The MGA With An Attitude
Wiring Harness Installation, Wire By Wire, Page 5 of 5 - ET-100M

ET-100I - Grounding, Tools, and Resistances
ET-100J - Primary Lighting Circuits
ET-100K - 1500 Type turn signals and brake lights
ET-100L - 1600 Type turn signals and brake lights
ET-100M - Everything that is not lighting circuits
- (you are here)

Everything that is not lighting circuits

Now back to the diagram for ET-101e - Switch On and Fused. For the 1500 you would already have green wires on the petrol gauge as part of the turn signal wiring. For the 1600 you need to connect the greens now. Use continuity tester to determine which green wire goes from fuse block to petrol gauge. Connect that green wire to FB terminal A4. Get behind dash panel, switch on, verify power on green wire from FB, switch off. Connect that wire to petrol gauge along with a second wire for heater blower supply. (There is no third wire here for the 1600 cars, as the TS switch takes input from the flasher unit). Switch on, verify power on the green wire near the heater blower switch, then switch off. If there is no heater in the car, install a snap connector on this wire to protect it from any short to ground.

If there is a heater or fresh air vent box, then use a snap connector to connect the green wire to the blower switch. Switch on, and also turn on the blower switch. (This is a pull switch on the slide control in most MGA, but could be a twist switch on the air control knob in some others. In particular, the fresh air ventilation option does not have a slide control for the water valve because it doesn't include a water valve or hot water core in the vent box. Some early MGA with a heater also had the blower switch on the air control knob). Verify power out from the blower switch, then switch off. Use another snap connector to connect the green/brown wire to the blower switch. Switch on ignition and blower switch, verify power on output end of green/brown wire near vent blower (at heater or fresh air box), and switch off. Use a snap connector to connect the green/brown wire to the bower motor, and also verify connection of snap connector for black wire ground connection of blower motor. Switch on key switch and blower switch, verify operation of blower motor, then switch off.

Note original blower motor has two black wires and is not polarized. It will turn only one direction regardless of which way the wires are connected. Reversing rotation of this motor requires disassembly and switching of wires inside. For that operation see ET-212 - Heater Motor Tech. Some aftermarket heater motors may be polarized and could reverse rotation by simply swapping the two wires. These motors should have colored wires. Black would normally be ground, but the wires might have to be swapped depending on polarity of the electrical system. Connect the reversible motor in the way that results in the most air flow.

Looking at the dash instruments, each one may have two or three black wires attached to the mounting bracket, as this is a convenient junction point for ground wires. The fuel gauge must be grounded to work. Other instruments are grounded for the lamp function. The fuel gauge has one additional wire connection for the fuel level sending unit. Connect the green/black wire to the "T" terminal on the gauge (T is for tank). Also go to the engine bay near the starter switch and use a snap connector to connect two green/black wires together. Then go to the right side of the fuel tank, just inboard from the RR wheel, and connect the green/black wire to the sender unit. When switched on the fuel gauge should read somewhere between E and F depending on fuel level in the tank. If there is a problem here, see separate tech article(s) on function, calibration, and repair of the fuel gauge.

Note that the sender unit is grounded on the tank, but the tank is mounted in rubber packing strips with a rubber fill pipe connector hose. The tank is therefore grounded through the steel fuel pipe between the tank and the fuel pump, and the pump body has a black ground wire to a chassis bolt. If the steel pipe is cut and reconnected with rubber hose (like aftermarket pump), the sender unit ground connection can be lost. Then you need to install a separate black grounding wire from one of the sender unit screws to a bolt on the frame nearby (where a p-clip attaches the wiring harness to frame above the rear axle).

For the screen wiper you can now connect the last green wire to the A4 terminal of the fuse box. Switch on and verify power on the green wire near the wiper motor, then switch off. Connect the green wire to the "2" terminal on the wiper motor, and also verify connection of the black ground wire to the "E" terminal. When you switch on again the wiper motor may run until it will stop at the park position (if it wasn't already at the park position). Then switch off. Connect the black/green wire to terminal "1" on the wiper motor. Then behind the dash connect the black/green wire to the wiper switch, and verify connection of a black ground wire on the other switch terminal. Switch key on, and pull the wiper switch knob to start the wipers running. The motor will run (single speed) as long as the switch maintains a ground connection. When the "W" switch is switched off the motor will continue to run until the parking commutator in the motor breaks an internal ground contact to make it stop at the park position.

If you turn off the key switch while the wiper motor is running it will stop immediately wherever it happens to be in mid stroke. Next time you switch the key on the wiper motor will start up where it left off to continue running, or may stop at the park position if the W switch has been turned off. If you have any problem with operation of the wiper motor, see other tech articles on function and wiring and adjustment of the wiper drive. If you mix up the three wires on the wiper motor there are 5 out of 6 ways to be non-functional and a few ways to blow a fuse. This was the last device to complete wiring of the green wire fused circuits.

There is one more fuse and a couple more wires behind the dash still to connect, those being for the horn. See diagram ET-101i - Fused Without Switch On. If you haven't done it yet, you can connect the last brown wire at the control box and at the A1 terminal on the fuse box. Also install the 25/50 fuse (or a regular 30-amp fuse) in this second fuse position, and verify power at the A1 and A2 terminals on the fuse box. Then connect the brown/green wire to terminal A2 on the FB, and verify power on the brown/green wire up front near the horn. Connect the brown/green wire to the horn (inside the domed cover). Once the power wire is connected, to test the horn you can connect a jumper wire to the second terminal in the horn and touch it to ground on the chassis to make the horn sound. The horn is not polarity sensitive, so either wire may be connected to either terminal. The horn is internally not electrically connected to the case, so it does not matter if the horn case is or is not grounded on the chassis. (Beware that aftermarket horns may be much different).

Connect the brown/black wire to the horn, then get behind the dash panel and use a LARGE snap connector to connect two brown/black wires together. (This is the only connection that connects the dash harness to the main harness, so the dash harness can otherwise be completely installed and connected on the dash before the dash is installed in the car). The last connection inside is to connect two wires to the horn push button, brown/black and solid black for ground. The horn push must be partially disassembled to do this, so it is (was) a good idea to make these connections earlier while installing the dash harness and before installing the dash assembly in the car. A push of the horn button makes a ground connection to complete the horn circuit and make the horn sound. You can remove the second fuse if you want to disable the horn.

We are down to the last few wires needed to make the engine run. See diagram ET-101d - Spark And Fuel. Connect the last two white wires to the A1 terminal on the fuse box. Switch on and verify power on the white wires near the ignition coil and near the starter switch, then switch off. Connect the white wire to the ignition coil, switch on, verify power at the second small terminal on the coil, then switch off. Connect the white/black wire to the second terminal on the coil, and connect the other end of white/black to the distributor side terminal. Switch on and check for power at the white/black wire. When distributor contact points are open this wire should be hot. If not the points are grounded when they shouldn't be (miss wired). When the points are closed the white/black wire should be dead (grounded). If not the points are not grounded when they should be (wire disconnected). When switched on and cranking the engine the coil should be making high tension spark. If not, then consult other tech articles on function and operation of the ignition coil and points. Disconnect the white/black wire to prevent spark or coil heating until you are ready to start the engine.

Switch off. Use a snap connector to connect two white wires near the starter switch. Switch on, verify power on the white wire near the fuel pump, and switch off. Connect the white wire onto the fuel pump and verify connection of the black wire to chassis at the fuel pump. Switch on and verify that the fuel pump runs (tickety, tick, tick). When fuel fills the carburetor float chamber(s) the pump should slow down or stop ticking, and there should be no fuel overflow at the carburetor(s). Then switch off. For more information on fuel pump and carburetor operation and de-bugging see other tech articles in the Fuel tech section.

The only wiring left is to connect the control box and generator. The generator connection is not required to make the engine run, but it is required to keep the battery charged. See diagram ET-101b - Charging Circuit. At the control box connect the yellow/green wire to terminal F, and connect two yellow wires to terminal D. At the generator connect the yellow/green wire to the smaller F terminal, and connect the Yellow wire to the larger D terminal. When you switch on the ignition warning lamp in the tachometer should glow. When the engine starts the warning lamp should go out. For more information on the charging system function and testing and adjustment, see the workshop manual, and also see this off site link: What Generators Do and Regulators Ought To.

That's it. By the time you make the last wire connection everything electrical should be working, because you were checking and verifying everything as you were connecting the harness wires. No problems, no issues, and no wondering why something doesn't work or where the fault might be when you switch on. If you follow this procedure during a full restoration or for a new harness installation, the first time you start the engine you should be able to take a care free pleasure drive or a cross country trip with no electrical problems. I hope you weren't too terribly bored with the reading.

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