Why don’t my non-driver door locks unlock?

Question

I have a 2005 Mazda 3 (SP 2.3) with roughly 133,000 miles on it. For a couple of years now, my non-driver door locks have not unlocked properly.

The problem is when I use my keyless entry to unlock the car, the driver door unlocks after the first press, but the rest of the doors don’t unlock after the second press (I can hear the driver door trying to unlock again, so I know the signal is getting there). Then I open the driver door, and press the switch on the arm rest and none of the other locks unlock. So I have to reach across and manually unlock them.

This problem has been going in and out, if I had to guess the symptoms seem to show up when the outside temps rise. I know the signal is getting to the locks in question because regardless of the symptoms above being present or dormant, any attempt to lock the doors from the driver seat arm rest switch or the keyless remote is successful.

Any idea what the root cause might be? And does anyone know how to fix it?

Answer 1

Sounds like we can assume the problem as follows: The unlock signal, whether it comes from the fob or the door switch, is failing to complete the circuit somewhere between the unlock module, and the first door. Whether the doors are wired in series or parallel, the fault is clearly upstream of, or at the point where they commonly connect to get the unlock voltage.

I would do some tests to isolate the fault. Looks like from your previous question you can at least take the door apart and know what's going on inside. Start with the passenger front door and find the wire that triggers the lock/unlock. There are a variety of wiring schemes for door locks, but typically you will have one wire that will open the lock when hot, and another that will close it when hot. Look up what your car uses so you know what to expect.

Run a 12v power source (grounded to the frame if it's not from the car's own power) and see if you can trigger the lock/unlock mechanism by applying +12v to each wire respectively, connecting as close to the lock mechanism as possible. My guess is that it will probably work fine, but if it does not, a fault at that point could also cause the other two doors "downstream" to fail.

If it does work on the other hand, you'll have to do some more continuity testing. etc to locate the break. (For example, hook up a multimeter to the lock/unlock wires and see if they get any voltage when you activate the fob or door switch.) Hope that at least gives you something to start with.