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