2nd Gen Tacoma battery draining overnight - solved

I have a 2009 Tacoma TRD Sport 4wd double cab. Not driving it very much at all due to COVID, but do need it for landscaping and dump runs this spring. A few weeks ago, it started losing the battery charge 100% overnight - or even sooner than that. There was a low/dead battery cell, and I got the battery replaced under warranty, but it turned out that was not the issue.

Long story short - there are not may things that can cause parasitic drain that will kill the battery flat overnight. A shorted alternator diode and the heater blower motor fan speed resistor are two. The alternator was working great - once we jumped the truck to start it, the alternator charged the battery very efficiently. Sure enough, the blower fan motor resistor was the culprit. I jumped the truck, drove it around, then reached under the dash to disconnect the resistor (it's in the passenger side footwell above the passengers left foot, just forward and to the left of the blower fan assy), and no problem with battery drain ever since.

i replaced this resistor about 5 or 6 years ago, IIRC, and I think I still have a spare floating around somewhere, so I have not changed it again as I keep hoping I'll come across the one I already have so I don't have to buy another one - though they are not expensive.

Posting this in the hopes it will help someone - this answer was not easy to find when I went looking online for help.

JIM in NH

 

I hope I can remember this if I ever run into this issue. Thanks for posting your solution

 

Not to be contrary, but it isn't possible for the Blower resistor to discharge a battery with the ignition switch in the Off Position. As you can see the relay coil is turned on by the IG1 No2 Fuse which only has power with the ignition switch on. With the relay coil not having power the 50 amp heater fuse is disconnected from the blower circuit and the blower power lead is grounded.

I'm not sure what the issue is/was, but that wasn't the blower resistor. Now if the Heater Relay was stuck on then that would do it if the fan switch was left on.

 

I think JimmyH is correct........diodes in general can and do leak.....