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Keyless entry - not working after car battery replacement

13 November 2021
SF bay area
New owner of 2000 NSX and recently my car battery died. Replaced the car battery it and now the keyless entry does not work.

The steps I have done are:
1. Replaced FOB batteries - the fob LED does illuminated when the lock/unlock button pressed
2. Found the fuse in the KES harness... its good.
3. Checked all other fuses in the car - all good.
4. Locking the car with the key does activate the alarm system

Any other things to check or ideas to rectify the situation?

I suspect the KES head unit took a crap...
The NSX key fob confuses the hell out of me all the time. Have you made sure you closed all openings including the hood? I don't think it will even lock the car with the hood not fully latched.
All points of access were closed. I locked the car with the key and the security system was activated. I then tried to unlock with the FOB, no success. Unlocked the car with the key and then tried to lock via FOB. Again no success.

Not sure how to test otherwise.
I also have a 2000 with keyless entry and have on two occasions experienced problems with the security system blocking operation of the keyless system. In my case the security system was indicating that the rear hatch was not latched (indicator light showing on the dash) even though I confirmed that the hatch switch was operating correctly and had a good connection. I could manually lock the doors; but, the security system would not arm. This problem always occurred after the car came out of storage and the battery was reconnected and it always eventually went away - but not because of me. On the third occurrence of this problem the battery died (9 years old) and since battery replacement the problem has not re occurred.

I am an ex electrical engineer and have no explanation for why battery replacement resolves the problem. However, four owners have reported similar problems with the security system which were resolved by replacing the battery. Read about the weirdness here!

My case is slightly different than yours because my security system would not arm, presumably because of the false hatch indication. But, that false hatch indication was originating within the security system module (definitely not an external wiring problem). You are getting a security system armed indication; but, the security system may not be providing the permissive to the keyless system / door control. This might be a case on flaky operation caused by the dreaded electrolytic capacitor problem; but, I have not seen any reports of capacitor failures in the security system or the keyless unit.

I have no analytical explanation for why battery replacement would fix this problem, just the empirical results from 4 owners where the problem went away after the battery was replaced. If your battery is more than a couple of years old, you might want to try replacement. Access to the keyless unit isn't bad; but, getting to the security module to check stuff is an absolute pain.

If the problem does appear to be the keyless unit @drew is kind of the resident expert on the OEM keyless system. You will need the factory electrical trouble shooting manual to check the associated wiring because the factory service manual does not cover the retrofit OEM keyless system.


Just re read your original post and noted that you said that you have replaced the car battery. Check to make sure that the battery post clamps are secure. The OEM battery post clamps are pretty flimsy and tend to get stretched form over tightening leading to flakey connections. If the clamps and battery are good, then your problem may be the keyless unit or the interface with the security system and the door lock control unit. @drew is probably your best source on how to diagnose the keyless module to confirm whether it has failed.
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The keyless unit merely sends un/lock requests to the alarm and door lock unit. There is no two-way communication anywhere, the whole thing is literally 'dumb'.

Using an Ohm meter OR a test light hooked to 12V to determine if the door un/lock requests are going to ground, via a back probe. (warning: do not use a simple wire, go down to Harbor Freight and get test lamp)

Wire function here

DM me if you need more help.
The keyless unit merely sends un/lock requests to the alarm and door lock unit. There is no two-way communication anywhere, the whole thing is literally 'dumb'.
I was thinking of the arm / disarm request that goes out from the keyless unit to the SCU; but, I don't think that would block open and close.

I do seem to recall that when the SCU was having problems I could lock the car with the keys; but, not the Driver's door lock switch. Try locking the car with the Driver's door lock switch. That is the same input as from the keyless unit. If the driver's door lock switch locks the doors then the problem is originating inside the keyless unit.

I can lock the doors with the keys. I can lock the doors with the passenger side door lock switch. I CANNOT lock the doors with the drivers side door lock switch. I initially thought the driver side inside switch was unrelated to the KES issue. But Drew, your comment makes me think there is another issue.

In showing my ignorance - is the SCU - the security module?

Should I test the alarm system by initializing the security by locking the door with the key then opening the door via an open window to see if it is really operating? Or is there something else in need to do?

BTW - thank you for the input...
This is a partial schematic from the 1997-2001 electrical trouble shooting manual showing part of the connections to the door lock control unit.

Door lock schematic.jpg

The triangles marked A and B are the connections to the keyless control unit which receive the lock and unlock requests from the keyless unit. You can see that the keyless unit uses the same input circuit on the door lock control unit as the driver side door switch to lock and unlock the doors. The passenger side door switch does not have this connection back to the keyless unit.

Locking and unlocking and arming and dis arming are two separate functions. The driver's (and passenger's) door lock switches only lock and unlock. The switch in the driver's door latch which includes the lock knob locks / unlocks and arms / disarms.

If you in the car, doors closed and you cannot lock the doors using the driver's side door switch, then something is wrong with that input circuit. The fact that the passenger's door switch and the switches on the key cylinder are locking and unlocking the doors suggests that the lock control unit is probably working. The problem may be the wiring between the driver's door lock switch and the lock control unit. The keyless unit on the 2000 is an add in accessory and uses an interposing wiring harness to connect to the SCU and the door lock unit. If the harness connections have come loose that might explain why both the keyless unit and the door switch have stopped operating.

The first thing I would do is actually disconnect the keyless module from its plug so that it is isolated from the vehicle. Now try the driver side door switch to see if it operates. If it does, that suggests that maybe the problem is in the keyless module. If the driver side door switch remains in operative, then I think you need to trace the wire harness from the keyless unit back into the car where it connects with the body harness and start looking for wire damage or loose connections. If nothing is obvious the next step might be to do an input test at the lock control unit on pins 10 and 14 to confirm that the inputs on the lock control unit are functioning. Lets leave that part for now because it is more involved, ugly to do because of the location and you should probably have a service manual with the pin out diagrams to do the test.

The SCU is the security control unit.

If the red LED on the door is flashing the security system is armed and I don't see any benefit to trying that last test you propose.
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Wow! thank you for all of this... It will be a little while before I have a block of time to devote to go through this.

I will disconnect the KES and see if I can lock & unlock from the driver side.

Also, looking at the schematic and realizing the issue may be in the drivers door latch. I can lock the door with the key but I cannot unlock the door with the ley. I suspect the actual mechanism may need either lubrication or a part has failed in the unlocking action. I need to pull the door apart to get to look it over.... not something I am looking forward to as I understand some of the plastic clips are easy to break... I just need time to go slow and take my time... sigh..

I will definitely circle back to you to let you know what I find out so you can add it to your diagnostic prowess.
If you cannot mechanically unlock the door with the key, that may or may not be a potentially serious problem. The key lock cylinder is mechanically connected to the door latch and you should be able to lock and unlock the door latch with the key even with the battery disconnected. If you cannot mechanically unlock the door with the key, that may be a sign of a potentially serious problem with the latch mechanism. It is a potentially serious problem because there are at least two anecdotal reports of failure of the latch mechanism to unlatch by any means meaning that the door remains permanently closed. In this case, in order to access the latch for repair you have to pull off the interior door lining without being able to access the screws that hold the door lining on. Once you have removed the driver's seat (T roof makes this easier) this typically involves cutting the front, lower and back door trim pieces so that you can pull the door liner off while minimizing (not avoiding) damage - a bunch of broken clips are going to be the least of your issues. Then, with the door card off you get to access the latch from inside the door to cut the latch mounting bolts. This is one ugly job!

If you really do have a mechanical latch problem, I would be inclined to leave the door open or at least tape the latch mechanism so that it stays in the open position and the door cannot be latched close until such time as you can investigate what is goin on. You will need to disconnect the battery to avoid battery drain. With luck, this may just be a problem with the connecting rod or the mechanism between the key cylinder and the actual latch, not an internal latch problem. However, I think it is best to play it safe until you know what is going on because you are in for a world of hurt if the latch fails with the door closed.
I'd remove the door card now, "world of hurt" is a quaint understatement.

If your SCU is not arming upon using key to lock door the driver cylinder switch is likely corroded.

Your KES un/lock requests should be going to ground regardless. I can't see how a bad interior door switch would interfere with that...maybe sending both un/lock to ground at same time?

I can see a bad door actuator/assembly causing issues...take the door card out immediately as a very cheap insurance policy.... even then it will be hard enough to open the door upon failure.
The SCU does arm with the key on the driver and passenger side.... I can unlock via passenger side but I cannot unlock via driver.... all that being said, I got the message to deal with the driver door mechanism earlier than later.... very valuable advice!!!

Again, thanks guys!!!
Follow-up.... the issue (as Drew strongly suggested) was corrosion on the battery cables. Cleaned everything, wire brush on the terminals and the lugs... then reassembled with a liberal application of dielectric grease. Now all works (both door lock switches, and the KES fob) except being able to unlock the driver door with the key. I can lock the door but not unlock. I suspect the key cylinder is buggered and will not rotate in the unlock rotation..