355 Fun new problem: Hazard flashers come on randomly after car is locked

Came out to the garage at work tonight and my car ('98 GTB) was sitting there with the hazard lights flashing. Pressing the unlock button on the fob stopped the flashing and unlocked the car. Drives normally, turn signals work normally, locks and unlocks normally. Drove it home, pulled into my garage, locked the car, went inside, and 10 mins later came out to check on it...and it's sitting there with the hazard lights flashing. They come on apparently on a delay after locking the car.

I assume this is immobilizer related since pressing the fob stops it, and they never come on when the car is unlocked. Car is stock except it's converted from F1 to manual. It's never had an aftermarket alarm or hackery on the wiring. I searched the archives and didn't see any threads. Anyone have suggestions on where to start? Thanks in advance.

 

Thank you for the guidance Ian, don't know what we'd do without you. You know your stuff.

I thought I had the problem solved just now. Went to the garage, locked the car, and sure enough after 60 seconds the hazards started flashing, like they did earlier in the day. I pulled the spade connector off the engine lid latch and found a surprising amount of dirt, grease, and corrosion (surprising in that my car has always lived in southern California). I left it unplugged, locked the car, waited a minute, and...no flashing! Waited a few more mins for good measure, no flashing. Closed the garage door, went inside, and five mins later glanced out the door...orange flashing from the garage windows. Damn. Cleaned up the connector, plugged it back in, and sure enough the flashing started 5+ mins after locking again.

So, in short, no real progress, but one new mystery is that the flashing appears to start at inconsistent times after locking the car. Does this provide a clue? -- if it was one of the latch switches, wouldn't it always start flashing at the same time? (BTW, I know you said to disconnect the ground, but there's only two wires -- wouldn't disconnecting either do the same thing?)

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That is very interesting...you know, something I just noticed is that the hazard flashing seems slightly more rapid than regular hazard button flashing. Didn't connect the dots, but now it makes sense -- the hazard lights aren't flashing, the alarm is going off, it's just that it's silent because I removed the siren. I would be happy to simply remove the anti-lift sensor, as my car only ever spends the night in my locked garage. Any idea if I can unplug it and put it next to the alarm siren in a cardboard box? Or does the immobilizer need to see it? Thanks again for the assist.

 

Ok, update: I removed the anti-lift sensor and the problem is solved, no more alarm going off after the car is locked. In terms of access, you don't have to remove the trunk carpeting. Lift up the hood seal in the right rear corner, remove the aluminum trim around the HVAC opening in the center top of the carpeting, and you can lift up the RR corner of the carpeting enough to reach under with a 10mm ratchet and remove the two nuts holding on the sensor, and the sensor.

The only difference in the car's behavior is that now when I lock it I get four flashes of the hazard lights, not two (I think it was two?). I opened the top of the sensor (the cover is held on by three screws) and the circuit board is right there. Will post pics later. Would like to be able to fix it if possible and put it back. But if not, I'm not replacing it with an expensive used sensor that will probably fail soon due to age. I just don't care whether the totally antiquated and irrelevant 90's anti-theft system is working on the car or stored next to the car in a box on my garage shelf.

 

In my experience, only after locking the car.

For posterity, here's what the anti-lift sensor looks like with the top off. Any guesses which of these doohickeys (the scientific term, I believe) went bad?

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Thanks as ever Ian for the tips. I did some digging and below is what is needed for a LHD car. Are we sure the later sensor is plug and play?

bracket 179371
sensor 203327 or 177149
wiring 180071

Good news is there are a million used sensors on ebay. Not so much the bracket and pigtail, but they're both cheap new.

 

Haven't had a chance yet, will try this week.

 

Update: Finally got the bracket and pigtail from Eurospares, and found a good used 360 sensor on Ebay for $20. Installed today and it all works perfectly. Single flash when I lock the car now.

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I haven't been able to get it off. It's held on by those three solder joints in the pic above (on the other side of the board there are three posts, that is what it sits on). I have a soldering iron but can't heat up all three at the same time to release the board from the posts. Any suggestions? I tried using a solder sucker as well, hasn't worked.