Rich9928p

I spend a fair amount of time assisting 928 owners with no-start of strange intermittent problems. One not so well understood problem is the interaction of the factory alarm system with the engine control. The alarm system has two purposes, make noise if something that shouldn't happen happens, and disable the vehicle from starting.

The factory alarm system control ages and can go faulty, so we need to understand how it works. Attached is the wiring diagram for the Porsche 928 1990 alarm. Note on the lower right corner of the wiring diagram below the "Control Unit." At the very bottom of the control unit diagram are connectors for terminal 3 (goes to EZK relay pin 86) - E85 yellow wire and connector 1 (input from central electric bus 15, ignition "on" voltage) - M74 Black - Red.

When the alarm control unit "opens" the circuit (in the case of an alarm situation) there is no voltage to the EZK relay socket 86, so the relay will not "turn on" and no voltage is provided to the LH and EZK modules. The car will not start even with the ignition key inserted and turned to the "start" position. If the alarm control module goes bad and this circuit is open, there will be a no start condition. In this case, the only way to start the car would be to use a jumper between EZK relay socket 30 and 87.

If you have a defective alarm module that opens this circuit at all times and do not wish to purchase a replacement, or need a quick fix - the fix for this situation would be to jumper the alarm module pins 1 and 3 together. This closing the circuit regardless of what the control unit out put is. I've never looked at this alarm control unit and do not know where it is located. Perhaps someone who has been there and done that can comment further.

If your 928 has an aftermarket alarm system with engine cut-out, it can be very difficult to diagnose problems because the person that installed the system decided where to cut the wires and insert the "alarm switch" ... they may have done a good job or they may have done a sloppy job. All that I can say is GOOD LUCK finding the connections and figuring out what was done.

Alan

Rich,
Alarm system varied in location over the years, in console, behind/above glovebox and under passenger seat (in LHD not sure about RHD).

Alan

Joe '87 S4

My factory alarm gave me nothing but problems. To start the car I would have to open the rear hatch, slam it shut, the car would honk, then I would be able to start it. It would then shut the engine off when it felt necessary. I had it disabled and what a relief. I really don't see the need for a car alarm. The first year I got my car, I had enough problems (for one reason or another) trying to start it with the actual key - I'd like to see a thief try and get it running without one.

Alan

Joe,
As I have noted in other posts - if its working correctly the factory alarm is actually pretty good I think. Yes a lot seem to have had problems, I suspect a lot of low tech solution to contact issues etc - very few understand Porsche alarms very well. My car was quite messed up when I got it - now its sorted its great. Aftermarket alarms are often even worse for the poor installation quality and effectivenes (usually anyway).

I think an alarm is fairly important especially one that can immobilize the car

Alan

Joe '87 S4

Hi Alan, I didn't mean to make light of all the hard work you put into the alarm and schematic. I was just so happy when I finally took my alarm out of the equation and the car worked! I'm gun shy of putting another alarm or having the factory alarm fixed.

Alan

Joe,
No offence taken - my point is it can be a very good system if its working the way its intended to. I do know its very frustrating when its not and "exorcism" seems the only way... but having a working alarm is a useful thing worth a bit of pain to get... Agreed though - there is a threshold of pain and maybe many have crossed that point.

I think there is a more than enough PO blame to go around here ... "don 't mess with what you don't understand" would have been a good mantra - probably should have applied to many alarm/ audio system installation "Technicians" also.

Alan

Rich9928p

I ask those who I give advice for their Porsche 928 problems to reply with the outcome of their attempts. This UK 928 owner was kind enough to provide some photos of his work.

The output of the Alarm relay was varying between 12 and 7 volts, and a clicking sound could be heard. If the voltage dropped low enough, the control current to the LH relay is not sufficient to keep the switch closed and the engine dies or sputters. He removed the plug and inserted a jumper, which bypasses the switching capability of the alarm module. His problem was cured.

Keep in mind that this 928 is a right hand drive model, so the glove box is a mirror image. The photos show the wiring that he found and the jumper that he constructed to bypass the alarm control module.

Alan

Hummm... PO / Alarm "Technician" blame abounds....

Herein lies a lesson - if you change it - document it!

Alan

Bill Ball

For the last few months if I lock my doors, which arms the alarm, the alarm goes off in 10-20 minutes despite no intrusion. So, I can't lock my doors. I'm not sure how to troubleshoot this (advice accepted).

In the meantime I would just like to disable the alarm so I can lock the doors. Rather than pulling the plug and jumpering as shown, can I just put a jumper in the backside of the plug from 15 to 87a? I would assume so.

What happens if I just pull the small 2 pin plug that is 30/Hn? I note that the doors locks and arming LEDS still work. I cannot try to start the car as my radiator is out for repair.

I am seaching for for previous troubleshooting advice, and this was the first thread I found.

Thanks!

Bill Ball

OK, I found comments about jumpering pins 1 and 6 of the Z plug on the central electric panel. Obviously, that is easier than fussing with the control module. That is, if I could find the Z plug! I see comments that it is the shorter of 2 horizontal plugs near the bottom center of the central electric panel. I see nothing resembling a horizontal plug in that area. WHERE IS THE Z PLUG! This thread gives its location - I guess I just need to pole around behind the panel.

https://rennlist.com/forums/showthre...=disable+alarm

And I guess troubleshooting the malfunctions involves checking door pins, hatch pin, hood pin. Anything else.

Thanks!

Bill Ball

Well, I think it finally dawned on me why my alarm has been tripping lately- the intake I have for my supercharger has been pushing the hood up enough to open the hood pin and trip the alarm. Pressed down on the hood. So far the alarm has not tripped tonight. Good.

I gather the Z plug is behind the central electric.

Stan.Shaw@Excell.Net

So, is there a simple way to disable the alarm in an '86.5?

Mike Frye

Stan,

My owner's manual had a long write-up about the alarm, and the last line was something like: If the alarm begins to malfunction, a temporary way to disable it is to jumper across pins X and Y in on the relay board. (Sort of a good reason not to keep it in the glove box I guess).

BTW- Stan I got the parts you sent, thank you very much!

BrianG

So, on a car that has no factory alarm installed, how does the factory blank off the wiring harness?

JEC_31

This thread confirmed what I had always suspected, that the "dealer-installed" (according to the window sticker) Alpine alarm system that I had to tear out of my ex-928 was in fact aftermarket and not a Porsche factory alarm.