So I am the second owner of a garage Queen 02 zo6 . I have owned this car about a year. I was driving the other night and I noticed my headlights would dim and then become bright again but only for a quick second. I parked the car and my battery was completely dead. I have been using a ctek charger recently to keep charged but this is a problem that driving me nuts. If I drive the car daily I have no issues, but after a few days unconnected it’s dead.

 

Read Post 4 above for troubleshooting steps. I would suggest, though, before doing anything to remove the battery, fully charge it and then take it to an auto parts store to have it load-tested. Most stores do it for free.

 

Well darn, the 99 vette I bought two weeks ago has a battery drain too. Letting car sit for a few days kills it. I had both seats out last week, repairing the loose track bars. Thought maybe the doors left open killed battery, nope. Time to go down this journey many of you have done, wish me luck, not a mechanic, but armed with a volt meter and some common sense. Will try these excellent suggestions.

 

These days it’s preferable to do a voltage drop check across the fuses instead of pulling fuses which may “wake up” a module and screw up your testing !!...video below and see if your DVOM has a millivolts setting and make sure your battery is charged before testing !!

 

Checked all mini fuses under footwell, and under hood. Next will be to see if headlight control module is warm to touch. Auto Zone checked my battery alternator and charger, says all is well there. WTH could it be. Would I still check diods on alternator?

 

Do you happen to have that list handy?

 

This thread is over a year old, my suggestion is to open a new post to get more replies. If you are having problems with the car not starting after a few days have you had the battery load tested? Next test to perform is a current draw measurement using a DMM configured as an ammeter and wired in series between the negative battery terminal and cable, normal sleep mode current should be 25ma after 20 minutes or so, verfy you actually have excessive current draw.

 

I most likely have the same issue, what wire should I splice the brown into for switched power?

 

You can tap onto fuse 22, large brown wire at the Ignition switch, and there is a three wire connector to the left of the BCM that contains a hot at all times, switched acc and a ground wire. I am not sure what color wire the acc is, but a simple DVM measurement will tell you what that wire is.

 

Orange is hot at all times; yellow is switched.

 

That 3 wire pig tail is an excellent source for accessory power. It's fused at 20 amps. NOTE! 97 to 03 have the pigtail. The 04 DOES NOT!
It also can be un,plugged from the harness to add or crimp on wires.
If it's virgin and never molested, it will have silver duct tape on the wires.

Battery draw: You need to obtain and learn to use a DC AMP METER to understand what is going on with your parasitec battery draw. The C5 has a constant 20 to 25 milliamp current draw. If the sleep/security mode draw current is higher than 25 ma. most batteries will drain quickly. Finding the excessive current draw offenders can sometimes be difficult. NOTE!! The C5 needs a battery that has a higher than normal RESERVE CAPACITY. These days, its difficult if not impossible to find a 75 group battery with the correct C5 designed 120 amp/hr RC rating. Switch to a quality 78 Group battery. The AGM batteries usually have a good RC but are a lot more expensive. The higher your battery RC rating is, the longer it will be able to sit without having a depleted battery. GM NOTE! Your owners manual states: If you need to store the vehicle for an extended period of time, disconnect and remove the battery.

Several ECU/PCM repair techs have noted and stated that leaving a battery on a battery tender for an excessive period of time is causing the electrolitic capacitors inside the ECUs/PCMs to swell and go bad.

 

Nice to hear from you Bill Curlee.

 

Yes, the glove box. When I bought the car new in fall 1999 (2000 Nassau Blue Coupe) I noticed light coming out of the glove box so like the poster below I added a couple
of layers of velcro dots to raise the level to engage the switch. It has worked fine up until the past year. My battery is now dying after a few days of inactivity. I looked at
the previous fix and noticed a slight indentation in the velcro dot fix from all those years ago. So I added a few more layers and that seems to have once again fixed the problem.

 

Absolutely. I have a 2004 Corvette C5 Commemorative Edition. I constantly have this issue. If it sits for 3-5 days or more, the battery is dead. I have to jump it to start, drive it miles to charge the battery again. I've even replaced the battery twice.
I just asked Grok AI about it and got quite a lengthy scenario saying it's not uncommon on C5's - with a number of things to try.

 

If you have no electrical diagnostic experience to find out where your parasitic drain is coming from seek out an automotive electrical shop...the more often your battery goes dead you will need another new one !!

 

You could have just read the sticky at the top of the forum or done a simple search within the forum and gotten an answer.

dan k.

 

I am going to be BLUNT and TO THE POINT! THIS IS A FACT! ALL C5 should be able to remain in the sleep mode for AT LEAST 30 Days (if not longer) and still be able to crank and start the engine. I have had my 02 ZO6 sit in storage for 40 days and the car cranked right up with zero issues. NOTE! This statement assumes you have a good, fully charged battery prior to testing the 30-day sleep mode function. If your battery depletes in hours or days, Fully charge your battery and have it tested. If the battery test good and still discharges in short order, YOU HAVE EXCESSIVE SLEEP CURRENT DRAW! PERIOD!

Your C5 (97-04) is designed to draw very close to 0.025 AMPS - 25 milliamps from the battery all the time when the vehicle is in the SLEEP MODE. There can be a few more or a few less milliamps depending on the car and all still will be normal. The vehicle enters sleep mode between 3-12 minutes.after the ignition is turned off and the doors are shut. The 12-minute time out mark normally occurs if the BCM detects something like ( leaving a door/s open or the hood open with the hood light on. At the 12 min mark the Retained Accessory Power ( R.A.P.) relay will time out, turn off specific interior/exterior lighting loads and put the BCM into sleep mode. Sleep mode provides battery power to keep the BCM in the monitor and security mode, the RFA in the monitor mode and to keep the PCM volatile memory alive.to maintain (fuel trims, last good throttle blade idle position etc..)

Your C5 needs a battery that has a large RESERVE CAPACITY to meet the needs of the cars 25 ma sleep mode needs and the ability to sit in sleep mode for an extended period of time. Finding a battery that has the correct RC and fits in the C5 battery tray can be expensive and sometimes difficult to find. On a car battery, RC means Reserve Capacity, indicating how many minutes a fully charged battery can supply 25 amps at 80°F before its voltage drops below 10.5 volts. A group 78 AGM battery can be found with 100+ RC. The higher RC the better!
NOTE!! You can purchase a $300 high RC battery but if you have a large sleep current draw, it will still go dead. It will just take a little longer. CHECK AND DETERMINE YOUR SLEEP CURRENT DRAW! If you are not TESTING, you are GUESSING!!!

• Example:
  A battery with 150 RC can run for 150 minutes (2.5 hours) at 25 amps before hitting 10.5V. Do the math. A 25 milliamp draw will allow the car to sit storred for a long time.

FINDING THE RIGHT BATTERY WITH HIGH RC:
Determine the foot print measurement of an OEM size battery (L x W X H.) Normally you can find a physically larger foot print battery that can still fit inside the C5 battery tray. It can be a little longer, a little wider and a little taller and still fit. The physically larger battery will normally provide a higher RC.
You can switch to an (AGM) Absorbed Glass Mat battery with that larger foot print. Unless you live in a VERY COLD climate or have extremely high compression, CA & CCA is not as important as finding a battery with a larger RC.
The only modification that I had to do to my C5 battery tray to fit a larger foot print battery was to elongate the slot in the battery hold down foot.

HOW DO I DETERMINE MY SLEEP MODE CURRENT DRAW?

You must have a multimeter that has the ability to measure at least 10 AMPS. The multimeter meter MUST be set up to measure 0 -10 AMPS. The Amp meter leads MUST be connected in SERIES between the negative battery terminal and the negative battery cable!
NOTE! The Initial AMP METER connection will show a high amp reading (approx. 7-8 amps) and quickly drop. Completely normal. After a few minutes, the BCM will start turning off loads (shedding) and between 3 min-12-min the finial sleep current reading should be displayed.
If you use an AMP CLAMP Meter, you just clamp it around the NEG Battery cable. The - & + symbols on the current reading can be ignored and is not important. You can ignore them.

IF, the Sleep Mode Current draw reading is high, you have some electrical load/s inside the vehicle that the BCM failed to turn off OR remans ON once the car is secured. (IMHO, a high sleep mode reading is over 30 milliamps.) COMMON EXAMPLE; IF, you turn the dash dimmer **** fully CW, it will force the mirror lamps to remain ON until the battery is DEAD!!

FINDING THE ROOT CAUSE OF EXCESIVE CURRENT DRAW:

There are several common causes that can cause excessive current draw. I will list a few BUT, please don’t think that this list is the only things in your C5 that can cause excessive current draw:
- Dimmer Switch turned fully CW.
- Boes Stereo Amplifier fails to turn off.
- Added accessory wired to a hot all the time source.
- Rear window defroster fails to turn off.
- R.A.P. relay fails to turn off/time out.
- Head light door motor control module fails to turn off when bucket reaches its fully closed or fully open limit. Feel the module. If it’s warm, its possibly faulty.
- Alternator bridge diode rectifier goes bad and continuously draws current.
- Interior lights fail to turn off after car is locked. Check car at night to look for lights that remain on
- Seat control motors are continuously trying to move the seat
- Seat Lumbar pump motor running
- Water inside a connector or on a circuit board causing corrosion (green crusties) and drawing current.

REMEMBER! CURRENT flow causes HEAT. If something is warm or hot to the touch when the car is in sleep mode, that component is drawing current.



PULL FUSES UNTIL THE CURRENT DRAW REDUCES TO SPEC:
Pull fuses until you see the excessive current draw to fall to spec. NOTE! Do not pull the BCM, PCM or RFA fuses. They are the main contributors to that expected 25ma sleep current reading.

This link explains how to read a millivolt reading across any fuse and calculate accurate current draw readings on that circuit. It’s very accurate and easily identifies circuits under load. There are numerous videos on how to conduct this test. This is just one example:


LAST BUT NOT LEAST! As C5 DIAG Stated, " If you have no electrical diagnostic experience to find out where your parasitic drain is coming from seek out an automotive electrical shop...the more often your battery goes dead you will need another new one !!"

I hope this helps someone.

BC

 

Here is a more in-depth video on finding a parasitic current draw using millivolt readings across circuit fuses. This video goes more in depth in how to use the required charts to extract the actual current draw reading from the millivolt reading across the fuse test points. This guy on You Tube is a very good troubleshooter and mechanic.