How-to Guide
How to Check Whether a Head Unit Is Draining the Battery
Quick answer
Your unit takes two supplies: a permanent one on pin 2 that holds your settings and the clock, and a switched accessory one on pin 3 that goes on and off when you turn the key. To find out whether it's the cause, measure the current draw at the fuse for the head unit circuit with everything switched off.
What this gives you
A number instead of a suspicion. If your vehicle sits for a fortnight between trips, or runs a dual battery setup where every milliamp on the crank battery counts, guessing which accessory is doing it is how you end up replacing the wrong thing.
Before you start
- A multimeter that reads DC current, ideally to ten amps.
- The fuse for the head unit circuit, worked out before you start.
- The vehicle switched off and left alone a while, so everything's gone to sleep before you measure.
- Some patience. Modules keep waking up for a few minutes after you shut the doors.
Step by step
- Understand what should be drawing anything at all
Pin 2 is the permanent battery feed, and it's meant to keep a small amount alive after you take the key out. Pin 3 is the switched accessory feed and should be dead with the key out. A little permanent draw is normal. A large one isn't.
- Shut the vehicle down properly and wait
Close the doors, lock it if that's how you normally leave it, then give it a good twenty minutes. Measure straight after you switch off and the number means nothing, because half the vehicle is still awake.
- Pull the head unit fuse and measure across the fuse holder
Set the meter to DC current on the ten amp range, pull the fuse, then put the meter across the two sides of the holder. The meter now completes the circuit, so you read exactly what that circuit is taking.
- Write the number down
This one number is what the whole conversation turns on. A reading you take and forget is a reading you'll have to take again.
- Repeat on other circuits to compare
Do the same at a few other fuses. If the head unit circuit isn't the outlier, the drain is somewhere else and your head unit was never the problem.
- Check the switched feed is actually switched
If your unit stays fully on with the key out, the accessory feed on pin 3 may have been connected to a permanent supply during the install. That's something you check in the wiring, not a fault in the unit.
If it does not work
- If the meter reads zero on every circuit, it's probably still on the volts range, or your leads are in the wrong sockets for current.
- If the draw is high but drops while you watch, something's still going to sleep. Wait longer and measure again.
- If your unit stays lit with the key out, check pin 3 against pin 2 at the main plug. The accessory feed should die with the key.
- If everything measures normal and your battery still goes flat, the battery itself is the next thing to test, not the accessories.
Worth knowing
Leave your vehicle for weeks between trips and a tired battery goes flat with no accessory at fault at all. Test the battery as well as the circuits before you change any wiring.
Still need help?
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