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Lexus Remote Keyless Entry Whodunnit
Posted to Technical Tips Forum on 11/16/2016 10 Replies

I wish I could construct this like one of James Avery's "whodunnits" but I'm not that good. It concerns a '95 Lexus SC-400 with a remote control (fob) that only HALF worked. The lock button would work, the unlock would not.

For those who don't want to read any further, it turns out the key fob itself was broken, and some careful micro-soldering repaired it. The microswitch had somehow separated from the circuit board. Fortunately it was still bouncing around inside the case of the fob.

(Hearing the car chirp twice in the driveway, unlocking itself, since I was holding the button down with my finger as I re-soldered it, was the best part!)

In retrospect, if I had taken the fob down to Autozone, I could have used their little "receiver" that ostensibly tests batteries in remotes (flashes when it sees ANY RF signal) to determine that one of the buttons worked, and the other didn't. Hindsight is 20/20, right?

Thinking the fob might be the problem, I'd already ordered a replacement off eBay, but ran into (yet unsolved!) difficulties putting the vehicle itself into "registration" mode, even with a Lexus tech supplying me detailed instructions and tips. Lexus used 3 different styles of keys and fobs on the SC's, the one button key (92-95?) the separate fob (96-97?) and the final multi-button key.

Perhaps the programming varies by year as well - key in, key out, cycle locks 5x, close door, open door, cycle locks again...blah blah blah... It would never cycle the locks to confirm (I read earlier, somewhere on iATN, that one step is missing in this procedure, as published)

Last night i sat upright in bed, and KNEW it had to either be the fob, or the unlock wire from the RKE module to the alarm/door entry module.

That was after about 4 total hours of thinking about it, an hour or two of staring at the schematic, and probably another hour turning keys, pressing lock/unlock buttons in the car, etc. I'm going to leave out the additional 1.5 hours I spent attempting to get the car into registration mode, where you train it to recognize a new remote.

I once had a boss ask my "Why aren't you working" as I stared at a schematic, motionless. "I am working! Go away and leave me alone!" was my reply.

Circuit problems are solved in the mind, or on paper, long before they're solved on the car. All you do on the car is rule stuff in or out, by taking a measurement or three, or physically examining the switch, connector, wire, whatever that your MIND has told has GOT to be where the problem lies.

Making a list of what you know --- for sure! - is a valuable time saver. Several times trying to figure this thing out --- I just ended up in a cloud of confusion. A list of what I knew to be true could have kept my feet better on the ground.

Sometimes you just have to step away for a while, work on something else, to allow the mind to clear.

As for what I knew for sure --- I knew, for instance, the RKE module wasn't COMPLETELY fried. It responded to a lock command, and that meant the antenna was connected, it was capable of "hearing" RF signals, it had power to it, and at least part of it's "guts" worked.

I knew the door lock motors worked. I could either turn a key in either door lock or use either the master or passenger door lock switches to lock and unlock the door. So all the switches worked too.

Eventually I figured out that I also KNEW that the "door open" switches worked --- why? Because the interior lights came on and went off properly when you opened the door.

What about the door latch mechanism that "told" the RKE module whether the door was locked or not? The module works in a "set/reset" mode --- if it sensed door was locked, it would unlock it. If it was unlocked, it would lock it --- this according to the manual, in a "theory of operations section" (this would go along a LOT better with the one-button remote on the earlier models)

My thinking was, "If it never sees the door as being locked, it may refuse to unlock it...."

But this didn't fly, because, if it didn't see door lock status changing state, it would disable itself --- again, according to the theory of operation section. I wasn't absolutely positive about this, and manuals HAVE been wrong before...

That only left two choices. One wire, from the RKE module, buried behind the rear seat kick panel, connecting to the Door Lock / Theft module at a connector under the front left kick panel.

Or the fob itself.

Removing panels and digging my way in to access these was NOT going to be fun...

Mind you, I was looking at a FACTORY manual for this vehicle. Colored schematics, connector outlines and pinouts, "Suggested test tips", theory of operation, I had it all.

Trying to diagnose this with JUST an All-data schematic would have been much harder. Especially online, instead of hundreds of pages all broken out, with things like connector pin #s in print so fine you could never reproduce it online.

The thing about electrical diagnosis is when you finally figure it out, all the things the circuit does, or doesn't do, suddenly make sense.

Of course I always go through this mental phase of wanting to give up, to take it to the dealer, convinced they have a magic scan tool that can read all the door and latch switches and a diagnostic tree that would solve this faster than I can...

Well, at least they know the correct way to pop those panels off without breaking all the tabs...

They don't of course, and my experience is they're not NEARLY as good as I am at reading a wiring diagram and deducing what MUST be wrong. They throw parts at it and when that doesn't work FINALLY get out the DMM and start checking things... and bill you for lots of hours as well as all the unnecessary parts... after all, YOU will never know they weren't needed and they're not about to admit to "Swaptronix!"

Unfortunately, on these older Lexi, NONE of the parts are available anymore, not door locks, actuators, modules....so it's salvage yard or nothing!

So what are the takeaways. You got an fob problem? First, replace the battery in the fob. 2nd, try it on one of those parts-store checkers to see if it's actually radiating. If you can get the vehicle into programming mode, try a duplicate fob. Check power & ground to the RKE module. Make a list of what you know for sure, what you suspect, and what you need to test out.

And if you really want to speed the progress on electrical issues, pop for a factory manual / wiring diagram. They're invaluable.

Wade from Colorado

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