boxsey911

For the last 3 weeks I've been battling with an electrical gremlin that stops me from disarming the immobiliser but I still haven't solved it.

First thing in the morning I can disable the immobiliser fine and start the car. If I then drive just a few miles and stop, the car starts acting up - once the immobiliser arms I have a real problem disarming it (I have to randomly press the fob many, many times). I also can't lock/unlock the car with the fob. If I remove fuse #1 (main HVAC fuse) all problems disappear. This leads me to believe that there is a big draw on the battery by the HVAC system that interferes with other electrical functions such as disarming the immobiliser or operating the central locking.

I have tried to methodically isolate the problem (without success). Here's the list of things I've tried which haven't removed the problem:

* removed the rear blower fuse
* unplugged the rear blower
* unplugged the rear blower relay (and tried a new one in its place)
* disconnected the rear blower temperature sensor
* disconnected the rear blower ballast resistor
* disconnected the ISV
* Unplugged the aircon and oil cooler blower relays
* Unplugged the final stage relay (and tried another one in its place)
* Unplugged the front cabin fans
* Unplugged and completely removed the CCU
* Checked the 'radio green wire' and made sure it is well insulated
* measured the battery voltage - 12.5 volts cold, 13.8 volts with engine idling

The other significant I've noticed is that I started to get poor radio reception when the electrical gremlins first struck. Good reception is restored once fuse #1 is pulled.

Other information that might be useful: car is a 90 C2, I have aftermarket aircon but it has never worked and I never use it, the alarm/immobiliser is an aftermarket remote system that requires me to press the fob to disarm the immobiliser once the key is in the dash with lights on. I don't hear the classic clink from the flapper valves when I shut the doors.

If I thought the problem was the alarm/immobiliser itself, I'd simply go to an autoelectrician and get it diagnosed/changed. However, it was the finding that fuse #1 was key to the problem (the fuse blew a couple of weeks ago) that leads me to believe it's HVAC related and needs to be solved rather than bypassed by removing the immobiliser.

Please suggest what else I should check or ask questions about the above. I'm happy to take more readings with the multimeter but you'll need to guide me through what to do as I'm new to checking out electrical circuits.

Black and blue

Stupid question, but how's the battery in the key fob?

boxsey911

I've put a new one in and in the spare. It didn't make a difference.

Sten

Is the alarm power tapped off fuse no.1? Just a thought..

boxsey911

Thanks for the suggestion Baz but presumably not because I can set the alarm with the fuse pulled.

After finding out all of the above have no effect, my next thought (after doing a lot of searching on here) is that I may have a bad servo and am planning to unplug each to see if they have any effect. Ideally I'd like to measure any change in the current drain as I unplug each one but at the moment I wouldn't know where or how to measure it with the multimeter - I've only mastered how measure battery voltage so far.

Sten

Depends on whether your meter will take the current - disconnect the battery +ve terminal and connect the meter (set to measure DC current) between the battery and the terminal. After an initial surge, the current reading should drop to ~30mA. Beware operating anything that draws a large current, it will pop the fuse in your meter. If you get stuck, give me a call tomorrow, I've got the wiring diagrams in a folder somewhere..

boxsey911

Thanks Barry, that sounds better way of testing the five different servos. Probably won't be able to get to do it until the weekend due to work getting in the way.

BIG'UN

[QUOTE=boxsey911;7075150]Thanks for the suggestion Baz but presumably not because I can set the alarm with the fuse pulled.

Steve, can't help with your problem but have you considered that if it's an aftermarket alarm it may have a battery back-up and you should still be able to arm your car.

Good luck in finding that gremlin!

Andy.

boxsey911

Thanks Andy, I'll check into that because as you say most alarms do have a backup. With the boxster I have to leave the keys in the ignition when disconnecting the normal battery otherwise the alarm goes off. It's managed to catch me out once or twice! With the 964 I can just disconnect the battery without any issues.

As I mentioned above, I doubt if I'll be able to do any further investigations until the weekend. Barry's guidance on current testing should be a lot better than what I've done so far because of the intermittent nature of the problem. I've had times when I thought I'd cracked it - I was convinced it was the blower relay last week because with the oil cooler relay put in there or with it missing entirely the problem appeared to be gone. I managed to start the car everytime on my drive. So drove off to the petrol station on my way to buy a new relay and I ended up having to push the damn thing off the forecourt because the immobiliser stayed on again. Very embaressing

The simple solution is just to get the immobiliser bypassed but at the moment I believe that would be just be treating the symptom. My guess is I would then just end up regularly blowing fuse #1. I hope to come back with more news this weekend

BIG'UN

I ended up having to push the damn thing off the forecourt because the immobiliser stayed on again. Very embaressing


Been there Steve.Everyone seems to be happy to stand and watch you push it as well when you've got a Porsche!! My immobiliser will be getting taken
out and replaced as soon as I get the chance, always lets me down in heavy rain.

boxsey911

Well the first measurements were:

With ignition off 1.73 A
With ignition on 5.60 A

Much higher than you were expecting unless I'm doing something wrong - Did it as described above i.e. I first removed the neg lead, then the pos lead, reconnected the neg lead and then put the multimeter probes on the pos lead and pos terminal on the battery.

boxsey911

Made a small amount of progress today.

After chatting off-line with Sten I first made a few current measurements. The first measurement was made with fuse #1 removed. The car had been cooling down for about 15 minutes, all doors were shut, interior lights off, bonnet switch isolated, etc. The current was a steady 1.35 A. With fuse #1 replaced and everything the same I didn't spot the next reading because of the smoke and sparks that came off the battery lead when I tried to measure it . At this pointed I halted any further measurements for fear of burning out some wiring.

Then I reconnected the battery to its normal state and fuse #1 blew straight away. I then disconnected the plugs from all five HVAC servos (pic of no. 5 servo unplugged below). With all of these disconnected, fuse #1 blew again!



Then I disconnected this plug that sits on the cabin fan housing on the right:



Here it is with the cap off showing all the wires that terminate inside the plug (the eagle eyed will notice that there is a green/white wire on the right that doesen't have a corresponding pin on the lower plug):



With this unplugged and fuse #1 replaced my immobilizer and starting problems disappeared. I guess I should have taken another current reading again at this point but the previous sparking had put me off doing it again. But suffice to say that the problem is gone with this connection unplugged (the problem with radio reception is also gone - see first post).

Since the cabin fans or the CCU do not work with this connection unplugged, I presume this is the main junction plug between the HVAC system that sits under the front hood and the CCU? Can anyone confirm this for me?

Now I have the dilema of trying to work out why there is a big current draw when this connection is plugged in.

Sten

Steve,
That looks like plug T34 - carries the earth and power connections for the interior fans and CCU, plus various control signals for Oil Cooler Relay. Do the internal right/left fans work OK, have they been replaced? Seem to recall another RLister having problems with F1 blowing because one fan motor had gone west. The Brown wires go back to MPV1, i.e. main earth point 6 which is on the side of the central electric box -make sure the connection is clean and tight. If I've read it the circuit rightly, the pins in the last pic are numbered as you look at the pic like this:-
1 2
3 4
5 6
7 8
9 10
11 12
13 14

If it's a bad fan then disconnecting the blower final stage should also stop F1 blowing as you're removing the earth feed. The fact that you're getting radio interference would also suggest electrical noise from something like a bad motor..

boxsey911

As far as I can tell both fans work well. If I unplug the final stage relay the fans don't work as you say but the problem remains.

Where abouts in the car is the central electrical box so I can check earth point 6? I presume you weren't referring to the main fuse box. I would really love this to be just a bad earth!

Any idea which side of the plug (causing my problems) the power arrives at? The fact that the problem remained when the CCU was removed but disappeared when this plug is disconnected has got me thinking that the problem could lie between the CCU and this plug.

boxsey911

Just updating this thread to say that after taking both sides of that terminal for the HVAC apart (see picture above) and dusting it down, the immobiliser problem disappeared! I was then able to measure the current drain with fuse #1 in place. No sparking this time but it was still just under 1.4 amps. Pulling fuses revealed that this drain was down to the interior lights still drawing current even though the doors were shut (as revealed by pulling fuse #11). With the interior lights clicked to the off position the drain dropped to 0.11 amps. I then, for good measure, cleaned up and re-tightened the earth point inside the right front wing and put on a new battery earth strap. Just need to hope that immobiliser problem doesn't re-occur now!