Dead battery after replacing cig lighter fuse

MSTCBRA · May 5, 2023

Ok here goes. Got this 96 Cobra in November from a car lot where it was sitting for 6 months or so. Battery was dead. They put in a new Motorcraft when I bought it. Over the winter I found the previous owner online. Looking to learn of its history I asked him what work he did to it. He said he lowered it, rerouted the plug wires to factory location, and replaced the alternator.

Fast forward to 2 weeks ago I decided to buy one of those Bluetooth units that you plug into the 12V port or cigarette lighter. Plugged it in the power port and nothing. Tracked down the 30A fuse under the hood was blown. Replaced it and the power port worked.

Today I tried to start the car for the first time since that day 2 weeks ago and it was dead. I charged it for a few minutes and it started. Pulled it outside and washed it. It started right back up and I backed it in the garage to dry it off. I put the charger on it while I was drying it. I had the charger on regular strength and it was showing 25% charged. If I moved it to trickle charge, it would show 100% charge.

Got it dried off and went to pick up my daughter from school. It’s 5 miles away. Everything was fine until the last mile where the volt gauge was fluctuating and started to drop pretty hard. I pulled it in and did not shut it off. All of the sudden the gauge dropped and the rest of the gauges dropped and the engine quit.

I asked another parent to jump me but as soon as we disconnected the cables it died.

My son had to come pick us up and I needed to get the Mystic Cobra home, so I picked up a battery from O’Reillys and drove back to put it in. At this time I pulled that 30A cig lighter fuse under the hood.

As I drove home I hit several stop lights. The volt gauge was at the R to M range (of NORMAL) for the first half of the trip. Then it started to drop again and fluctuate. I panicked and took some back streets to avoid stoplights. I got home just fine and it idled in the garage for 5 minutes until I shut it off.

I haven’t done anything but check voltage on the Motorcraft battery that I just replaced. It shows 11.25V. Now I’m imbibing on some whiskey.

If anyone has read to the end of this and is kind enough to respond with some suggestions, I’d appreciate it!

white95 · May 5, 2023

Check the bulb in the cluster for the alternator/battery light. If it blows, it won’t let the alternator charge. Although the resistor should permit it to do so.

You mentioned he replaced the alternator. Check the connector and make sure it isn’t somehow slightly melted not making good contact. You could always take the alternator in to be be tested. It’s not uncommon for brand new parts to be faulty.

ttocs · May 5, 2023

def get the alt checked. After that there are some other little things we can check

cobrajeff96 · May 6, 2023

Make sure the wires in the alternator plug are still connected too. They are real thin gauge wires and if that portion of the harness is man-handled too much they can easily sever or short or do anything we don't want them to do. Make sure they don't have cross-continuity to one another with a simple ohm/diode check (with a multimeter). Make sure the engine still has good ground to the chassis. Loose/rusted/non-existent grounds will cause all kinds of electrical and electronic issues.

MSTCBRA · May 6, 2023

MSTCBRA said:
Ok here goes. Got this 96 Cobra in November from a car lot where it was sitting for 6 months or so. Battery was dead. They put in a new Motorcraft when I bought it. Over the winter I found the previous owner online. Looking to learn of its history I asked him what work he did to it. He said he lowered it, rerouted the plug wires to factory location, and replaced the alternator.

Fast forward to 2 weeks ago I decided to buy one of those Bluetooth units that you plug into the 12V port or cigarette lighter. Plugged it in the power port and nothing. Tracked down the 30A fuse under the hood was blown. Replaced it and the power port worked.

Today I tried to start the car for the first time since that day 2 weeks ago and it was dead. I charged it for a few minutes and it started. Pulled it outside and washed it. It started right back up and I backed it in the garage to dry it off. I put the charger on it while I was drying it. I had the charger on regular strength and it was showing 25% charged. If I moved it to trickle charge, it would show 100% charge.

Got it dried off and went to pick up my daughter from school. It’s 5 miles away. Everything was fine until the last mile where the volt gauge was fluctuating and started to drop pretty hard. I pulled it in and did not shut it off. All of the sudden the gauge dropped and the rest of the gauges dropped and the engine quit.

I asked another parent to jump me but as soon as we disconnected the cables it died.

My son had to come pick us up and I needed to get the Mystic Cobra home, so I picked up a battery from O’Reillys and drove back to put it in. At this time I pulled that 30A cig lighter fuse under the hood.

As I drove home I hit several stop lights. The volt gauge was at the R to M range (of NORMAL) for the first half of the trip. Then it started to drop again and fluctuate. I panicked and took some back streets to avoid stoplights. I got home just fine and it idled in the garage for 5 minutes until I shut it off.

I haven’t done anything but check voltage on the Motorcraft battery that I just replaced. It shows 11.25V. Now I’m imbibing on some whiskey.

If anyone has read to the end of this and is kind enough to respond with some suggestions, I’d appreciate it!

I’ll add one more thing…I’ve driven it for 600 miles with zero issues until I replaced this blown fuse under the hood.

ttocs · May 6, 2023

ah it sounded like you just made it back from the autoparts store. I was thinking that 30A sounded really high for just a cig lighter and in the schematic above it shows a 20A. How long after you replaced the fuse did it take to kill the battery?

MSTCBRA · May 6, 2023

ttocs said:
ah it sounded like you just made it back from the autoparts store. I was thinking that 30A sounded really high for just a cig lighter and in the schematic above it shows a 20A. How long after you replaced the fuse did it take to kill the battery?

Yeah me too. It had a blown 30A in so that’s what I replaced it with. I did that 2 weeks ago and just tried to start it yesterday for the first time and battery was dead.

To note: the 30A did not blow again but I pulled it out. I’m going to check some battery and charging voltages today.

ttocs · May 6, 2023

one normal slow battery draw is the mach amps that were in all cobras. Even if all the amps in the back were pulled there is one under the dash that is normally left in place and connected. They normally take a few days to kill a battery but I would not be surprised if one could do it faster. I is located below the radio. On my 95 I can pull the carpet back from the console on the drivers side under the radio and see the amp. It is an octagonally shapped metal thing with a bunch of holes in it. If you can disconnect the harnesses going to it. There is a chance you could lose the sound from the radio, but we can fix that with out too much trouble if the battery stops dying.

MSTCBRA · May 6, 2023

ttocs said:
one normal slow battery draw is the mach amps that were in all cobras. Even if all the amps in the back were pulled there is one under the dash that is normally left in place and connected. They normally take a few days to kill a battery but I would not be surprised if one could do it faster. I is located below the radio. On my 95 I can pull the carpet back from the console on the drivers side under the radio and see the amp. It is an octagonally shapped metal thing with a bunch of holes in it. If you can disconnect the harnesses going to it. There is a chance you could lose the sound from the radio, but we can fix that with out too much trouble if the battery stops dying.

Here are my tests so far. I’m a little rusty with this but it seems the Bluetooth dongle that I left plugged into the power port for 2 weeks killed the battery. Let me know your thoughts. See anything else?

Battery on standby 12.5v

Idling with no accessories 14v

Every acc on 13.35-13.65v

At 2k rpm no big change

Amp draw

20 amp Fuse in - 60a

Fuse in and Bluetooth dongle in power port - 115a

ttocs · May 6, 2023

oh hell yes people do not realize how much power bluetooth uses. I turn it off on my phone when I am not using it because it will last much longer with it off. It is always on, broadcasting waiting for another bluetooth item to come close. When it does, they do a quick electronic "hello/fist bump" so that if you want to connect to it automatically, it will.

MSTCBRA · May 6, 2023

ttocs said:
oh hell yes people do not realize how much power bluetooth uses. I turn it off on my phone when I am not using it because it will last much longer with it off. It is always on, broadcasting waiting for another bluetooth item to come close. When it does, they do a quick electronic "hello/fist bump" so that if you want to connect to it automatically, it will.

Yep. Everything else look good with my numbers?

MSTCBRA · May 6, 2023

MSTCBRA said:
Yep. Everything else look good with my numbers?

That dongle has a power button too and I turned it off. Guess it doesn’t matter.

cobrajeff96 · May 6, 2023

The voltages are correct, so the car as-is looks good. But if the car was blowing fuses on that circuit, it either means the circuit was overdrawn or was shorted to ground. That it doesn't blow immediately when you plug a new fuse in means you can forget about a short to ground (unless it's a chafed wire that only occaisionally comes into contact with bare metal that is part of the overall car's ground and dynamic forces like accel or braking is what brings them together to pop the fuse). So more than likely something was over-drawing it and maybe the previous owner installed a higher amp rating fuse (a big no-no) as a band aid. You generally don't want to up the ante on a circuit's fuse because the gauge of the wire then possibly takes on more current than it can handle, making the wire take on the duty of a fusable link which heats it. If it takes on enough current flow over its threshold, it'll heat so much that it melts the insulation and now you've got a potentially life-altering problem if that actual fuse doesn't first do its job.