Lexus RX300 Battery Drains Parasitic Draw

This 2001 Lexus RX300 would drain the battery periodically when the vehicle sits. After testing for a parasitic draw and none found we performed the normal electrical system tests and returned the vehicle. The battery load test was weak but not completely failing, I was confident that it was not the cause of the problem. I have seen battery failure that causes them to short internally and drain themselves. This did not seem to be the case but recommended the battery to replaced if we cannot find any other problem. Sop the customer took the vehicle then called us about two weeks later letting us know the battery drained itself again. After testing the vehicle over and over we could not find the problem. Then one day I used the vehicle for a parts runs and was listening to the radio, the next day after parking the vehicle the battery was dead. I charged it up, pulled it into the shop and tested for a parasitic draw.


Parasitic draw test showing 1,211ma draw, we would like to see less than 100ma

I begin to trace where the parasitic draw is coming from by isolating the circuits in the fuse box one by one. The circuit is traced to the RAD1 20 amp fuse. I marked it with a white paint pen for easy reference.

Fuse Box Fuse RAD1

Under hood fuse box RAD1 20amp fuse, white paint pen marked

I know what circuit is causing the parasitic draw so now I need to look up the power distribution for that circuit. Luckily we can see that the RAD1 fuse only powers the power antenna motor and the factory radio amplifier. So it’s all starting to make sense to me know. I could never duplicate the problem because every time I would get the vehicle for a test drive I just turned off the radio. The customer always listened to the radio when they drove the vehicle. Now which one of these circuits are causing the problem?


The wiring diagram showing RAD1 20amp fuse power distribution

At first I started to look for the amplifier because I thought that was the problem. I have seen this problem before but just factory radios causing parasitic draw not an external amplifier. The amplifier wasn’t easy to get to either. Then I noticed the power antenna was broken off and has been that way for a long time. The power antenna probably stuck on years ago and finally just stopped working but found a way to draw power from the battery.¬†When removing the antenna motor it was warm to the touch from the power draw, I knew I found the problem. I then dis-assembled the power antenna motor to see a shorted out circuit board from the water getting into it. The broken antenna also cased a water leak, so keep that in mind if your power antenna is broken off.

This post was written by: Martin Hand


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Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.

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