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Skyvisions
Skyvisions, ASE Certified Technician
Category: Toyota
Satisfied Customers: 14505
Experience:  Toyota Master Diagnostic
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My 1996 Toyota 4 runner has the battery light coming on. I

Customer Question

My 1996 Toyota 4 runner has the battery light coming on. I replaced the alternator, checked the battery and all checked out fine. The place I took it to that replaced the alternator for me were at a loss as to why the light would come on, all systems checked out according to them (they are are excellent shop). Today the light came on again and went out after driving for 10 miles. After sitting for 6 hours I started it up and the light came on again, no change in the blower fan speed, lights didn't dim and brighten with revving up the engine, I am at a loss. ? problem in the computer system, any ideas?
Submitted: 1 year ago.
Category: Toyota
Expert:  Skyvisions replied 1 year ago.

The battery warning light on this vehicle will only come on for one of two reasons either that wire going from the instrument cluster and light itself to the alternator is getting shorted to ground which would be extremely unlikely and virtually impossible or the field circuit in the alternator is not charging. How the system works is the gauge fuse sends 12 V up to the cluster through the indicator bulb then back out to the connector at the back of the alternator. Within the alternator if it is not charging current the field circuit that allows that voltage to go to ground thereby causing the indicator light to illuminate. Once the alternator starts to produce current in the field circuit voltage is fed back that line which negates the circuit to ground and causes the warning light to go out indicating that the alternator is charging. Either the shop did not check this correctly or when they looked at it it was charging. The bot***** *****ne is 99% of the time this problem is resolved by replacing the alternator. Make sure you buy quality alternator I've gone through two alternators because they wanted to go cheap aftermarket and found that the aftermarket one was not working correctly. Once I installed a good remanufactured Toyota or DENSO alternator the problem was resolved.