Battery slowly running down

[randyveach]

I've had the battery checked at 3 different places. All cells are good. I've got a slow leak somewhere and before I start looking for it, I thought I'd ask if this is a known problem for 1986 asc McLarens.

Thanks in advance.

[5280Capri]

I am not aware of any particular problems with these cars.
First suspects would be aftermarket accessories, alarm, audio, etc. These are known to create parasitic losses.
Otherwise pull a battery cable and put an ammeter from the battery post to the cable you just disconnected. Next start pulling fuses to isolate which circuit is the problem and go from there.

[svo413]

I often have a problem with my knee bumping the interior light switch. Check the hood light too.

[plumkrazy]

Seen a couple of Fords that have Alt. drain's

[negusm]

I am not aware of any particular problems with these cars.
First suspects would be aftermarket accessories, alarm, audio, etc. These are known to create parasitic losses.
Otherwise pull a battery cable and put an ammeter from the battery post to the cable you just disconnected. Next start pulling fuses to isolate which circuit is the problem and go from there.

Exactly right. This will take you a small amount of time and you will zero right into the culprit.

If it is not a parasitic drain, you have a charging problem.

Is the battery draining with the car just being in the garage? What are the symptoms?

-Mike

[crewleader56]

I had a similar problem. I ended up finding a poorly crimped positive battery cable (factory crimp). It only acted up when I had everything on including the fog lights. Alt tested good and previous owner had installed a new battery a week before I bought it. It grabbed the negative cable and pulled it out of the replacement end with little effort so reattached that first. Then I yanked on the pos factory end and pulled the cable right out with less effort. Had about two strands with arc marks that were powering the whole car. Install new positive cable and the starter whirled the engine over like the day it came from the factory.

It may not be your problem but worth looking at.

[heat4copper]

I also had a similar problem, like Crewleader56 noted. In my case, I ended up replacing a battery https://www.carid.com/batteries/, as something was wrong with the terminal. I must say, I was so irritated with constant electrical problems, that was even glad that new battery solved it all.

[negusm]

But those are not parasitic drains.

What you would see is that the battery is fully charged but yet, won't crank the starter motor.

*BUT*, it is EASY to misdiagnose a bad cable as a parasitic drain so you have to do the ammeter check to be sure.

*IF* the ammeter (while the car is off) shows NO drain (or only a milliamp or two for something like a clock) then we are not dealing the a parasitic drain and instead we're talking bad battery cables.

Years ago I had the issue where the car would start all week...except once and a while I would get "click click click"....like a dead battery.

I freshened up the terminals on the battery, and it would start. Next time, that didn't work so I had the battery tested...it was fine. But a week later had the same issue. Tested the alternator...it was fine. Probably tried a new starter Solenoid. It would fail to start more often and I somehow figured out if I turned on the headlights for a minute...then the car would fire up.

Finally my dad, took a look and he grabbed the positive battery cable and bent it...and all you heard was crackling inside like dry leaves...internally the cable had corroded so bad it was down to a couple of good strands. Outside, the cable looked perfect.

Later I found the starter battery cable doing the same thing.

Yeah, if you still have original cables, SUSPECT them. Pos/Neg battery AND the starter cable.

BTW turning your headlights on, will temporarily REVERSE corrosion in the electrical system. It was enough to allow me to get enough juice through to get the car started.

-Mike

[heat4copper]

Wow, I haven't checked the cables then and simply threw the battery away. Honestly, I would never think, that if the cable looks fine, it may be bad inside... Bad for me, looks like the battery was not that bad then. Anyways, thanks, next time will watch it more carefully.

[negusm]

Yeah, and that's the insidious thing about it. You pull the cable off, wiggle it a bit, clean it, put it on and things work for a bit. So it's hard to tell what is messed up where.

-Mike

[svo413]

I had a Motorcraft battery with a loose connection inside. Put jumper cables on it, was ok, took the off and it was bad.

Take the bulb out on the hood light too. No trunk light on our cars.