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Repairing Electrical Problems by Draining Storage Voltage

Short Circuit: A Guide to Repairing Unusual Problems by Draining the Stored Voltage From Capacitors

James Berry Apr 23, 2018
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I have received numerous questions over the last few years about things that I think fall into a communications issue between modules, usually caused by a low voltage situation or after component replacement, or sometimes it’s just an unusual occurrence.

These are just a few samplings of the questions that I have received that were cured by the procedure we will cover below.

Question: I own a 1999 C5 Corvette with a six-speed transmission. I noticed that the engine had been turning over slowly for the last few weeks so I replaced the battery and now the Welcome Corvette display will not illuminate and the engine will spin over but not start. I am suspecting PassKey issue disabling fuel pressure, but that does not explain no display on the dash.

Question: I own a 1998 Corvette. The gauges and the Drivers Information Center (DIC) are not working after a jump start.

Question: The temperature would not adjust so I replaced the climate control head on my 2000 Corvette and now nothing will work on the new climate controller.

After working with the owners of these vehicles, a simple procedure resolved their problems.

The first clue that you may have a communication issue on a Corvette is that the DIC is not working. Remember, computers hate low voltage and will sometimes go into a logic lockup condition and cause confusion on the data bus. This low voltage or lack of communication can also cause a U code to set, which can be checked with a scanner.

When I think there is a communication issue on a C5 Corvette the first thing I do is drain the capacitors. The safest way to perform this is to remove both battery cables from the battery, isolate the cables and leave them disconnected overnight.

I have also been asked several times about touching the battery cables together to drain the capacitors. I have not had any problems with this procedure when it is performed correctly. Here’s that procedure:

1) Remove the key from the ignition.
2) Disconnect both battery cables from the battery, removing the negative cable first.
3) Isolate the battery from the battery cables. If it makes you more comfortable, remove the battery from the vehicle.
4) Connect the positive and negative battery cables together for approximately 5 minutes. This will drain all of the capacitors in the computers and forces them to cold reboot.
5) Reconnect the battery cables to the battery, installing the positive cable first.

Cold rebooting should not affect the programming of any of the modules because it is very rare for this software to become corrupt. It is possible that you may have a computer or module that was having an underlying problem prior to performing this procedure. It is also possible to have a failed computer or module after a low voltage situation, such as a weak battery or an alternator that is not charging correctly.

Whenever you touch the battery cables together and drain the capacitors the clock will lose its memory, the radio stations will need to be reset, all fault codes will be cleared, the outside temperature may take some time to relearn, any systems with security codes will need to be reset and the computer will need to relearn the adaptive values.

Because the computer has to relearn the adaptive values, the engine may take some time to run normal again. You may experience low idle, or possibly stalling, until the system values are relearned. Most vehicles will go through the relearn procedure automatically. For the procedure to work you will need to keep the engine running until it idles on its own then drive the vehicle for approximately 10-20 minutes.

On rare occasions I have needed to use a scanner for the computer to relearn specific values like throttle body position, so it’s possible you may need a good scan tool for the relearn procedure.

If you are uncomfortable with touching the disconnected battery cables together you can always leave them isolated overnight and it should accomplish the same thing as touching the battery cables together, it just takes longer to drain the voltage from the capacitors. I know this sounds scary to a lot of you out there, so I would recommend if you are uncomfortable with this procedure to take your vehicle to a mechanic you trust. ( Note: DO NOT USE THIS METHOD THINKING IT WILL MAKE THE AIRBAG SYSTEM SAFE TO WORK ON. To avoid personal injury or death, on vehicles equipped with airbags you must disable the supplemental restraint system before attempting any steering wheel, steering column, airbag, or instrument panel component diagnosis or service following the service manual for your vehicle.)

If the problem persists, the next step is to check all the fuses.

You may also want to use a scan tool to see if one of the computers in the vehicle is not communicating. If a computer needs replacement, most will require the Vehicle Identification Number to be specifically programed into them. Any computer programming will need to be performed by an authorized technician with the proper tools for programing a module.

Now, before you start sending me letters telling me that this procedure will cause other problems I will refer you to the GM Tech Link, a monthly publication for the GM Dealership Service Professionals, specifically the Programming Tips section of the August 2005, Volume 7, #08 issue. “Some components contain capacitors, which can store voltage after being turned off. If stored voltage is released by a module while you are programming another module, it could cause confusion on the data bus, causing U codes to set. To prevent this, you may be instructed to disconnect the battery cables and touch them together. This drains the stored voltage from capacitors.”

I use this technique, and it is a very common fix for problems like the ones mentioned at the beginning of this article. Hopefully, this simple solution will allow you to repair unusual problems without pulling your hair out. Good luck and let us know if this helped you out. Vette

Disclaimer: While all the contributors and editors at Vette magazine try and help with problems you are having with your Corvette, we cannot guarantee that you will not damage your vehicle more while trying to make a repair. While we hope you can fix your vehicle, ultimately you are responsible to research your problem and the advice provided by us here at Vette magazine. You and you only are responsible for the consequences that come from working on your own vehicle.

02 Electrical Short Circuits 2/2

Connecting the positive and negative battery cables together for approximately 5 minutes will drain all of the capacitors in the computers and forces them to cold reboot. This is a very common fix for communication problems.

Photos by James Berry

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