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davehop, Ford Technician
Category: Ford
Satisfied Customers: 1312
Experience:  Fully certified in all phases of automotive repair as recognized and certified by Ford Motor Company
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Ford Mustang I have a 99 Mustang. The battery keeps draining

Customer Question

I have a 99 Mustang. The battery keeps draining down (sometimes). I used a Fluke DMM and measured the amp draw with the car switch turned off and all lights (interior lights, trunk light, etc...turned off. Is 135 milliamps too much drain on a car battery with everything turned off? I have isolated the amp draw to fuse 29 (GEM, 5 amp fuse). I read the TSB's and disconnected the AC Compressor and there was no change in amp draw so I don't think the AC clutch sticking is the problem.
Submitted: 6 years ago.
Category: Ford
Expert:  davehop replied 6 years ago.

davehop :

Hello and welcome to JA, the factory spec for battery draw is .050 mA or 50 miliamps, the first question I have is there any aftermarket accessories installed? Radio, guages, alarm, remote start?

Customer :

There is an aftermarket radio, that is the only aftermarket add-on.

Customer :

The radio was installed before we purchased the car 2 years ago.

Customer :

This problem started about a 6 weeks ago.

davehop and 2 other Ford Specialists are ready to help you
Expert:  davehop replied 6 years ago.
I would disconnect the radio completely and then recheck the draw, remember, after connecting the meter to wait 45 minutes for all the modules to go to sleep to obtain an accurate reading
Customer: replied 6 years ago.

Sorry for the delay...this problem apparently comes and goes. I have tried to see excessive amp draw on the battery and apparently it is not occurring now. I took the car to Auto Zone yesterday and had the electrical system tested. They confirmed the battery and alternator are functioning properly. I explained what has been happening, and mentioned that I had heard that possibly the voltage regulator could be sticking and draining down the battery, and he confirmed that was a possibilty. I put a meter on the battery last night after running the car for a few minutes and saw the battery voltage drop from 12.72 volts to about 12.62 volts after monitoring for about an hour and a half. It stabilized and there was no further drain on the battery. The car started with no problem this morning. I plan to continue driving and testing the car for a couple more days and see if I can get the problem to re-occur. Do you have any suggestions on what I could do to accelerate the possible re-occurrance of this problem? How could I test and confirm that the voltage regulator is not occasionally sticking? How can I confirm that the AC clutch is not occasionally stuck on? The aftermarket radio is definitely not the issue...

Expert:  davehop replied 6 years ago.
Having worked on these cars for over 22 years, anytime I have an issue with a battery draw, the first thing to do is to disconnect any aftermarket devices, even if they may not be causing the problem, just to get the car back to factory so our diagnostic procedures and processes can be utilized without being hampered ar need to be altered to work around an aftermarket installation. Also battery drain must be tested in miliamps, not by monitoring the voltage of the battery, and as such the correct amount of draw cannot be seen in volts but in miliamps, So would you like to figure this out properly?
Customer: replied 6 years ago.

Merry Christmas and Happy New Year! I would like to figure it out properly, but I have been driving the vehicle for several days now with no further problem. The problem may come back at a later date, but for now it seems to be OK. In the event it does return I will reach back out to you. Thanks for your help.

Expert:  davehop replied 6 years ago.
same to you, happy holidays