pilotincommand

Hey folks,

I know a number of threads have been posted on immobilizer issues, but I can't seem to find one that matches mine. If I've simply missed it, I'd appreciate a pointer....

Initial symptoms. Car had been sitting for about a week, unlocked in the garage.

Neither fob would activate central locks or immobilizer. Red light shines on both fobs.
Disconnecting battery for a few minutes and reconnecting resets the system and immobilizer/locks work, but only temporarily (few minutes). Charging the battery overnight had no effect, and I have greater than 12 V across the battery terminals.

Disconnect battery again, and fob works, but notice that door LEDs are double flashing.
Noticed that luggage compartment light is inoperable. Bulb is good, 12v at positive bulb terminal. Dome lights work and come on when hood is raised. Suspecting a faulty ground on bulb wire, but others report that should cause a double flash, not a complete immobilizer/ central locking failure. Now, after disconnecting and reconnecting battery, fob works briefly and then stops with no flashing LEDs on doors.

Questions:

Where does the ground from the luggage compartment light connect to the ground for the dome lights? I want to check continuity but I can find the connector.

Would a faulty luggage compartment ground cause a complete failure of the immobilizer and central locking? Does this imply a failure of the alarm control unit instead of just a faulty ground?

Thanks!

nine9six

Ques 1: I dont know.

Ques 2: They are on the same circuit; sounds like a ground fault to me. Alarm ctrl units rarely go bad.

hoggel

From another thread.

pp000830

This may help:

pilotincommand

Thanks for the responses. I had seen the diagram noting the luggage light meeting up with the door lights at "41", but I was hoping someone could provide a specific location in the passenger compartment to limit disassembly....

That said, I have some new data. Based on the diagram above, I ran a wire from the ground side of the luggage light to pin I/7 on the immobilizer and the light comes on, so that confirms that I've got a ground problem on the luggage light.

Unfortunately completing the ground to the luggage light doesn't fix the immobilizer issue. If I disconnect the battery for 20min or so, the fob works and the doors lock/unlock and the alarm arms/disarms. I still get the double flash on the door LEDs. After an hour or so, the immobilizer stops responding to the key fob and the only way to reset it is to disconnect the battery.

I tried disconnecting the immobilizer from both connectors for an hour or so and that didn't have any affect. Only disconnecting the battery resets the system.

Any suggestions appreciated.

hoggel

Strange immobilized/interior light problems can be a result of aftermarket alarm system installs. They sometimes tap into the interior lights circuit in the cabin splice area on the outboard side of the rear passenger footwell.

pilotincommand

Thanks Hoggel. I'll take another look, but there's no immediate evidence that any modification has been made to the system and there's no aftermarket alarm system in the car now.

hoggel

Check out oldman40's post #9 in this thread. https://rennlist.com/forums/993-foru...gram-help.html

nine9six

Im with hoggel on this thought, but OEM immobilized/interior lights ARE on the same circuit and a ground fault will indeed result in the same issue. The fact that door top LED lights are double flashing proves there is a system issue that needs to be resolved.

Can the OP provide actual numbers of battery voltage, rather than "over 12 V"?

The inop trunk light is also on the same circuit, and a ground fault there will indeed cause the reported issue.

pilotincommand

I'll double check the battery voltage tonight and post an update.

As for the luggage compartment light, running a new line from the negative terminal of the luggage compartment light to pin I/7 on the immobilizer control unit should resolve the problem, but that's not what I'm seeing.

mpruden

Pilotincommand - care to post where you are located? A fellow RLer may have a Porsche scan tool that can communicate with the alarm module to pull codes, check statuses, run drive links, etc. It may help you isolate the fault.

Also, what year is your car? US or RoW?
How long was it working before it started acting up?

Gbos1

that diagram just gave me a headache! Dang immobilizer!!!!

pilotincommand

Mpruden, I'm located in Rockville, MD and the car is a 96 993 Carrera. US Spec. Bone stock, no mods. Car worked flawlessly for over a year. Only issue was a dead battery in a key fob three months ago

Nine9six: Battery Voltage across the terminals is 12.24 volts.

pp000830

Here are some pages that can help you locate items on the above wiring diagram.

NC TRACKRAT

Battery voltage is marginal. 12.6 volts is optimum. Have you checked for a good, solid ground at the microswitch on the trunk lock? Are you sure that the microswitch is closing properly every time you close the trunk?