1992 SC400 Won't Start: Immobilizer Issue?
 

I didn't realize the trickle charger on my 1992 SC400 failed and now the battery on my 1992 SC400 has died. In the past when this has happened and the car had previously been locked via the remote on the key (factory alarm system), I would have to unlock the car with the remote once the jumper cables were connected in order to disable to alarm/immobilizer and start the car. This time, I cannot get the key remote to unlock the car and turn off the alarm/immobilizer. Because of this, I can't start the car. All the lights come on, but b/c the alarm is set, the immobilizer won't allow the car to crank/start. I also cannot unlock the doors with the switch inside and I assume this is part of the alarm deterring theft. Any ideas what could cause this? I'm waiting for a new key battery to be delivered and am hoping this solves the issue (I have original remote keys but neither was working - can't remember the last time I replaced the battery in either key). If the key battery doesn't fix the issue, does anyone have any idea what else could work to override the factory immobilizer? I'm worried this might mean the ECU has failed/is failing, though I've never had any symptoms leading up to this issue. I know in modern cars there's ways to hold the key next to the ignition and it will override the immobilizer but not sure if there was something similar back in 1992. Any thoughts?

Gonna have to look it up, but may work. Leaving the key in the ignition 30 minutes

The SC300/400 do not have a factory immobilizer. The ignition keys and ECU are not directly coded to each other in any way. The factory alarm system has some limited ignition lockout functions as you've described but it's not like an immobilizer system you'd see on late 90's and newer vehicles.

Interesting. Thanks for that information. Are you aware of a way to override the lockout functions? I'm still waiting on the batteries for the keys to arrive. I opened up the key and it's a very old battery, so hopefully replacing will solve the issue.

Hello,

That is true, but only partially..

It is correct that the Key is in no way linked to the car, it doesn't have a Chip built into the key to be recognized by the ECU (so holding the key near the Ignition won't really do anything). That said, it doesn't mean that the Theft Deterrent ECU has no other useful nasty tricks up its sleeve, as it still has the power to Disable the Starter if it so chooses, usually in cases when the car suspects that someone is trying to break into it, like Unlocking the Doors with Battery Disconnected, or Unlocking the Doors Manually (aka without pressing the button on a Remote). It is a relatively common issue lately from the number of threads about it, especially for people that are missing said Remote for any reason, here is one of the threads on this subject.

The good news is that the thread linked earlier also contains the details on eliminating the Theft Deterrent ECU from the Starter Circuit, such that this issue won't keep coming back, while still maintaining the Full Door Lock functionality.

I am usually not a fan of modifications to factory systems, but this is one of those cases where even I would suggest to follow the bypass procedure for the aforementioned Theft Deterrent ECU, mostly because it's one of those systems that would cause a lot more drag to the Owner than to any potential bad actor that possesses at least the basic motor functions, as all it takes to bypass it is a Paperclip in place of a Starter Relay. There is simply no reason that I can see to keep it in anticipation of a new glitch still in store for you, seeing how easy it is to Bypass with absolutely no irreversible changes.

Hope this helps and best of luck!

Key battery wasn't the issue. I'm at a loss now on how to proceed. My mechanic doesn't want to touch the issue. I wonder if the Lexus Dealer would still have employees who understand how these old alarm systems work on the SC? Frustrated that I didn't catch that the trickle charger wasn't working which led to the battery failing.

Did you read the previous reply, Post #5, by any chance..?

You can also try, just for the sake of it mostly, to Lock the Car with the Remote, if that still works, then Disconnect the Battery for at least 10 Minutes. After that, Connect the Battery again and try Unlocking the Doors with the Remote, see if the Alarm System would accept it and the Alarm would turn Off.

While at it, ensure that All the Locks are working properly, sometimes if the Door Lock Actuator is a little worn, it would only pull the latch about 1/2 or 3/4 of the way there, but not Lock it completely, which is what Alarm Systems struggle with a lot. To avoid this issue, you can Manually pull the Latch above the Door Handle to "Locked" and only then close the Door, ensuring, of course, to keep the Key in your Pocket and not somewhere inside the car. Also, use a Screwdriver to Lock the Hood Latch while keeping the Hood open, as I am pretty sure those cars already had Hood Switch. Only after all that is done, the Doors are Locked securely and the Hood is "Latched", press the "Lock" Button on the Remote, wait for at least a few minutes, and only then disconnect the Battery.

During this procedure, observe the Security Light on the Dash very carefully, and after the procedure is done, describe it here.

If that doesn't work, you may have an issue with the Theft Deterrent ECU, similar to the one described here.

Hope this helps and best of luck!