How to measure the battery current draw in a Porsche 964 or 993

How to measure the battery current draw in a Porsche 964 or 993

General

Reliable and safe measurements of the battery current in a 964 or 993 needs a bit of preparation. 
Normal current draw in a Porsche 964 or 993 with ignition off is about 25mA. (0.025A) An aftermarket alarm or other equipment can introduce an additional 5-50mA. Theoretically, a fresh 75Ah battery would keep a 25mA current alive for about 100 days. Taking into account the needed power for cranking the engine, self-discharge and battery age, one could expect the battery to support at least a month of standing still and locked.

Known and common problems

These are the most common causes for excessive current draw at ignition off:

Door switches: Turning off the interior lights masks this fault. The door switches are to connect to ground when the doors are open, and are often corroded. The switches are connected to the lock/alarm control unit, and do not only control the interior lights, but also other functions such as keeping the ability to operate the power windows and sun roof.

The Climate Control Unit: The CCU is supposed to stay on for up to 20 minutes after ignition off. Sometimes the internal power circuit fails to turn off the unit, causing a drain of about 200mA. See here for details on this topic.

Tachometer OBC display: Sometimes the On-Board-Computer in the tachometer (if fitted) does not turn off when the doors are locked. This will cause a drain of about 50-100mA. See here for details on this topic.

Test procedure

Use an amperemeter that can withstand about 10A. (in case the interior fans start at ignition on)

  • Cold engine and CCU A/C off, minimum temp and fan speed 0. (no cooling/interior fans shall run, and possibly damage the amperemeter with over 25A current)
  • Open bonnet and depress lock mechanism to turn off lights. Put a rag over the lock to avoid slamming the bonnet into a closed lock. This applies also to the engine lid. Open a window to avoid problems getting into the car if any problems.
  • Close and lock doors.
  • Disconnect positive battery lead and connect an amperemeter in series. 

The brief battery disconnect shall cause all units in the car to briefly wake up, but go to sleep.

After 5-10 seconds you should measure a current of about 25-50mA.
If not, pull fuses one by one to determine the circuit, and any problematic unit.

Turn on the ignition briefly for one second. Current draw should rise to 0.5-1A. Many units will briefly activate, but go to sleep again and reduce current to about 25mA again.
Please observe that in 964 models, the two interior fans never completely stop at ignition on. CCU fan speed setting 0 is the same as 1. This will cause a higher current draw at ignition on.

If the current does not go down to normal 25-50mA again, check the following:

  • Try to operate the electric windows. If they work with doors closed and ignition off, one or both door switches have failed. 
  • CCU active? The CCU fan on the back runs when it is active, but this can be masked by a worn-out and inoperative CCU fan. 
  • OBC backlight on? Could be difficult to spot if the display is not activated