Rear Door Power locks not working

jollymon · December 29, 2005

Hello, I am hoping someone can help me. I have a 2003 Chevy Suburban, 66,000 miles. It has power locks, and a alarm system.

The rear doors and the back hatch will not lock, with the power locks. If I lock the rear doors manually, they will unlock. It does not mater if I am using the remote, or if i am using the lock/unloc button on the drivers door.

I have tried switching the lock and unlock relay, that is located in the fuse panel and it still does the same thing.

Just as a side note, about the same time that this happen, my turn signals started acting real strange, IE turn on the left turn signal, and the hazards would come on, turn them off and the right turn signal would come on. I repaced the turn signal module and that fixed the problem. and right before that, I was having problems with my 4WD and the dealer replaced the 4wd switch on the dash.

If anyone has any suggestion It would be a great help.

Thanks.

GM-Tech · December 30, 2005

I'm not really sure what happened here, but the above post was really from me. I have no idea why it appeared as being posted by Jollymon.

Hmmm, I wonder if when I meant to respond to your post, I clicked on EDIT instead of REPLY. I wouldn't put it past me!

mmmikkke · December 30, 2005

It's possible that the back doors think they're still open so they won't lock. Gotta love those engineers. They know what we want better than *we* do!

GM-Tech · December 29, 2005

Can you feel (or hear) the lock relay click when you use the power locks to lock the doors?

The two relays share power and ground and they perform dual duty as the power for the actuators and ground for the actuators. So what this is telling us is that if they unlock okay, it's not a power or ground issue and the wiring from the relays to the actuators is okay.

jollymon · December 29, 2005

I did not think to check it with the remote.

Shouldn't make any difference. The body control module operates the relays whether it's you pushing the door lock switch on the door panel or you use a remote.

We're missing something here because this doesn't make sense. You swapped the relays (assuming the right ones) so it's not the relay. The wiring must be okay because it's the same wiring from the relays all the way to the actuators for either lock and unlock. If it was wiring, neither lock or unlock would work. Same thing for power/ground.

Here's a schematic. I left this at 1024x768 so it'd be legible. Sorry if it overwhelms your screen. You'll just have to scroll. I'll explain below the image...

For the sake of simplicity, try to imagine a motor (lock actuator) with the two leads attached to "C" and "D". C & D are indicated in the little triangular boxes that come off the wire from each relay. This would really have all the rear actuators attached, but lets not confuse matters.

Now, in the relay state shown, neither lock or unlock is being commanded. When you press LOCK, the BCM provides a ground to the coil in the LOCK relay (the one on the left), pulling the little arrow that's currently pointing at lock relay terminal #87A and making it point at #87 instead. That connects terminal 87 to terminal 30 and power then flows from the LOCK fuse through the relay to "C" which is connected to the actuator.

At the same time, the UNLOCK relay remains in it's un-energized state, i.e. the arrow stays pointing at #87A in the unlock relay. If you follow the wiring coming out of terminal 87A, you see it leads to ground.

Now you have power on one side of the actuator motor, ground on the other, and the motor moves in the direction needed to lock the door.

When you press UNLOCK instead, the same stuff happens, just swap LOCK relay with UNLOCK relay in the explanation above.

So as you see, there is nothing electrically wrong with your locks. Must be a little guy in there preventing the actuator from moving to the lock position.