The MGA With An Attitude
Wiring Harness Installation, Wire By Wire, Page 3 of 5 - ET-100K
ET-100I - Grounding, Tools, and Resistances
ET-100J - Primary Lighting Circuits
ET-100K - 1500 Type turn signals and brake lights - (you are here)
ET-100L - 1600 Type turn signals and brake lights
ET-100M - Everything that is not lighting circuits
1500 Type turn signals and brake lights
Okay, so general lighting is all working, and now we get into brake lights and turn signals which are fused, so start with power to the Fuse Box. See with ET-101 - Basic Circuits. You can connect both (2) white wires to output side of Ignition Switch. One goes to the Ignition Warning Light, which is still not connected on the other side, so is of little concern whether that white is connected or not. The other white wire goes the Fuse Box.
Now see here: ET-101e - Switched On and Fused. Once connected at the switch, turn on the Ignition Switch, use test light to find white wire near FB that is hot, and connect that one wire only to terminal A3. (There is no need at this time to have power on white wires for ignition coil or fuel pump). Install the 17/35 fuse (or a regular 20-amp substitute fuse), switch on, use test light to verify power on the A4 terminals of the FB, then switch off.
From here I will address the 1500 type turn signals and brake lights first, being the earlier (and more complex) of the wiring flavors. There are two A4 terminals on the FB holding four green wires. Three of these wires need to be connected to make the 1500 turn signals work. Use a continuity tester to discover which green wire goes to the back of the fuel gauge, and connect that to FB terminal A4 first. You might alternately use trial and error to connect one wire at a time, switch on, and check with test light to see which wire has the power at the output end. Leave wiper motor, brake switch and flasher unit green wires disconnected from FB at this time.
Then get behind the dash panel, find three green wires together (one from the main harness and two from the dash harness). One green is power input from the FB (main harness wire). Switch on, verify power on this wire, and switch off. Then connect those three greens (with ring terminals) to the "B" terminal on the petrol gauge. A second green is power feed to the heater blower switch. Install a snap connector on the switch end of this green wire to protect it from any ground short, but do not connect the switch.
The third green goes to the Turn Signal switch. See here: ET-101f - Switch On and Fused, 1500. Switch on, verify power at the output end of that wire, and switch off. Connect this last green wire to the "F" terminal on the TS switch (F is for "flasher", but not on the 1500). Switch on, operate the TS switch left and right, verify power on the output terminals of the switch, and switch off. Then connect the green/yellow and green/blue wires to the L and R terminals respectively. (You should notice that the L and R terminal labels are swapped for the MGA, where L actuates the right signal and R actuates the left signal. This is entirely due to orientation of the switch manual actuator lever hanging down on the MGA where it extends upward on some other cars). Also connect the Light Green wire to the TS warning lamp (permanently connected with original parts), and plug the lamp into the dash socket. The lamp socket needs to be grounded on the dash panel, so either scrape off some paint or install a separate ground wire for this lamp mount.
In the engine bay, find the green/yellow and green/blue wires near the TS relay. Switch on, operate the TS switch, verify power on these wires, and switch off. Then connect these wires to TS relay terminals 4 and 8 respectively. Switch on, operate the TS switch, and listen for the click of the TS relay for left and right application. If it doesn't click for both directions, fix it before you go on. The TS box must be grounded on the bulkhead (the ninth terminal) for the relays to operate.
Use a continuity tester to determine and connect the green wire from FB "A4" terminal to the Flasher Unit. Connect three wires to the FU, green, light green, and green brown, to terminals B,P,L respectively. Switch on, verify power input to the FU on green wire (terminal B). Also verify power output from the FU on terminal L. This may show slightly dimmer light output for your test light due to internal resistance of a heater wire. You can also test this with an ohm meter on FU terminals B and L. The panel lamp terminal P should be open circuit at rest. Refer to another tech article for explanation of how the flasher unit works. Connect green/brown to TS relay terminal 1, switch on and verify power delivery here, then switch off.
Use a continuity tester to determine and connect the green wire from FB "A4" terminal running over to near the starter switch. Near the starter switch use snap connectors to connect two green wires and two green/purple wires. Also connect the green and green/purple wires to the brake switch (lower on frame). At the TS relay connect green/purple to terminal 5. Switch on, verify power at BS green wire terminal and open circuit on the other BS terminal. If no power here, investigate snap connector in green wire. Have a helper step gently on brake pedal, and verify power on both terminals of BS. If it requires heavy pedal pressure to operate BS, replace the switch. You can do that BS test at any time (now or later). Install a jumper wire temporarily across the terminals of the BS so you don't need a helper to hold the pedal down. Switch on and verify power (from BS) at TS relay terminal 5. If no power here, investigate snap connector in green/purple wire near starter switch. With power still on, verify that power at TS relay terminal 5 is also getting to relay terminals 3 and 7. If not, then investigate the normally closed internal relay contacts. When this works you may remove the BS jumper wire, but keep it handy for more use.
With power still on, operate the TS switch to the left turn indicator position. Relay will click, then verify continuity for relay terminals 1,6,7. Otherwise use test light to verify power out at terminals 6 and 7, same as reduced voltage at flasher unit terminal L. Operate the TS switch to the right turn indicator position. Relay will click, then verify continuity for relay terminals 1,2,3. Otherwise use test light to verify power out at terminals 2 and 3, same as reduced voltage at flasher unit terminal L. If any of this does not work, investigate relay internal normally open contacts.
For the purpose of TS relay internal inspection you need to remove the cover. For this the relay may have to be dismounted from the bulkhead for access to un-bend some bent over retaining tabs. If you are a little crafty you can leave these tabs straight, or form them just slightly inward so the cover will snap into place and stay there, but can be removed with a firm pull. Be sure to always replace the cover in the correct orientation with terminal marking 1 at top. If you ever have to R&R the relay from bulkhead more than once you may also find a crafty way to install captive studs in the bulkhead so you don't need two people to work the fasteners.
With four wires connected you have verified correct operation of the TS relay. Next is to connect and verify operation of the brake lamps. Temporarily install a jumper wire from FB terminal A4 to TS relay terminal 5 (so you don't need a helper to press the brake pedal). Connect white/purple and white/brown wires to TS relay terminals 7 and 3 respectively. Switch on and verify power on white/purple and white/brown wires near the starter switch. Use snap connectors to connect two white/purple wires and two white/brown wires near the starter switch. Switch on, verify power on white/purple and white/brown wires at RR corner, then switch off.
Use a snap connector to connect two white/purple wires at RR, then verify power on white/purple wire at LR corner, then switch off. Connect white/purple inside the LR lamp, then switch on and verify operation (light up) of the LR brake light. Assuming the low light parking light was already working, this should be a cinch to give bright light for the brakes. If not, fix it before you go on. Use a snap connector to connect two white/brown wires at RR, then verify power on output end of white/brown wire at RR corner, then switch off. Connect white/brown inside the RR lamp, then switch on and verify operation (light up) of the RR brake light. Assuming the low light parking light was already working, this should be a cinch to give bright light for the brakes here as well. If not, fix it immediately.
Then remove the jumper between FB and TS relay. Switch on, and there should be no lights. Step on brake pedal and both brake lights should light up bright. If not, fix the brake switch or the BS circuit wiring before going on.
Just two front lights to go. At TS relay connect green/red and green/white wires to terminals 6 and 2 respectively. Install a jumper wire from FB terminal A4 to TS relay terminal 6, then switch on, verify power on green/red wire at LF corner, and switch off. Use a snap connector to connect two green/red wires at LF corner, then switch on and verify operation (bright light) of LF turn indicator lamp. If not, fix it. Move jumper wire from TS relay terminal 6 to terminal 2, then switch on, verify power on green/white wire at RF corner, and switch off. Use a snap connector to connect two green/white wires at RF corner, then switch on and verify operation (bright light) of RF turn indicator lamp. If not, fix it.
Very close to finish now. Remove the jumper wire from FB to TS relay. Switch on, and you should have no lights. Operate the TS switch to the left turn signal position, and verify two left side bright lamps flash in unison with dash indicator lamp. Operate the TS switch to the right turn signal position, and verify two right side bright lamps flash in unison with dash indicator lamp. If the flash rate is too fast or too slow the flasher unit may be faulty. If one side flashes at a different rate than the other side (or lights up but doesn't flash), check for poor ground connections at the lamp fixtures on the slower flash side. If one corner lamp (front or back) lights up but the other one on same side does not, the flasher unit is not supposed to flash but should keep the one lamp illuminated. If only one lamp lights up, and it does flash, you have the incorrect flasher unit (probably a heavy duty flasher for 4-way hazard flashers, not a normal MGA function).
Then switch on, step on brake pedal and verify that brake lamps light up. While holding brake pedal down, operate the TS switch to the left turn signal position, and verify two LS bright lamps flash in unison while RS brake lamp stays on. Still holding brake pedal down, operate the TS switch to the right turn signal position, and verify two RS bright lamps flash in unison while LS brake lamp stays on. This is the whole reason for existence of the TS relay. It disconnects one brake lamp from the brake light circuit and connects it to the front lamp and flasher unit to make turn signals work using the same rear lamp as is used for the brake lights.
One last test. Turn on the parking lights and verify low lights at all four corners. Switch on ignition and step on brake pedal to verify that both brake lamps light up. If one brake lamp comes on and the opposite side tail light goes out, look for bad ground connection on the tail lamp fixture. This would also be accompanied by a slightly dimmer tail light on that side (when brakes are not on). Reason this can happen is because the combination brake and tail light bulb has only three connections with a shared ground contact. When the lamp fixture has a bad ground the tail light filament in the bulb can find a low resistance ground path through the brake light filament, then across to the opposite side and through the other brake light filament to the good fixture ground connection. But when you hit the brakes power is also applied to the brake light filament. With equal system power on two input contacts, and no ground connection in between the filaments, the lamp will go out with no light from either filament.
All jolly good fun. Keep reading?