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I am having a problem I have not been able to figure out yet. But I am continuing to try. Maybe you have had this problem??
I have a 89' L6 F-150 Lariat , the problem is my truck battery dies in 2 days if I do not drive it. The battery is brand new. It has been doing this even with the old battery, I noticed that when I turn the ignition key switch to go to start the truck.. That I do not hear the fuel pump go on.. That is because I believe it is already on... I just recently replaced the alternator w/ voltage regulator... This is because my truck charging capacity died so I replaced the alternator...
I had a similar problem and it turned out that the hood light was not shutting off. Check yours at night with the hood shut. My truck would be ok during the work week, but if it sat unused over the weekend the battery was drained dead.
If the fuel pump is staying on, you can hear it running by listening under the truck. Check the fuel pump relay, you might unplug it and see if battery stays chgd overnight.
You can speed up the whole testing process by disconnecting your negative battery cable and using a test light. Disconnect the negative battery cable and set it away from the battery post and put one end of the test light one the bare battery post and the other end on diconnected cable, if there is a current draw the test light will light. The hood light will be more than enough draw to light the lght so you will have to pull the hood down enough for it to go off or just remove the bulb. Good luck finding the problem.