How to check whether a head unit is draining the battery
The unit takes two supplies: a permanent one on pin 2 that holds settings and the clock, and a switched accessory one on pin 3 that turns it on and off with the key. To find out whether it is 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. On a vehicle that sits for a fortnight between trips, or one running a dual battery setup where every milliamp on the crank battery matters, guessing which accessory is doing it is how people end up replacing the wrong thing.
Before you start
- A multimeter that can read DC current, ideally to ten amps.
- The fuse for the head unit circuit identified.
- The vehicle switched off and left alone for a while first, so everything has gone to sleep.
- Some patience. Modules keep waking up for a few minutes after the doors are shut.
Step by step
- Understand what should be drawing anything at all
Pin 2 is the permanent battery feed and it is meant to keep a small amount alive with 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 is not.
- Shut the vehicle down properly and wait
Close the doors, lock it if that is how it normally sits, and give it a good twenty minutes. Measuring straight after switching off gives you a number that 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, and put the meter across the two sides of the holder. The meter is now completing the circuit, so it reads exactly what that circuit is taking.
- Write the number down
This one number is what the whole conversation turns on. A reading taken and forgotten is a reading you will have to take again.
- Repeat on other circuits to compare
Do the same at a few other fuses. If the head unit circuit is not the outlier, the drain is somewhere else and the head unit was never the problem.
- Check the switched feed is actually switched
If the 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 is a wiring check, not a fault in the unit.
If it does not work
- Meter reads zero on every circuit: the meter is probably still on the volts range, or the leads are in the wrong sockets for current.
- Draw is high but drops while you watch: something is still going to sleep. Wait longer and measure again.
- 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.
- Everything measures normal and the battery still goes flat: the battery itself is the next thing to test, not the accessories.
Worth knowing
A vehicle that sits for weeks between trips will flatten a tired battery with no accessory at fault at all. Test the battery as well as the circuits before changing any wiring.
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