Electrical problem, battery drain

[nsx4fun]

OK, I don't know if this belongs in electronics or not, I guess that depends upon your definition for electrical vs. electronics.

anyway, a few weeks ago, I drove to work and when I went to leave, my battery was completely dead. I got a jump, got it started, drove it home.

Parked it in the garage, re-started it to see if the battery had re-charged, it started just fine. I then began monitoring the battery voltage from the fuse block in the engine compartment and it was dropping quickly enough over the next 2 hours, I knew by AM, it would be dead. So I drove a different car the next day on a business trip. I later pulled the battery out of the car, charged it up and let it sit in the garage for 3 weeks. It seemed to hold a charge, small drop in voltage over time, but not much. I put it back in the car today, it started right up, but I noticed what I thought was a significant spark when I connected the negative battery cable. I drove it to a gas station, filled her up, drove home and checked the voltage, it was back up to 12.3

I've been checking it periodically and it is dropping fast enough that I can tell it is being discharged. when I disconnected the negative cable to stop the discharge, it didn't spark as bad as before, but that could be the big spark was charging up all the discharged capacitors.

Anyone have their car do this to them? Any knowledge what item on the car is most likely to fail in a manner that then becomes a drain on the battery while the car is turned off? I'm hoping for some ideas before I just start pulling fuses to see if I can find the problem area that way.

I did disconnect the 2 things I recently did to my car, to eliminate them as the culprit.

 

[rbirling]

You can measure the current being drawn while the car is off. Pull the negative battery cable and insert an ammeter between the cable and battery terminal. Then if it's excessive, narrow it down by pulling fuses. Or it could be a weak battery.

 

[Briank]

The normal drain in an NSX is 40 to 65 mA which will run down most batteries in 2 - 4 weeks. It sounds like you have a larger drain then that so an amp meter connected between the negative terminal and the negative cable to see what the draw is and if its high then start pulling fuses till it drops. You can also just connect a test light between the terminal and the cable if it lights the draw is large and then you can pull fuses till it goes out, I would start with the clock fuse.

 

[westernb4]

Apparently a rubber bushing at the brake pedal can wear out, causing the brake lights to stay on continuously.
This is however probably not the problem since you're likely to have noticed the lights by now.

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Apparently a rubber bushing at the brake pedal can wear out, causing the brake lights to stay on continuously.
This is however probably not the problem since you're likely to have noticed the lights by now.

 

[nsx4fun]

Yes. I've checked that no lights were on.

I know how to do the troubleshooting, but thank you BrianK for the normal drain current. That will definititely help in knowing when I've found the problem.
I guess I need to check my Fluke and make sure it has a good 10A fuse in it's ammeter circuit. As quick as the battery is discharging, I know it's a substantial current draw.

I posted hoping there was a well known problem that could be my first place to check. I did find a similar post on here where Paul did mention their stereo head unit went bad and replacing their head unit fixed the problem. Seeing as how I recently had my head unit repaired, I think I might start with that fuse first.

- - - Updated - - -

OK, I just took my first measurement. 7.2 Amps

- - - Updated - - -

OK, checking my Service Manual, I quickly discover much to my horror, there are 3 fuse boxes, not just 2.
But wait, there is one wire feeding the underhood fuse box. So I disconnect that wire.
Current draw now, 0.01 amps. Well, at least I don't have to deal with the other 2 fuse boxes now, especially the under-dash one.

- - - Updated - - -

Apparently I have been a victim of inconsistency in my ammeter connection.
Further testing, I still have a current drain problem when the 120 amp fuse is pulled.
I'm pretty sure this points to the starter motor being the problem. Has anyone heard of a starter motor going bad such that it drains the battery but still works when needed to start the car if the battery has a charge?

 

[Briank]

Bad starters are rare, pinched wire ?

 

[ralph cerbone]

7 amps is alot ,should not be hard to find , first disconnect alt at main terminal ,they can short internally ,if not reconnect , then check any motor for heat . look at things that are powered up with key off , I have seen door lock motors and power seat motors cause real headaches .

 

[comtec]

Apparently I have been a victim of inconsistency in my ammeter connection.
Further testing, I still have a current drain problem when the 120 amp fuse is pulled.
I'm pretty sure this points to the starter motor being the problem. Has anyone heard of a starter motor going bad such that it drains the battery but still works when needed to start the car if the battery has a charge?

The 120amp AGC fuse in the engine compartment fuse box does not power the underhood fuse box. The +12v battery connection feeds both fuse boxes.

Mike

 

[ftuhy]

7.2 A would not be the starter motor itself; that would be greater than 100 A. I would need to look at the wiring diagram to see if the starter solenoid could do that - but my WAG is the starter circuit is not your problem.
7.2 A (86 W) is way too much for a trunk light being stuck on. I'd pull fuses on the stereo & climate control. BrianK might know if blown CC capacitors or other CC component could result in the unit drawing current all the time, even when the display turns off.

Another random idea - could it be a defective ignition switch leaving something turned on, or a stuck contact (instead of the usual PCB open) on the master relay?

Non-electrical trouble shooting idea: 86W is enough to make something quite warm. Turn the car off, disconnect the battery, let everything cool down, maybe even overnight. Then, re-connect the battery, wait 10 or 15 minutes, then carefully feel around for something warm. Unfortunately, it takes a bit of work to be able to do this feel test on things in the center console. Be sure to check all computer/control modules.