acme15

Assistance with LS400 charging issue ('98 UCF20)

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Need a hand please! '98 UCF20. It is a Toyota Celsior but that shouldn't make a difference.

Car died, Battery wasn't getting any charge. Putting a new freshly charged Battery on resulted in the Battery quickly discharging. I therefore diagnosed as a dead alternator.

Alternator and its connectors were covered in oil from a leaky valve cover gasket. Alternator and gasket both replaced, and connectors cleaned with contact cleaner.

The car now works and the alternator is maintaining the Battery voltage, but it only sits at around 11 volts. I can drive along with AC, heated seats, all lights, etc, and it regulates to 11 volts.

When I switch the car off, the Battery voltage slowly recovers to over 12 volts again. I have tried a known good Battery - Two in fact!

Tomorrow I am going to check the output at the alternator.

My questions are:

a.) If the alternator is outputting 11V instead of 14V, is there anything other than the alternator which can cause this, given that it is a new unit, and the regulator is built in?

b.) If the alternator is outputting 14V - What could be causing that voltage loss between the alternator and the Battery? A short or bad ground etc? Are there common places for this to occur?

Many thanks!

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The alternator should output around 14.3v to Battery with no load if you are dropping to 11 volts with ancillaries on there is a fault with one of them either a bad ground or a faulty circuit within a individual unit. connect a voltmeter on the postive and negative poles of the Battery and check the drop in voltage as each ancillary is switched on another other check is to connect a wire from your alternator ground connection to the engine to make sure your ground connection is ok check the voltage at the Battery when you have this connection in place.

The Battery will be reading around 11.5 volts when all your ancillaries are on as none of the power from the alternator will be charging it and some load will be taken from it.

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OK so at idle the alternator is putting out about 11.5V.

I measured this voltage from the positive output on the alternator to the casing of the alternator with the engine running.

So my attention then turned to the loom pigtail with its three wires. The two outer wires have continuity to ground and 12v with ignition on, but the middle (black and red) wire has neither.

The middle wire is switched ignition. The 7.5A switched alternator fuse is fine.

So I'm assuming that is my issue.

I need to know where that red and black wire goes after it disappears off into the loom. On some cars it is yellow and black.

Help?? 🙂

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You will probably end up with a requirement for a Auto electricians input as the wiring diagram for these cars are very difficult to source unless you are prepared to shell out for the Lexus LS400 manual (electrical).If you have tested everything at source and the problem is now in the electrical innards of the car it is going to be very difficult to find the fault without a illimination programme of possible causes.

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Double check the alternator is working properly.

It did happen for me before when replacement  alternator that I bought was faulty .

If there is enough room  you could try and connect the positive lead of the multimeter to the terminal on the back of the alternator and find a good ground ... see if the voltage is same low or higher.

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In the end I rewired it. New positive, new earth, new switched live feed, new fuse, new relay, and spliced it into the wire corresponding to the alternator middle wire. Working fine now. 

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