One of the common 'faults' with the 4 wheel drive 964 is triggering of the 4wd/abs warning lights for no apparent reason. The sequence is well known to anyone who has had the problem. The 4wd drive and abs lights go on together, the buzzer then goes on to say it is a category 1 alarm, and the warning light light comes on to say a warning has triggered - all this in under 1 second - and very disconcerting if at speed. Having had this fault myself (hopefully now fixed) I believe I have a good view on all current probable causes (but no doubt someone will e-mail me with another!). Here is my guide to this problem:-

1. Poor earth connections leading to insufficient current in the circuit. Look for corrosion and check for current draw. Clean all earth points.

2. Faulty spark plug boots causing high voltage spikes to earth that trigger the alarm. Look for visible cracking, and/or incorrectly fitted boots. Make sure all boots and insulation are correctly fitted and operational. Change if needbe.

3. Cracks in the twin distributor unit causing voltage spikes to earth that trigger the alarm. Difficult to see hairline cracks - needs proper checking. Make sure the caps are not cracked, change if needbe. Look for hairline cracks and moisture in the caps.

4. Water ingress on the electricals, especially in the front boot area where the pump etc is. Any water can lead to circuit shorting. Make sure all connections, etc, are dry and that no moisture is present. Refix seals, etc as needed. WD40 can sometimes help.

5. ABS brake pressure switch (on the pump unit) is leaking pressure, and/or other pressure leaks in the system. Usually visible and/or takes a long time (3-4 seconds) for the warning lights to go out when starting. Re-tighten connections (take care) and if needed change switch/unit. Expensive (£700ish with labour).

6. Faulty sensors on the wheels. Either faulty sensors, or sensors with dirty contacts. The Bosch hammer may pick up a "missing abs" or similar result. Clean all contacts. If the sensors are at fault then will need to be changed (on early cars this will mean getting a new set (4 off) fitted since the sensors were upgraded over the life of the 964. Expensive (£500ish with labour).

7. Incorrect or faulty relay. Fit new relay (it is marked in the fuse box).

8. Tyres of the wrong radius fitted. This messes that 4wd logic for wheel spin causing the alarm to trigger. Fit correct tyres.

From the correspondence I have seen on this matter, the two most likely causes are: leaking abs pump/switch, and poor earth connections - Alex D