I have a 92 SC400 and the door lock system only responds when the unlock function is pressed on the door switchs or using the key. In other words locking mechanism doesn't function. You can manually lock and then use the system to unlock. I have an aftermarket radio, no OEM amp or cd is still in the car. I have no working key fobs. The rest of the electrical works fine. My question is, is this related to the Theft ECU because of the aftermarket radio? If so can I bypass the the theft ECU to get the locking system to work. Because of the age of the car and the aftermarket installed on the car, would like to avoid a visit($) to the dealer for new key fob. Any help?

 

Mine has the exact same problem, and have never found an answer. Subscribing though in case somebody knows.

 

Does this happen with both keys, or only one of them? Would be unusual for both keys to fail at the same time. But the keys do fail with time (I am the original owner of the car). One of my 92 SC400 keys failed early this year. The other key body was nearly worn out, so I decided to replace both. You have to purchase new keys, have them cut to your VIN number, and install a new chip in the ECU in the car. The ECU is located beneath the right rear window. There's a thread here somewhere that shows photos on how to get to the ECU. Sewell is the place to order keys from for the best service and price. They are sold as a pair with the new chip included. The cost was $334.

 

My original key is a goner. So you are stating that if I get the new key with fob and new chip, the lock switches on the door will work?

 

After weeks of tracing wires and looking at schematics, found cure to the doors not locking using the door switches. I believe the key turn switch sensor in my pass door is bad. Once I unplugged the white plug of the gray and white plugs that connect to the door lock actuator inside the rearward part of the passenger door, the door lock mechs on both doors work. The key sw on the drivers door works, but the passenger door does not. Can still lock the passenger door with with the key, but the auto lock feature, (turn once locks the door, turn twice locks both) does not work. I can live with that. I installed a $25 wireless lock with FOB to the system and saved myself the $350 for the new chip and key with FOB. I can't guarantee this will work with others that have this problem so you might want to check both plugs, driver and passenger. What I did was take a voltmeter and set on resistance and ground common(black) and and use the pos probe to touch the contacts in the white plug on the actuator side with it unplugged. If you get the actuator to lock and unlock touching the outside contacts then you have found your problem. I will post more info as I dive into the system further.

 

Final post. There are dime sized plastic sensors on the inside end of the keyed locks on the doors. They send a pulse to the actuator that either locks or unlocks the door. Unplugging the white plug disables this sensor. My problem, the sensor is cheap and is held in place by 1 plastic tab. If this tab breaks then the sensor, which is mechanical in nature, falls away causing the door to stay in either locked or unlocked mode depending on the last position it was in. If in the locked position, the door lock switches will only unlock, if in unlocked position vice versa. I jb welded the tab but will probably have to pony up for the new cylinder. The good news, when i fixed the sensor, the 20 year old security system awoke. Unbelievable, the sc400 is a wonder. If you only want the door switches to work, unplug the sensors, but you need them for the theft deterrent to work. Sorry for the long post.

 

Good stuff.

 

Wanted to add one more item. Through out the site it is stated that the small plastic clip that breaks on the hinge to open the door is hard to find by itself. That is correct, but... I went to a local autozone and they had door clips for various cars foreign and domestic. There was one clip that matched the diameter of the hole in the handle but has a square receiver for the rod from the actuator. You can use this clip, but you need to add a 1/8 tie wrap below the 90% bend around the clip and actuator rod. Works perfect so far, little hard to get your hands in the door to make sure the tie wrap is secure but a little patience and you can save yourself $200. Also, while I had the actuator out, I cleaned it up added some silicone sealant to some of the plastic areas that were opened because of age and sprayed the lock assembly cables with silicone to help the rubber last longer. This took about 2 hours, but I learned a tremendous amount about the electronics, some DIY's to get the door system working properly.