My circuit is the best compromise I can think of without building a custom circuit that actually senses the audio system load. This is a manual/automatic system for overriding the ELD signal. It works like this: The resistance seen by the PWM at the ELD is what determines the voltage drop from the 5 VDC produced by the PWM. I have a relay that switches between the ELD resistance, and a fixed 820 ohm resistor tied to ELD ground (NOT CHASSIS GROUND). Do not ground this resistor to chassis ground. Do not ground this resistor to chassis ground. Got it? Good.
This relay could just be controlled with a switch, but that isn’t enough for me. I am powering the relay coil from the Head Unit remote turn-on (blue/white wire) and grounding the relay coil with a toggle switch at the dash. This means that in order to bypass the ELD, we need both the Head Unit on and toggle switch closed. The idea is that we are doing this for our sound system but we still want fuel and battery savings when we’re not cranking tunes. This is common for me with kids and phone calls (Bluetooth). We could just control the relay from the head unit, but the toggle switch lets us also save fuel with low volume listening sessions (kids, family, coworkers, etc.). It requires that you are paying attention to the vehicle, the voltage, and your system usage. Later, I will develop a solution that monitors the sound system and triggers the relay with 20-40A of usage.
Onto the circuit design.
(1) 20A 12VDC relay, with 12VDC coil and N.O and N.C. contacts (5-pin) – I recommend getting a 5-wire socket with it.
(1) 820 Ohm Resistor (Gray/Red/Brown)
(1) 12VDC SPST Toggle Switch (unlit – a lit toggle would need separate terminals for the lamp)
Approximately 10 feet of 18awg wiring – each 10 ft. in three colors for a total of 30 ft. I found that 10 feet of 18-4 CL2 speaker wire works great.
Digital Multi Meter (DMM) or volt-meter.
30-40W Soldering Iron and solder (soldering is recommended over crimp terminals)
Heat shrink and electrical tape.
Wire connectors - ring terminal for ground, butt connector for remote.
Haynes manual with wiring diagrams to verify your specific system wire colors
Areas of access:
Engine bay – main fuse panel
Dash – removal of head unit and access under dash
Firewall – path for cabling (determines wiring length)
Some DMM images to explain what is going on:
Gray DMM on the Right is Battery Voltage
Yellow DMM on the Left is the ELD voltage seen by the PCM
In these, the Accord is idling at 800 RPM in the driveway after sitting for a day. I turn on and off loads to monitor the ELD sense voltage and the battery voltage. In this example, the alternator did not turn off, that requires more time and some driving around.
Image of tach position for all of these photos:
Example while driving, alternator is on, 3.3 VDC indicates little ignition load
Then I turn the A/C on and the ELD sense voltage drops to 2.0 VDC. The alt is on because the car just started and the ECU waits a bit to charge the battery.
Now the A/C and rear defrost are on, and the ELD sense voltage drops to 1.16 VDC and the battery voltage is down to 13.96 VDC with the alt still on. The stereo is off – this is why a stock alternator can have a hard time keeping up with a sound system at idle.
Now I engage my circuit, and the ELD sense voltage drops to the design ~2.1 VDC (due to the 820 ohm resistor). This will always turn the alt on and keep it on.
Now the A/C is on, the stereo is on (high listening volume) and the engine at 800 RPM idle (with ELD bypass engaged)