire out of the connector to check for power on the bullet connector. Use a volt meter or test lamp from bullet to ground. Get either 12 volts or a bright light before you go any farther. Then plug the 3 reds into the connector and see if all 5 lamps light up (4 corners and the rear license lamp).

If you get front lamps only, or rear lamps only, then you may still have a bad contact at the same connector, or you may have a bad ground on the chassis at one end of the car. If you have a bad ground, then a volt meter will show 12 volts on the ungrounded lamp housing, or a grounded test light will glow dimly when touched to the ungrounded lamp housing.

For power connections you have to follow the harness one connector at time, starting at the power source. Be sure you have power on all three wires at the engine bay connector before going farther. Then proceed to another connector with 3 reds at the right front corner of the car, and treat that the same as the last one. Check for incoming power, make a good contact with the connector, then check for outgoing power on the other wires. Find another 4 red wire connector at the right rear, just behind the rear wheel. Repeat tests and verification for power connections. When you have power on all wires at these connectors, then proceed to check for power at the red connection at the lamp fixtures.

On the ground side, when checking the black wires, in the front you have 3-way connectors at each corner where the parking lamps and the headlamps share a common junction connector. If the headlamps work you can figure to have a good ground at that junction connector and from there to the chassis. Then you're only concerned with the black wire connection from the parking lamp body to the junction connector. When one headlight glows dimly, that lamp will have a bad ground connection. It glows dimly because it finds a ground return path through the second filament of the same bulb, then through the harness to the bulb on the other side of the car, and then through one of the bulb filaments there to a good ground connection.

Another oddity here is that if you switch from high to low beams the dim bulb will go completely out, because without a ground connection it will end up having 12 volts on both sides of the bulb filament. The same condition can apply to a tail light. If the tail lamp has a bad ground it may glow dimly. Then when you hit the brakes and the brake lights are supposed to light up, the one with the bad ground will just go out.

When you have power on the red, and a good ground connection on the black, and it still doesn't light up, then you either have a burned out bulb or a bad contact on the base of the bulb inside the socket. Check for free motion of the spring loaded contact plate inside the socket. Clean these contacts with a little CLR on a Q-tip. If you don't have 5 parking lights working by then, maybe you can hang up your jock and sell the car. Get the parking lights working first to be sure you have good ground connections on the lamp fixtures before you even think about chasing turn signal problems.

Testing of the optional fog lamp is similar to a tail light. Verify good ground connection at the lamp first, then check the power connection starting at the source. The fog light switch gets power from the parking light terminal on the headlight switch. Then there is a single continuous red/yellow wire from the "F" switch all the way to the fog light connector. This "F" switch and the red/yellow wire was installed in every MGA, even when the fog lights were not fitted at the factory (most commonly a dealer installed option). If you have two fog lamps installed there will be a 3-way connector where the red/yellow wire connects to the first fog lamp.

If you have one fog lamp and one driving lamp installed, these will need to be on separate switches. Then your car will have to have one additional switch installed and another wire run from the new switch forward to the additional lamp. Broad beam fog lamps may be powered from the parking light circuit, so the fog lights may be used with the headlights turned off. Pencil beam driving lamps should be powered from the high beam terminal of the dipper switch (blue/white wire). Then the driving lights will be used in conjunction with the normal high beam headlights, and the driving lights will go out when the headlights are switch to low beam for oncoming motorists. If you are installing only driving lights and not fog lights, then you can use the existing red/yellow wire in the harness, but you should install one new wire to supply power from the dipper switch high beam terminal to the "F" switch on the dash (and disconnect the red wire from the "F" switch).

The map light switch is likewise powered with a red wire from the parking light circuit, so the map light will only work when other lights are on. If you are replacing the wiring harness, the single wire running from the map light switch to the map lamp may not be part of the harness. The map lamp originally has electrical ground on the dash panel. If a wood dash is installed a ground wire must be added for the map lamp.

Original panel lights consist of four small bulb sockets attached to the harness, one for each of the round instruments. Ground return is through the instrument body (larger instruments) or through the lamp mounting bracket (smaller instruments). Each instrument has (or should have) a black ground wire (at least one) with a ring lug attached to the mounting bracket. The four panel lamps are powered from the rheostat (commonly called "panel light switch") and are connected with red/white wires. These are 4-watt bulbs for a total of 16 watts or 1-1/3 amps maximum power running through the rheostat. The original bulbs are not particularly bright and may usually be used at or near full intensity.

The rheostat is a variable resistor controlled by rotating the "P" switch knob. In the vertical position the switch is off. Just slightly clockwise from vertical the switch will make a direct connection to the red/white wires to supply full voltage to the lamps. As the switch is rotated farther clockwise it introduces increasing resistance to the circuit to reduce the voltage to dim the lamps. One characteristic of this type of dimmer is that current passing through a resistor generates heat, so the switch gets hot. The amount of heat is directly proportional to the current and directly proportional to the voltage across the switch. The current is inversely proportional to the total circuit resistance. If you install an additional dash lamp or two for added instruments, the circuit resistance drops and the current and power increase, which generates more heat in the rheostat. The rheostat can generally tolerate two additional 4 watt bulbs (24 watts total circuit load) with no problem.

But, if you increase the power draw to higher levels the rheostat may be overloaded and overheat, causing premature failure of the switch. Because of its location in open air behind the metal dash, this is generally not a fire hazard, but it can melt a little insulation off the connected wires for a short distance from the connection terminal, and can also burn the contact in the variable resistor and may also burn out the resistor. It is becoming common practice to install 10 watt halogen bulbs in the two larger instruments, and sometimes 5 watt halogen bulbs on the smaller instruments for better illumination. That increases the original 16 watt circuit load up to 28 to 30 watts, and additional bulbs would increase the load even more. Running this much additional current can be a BIG problem for the original rheostat. This is not a problem if the dimmer is set to maximum intensity where the current does not pass through the variable resistor. But when the dimmer is set just slightly more clockwise to dim the lamps just a little, this is where it gets maximum current flowing through a very short length of resistor. With the resistance fairly low this does not generate a large amount of heat, but the heat that is there is developed in a very small spot in the resistor so the local temperature goes quite high. This is the most stressful setting for the rheostat, and most likely to cause failure of the resistor element in the switch.

There is also an admonition from the makers of the halogen bulbs that reducing the voltage may shorten the life of the bulb. In this respect the halogen bulbs are not like the original incandescent bulbs. Incandescent bulbs will last much longer if they are run just a little dimmer at lower voltage. The halogen bulbs will have maximum life when run at full voltage, or otherwise dropped to such a low voltage that they hardly light up at all. So using the dash switch dimmer function to lower the brightness of halogen bulbs could be detrimental to both the dimmer and the bulbs. If you want to be sure to avoid this condition, or if the dimmer resistor gets burned out, you can add a jumper wire between the two terminals of the "F" switch to bypass the switch all together. This will also defeat the "off" position of the panel light stitch.

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