Electrical 95 Mustang GT battery discharges

[Bill hot cam 96]

I have a 95 Mustang GT and the battery is going dead overnight.I traced the draw down to the HEGO fuse.I pulled the fuse and my draw went down to.17 millamps it was at .66. What is hego? Why does it draw with the key in off position?

 

[jozsefsz]

That's your O2 sensor circuit. It shouldn't draw with the key in the off position.

http://www.veryuseful.com/mustang/tech/engine/images/94-95_5.0_EEC_Wiring_Diagram.gif

According to this diagram, it looks like it's fed fairly directly from the ignition switch (through a fusible link) - it doesn't go through the CCRM or other relay. So either the ignition switch has a problem, and it's keeping the circuit energized, or something else is back-feeding voltage into the circuit when the ignition is switched off. And the O2 sensors are taking advantage of the available voltage. In this case, disconnecting them via pulling the fuse is treating the symptom and not the problem.

Most likely someone has hacked the wiring harness and caused a problem. If you have an aftermarket alarm, remote keyless, remote start, or radio, suspect any or all of them.

 

[Bill hot cam 96]

That's your O2 sensor circuit. It shouldn't draw with the key in the off position.

http://www.veryuseful.com/mustang/tech/engine/images/94-95_5.0_EEC_Wiring_Diagram.gif

According to this diagram, it looks like it's fed fairly directly from the ignition switch (through a fusible link) - it doesn't go through the CCRM or other relay. So either the ignition switch has a problem, and it's keeping the circuit energized, or something else is back-feeding voltage into the circuit when the ignition is switched off. And the O2 sensors are taking advantage of the available voltage. In this case, disconnecting them via pulling the fuse is treating the symptom and not the problem.

Most likely someone has hacked the wiring harness and caused a problem. If you have an aftermarket alarm, remote keyless, remote start, or radio, suspect any or all of them.

As I can tell far as I can tell you why not enough but I did put a radio in it and it worked fine until till last week been in the radio has been in there for months

 

[jozsefsz]

Well, 0.66A (if I read what you wrote correctly) draw would progressively weaken your battery to the point where it dies overnight now, whereas before it could have had enough juice to start up in the morning and you just didn't notice the symptoms.

Electronics (your radio) can also short and malfunction at some point in time - I've had that happen where a previously functional radio started completely killing the battery within days. As you no doubt noticed when wiring the radio, it takes both an always-on (battery) voltage source and a switched (ignition) voltage source. If those two cross, due to an internal short in the radio or mis-wiring, then the rest of the harness beyond the ignition switch will now have voltage even when the switch is off - the HEGO circuit included, which is happy to apply the voltage to warming up the O2 sensors while slowly draining your battery.

I'd disconnect the radio wiring harness (if that's the only aftermarket thing in there) and see if you're still getting voltage at the HEGO fuse when the key is off - you shouldn't. If it doesn't make a difference, something else is backfeeding into that circuit, or you have a problem with the ignition switch.

 

[Bill hot cam 96]

Well, 0.66A (if I read what you wrote correctly) draw would progressively weaken your battery to the point where it dies overnight now, whereas before it could have had enough juice to start up in the morning and you just didn't notice the symptoms.

Electronics (your radio) can also short and malfunction at some point in time - I've had that happen where a previously functional radio started completely killing the battery within days. As you no doubt noticed when wiring the radio, it takes both an always-on (battery) voltage source and a switched (ignition) voltage source. If those two cross, due to an internal short in the radio or mis-wiring, then the rest of the harness beyond the ignition switch will now have voltage even when the switch is off - the HEGO circuit included, which is happy to apply the voltage to warming up the O2 sensors while slowly draining your battery.

I'd disconnect the radio wiring harness (if that's the only aftermarket thing in there) and see if you're still getting voltage at the HEGO fuse when the key is off - you shouldn't. If it doesn't make a difference, something else is backfeeding into that circuit, or you have a problem with the ignition switch.

Thanks it is the factory amp feeding back

 

[jozsefsz]

Nice find! Make sure the wire that triggers the amp is the (typically blue) 'power antenna' output from the radio. That should shut off when the radio's off. If it's hooked to battery voltage, it'll never turn off.