Why is there a 0.12 amp power draw on the room fuse of my Mazda RX-7 FD that is slowly killing the battery?

A 0.12 amp (120 milliamp) parasitic draw on the "Room" fuse is definitely enough to slowly drain your battery over a few days. The Room fuse provides constant power to maintain memory settings and power accessories while the ignition is off. On the FD RX-7, this circuit typically powers the dome lights, hatch lights, radio, and trip computer.

Difficulty: Moderate Tools Required: Multimeter (with ammeter capability)

Here is a breakdown of common culprits and how to diagnose the drain.

Common Causes for Room Fuse Draw on an FD

The factory harness rarely develops issues unless it has been damaged and is shorting against the frame. Most parasitic draws on this circuit are caused by:

• Corroded Audio Relay: Moisture can cause heavy corrosion or rust buildup inside the audio relay, creating a high-resistance short that drains the battery without immediately popping the fuse ².
• Aftermarket Audio and Electronics: If your car has an aftermarket radio, alarm system, or component speakers, these are prime suspects. For example, a loose wire on an aftermarket speaker crossover mounted inside the door can make contact with the metal door frame, causing a draw ².
• Internal Component Failure: One of the modules on the circuit (like the trip computer or CPU) may have an internal electrical issue drawing constant power ¹.

How to Isolate the Draw

To locate the exact source of the 0.12A draw, perform a parasitic draw test:

1. Verify Lights are Off: Double-check that none of the lights powered by the Room fuse (interior dome, hatch, key cylinder illumination) are staying on due to a faulty door switch.
2. Set up your Multimeter: Disconnect one of the battery terminals. Connect your multimeter (set to measure amps) in series between the battery post and the disconnected battery terminal so you can monitor the live current draw ¹.
3. Inspect the Audio Relay: Pull the audio relay and check for internal rust or corrosion. If the draw drops immediately when the relay is removed, you have found the culprit.
4. Unplug Components One by One: If the relay is clean, start unplugging individual components on the Room circuit (radio, trip computer, warning CPU, etc.). Monitor the multimeter as you disconnect each item.
5. Identify the Culprit: When the current drops from 0.12A down to a normal sleep level (typically under 0.05A), the last component you unplugged is the source of your drain. You can then repair or replace that specific component ¹.