Moffat

My toyota crown majesta V8 which is basically the same with the Lexus LS400 made in 1999 is having power issues. I changed the alternator and battery but just two weeks after, the battery was flat and the new alternator was already undercharging. My mechanic advised me that something was draining power, he however failed to trace the exact thing that was draining power. The car can only be started by jumping the battery, and if I stop I have to keep raving so that it does not switch off because if it switches off i will have to jump it to get it on again. How can i deal with this problem?

RA40

iTrader: (6)

If you have a multimeter this test for your alternator:

This to check for battery drain:

Moffat

thank you.

python

iTrader: (4)

https://en.wikipedia.org/wiki/Toyota_Crown_Majesta
first cars looked like a infinity q series..then morphed into a buick
first i ever heard of this car

Superfast1

Python- You're spot on. The older models look just like the Infinity Q's & the newer models look like the Buicks.

Moffat- Looks like you may have a short to ground. To check for drain, with the car off, disconnect one of the battery terminals then insert a voltmeter in the current (amp) setting between the battery & the cable you had just disconnected then monitor how much current the car is drawing from the battery. Make sure the voltmeter is switched to the amp setting to measure current. Depends on which battery cable (positive or negative) you disconnected & how you connected the amp meter, the display may show positive or negative current, it should be fine so don't worry about it. Once you've it hooked up & w/ car remains off, the ECU, clock & a few other accessories should only draw a small amount of current (mA). If it is then your battery or alternator may be faulty even though it's new. Here's how to do a quick check: With car running, disconnect one of the cables. If you remove the positive cable, make sure your wrench is not touching any metal parts on your car i.e. fender otherwise you will get zap. Anyway, if car is still running then most likely you have a bad battery. If car die as soon as you disconnect the cable then it's your alternator. If you're not sure how to go about it then remove & take your battery & alternator back to the store you had bought them from for a test or take your car to another shop. Let us know what you've found out. Good luck.

python

iTrader: (4)

never perform this test with the car running as it sends an infinite surge/spike to the system and could do some harm.
http://www.troubleshooters.com/dont_...ct_battery.htm

Superfast1

The diode assy inside the alternator is a four-way rectifier which is used to convert AC to DC voltage & the output of the alternator is clamped to around 14 Vdc via the internal (for newer cars) or external (for older cars) voltage regulator. There are also plenty of filtering protection to filter out voltage spikes or transients riding on the DC line so it's very unlikely that you will damage to any of the electrical components on your ECM. Of my 40 years as a car enthusiast, I haven't seen one that was brought down by disconnecting the battery. Anyway, as stated, it's very unlikely but anything can happens so it's also possible so exercise it at your own risk.