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66 Mustang Battery Drain

New User
Posts: 2
Joined: 07/08
Posted: 07/29/08 03:38 PM

Our 66 Coupe (289-2V, auto, pwr steering/brakes, after market security alarm) keeps running down the battery, overnight.  Auto Zone tested it; first-found the battery was bad (bought a new one), second-checked alternator, charging okay, third-found that the voltage regulator was bad (replaced with a new one from NAPA); each time, the next day the battery was dead and car had to be jump started.  Can anyone tell me where to start/what procedures/tests to run (with a multimeter?) to find the electrical drain/short/or?  Thanks.  

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Posts: 440
Joined: 03/08
Posted: 07/29/08 03:54 PM

I am no electrician, but I might help you isolate the problem.  First, you want to make sure that no lights are left on such as the glove box or interior. I am no electrician but you want to split the electric wiring in half to find out if the drain is coming from the engine compartment, or from the dash, or from the rear.  To the left and up from your Master Cylinder is a wiring plug.  Pull that out and let the car sit overnight with a full battery charge.  Plug that back in and if the car starts the next moring, the the short is not in the engine compartment.  If you take the kick panel off the driver's side, you will see the connections for the rear part of the car.  Unplug them and repeat the same thing.  If the car starts the next day, then you now know where the short is.  If it doesn't, then your brake lights have been on all night.  Yeah, mine started sticking a few months ago, and that was my problem.  Hit the brake switch attached to the brake pedal with some WD-40 or get a new switch. If that ain't it, it's under the dash.  Do you enjoy lying on your back with a multi-meter?  I didn't think so.  But, that is what you are going to have to do.  Take the gauge pod out, get yourself a good wiring diagram and start testing.  

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New User
Posts: 12
Joined: 07/08
Posted: 07/29/08 08:14 PM

I think you can disconnect the battery cable and put a test light between the cable and battery post and then follow the isolating procedures that soaring3 mentioned above. That way you won't have to wait until the next day to see if the battery drains.
Unless I'm mistaken (and occasionally I am) the test light won't light up unless there's a draw on the system. If everything is off it shouldn't light up.
Someone please correct me if I'm wrong.  
07 Shelby GT500
68 fastack 427 Tunnel Port
68 coupe
95 SVT Cobra
77 Cobra II drag car

Post Reply
Posts: 1410
Joined: 03/08
Posted: 07/30/08 12:23 PM

Good advice from the other two posters. I'll jump in too.  If the battery goes dead overnight thats a pretty good drain and suggests that it's either a short or a light bulb on all the time, Glove boxes and trunks are good places to look. take a walk around the car when dark to see if you can spot any lights that are on that shouldn't be.  With that completed, the best way to find this is with an ammeter that can read at least 25 amps hooked in series with the battery cables (either one is ok, I prefer to use the neg terminal) Determine if you havea drain greater than .500 milli-amps, (If below .5A, the problem is in the battery and it should be returned for a comprehensive load test).

I would expect to see 10 amps or higher based on your data. Go to the plug that Soaring described and unplug it, if the amperage drops the circuit in question goes into the interior of the car. You'll need a wiring diagram to troubleshoot also. From there it's just a matter of isolation by removing the fuses one by one until the load drops. That's the affected circuit.  

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New User
Posts: 1
Joined: 01/13
Posted: 01/11/13 02:32 PM

i have that exact same problem, but non of my interior lights work so  is there any other way i could test it?  

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Posts: 455
Joined: 04/12
Posted: 01/11/13 10:04 PM

a few tests...

do you own a DIGITAL volt ohm meter...??? almost any Digital volt meter will work if it has a 10 amp DC  or 15 amp DC or 20 amp DC setting...  below is a HF37772.. about 20 bucks if you don't have one..


print this chart..
run this test first to verify the car is properly wired.. takes just a few minutes..


post your results by number of the second half of the test..





now...  lets get to a parasitic load test...

engine off.. hood open.. drivers window down.. doors shut..  

change meter setting to 10, 15 or 20 AMPs DC..

change your test lead connections into the proper holes.. as they are different than the voltage test...  one will be labeled DC AMPS... the other is the same. COMmon

on PRE OBD2 cars. (earlier than 1996..)   you can now.. just disconnect one battery post..  the engine is OFF...

hook one test lead to the battery post... the other to the battery cable end you disconnected from that post and read the numbers....  how many amps and milliamps do you have..

if your car is stock.. without any late model electronics... it will read 0.000  or 0.001..

any thing more than 0.030 DC AMPS is TOO MUCH... thats 3/100ths of an amp...

i have had way too many car alarms drag batteries down...

i have had some car stereos drag batteries down...

Current Draw


pay attention to the first part of the voltage drop test above... if your battery is NOT FULLY Charged .. the minimal draw of the car alarm overnight will discharge your battery.. as its not fully charged.. so it has less reserve to discharge...

post your current draw reading....

practice the tests above on other cars...

please follow the instructions carefully....

NEVER NEVER pull a battery cable off with the engine running..  what happens is the current is flowing from the alternator into the battery.. if you remove the battery.. you loose the load... the electrons can build up before the voltage regulator can turn down the alternators output.. the voltage spike for a fraction of a second can be OVER 100 volts DC...  this can damage the alternator diodes... and can damage other electronics...

let me describe for the 1996 and newer cars...

same procedures above... but  you will need one of these or something similar..


so you can with the battery connected.. start and warm the car up to operating temp... shut it down.. get out... lock the doors as normal...   look at your watch... wait 7 minutes... while you are hooking up your amp meter to either side of this device... so after 7 minutes.. you can loosen the knob disconnecting the main battery.. but the battery is NOT disconnected.. its run through your amp meter ... so you can see the tiny parasitic draw the computerized electronics require in the off position.. but  OBD2 cars have various circuits that can stay active for 6 minutes after shut down and locking the doors...  you don't want to be chasing the wrong draws.... so you have to wait...  

just disconnecting the battery after shut down completely disconnects the voltage into the car..  this shuts down the electronics in a different way than what normally happens when you shut down..   fooling your test into SOMETIMES an IMPROPER reading...  sorry.. newer cars are really complicated..

please post results.. i do drop in a few times a day...


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