Why when the key is off is my RAP fuse under the hood drawing power causing a new battery to drain overnight?

Tiny
SHANEALLEN27
  • MEMBER
  • 2000 CHEVROLET BLAZER
  • 190,000 MILES
Brand new battery drains overnight.I hooked up test light after pulling negative cable off battery by pos end of test light to cable and neg end off test light to ground on battery.I then pulled fuses one by one and nothing. Mind you the light on the test light here never went out yet or at all. Withh the ctsy lps fuse pulled and radio fuse pulled and the RAP fuse under the hood pulled the test light goes out. HOPE I HAVENT LOST YOU YET. Now with the RAP fuse pulled I can put the ctsy lps fuse back in and the test light comes being dim not bright then starts blinking. While this is going on I can touch the radio fuse back to its socket and light goes back to bright and not blinking. THE RADIO THIS WHOLE TIME HAS BEEN PULLED AND SITTING IN THE PASSENGER SEAT. With everything plugged back in and hooked up and key off I tried the power seat (driver) and nothing even with the key on or off. Could the switch on the driver seat be causing all the crap above as I have describe.I am a mechanic but not a very seasoned mechanic. Would appreciate the help. Like I said I hope I didnt lose you anywhere
Wednesday, September 26th, 2012 AT 3:10 AM

3 Replies

Tiny
ASEMASTER6371
  • MECHANIC
  • 52,796 POSTS
Sounds like a wiring issue. Trace the radio wire back to the plug.

By the way, RAP? Please explain

Roy
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Wednesday, September 26th, 2012 AT 3:17 AM
Tiny
SHANEALLEN27
  • MEMBER
  • 8 POSTS
Retained Accessory Power (RAP)
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Wednesday, September 26th, 2012 AT 3:20 AM
Tiny
CARADIODOC
  • MECHANIC
  • 34,330 POSTS
Hi guys. Allow me to add some insight that is likely involved. You can't use a test light as you described to test for a drain. Unless specified otherwise, only 35 milliamps, (.035 amps) is allowed for the radio and computer memories. That's WAY too small to light up a test light so you can have an excessive drain without the light being on. The test light routine worked many years ago when the only drains were stuck trunk and glove box lights that drew a half amp or more.

Second, you have computers that have to time out and go to "sleep mode". Until that happens, often up to 20 minutes or more, those computers can draw as much as three amps in total. Your test light won't allow that much current to flow so the computers will try to shut down, wake up, shut down, hence the flashing light.

You have to do this with a digital volt - ohm - amp meter but there is still a problem. If you use the 200 ma range for accuracy you are likely to blow its internal fuse. You can start out on the two amp scale, then after 20 minutes switch it to the lower scale but just turning the switch will momentarily brake the circuit and wake the computers up again. That will also blow the meter's fuse. Hmm!

Check out this page for the solution. Go half way down to "Computer-Controlled Cars".

http://randysrepairshop.net/testing-for-battery-drain.html
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Wednesday, September 26th, 2012 AT 4:15 AM

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