Thanks for the schematic, its a LOT clearer/readable than what I have.

My issue with the green wire is that once connected to that spade connector on the head unit sleeve, there doesn't appear to be any electrical link to the sleeve, the head unit or even that other metal tab. Resistance measurements show no connection to any of these items. So unless there is a circuit/relay embedded inside that black plastic rectangle the spade and tab are mounted in, I don't understand how the green wire ever sees ground.

The metal tab initially connects to nothing when the head unit is removed. As the user inserts the head unit, the tab makes contact with, and slides along, the bottom of the head unit metal case until the head unit is fully seated into the sleeve - at which time the tab loses contact with the head unit case and rests on that small piece of "isolation tape" (looks like thick scotch tape) stuck on the bottom of the head unit. A picture of mine is shown below.

When I measured resistance between that spade connector, the metal tab, and the head unit case, there was zero continuity. That was true whether the head unit was installed in the sleeve or not. How does the green wire ever see ground?

 

I don't have my manuals here with me but I think the head unit case is grounded through the head unit rear connector. The metal tab would ground the green wire as soon as the head unit is slid out enough that the tab is no longer on the tape. Check to see if the head unit case is grounded when the electrical connector is connected?

As the alarm system inputs are normally electrical open (no connection), and ground to trigger the alarm, the system is relatively easy to troubleshoot. If you think the green wire is the trouble just disconnect it (or remove and unplug the head unit), make sure the wire is not touching anything, and lock the car with the fob. If that doesn't correct the double flash then move on to the next thing on the list.

The engine and luggage latch switches and glove box switch can just be disconnected (unplug one at time and test). Since you have a cab it could be the latches in the RSD? I've never seen those but they are on the list of alarm inputs.

Checking the door open switches and the door lock switches takes a bit of care. You can disconnect the door open switches but the door latch has to be in the closed position to lock. I take the door open switch out and disconnect, tie a long piece of string around the wire at the connector, put some tape over the connector, close the door, then test. I've never checked the door lock switches.

Alternatively you can disconnect the connector from the alarm control unit and make sure there is no continuity to ground on any of the alarm inputs. Mind that the door lock switches may be ground if the doors are not locked.There is a pinout in the repair manual.

If your local shop has the PIWIS, Porsche Hammer, or the Durametric they can test all of the inputs.

 

Yup the head unit is grounded via PLUG A - otherwise the head unit doesn't have a complete circuit to operate. Got that part.

But the green wire has no apparent connection to that larger metal tab pressed up against the case - that's where I'm confused. Either that black plastic thingy is "broken" in my car, making for a permanent "no ground" signal, or there's some other connection between the spade and the tab that I can't see.

If you (or any other poster reading this) has easy access to the OEM head unit cage, could you do a measurement between the spade connector and the larger metal tab? Is there a solid connection between them? If so, THEN I get what hoggel is saying and it makes perfect sense.

The electrical drawings show the "Ground Radio Case" circuit as a normally open single pole switch (see image) - which is completely consistent with hoggel's explanation, if the spade is the incoming green wire and the metal tab is the "switch" that makes or breaks contact with ground on the head unit sleeve. But it only works if the two metal pieces are just two ends of a continuous conductor.




Rod

 

Ok, folks - nevermind my message above. I just re-tested those two metal tabs on my CR-210 sleeve with a different multi-meter... and this time the resistance measurement showed them to be connected inside the black plastic mount they're in. I checked the first meter I used, and its got some problems, it turns out. Cripes!

So hoggel's description of how the green wire functions is right on... at last I know what's real.

Once the head unit is installed in the sleeve, the large tab is insulated from the unit's metal case by the small piece of clear "tape", and the alarm unit sees an open circuit - which it considers a non-alarm state.

If the head unit is removed while still connected to PLUG A in the back, which grounds the unit, then sliding the head out of the sleeve allows the large tab to make contact with the case, and changes what the green wire sees from an open circuit to a grounded input, triggering the alarm.

The only thing that bothers me now is that a thief isn't likely to carefully remove the head unit while the car is locked and alarmed... they're far more likely to take a crow bar to the dash after breaking a window to get in... its possible you could rip out the whole head unit, sleeve and all, and if done correctly the alarm system wouldn't notice the head unit is missing - of course, meanwhile its going nuts due to entry without disarming first.

Sorry all for wasting so much time on this non-issue...
Rod

 

Mine is an Euro spec '94 C2. A few years ago I had a car stereo shop replace my old, no CD player factory radio with a new Panasonic radio with CD player. But before doing that I did not know new radio needed a handshake wire to the alarm system. Only after the alarm went off a few times in the middle of the night and I had my maint/repair shop check on it, I finally knew why. So I had the shop install an OMP battery kill switch. (In Taiwan, parts and labor cost about US$100.)

Now, every time I am done parking my car, I switch off the battery completely. When I want to drive my car, I switch it back on. Although the alarm system starts functioning immediately after I switch on the battery, I can easily disable the alarm system by unlocking my car door with my car key.

Merits of the battery kill switch are:

*You can park the car for 6 to 8 weeks and the battery will be still juicy.
*Except yourself with your car key, nobody can start your car because once the battery is switched back on the alarm system and immobilizer will start functioning right away. (Thief would need a tow truck to steal your car.)

 

Darn - I just sent a message and I think it got lost. Trying again.
I had my alarm going off at random times after I locked the car. The mechanic seemed to fix it by making sure there was no short to ground with the cable in the windshield going to the interior light (97 C2 Cabriolet). This seemed to work for a while. I also replaced the old battery to get voltage up to 12.64 from previous 11.95.
But now the alarm issue has recurred. He will check to see if the code shows a short again. If it does not, then I have the following to report and ask about. I now notice that the alarm goes off between 80-120 minutes after locking. Might be 120 mins with top up and less with top down. Anyway, when it does NOT sound, I found the remote will not unlock the car and I have to insert the key to open the door. Then the car goes nuts with alarm and spasmodically flashing red LEDs on the doors until I can quiet it down. Once I think I had to start the car to quiet it. Anyway, this issue of 993s not unlocking remotely is not uncommon, my dealer says. But the correlation with an alarm issue is not normal. Any thoughts??