Hello all. This is my first post about my first experience with a Lexus so I will be brief. I just purchased a 99 RX 300 that had a bad transmission. Had the trans. rebuilt and all is good there. I noticed though after letting the car sit 2 days that the battery was stone dead when I went to start it. No biggie I got out the charger, put it on boost and started the car. Let it sit overnight and same thing the next morning, dead. Get a new battery and was good for a couple of days then go to start it one morning and dead again. Something is draining my battery.
In the mean time a cel came on so I went to check it with my scanner and my scanner powered up with the key in the off position. Now this is not so unusual because some cars have power to the obd port at all times. What was unusual is that the scanner read the code without ever having to turn the key to the on position. Is this normal? Is the port supposed to have power with the key off?
Furthermore is this the culprit that could be causing the drain on my battery? If it is not normal how do I fix it? I have done some research and cannot find a definitive answer. Please help!

 

Welcome to CL

When something like that is draining the battery every 24-48 hours, before you do anything else, check and see if the light-bulbs inside the glove box, underhood (if applicable), trunk, and other hidden areas are actually going off when the covers are shut. This may not always be easy to do....and you may have to peer inside a tiny crack just before or as the lid closes. But, if there is a short in the system, or the on/off switch or sensor for the lid or door-closing that triggers the light is not working, then there's your problem.

If the lights are all going on and off OK, of course, then you can take it from there and start probing other things.

 

Did it start right after you got the car back from the transmission shop???? Take it back to them and tell them what's going on, maybe they forgot to connect a wire, sensor or broke something when they removed or reinstalled the trans.

 

I remember my OBDII reader would light up when the key was in the ACC position but I could not read the codes. For that I had to turn one more notch to be in the RUN/ON position. It sounds like your vehicle is not behaving properly.

 

Please read up on 'parasitic draw' and compare it with what is going on in your RX.

I would start by looking for any after market add on accessory and to begin with disconnecting it.

You can connect a VOM in current (Amp) settings and start removing fuses one by one and see if find which circuit is drawing the current.

Salim

 

If you have a digital multimeter here is an easy way to determine what circuit is drawing the extra power. Charge the battery to its proper state of charge. Set the dome lights off and make sure the key is not in the ignition.
Attach your multi-meter to the battery and have it show voltage. The cables should be long enough so that you can see the display from the drivers seat. Since your car is pulling excessive voltage from a circuit you should see the voltage readout constantly dropping at some level. Remove 1 fuse at a time from the fuse box while watching the readout. Once you pull a fuse and see that the voltage is stable or not dropping as before then you have found at least what circuit is the problem. They will put several components on a circuit. If you post what fuse/circuit is the problem I can tell you exactly what all is on that circuit.
Hope this helps.

 

When I worked in a shop we would disconnect a battery cable and connect a test light in series with the battery. Any significant parasitic current draw would usually be sufficient to light the test light.
Prop up the light where you can see it. After that it's trial and error until the light goes out.

 

Those test lights are great.

My neighbor had a boat trailer and his tail lights were not lighting up. He pulled his nifty Fluke vom and showed me 12v at the bulb socket. I told him to test it with e test light bulb and sure enough it would not light. He had a corroded splice in the wiring and the vom drew no current and reported 12v.

Salim

 

Yep. Best way to find parasitic draw.