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Toyotapro
Toyotapro, ASE master tech
Category: Toyota
Satisfied Customers: 9973
Experience:  ASE Certified, 10+ years Lexus and Toyota Certified
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2002 TOYOTA TUNDRA: REPLACED BATTERY..BATTERY LIGHT STAYING ON..VOLTS

Resolved Question:

BATTERY OVERCHARGING ON 2002 TOYOTA TUNDRA I REPLACED BATTERY AND HAD THE ALTERNATOR TESTED IT TESTED GOOD. BATTERY LIGHT STAYING ON AND SHOWING 15+ VOLTS WHEN RUNNING.
Submitted: 4 years ago.
Category: Toyota
Expert:  Toyotapro replied 4 years ago.

Thank you for using just answer.

Was this an aftermarket alternator or the factory one on the truck? What about the replacement one?

When you put your volt meter on the battery when running is the exact volt reading 15.000 volts?

Any aftermarket equipment on the truck?

Any recent repairs before this issue?

Customer: replied 4 years ago.
it is the stock alt. i bought a new one also from toyota dealer. sometime it shows 15.5 sometimes 16.8 it goes up and down.
Expert:  Toyotapro replied 4 years ago.
Thank you for the reply, please answer all my questions from above so I can best asssit here
Customer: replied 4 years ago.
just a trailer brake but its un hooked.
Expert:  Toyotapro replied 4 years ago.
And did this sudenly appear for no reason or has someone been under the hood recently?
Customer: replied 4 years ago.
no it was setting in drive for a couple days with out driving it i'm the only one in the household.
Expert:  Toyotapro replied 4 years ago.
Does your truck have the optional daytime running lights?
Customer: replied 4 years ago.
yes it does. i cleaned all battery terminals and alt terminals as well.
Expert:  Toyotapro replied 4 years ago.

Have a volt meter handy?

Does this light stay on solid all the time now or does it come and go?

Customer: replied 4 years ago.
yes i have a meter handy. the light comes and goes. also the abs light will come on but not as muchas the battery light. the volt gauge on the dash goes up and down the head lights get real bright and dim.
Expert:  Toyotapro replied 4 years ago.
ok, I have a hunch that the battery you installed might be the issue here, can you get teh light to come on when at idle?
Customer: replied 4 years ago.
the light comes on at idle and even comes on at 3000rpm it seems it does not matter.
Expert:  Toyotapro replied 4 years ago.
ok, start the truck with the engien at idle until the light comes on, then I want you to get an assistant or carefully yourself remove the positive terminal from the battery and tell me if the light goes out.
Customer: replied 4 years ago.
the light was on at idle i removed the positive cable and the light is still on.
Expert:  Toyotapro replied 4 years ago.
ok, shut truck off, have you verified that the wire that bolts to the alternator is clean and tight?
Customer: replied 4 years ago.
yes i cleaned the cable end and both nuts. the nuts are tight. i'm a forktruck mechanic but i can't figure this out.
Expert:  Toyotapro replied 4 years ago.
ok, on the 3 wire connector at the alternator, can you get the volt meter here to see what the voltage reading is on the connector side with the key in the on (not running) position?
Customer: replied 4 years ago.
yes with key on not running one terminal is 11.7 the other 12.0 and the last one 9.3
Expert:  Toyotapro replied 4 years ago.
What is the wire color on the one tha tis 9.3?
Customer: replied 4 years ago.
its white testing 9.6 now.
Expert:  Toyotapro replied 4 years ago.
There is a 7.5 amp fuse labeled ALT-S, what voltage do you read across this fuse?
Customer: replied 4 years ago.
12.1
Expert:  Toyotapro replied 4 years ago.
ok, looks like you have a wire issue here, the fuse I mentioned powers the white wire at the alternator for the voltage regulator. If it is only at 9.3 volts it is telling the regulator the voltage is low. This is a solid wire from the fuse block to the alternator so either a junction block issue or a wire issue here.
Customer: replied 4 years ago.
what junction block would it be?
Expert:  Toyotapro replied 4 years ago.

Junction block is another name for a fuse block, what I am teling you here is that you have voltage at the top of this white wire where the fuse powers it but not at the end of the wire where it connects to the alternator, you either need a new fuse block here of the wire is bad somewhere. The alternator and battery you installed are not the issue here.

Toyotapro, ASE master tech
Category: Toyota
Satisfied Customers: 9973
Experience: ASE Certified, 10+ years Lexus and Toyota Certified
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