Here is a TSB on that problem
DCS NO. 88-01 GROUP NO. 6 October 8, 1987
SUBJECT: 1988 CENTURYS EQUIPPED WITH LG3 3.8L ENGINE (VIN CODE 3) EXPERIENCE RUN DOWN BATTERIES
TO: ALL BUICK DEALERS
ATTN: SERVICE MANAGERS BE SURE TO REVIEW THIS INFORMATION WITH YOUR SERVICE DEPARTMENT PERSONNEL.
We have received comments of 1988 Centurys equipped with the LG3 3.8L engine (VIN Code 3) and V08 H/D cooling option on which the primary engine cooling fan continues to run after the ignition is turned off, which can ultimately cause the vehicle's battery to "run down". This condition occurs when the Climate Control selection is in any position except "OFF" when the ignition is turned off. Investigation indicates that a relay in the engine cooling fan system does not have a needed internal diode. Repairing this condition will involve installing a diode assembly kit into the 250 circuit (brown wire) at the affected relay location. For emergency situations, T.A.C. should have a small quantity of diode assembly kits available.
The condition was corrected at the assembly plants effective with the following VINS:
Oklahoma City J6408477
We plan to publish a Dealer Service Bulletin in the near future.
Wednesday, December 15th, 2010 AT 8:10 PM