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I have a 1992 F150, 300 6cyl, that I've had forever. Always a good truck, but for the past 6 months or so, if it sets for a couple weeks, it will drain the battery. The truck isn't driven much, so it's dead often. The old battery was weak when tested, so I replace it, and it still kills the new battery in a few weeks. I tested for parasitic drain at the negative battery cable, and found the drain to be 110 ma, so I started pulling fuses. I found that the problem is the fuse for the O2 sensor heater, b/u lights, and camper battery power, which I'm not sure it has. I unplugged the O2 sensor and the drain is gone. I was under the assumption that the O2 sensor heater was operated by the EEC, and only with the key on. Is this correct? This makes me wonder about the relay that powers the EEC sticking. Has anyone had this issue, or have any suggestions?
Fuse E should be hot only when the key is in the Run position. If it is hot with the key in the Off position and/or removed you have an ignition switch problem. May need adjusted or replaced.
The O2 sensor heater circuit is not controlled by the computer, it is supposed to get power only when the key is in the Run position.
The heater circuit is powered on the Pink/Orange (P/O) wire and the ground is the Black (BK) wire.
I appreciate the information. I can see how this could be. If it is the switch causing the issues, then I would assume the back up lights would also have a small amount of current to them with the key off also, since they are on the same fuse as the O2 sensor heater. I haven't had a chance to check it yet, but I will be shortly, and I'll leave a post of the results. THANKS!
I checked the backup lights, and I do have a small amount of voltage with the key off. I’ll be changing the ignition switch in the next few days. Thanks for the help!
I replaced the ignition switch, and it changed nothing. It still has a 110 ma draw at the negative terminal to negative cable. As I unplug the O2 sensor it drops significantly. Or.. if it put the truck in reverse with the key off, it will also give a 110 ma draw when the O2 sensor is unplugged. Both off the same fuse. Any ideas??
check your reverse light circuit wiring for chafing and or the switch that activates the reverse lights? check the socket the lights go in with the meter and see if you can detect a draw there.
I replaced the ignition switch, and it changed nothing. It still has a 110 ma draw at the negative terminal to negative cable. As I unplug the O2 sensor it drops significantly. Or.. if it put the truck in reverse with the key off, it will also give a 110 ma draw when the O2 sensor is unplugged. Both off the same fuse. Any ideas??
This goes back to the whole concept that circuit is not energized unless the key is in the Run position. Before you replaced the ignition switch did you verufy there is no voltage output with the key in the Off position or completely removed???
The power to Fuse E comes from the ignition switch on a Gray/Yellow wire. With the key Off or removed there should be zero volts on that wire. Since you are still getting current draw, with your meter swt to amps and inline with the negative battery terminal and cable, that implies there is still voltage on that circuit when there is not supposed to be.
Just to keep both of us from going crazy set your meter to DC volts, place the black meter lead on the battery negative terminal (or a known good ground) and the red meter lead on either side of Fuse E. There should be zero volts on that circuit. If there is real voltage, which would trigger the current you are measuring, something is keeping the ignition switch from opening completely in the Off position. The nylon actuator comes to mind as a possible root cause.
With the switch off AND the key removed, there is a trace voltage (approximately .5 volts) present on the switch side of the fuse, with the fuse REMOVED. This tells me that it isn't a wiring issue backfeeding to the fuse. I replaced the switch and no change. I did not check with the switch removed, but the switch is in the off position out of the box. I've even turned the key back and forth a little, while looking for a change in current draw. The tumbler assembly has some play in it, but that doesn't seem to have any effect on the current. I will remove the harness plug from the switch and check it again in the next couple days. I'm usually pretty good on wiring issues, but this has me going crazy now.
IIRC the O2 wiring is in the same loom as the battery cables. Any corrosion or acid that has migrated down the cable can backfeed voltage through the O2 wiring. I would put the O2 wiring in its own loom and clean it with baking soda/ water to neutralize any acid.
If that were the case, there would be voltage showing on the load side of the fuse socket, instead of the line side, with the fuse removed. I'll double check today.
THANKS!
