I bought a1999 Mercury Sable LS with 60,000 miles. The previous owner had replaced the battery 6 months before. Then, 6 months later, the battery went dead again. For a few days before this happened, the wipers would occasionally come on by themselves and the "cruise control" did not work.
The repair shop said the charging system was good and sold me a new battery. Three months later the battery died again. A Ford dealer tested the alternator and said all looked okay, but installed a new battery saver relay, and said they couldn't be sure the problem was solved. The battery went dead again in less than 30 days. The dealer had the car for a week and could not find a problem. They finally installed a "Battery Buddy" cutoff device on the positive battery terminal which requires a remote in order to start the car.
Last weekend I took the car on a two hour trip on the interstate. About 20 minutes into the trip the engine "coughed" once, the RPMs went up to around 2400 to 4000 briefly, and then I lost power. When I had to slow down rapidly to exit the interstate all the "idiot lights" flashed on & off and the car bucked as if the engine was shutting off. Then the lights ceased flashing and I was able to take control of driving. I would much appreciate any suggestions you may have. I just can't trust this car on the interstate again.
The "battery buddy" which the service department installed didn't repair the problem; it simply bypassed the dead battery symptom. There is most likely an excessive amperage draw on the battery while the vehicle is at rest. After all interior lights and electronic devices have powered down, anything above a 50 milliamp draw in current is considered excessive.
The draw may be intermittent and the Ford technician attempted to duplicate the condition for a week without success. The Generic Control Module is the brain behind the interior lights delay and the vehicle's battery saver relay. The Generic Control Module and all its related circuits and components may warrant further inspection.
Adding drivability issues and a light show at the instrument panel to the situation may indicate an intermittent loss of a significant power or ground circuit, or the failure of a crucial electronic component such as the Powertrain Control Module.
It can be a tough job that requires a Ford technician who specializes in electronic diagnosis. If you're not happy with the service department you're working with, take it elsewhere. Ford service does have the benefit of consulting with technical support engineers when hard to diagnose jobs come along.