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Topic: Parasitic Battery Drain (Read 1574 times) previous topic - next topic

Parasitic Battery Drain

Hi Everyone,

88 5.0 V8 Thunderbird

I have a parasitic battery drain that I am having issues with. I tested the battery drain with a Multimeter set to 10A and its drawing a 3.7 from somewhere. I tried unplugging each fuse one by one to try to isolate the issue. No change, so thinking that I possibly missed one, I decided to remove ALL of the fuses and the relays and flasher. Still the same reading I get.

The car was working fine a few days ago, no issues, no changes in parts. It just started draining overnight. I charged the battery to Full several times and leave the neg cable off now when I park. WTF :flip: ? I am asking here before I consider taking it in to a garage.

Any ideas on what to try next? Could it be the Alternator? Ignition Switch? Could it be the battery even if it less than five years old and only summer used?

Parasitic Battery Drain

Reply #1 –
If you can see the voltage drop with the battery Un hooked, it is junk.

There is a fuse able link harness that connects to the starter solinoid. With the battery unhooked, check each fuse able link individually and check resistance to ground. The lower the resistance, the higher the drain and should tell you at least which group of wires to start checking. The fuse able links also power several always hot wires like the brake lights ect.
Quote from: jcassity
I honestly dont think you could exceed the cost of a new car buy installing new *stock* parts everywhere in your coug our tbird. Its just plain impossible. You could revamp the entire drivetrain/engine/suspenstion and still come out ahead.
Hooligans! 
1988 Crown Vic wagon. 120K California car. Wifes grocery getter. (junked)
1987 Ford Thunderbird LX. 5.0. s.o., sn-95 t-5 and an f-150 clutch. Driven daily and going strong.
1986 cougar.
lilsammywasapunkrocker@yahoo.com

Parasitic Battery Drain

Reply #2 –
My first check for any parasitic battery drain on an older vehicle is to unhook the wires from the alternator and see if that is it.

A five year old battery is by no means new. The fact that it is summer use only is possibly even harder on it unless you take it out of the car and keep it from the freezing, thawing cycles.

Parasitic Battery Drain

Reply #3 –
FWIW:  I experienced the same thing with my '86 Cougar a few years ago and it turned out to the inertia switch for the fuel pump.  Would have NEVER figured it out but parked the car one evening and everything was VERY quite and I heard a slight 'hum' from the rear right quarter panel and was able to figure out what it was from that (thanks to the nice folks on this forum that were able to tell me what it was from my description).  Hope you find a solution soon!
1986 Mercury Cougar XR-7 Turbo-charged 5-speed.
Owned 22 years.

Parasitic Battery Drain

Reply #4 –
I prefer to use a test light between the disconnected negative battery cable and the negative battery post.Then isolate things until the light goes out.
If you accidently ground the positive circuit when disconnecting stuff it may damage or blow a fuse in your meter. With the light, it will just get brighter.

Anyhoo, check simple stuff first. Is there a light on somewhere? Underhood, trunk, glove box. Check the car in the dark.
Then remove the wires from the positive battery cable terminal  of the starter relay.
If the light is out, reconnect the positive battery cable. If the light is still out, touch the other wires to the terminal one at a time to see which one has the problem.

Here is a diagram to help get you started. Let us know what you find.

Parasitic Battery Drain

Reply #5 –
Thanks very much for your help, appreciated all around

It was the alternator and I also switched out the battery for good measure. Alternator has never been replaced before
The battery was over 6 years old and was always removed over winter and stored indoors.

The never ending money pit but I love driving this mofookieX