North American Turbocoupe Organization

Installing Headlight Relays
andrewjs18 Offline
Installing Headlight Relays
by Keith Kibler - aka (kibstone)

Installing Relays to control Headlights (87-88 Thunderbird Turbo Coupe - others may be similar)

Why? If you suffer from worn out headlight switch circuit breakers, headlights going out for no apparent reason, and you have tested and reground the entire electrical system, this mod is for you! Ford designed the headlight switch so that all current for the headlights high and low beam) and fog lights runs through the headlight switch. There is a self-resetting circuit breaker in loop, that tends to wear out. Also, if you run a non-stock headlight bulb, this circuit breaker will wear out faster, since there is typically more current drawn for the non-stock bulb.

Again, I emphasize this mod is not a fix for a faulty wiring system. This mod is a fix for a poor design by Ford. If you have a bad wiring system, all you will have after this is a bad wiring system with a couple of relays, probably masking the real problem.

How much? ~13.50 for the 5 pin relay, 8.00 for the 4-pin relay, a couple of bucks worth of connectors, and a couple of hours of time


* Two 30 Amp fuse holders. I used the blade-style 10-gauge holders from radio shack. Make sure they are weather-tight.

* Two 30 Amp fuses.

* Two 30 Amp relays. I used a regular "4 pin" relay for the low beams, and a "5 pin" relay for the high beams. The guy at NAPA did not have a "fog light" relay, but he pulled out the book to find the right kinds of relays. The difference between the two relays is the 4 pin has one pin for pin 87, the other has two.

* A bunch of blue (12-16 gauge) female spade connectors

* Two large yellow (8-10 gauge) ring lug connectors (to go on the battery side of the starter relay)

* Two blue ring lug connectors

* A couple feet of 14 gauge wire (preferably two different colors like red and black - some will be used to create a ground wire, some will be used to extend power wires to the relays). The amount you will need depends on where you mount the relays.

* solder, flux, heat-shrink tubing (and soldering gun - my iron would not heat the 10 gauge wire)

* two sheet metal screws (to mount the relays)

* Drill with suitable size bit for sheet metal screws, center punch, hammer (optional if you have an automatic center punch).

* A heat source - heat gun, propane torch, lots of matches, etc. to shrink the heat-shrink tubing

* Wire ties, black electrical tape

* Lots of patience

* A second pair of hands (optional, but may be necessary for the next item) - helps for soldering the wires back together.

* One 12-pack of a beverage of your choice


1. Remove battery.

2. Remove battery tray.

3. Remove the plastic cover over the starter relay and coil.

4. Open a beverage - one for you, one for your friend.

5. Find the necessary wires - unwrap the wiring harness that runs under the battery box and find the red wire with black tracer (R/B) and the green wire with black tracer (G/B). Low beams are controlled by the R/B wire, and high beams are controlled by G/B wire. The R/B wire runs from this harness, across the radiator support to the passenger side, then back to the driver side. The G/B wire is "Y'ed" in the harness under the battery box, one wire going to the passenger side, the other going to the drivers side. I removed most of the wire loom for this project to make sure I was working on the correct wires.

6. Say a prayer...

7. Find a suitable location for the relays. I mounted my relays on the inner fender well near the radiator support, behind the battery. Center punch and drill the holes to mount the relays. Mount the relays, temporarily so the wire lengths can be determined.

8. I started this wiring project by hooking up the "30" pin on the relays to the starter relay. I don't like to splice 10 gauge wires, so I ran two wires to the starter relay. Crimp the yellow female spade connectors to one side of each of the fuse holders. Optionally, but recommended, apply solder to the connector as well. Remember when soldering to HEAT THE WORK - NOT THE SOLDER!

9. Connect each fuse holder to the "30" pin of each of the relays. Extend, if necessary, the wires from the other side of the fuse holders to reach the battery side of the starter relay (An aside - I have two 87/88 T/C's - one was wired with the battery connection toward the front of the car, the other wired toward the back of the car). I used 14 gauge wire for these extensions. DON'T FORGET TO USE THE HEAT SHRINK TUBING! Crimp and solder the ring lug connectors to the extension. Remove the nut on the battery side of the relay, and install the lugs onto the starter relay. Replace nut and tighten.

10. Create two ground wires with 14 gauge wire and the blue ring lugs and spade lugs. I hooked my grounds to grounding bolt that is on top of the radiator support near the battery. Hook one each of the ground wires to lug 85 on the relays. Remove the ground bolt from the radiator support and put the ring lugs on the bolt and re-install the bolt in the hole. I did not use any loc-tite on the threads, but I did put a dab on top of the bolt again, to make sure it does not wiggle loose. For these wires, I used black wire.

11. Here would be a good place to take a sip (or chug) of that beverage you opened earlier.

12. Find the R/B line again. Cut (yes, cut) the wire about 2" from where it comes out of the harness that runs under the battery tray. Crimp and solder a blue spade connector to the wire that continues to the lights. Hook this to lug 87 on the 4-pin relay. You may have to untangle this wire to get enough wire to hook it to the relay. You may also have to extend this wire if you did not mount your relays in the same location as I.

13. Extend the other end of this wire (the one still in the harness below the battery tray) to reach the relay. Again, DON'T FORGET THE HEAT SHRINK TUBING! Crimp and solder a blue spade lug onto the extended wire, and hook this wire to lug 86 on the relay. I used red wire here.

14. Install a fuse in the fuse holder.

15. At this point, I would set the battery back in - carefully - and reconnect. Turn the headlights on - you should get low and high beams. This time, however, there will only be about 10mA being pulled through the switch.

16. Congratulations! Since there were no flames, finish all open beverages and open another!

17. Remove the battery again, and proceed to re-wiring the high beams.

18. Now, find the G/B, near where you cut the R/B wire. Follow toward the lights to the splice. Cut the splice out - leaving two wires running to the headlights and one wire in the harness.

19. Crimp and solder spade lugs onto the wires running to the headlights. These should be long enough to reach the relay with out extension, if you untangle them from the rest of the harness. Extend if necessary. Connect each of these wires to the 5-pin relay. Hook up to the lugs marked 87 - both will be marked 87.

20. Extend the G/B wire to reach the relay - REMEMBER THE HEAT-SHRINK TUBING! Terminate with a blue spade lug. Connect to lug 86 on the 5-pin relay. I used red (I know - the base color is GREEN!) wire for this.

21. Finish all open beverages.

22. Again, I would set the battery back in and test the high beams before buttoning everything back up.

23. If everything works as planned, celebrate! If not, check your wiring and the solder connections.

24. Re-install any wiring looms removed, use some wire ties to neaten things up, and wrap any exposed wiring in either wire loom or black tape (duct tape works too, depending on the color scheme under your hood)

25. You should now have high beams and low beams - just like Ford intended! This time, however, you will not burn your finger when you go to shut the headlights off! The door is now open to run a higher intensity bulb in the headlights, since the switch is no longer the bottle neck.

26. I just finished this on my own 88 TC. I ran the car for an hour afterward with the high beams, low beams and fog lights on, and switch never got even warm. Prior to this mod, my switch would get really warm after the headlights had been on about 20 minutes. The only problem (if you want to call it a problem) is the "Low Beam Out" indicator on my system sentry panel illuminates when I turn the headlights to low beam. I guess the circuit is not drawing enough amps through the switch to keep the computer happy.

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