Why won't the headlights on my Mazda RX-7 FD turn on if the high beams still work?

Based on the symptoms described for your Mazda RX-7 (FD), this is a known issue generally traced back to a failed headlight relay, a broken contact inside the headlight switch, or a bad ground. Because the high beams are on a different circuit within the switch, they will often continue to work even when the low beam circuit fails.

Difficulty Rating: Moderate (Requires a multimeter and basic electrical troubleshooting)

Here are the most likely causes and how to diagnose them:

1. Faulty Headlight Relay (Most Common)

The most frequent cause of this problem is a corroded or failed headlight relay. In a stock FD setup, this relay is covered with a black rubber boot and is mounted on a standalone bracket just forward of the driver's side shock mount ¹. It is highly susceptible to water ingress and internal corrosion, especially if it has been relocated or is hanging sideways or upside down.

How to test and fix:

• Check the relay by jumping the wires. The headlight switch grounds out when turned on, which closes the coil and powers the headlights.
• Locate the two red wires (constant power) and the larger white/green wire (connected to the headlight).
• Jumping a red (or double red) wire to the larger white/green wire should activate the headlights. If they turn on, your relay is bad and needs to be replaced ¹.
• An over-the-counter replacement option that has worked for this is BWD part number R4136Z (or R4136). When installing the new relay, adding dielectric grease to the terminals and ensuring it is mounted upright can help prevent future water damage.

2. Broken Headlight Switch

If the relay is functioning properly, the internal soldered connections inside the steering column headlight switch may have broken.

How to test:

• Disconnect the clock spring harness under the steering column.
• Locate the three wires coming from the headlight switch: green, red, and black.
• Grab a multimeter and set the headlight switch to the "headlights" position.
• You should have continuity from the green wire to the black wire, and from the red wire to the black wire.
• If you only have continuity on one of those pairs, the headlight switch itself is bad ¹.

3. Bad Ground or Connection at the Plug

If both the switch and relay check out, you may have a bad ground wire or a poor connection at the headlight harness itself. The stock FD headlight circuit uses three wires: a low beam 12V, a high beam 12V, and a ground ¹⁰.

How to test:

• Use your multimeter to confirm whether you are getting 12V power directly to the low beam pin at the headlight plug when the switch is turned on.
• If you have power at the plug but the light won't illuminate, you likely have a bad ground. Splicing a new ground connection to the ground wire on the plug will resolve the issue ¹.