Dave,
Try disconnecting the leads that engage the clutch on your air conditioning compressor clutch. With your climate control in the defrost (or bilevel) postion or rear window defrost on, most manufacturers have the A/C pump cycle to keep the pump excercised and seals conditioned. Being from Michigan, I'll disconnect the wires from my A/C clutch if we are having a real cold spell. These wires should be at the front of the pump right by the belt and pulley. Hope this helps.

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Thanks,
Dave

1998 BMW 540i Sport
1995 E320
1992 400E
1989 BMW M3
1979 Triumph TR7 (1st car)
1986 BMW 735i (Sold)
1980 BMW 320i (Sold)