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1995 Ls400 Will Not Charge


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Hello all, I am working with a 95 LS 400 that is not charging. I have replaced the alt and checked all fuses, links and wireing harnesses, but still she will not charge. any ideas are greatly appreciated

Provided that a wire has not inadvertently fallen off or become loose, it sure sounds like your voltage regulator is not providing field current to the alt.

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I replaced the alt twIce on a hunch I may of recived a faulty one, but I stIll have the same no charge condition. I looked at wire harness connectIons and I Inspected the wIres all seems good no dry rot plastIc or wIre Insalator nothIng Is chafed or burnt rIpped or torn. I'm perplexed

First of all, welcome to the club! :cheers: Next, do things right! Replacing anything on a hunch is like p1ssing in the wind. You don't know where it's going to go or if you'll get soaked. As Bali asked, what readings are you getting?

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Is the battery shot and not taking the charge? and is the belt tension wheel holding the belt tight enough?

As far as I know the voltage regulator is built into the alternator. The connector may look good but maybe the wiring is cracked or shorting. check for worn insulation along all of the wires and battery cables and ground point.

Also the ls uses a 100 amp alt if I remember correctly

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Yeah, I would get a volt meter and test the battery terminals. First with the car on, then with the car off.

Precision, don't want to seem harsh, but if you're going to diy, then instead of buying parts you may not need, invest in some basic tools. As Vander suggests, a mutimeter would be your first step.

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I tested voltages running and off, and running with battery terminals off . same results every time, 12 volts, I tested all the wires in the 3 pin harness @ the alt and all 3 are at the same voltage 12 volts. there is a new belt and tension pulley. the battery is in top condition passing a draw test and hydrometer reading so all your basics are covered any other bright ideas

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I tested voltages running and off, and running with battery terminals off . same results every time, 12 volts, I tested all the wires in the 3 pin harness @ the alt and all 3 are at the same voltage 12 volts. there is a new belt and tension pulley. the battery is in top condition passing a draw test and hydrometer reading so all your basics are covered any other bright ideas

P;

12 Volts?...........This may sound really screwy, but bear with me.

Your battery, all by itself, is rated at 12 Volts. Let's say it is disconnected from everything and it does indeed have exactly 12.00 Volts on it, + to - terminals.

Now, lets say that your alternator puts out exactly 12.00 Volts on the test bench at the local Auto Parts store.

In order for the battery to be charged by the alternator, the alternator's output voltage MUST be higher than the battery's terminal voltage.

With 12.00 Volts output from the alternator, it will not charge your battery.

Usually, an alternator outputs somewhere in the neighborhood of 13.8 to 14.6 volts, depending on design and manufacture of it.

In order to charge a battery, the applied voltage to the battery must be higher than the battery's terminal voltage in order for charging current to flow from the alternator to the battery.

A really good analogy is you want to put some air in your tire. The tire pressure is 32 psi. You have an air compressor that has 32 psi in the tank, but no more. What happens when you try to put more air in the tire? nothing..........there isn't any excess air pressure to cause air to flow from the compressor to the tire. Volts and amps works the same way. It is the flow of current from the alternator to the battery that re-charges it and causes the terminal voltage to increase. Once the battery terminal voltage equals the alternator's output voltage, charging current ceases to flow........Oh, BTW, that also means your battery is fully charged. Another subject for another day, but this condition doesn't happen very often.

When you had the auto Parts store check your alternator, did they load test it? Meaning, did they connect it to load resistors and make sure it produced rated current (105 amps rings a bell here(?)). If they just spun it up with no load and said "it's OK, it's putting out 12 Volts", go somewhere else, they don't know what they are doing.

Also, anytime I hear default numbers like "12 volts", I'm very suspicious that corners were cut. I haven't measured anything on my 400 or 430 lately, but my alternator on my Suburban puts out 14.6 volts. As far as I'm concerned, dash voltmeters are completely worthless.They tell you nothing in terms of your charging system's health. The only thing that tells me the information I want is a charging ammeter. It will tell you when the battery is being charged by the alternator and when the battery is being discharged by the vehicle.

Question; Are you really sure that all the voltages you measured were indeed 12.00 volts? It's important........

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Hi,

You say you ran the engine and therefore the alternator with the battery connections off? This will damage the alternator.

I cannot, as the other posts also say, understand how you are getting 12volts with the engine off and still 12volts with it running, are you using a digital multimeter to measure the voltage?

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Hi,

You say you ran the engine and therefore the alternator with the battery connections off? This will damage the alternator.

I cannot, as the other posts also say, understand how you are getting 12volts with the engine off and still 12volts with it running, are you using a digital multimeter to measure the voltage?

Now that I think about it, if he's measuring 12 volts, his alternator isn't working. He's just measuring battery terminal voltage. And if this keeps up, it's probably not long for this world either........

I bet the Auto Parts store didn't put a load on the alternator when they tested it. Open circuit voltage measurements are meaningless.

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The alt came from Napa, they use a computer tester that spins the alt up to 2200 rpm while measuring voltage and current across a load curve, they tested It correctly. the vehicle Is completely capable of running every accessory with out the battery connected with and damage to any part of the vehicles electrical system. the whole story about how alts work was cute, but still dose not explain why the vehicle will die If left running @ Idle. all wires at the alt have the correct trigger voltages to activate the alt. actual voltages were In the neighbor hood of 12.26 - 12.48 volts dropping the longer the vehicle runs until It dives out and stalls at about 9 volts. at this point I reset ( disconnect the batt for 10 min) and charge the battery back up with a brand new snap on logic charger. It seems to be something such as Idle speed...

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