Strange alarm control unit problem M534 after water damage

[Smallblock454]

I have a customers' 9x6 with a M534 alarm ecu water damage. European car, so M534 and 433.92 MHz system.

Car was fitted with a 996.618.260.00. I received a 996.618.262.02 from Porsche as a replacement.

Everything has been drained and dried. Wiring harness has been checked. Connections have been checked and cleaned. Antenna has been checked. Everything seems fine so far.

Programmed the new unit and everything works fine for about 2 days. Then suddenly the keyfob where only recognized from when you put them directly beside the drivers window,

No error messages / DTCs.

Measured the keyfob signal strength. Perfect.

Measured antenna and tested against ground, no abnormalities. Replaced antenna with new original antenna. Problem still exists (this M534 has an extra antenna connector).

Installed M534 with the reception problem in another car - same phenomenon - handheld transmitter is only recognized directly on the windscreen / driver side window.

Put the M534 from the other car in the car that had the water damage. Everything works as expected. Key fobs are recognized from 5 to 6 meters.

Ordered another M534 from Porsche, programmed it to the car. Now everything works fine, but…

now 2 days later the key fobs are again only recognized, when you put them directly in front of the windscreen / driver's side window. Aaaargh! It's so bad that they aren't recognized when you stand in front of the trunk or the passenger's side door.

I can rule out any interference problems.

I'm running out of ideas.

Any ideas what can cause the M534 receiver to "die" within approx. 2 days?

Anyone had a similar experience?

Thanks in advance…

[spinjockey]

Is the car going into deep sleep mode? If so putting the key into the door lock to unlock should wake the car up and return to normal operation.

[Smallblock454]

Thanks for the idea.

The car isn't in sleep mode. If the car wouldn't be in sleep mode after 2 days, the key fob wouldn't be recognized at all.

But the key fobs are both recognized. They now only work from very low distance. The fobs are OK - measured the range - everything is fine. If you put the replacement alarm ecu in another car, you have the same problem. So something in that car seems to make the alarm ecu receiver to "die" / insensitive after approx 2 days. As the alarm ecu in this car has a separate antenna input, i for sure measured the antenna against ground, to test if there is a short. But there isn't. And i couldn't find any other anomalies.

I don't think 2 new alarm ecus have the same defect from the factory.

[spinjockey]

Originally Posted by Smallblock454

Thanks for the idea.

The car isn't in sleep mode. If the car wouldn't be in sleep mode after 2 days, the key fob wouldn't be recognized at all.

But the key fobs are both recognized. They now only work from very low distance. The fobs are OK - measured the range - everything is fine. If you put the replacement alarm ecu in another car, you have the same problem. So something in that car seems to make the alarm ecu receiver to "die" / insensitive after approx 2 days. As the alarm ecu in this car has a separate antenna input, i for sure measured the antenna against ground, to test if there is a short. But there isn't. And i couldn't find any other anomalies.

I don't think 2 new alarm ecus have the same defect from the factory.

I agree there is a problem with the configuration. It’s not clear that the ecus are damaged. You swapped our two units so it’s clear that it is a car specific issue but have you tried reprovisioning the ecus to see if they are permanently damaged. This is almost the kind of behavior I would expect from an electrical short but you say you’ve cleaned up the residual moisture. Have you looked closely at the integrity of the wires in the loom or connector to see if there might be a mechanical short or degradation of the wiring due to moisture or just handling?

[Smallblock454]

Problem solved. It was caused by the key fobs, not the alarm ecu.

Reworked the key fobs again (they had been reworked by an unknown person before, who apparently didn't know what he was doing). Now everything works fine and reliable.

Already doubted my sanity.

Thanks for the ideas for finding a solution.