Battery Warning Light

[juicestop]

I had a new Battery - Bosch S4 - fitted about 3 weeks ago, and a new alternator today. I am still getting the red Battery warning light on the dashboard. My mechanic says that he's checked that the alternator is charging the Battery and that everything else seems to be fine. Also had a new idler pulley, water pump and serpentine belt fitted in the past two months. Does anyone know why the warning light hasn't disappeared?

[true blue]

Hi try the obvious first , the Battery leads need to be pushed right on the terminals and fastened tight ; the alternator drive belt needs to be in good condition ,tensioned correctly and not slipping ,sometimes if the belt has slipped previously it hardens and does not drive properly .Take the car to an auto electrician , electrics confuse most 'mechanics' ,he will check the Battery which should give approx 12.7 volts as it is new and the alternator should charge approx 14 to 14.5 volts when running .A load test can also be carried out on the Battery as it is not unknown for a new Battery to have problems .

Dave

[juicestop]

Thanks for the advice Dave. I can confirm the Battery leads are tight, the drive belt is brand new and tensioned correctly. I agree that the next step is to seek an auto electrician for further analysis.

[jackcramerr]

first thing to find out is the Battery charging at all ?

silly one but checked the fuse and clean any contacts of alternator cable / connector ?

how many miles has the car done ? I am not sure if these aternators have brushes. any chance oil or coolant leaked onto brushes and now there is crud in there ? if the are brush type then slip ring and brushes need to be cleaned. the other thing left to see is and damage to any harnesses.

[juicestop]

Hi jack. The alternator is brand new. I had a total loss of power previously, RAC man confirmed dead alternator. It could be the harness I'll get that checked next. The car's mileage is 64k.

[juicestop]

So after taking the car to the local and well respected Beds Auto Electricians, they can't work out why the Battery warning light is still lit. The checked out fuses, harness and plug, earth straps, Battery and alternator. They confirmed everything is working fine and the Battery is charging above 14 volts. They said the only thing they can think of is that the car needs to go to Lexus or Toyota for full diagnosis and that it may just be a reset is needed for the warning light to dissapear. Any thoughts anyone?

[ChumpusRex]

The light does not need resetting. It goes out instantly as soon as the fault is fixed. If the alternator is charging correctly, then it sounds as if the problem is with the wire that connects the alternator to the light (or that the alternator isn't compatible, or has an internal fault which causes it to send the wrong diagnostic signal).

On the IS220, the alternator the light is controlled by the blue/black wire from the alternator which runs to the dash. If this wire isn't connected, then the light will come on, or if the alternator is putting the wrong voltage on this wire, then the light will come on. It should be fairly straight forward for an auto electrician to check if the alternator is putting the correct 12 V signal on this wire.

On the IS250, the alternator communicates with the ECU directly, and the ECU communicates digitally with the dash via CANbus. In this case, there is a red wire running from the alternator to the ECU which carries the "alternator working correctly" signal. 

[juicestop]

Thanks for the detailed reply ChumpusRex. My car is an IS250 and will get the red wire checked out. I have been driving the car, only very locally just in case it does lose all power again. I've done about 30 miles and it seems okay so far. The red warning light is still on, but somewhat strangely does sometimes go out for about 10-15 seconds and then back on again. So I'm thinking that maybe the "alternator working correctly" signal is not connected correctly, but that the alternator is charging the Battery correctly.