BACKYARD MECHANIC: Faulty computer behind door-locking problem

Hey there, time traveller!
This article was published 30/06/2011 (5323 days ago), so information in it may no longer be current.

QUESTION: I have a 2005 Impala and recently I am not able to lock any of the four doors. To open it is not a problem. When I try to click to lock, I can’t. I’m a professional electrical engineer so I checked the fuse-box relay, which I even hear to click, and after the third click on remote the horn alarm sounds to let me know that the doors are shut, but they are not.

I cleaned all contacts and checked for loose wiring. I even checked the fuse box and card module in it (to check if the contacts are working) and it seems that they are and the module looks in perfect shape. I took the car to a Chev dealer and they suggested that the breaker module needs to be replaced. When I called to buy it, their answer was that they do not have it and that this part is not available anywhere in the country and I need to wait at least three weeks. At that moment it got to me that a component like that is not often faulty. On the Internet I cannot find any schematic drawing in order to find where the dividing point is for all doors to be open or shut.

Could you direct me where I should check or where I should measure output impulses for closing doors? Maybe where to find the proper schematics?

ANSWER: Start your diagnosis by checking the base door-lock system. By this I mean the manually-operated power locks using the interior door switches. The door-lock switches are wired into a loop circuit that goes to the body computer. When you press unlock, the switch completes the circuit. When you press unlock, there is a 470 ohm resistance in the switch that provides a different voltage drop to the body computer. The body computer operates the door-lock motors through internal relays based on the voltages it sees from the door-lock switch circuit. If the doors don’t work from the interior switches, then the problem is either with the body computer or the lock-switch wiring.

If the doors lock from the interior switches but not from the remote key fob, then the problem is with either the keyless entry receiver or the body computer. You say the horn honks after pressing the lock button on the remote three times. This would indicate to me that the remote receiver is fine and communicating with the body computer, because it is the body computer that operates the horn, too. That would lead me to think the problem is in the body computer.

The wiring to the door-lock motors should be fine, because they do unlock. Current flow to lock and unlock the motors is simply reversed on the same wiring. The body computer contains the actuators (relay contacts) that switch the current flow direction.

All the diagnosis leads back to a faulty body computer. I am not sure what you are referring to as a “breaker module” but perhaps it is the body computer you are referring to. The relays you hear clicking in the fuse box are probably for the lights. They provide a visual indication when doors are locked or unlocked.

QUESTION: I am having a problem with the clutch on my 2001 Mazda. When I step on the pedal, the clutch disengages and the car will come to a stop as it should. However, when I am sitting at a stop light with the pedal pressed, gradually the car will want to pull away just like I was taking my foot off the pedal. I can’t see any leaks and I had a friend help me bleed the clutch system so there shouldn’t be any air in it. Do I need a new clutch?

ANSWER: You don’t need a new clutch. The problem is with a worn cup seal in the clutch master cylinder. When you are stopped with your foot on the pedal, the fluid is forced back past the cup seal and returns to the master cylinder reservoir. This internal leak acts just like you taking your foot gradually off the clutch pedal. Replace the clutch master cylinder, bleed the system again and it will work fine.

Jim Kerr is an experienced mechanic, instructor and member of the Automobile Journalists Association of Canada.

kerr.jim@sasktel.net