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Toyota Tacoma with 3.4 L engine battery light on

A freinds Tacoma had the battery light come on so she went to AZ for a test of the charging system…They told her it was the battery. She got a battery and directly went to my house as I told her I will install it for her,

I checked the terminal voltage and it was 9.5V. When the engine was reved it did not change. It may have gone down a tenth or two.

I told her it looks like the alternator may be bad but and the battery. I proceeded to put in the new battery. I tested the battery and it was new this month and rang in at 12.65 volts. Perfect I cleaned all of the terminals including the positive on the alternator. There were not bad looking but I wanted to be thorough. It fired up and the voltage at idle was 13 volts. When she put the lights and blower on it dropped to 12.8. I told her there is still a charging issues and I dont know how the guy at AZ could have concluded the Alt was fine. I guess she trusts a guy with a uniform and a fancy looking machine over me. So, She wanted to drive it and see what would happen…The battery light would come on intermittently…by day 3 the battery was dead.

She limped to my house and I went to put in a new Alt thinking this is the root of the problem. After installation and jumping the battery to get it started it ran but again was only putting out 13 volts at idle. I cleaned the contact points good with a wire brush before I installed the new Alt. When reved it put out 13.2V. I know it should be up over 14 volts.
What is going on? I am looking for maybe a bad grounding strap. I need to remove the skid plate to really see if there is one below and Have not had time to do so.
I suppose the new Reman from NAPA could have a bad voltage regulator. But what are the odds?
The voltage drops from the Bat terminals to the AlT are almost 0. Same with the resistance. Milli ohms.
She has been driving it today and so far the light is not coming on. Could she have killed the new battery this early and that is what is going on now?

Does anyone know if there are engine grounding straps accessible from below. Thanks for ideas and shared experiences with this engine and issues.

You do realize that a bad battery can damage a perfectly good alternator?


That discharged battery will draw more current than the vehicles lights and accessories, the combined load may be more than 100 amps and can damage the new alternator. You must recharge the battery before starting the engine.

That is all very interesting information but it is too late now…As I stated in my post this has already happened. Other than a lesson for future reference it does not help me currently. I am thinking the only thing I can do is get both the battery and alternator bench tested to confirm they are working properly.

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It’s possible the “dead” battery damaged the new alternator. The expert’s posts above are correct, a new alternator shouldn’t be installed with questionable battery. The battery should be charged using a battery charger first. Then hook up the new alternator and test it out.

But I don’t think that’s the most likely explanation for what is happening here. I presume the battery couldn’t crank the engine. That’s why it was called dead. But that doesn’t mean it was completely discharged. If a battery can light up the taillights, the stop lights, run the radio, and the headlights work but are maybe a little dim, it still has quite a bit of charge left in it. I’m guessing that’s probably the case here.

More likely the cause is one of these

  • The replacement alternator is faulty. Most parts stores have an alternator test fixture. Take it back and try that. Voltage regulator failure is rare, but a failed diode isn’t.
  • Wiring problem either in the positive or grounds between the battery and the alternator. Make sure both are grounded securely to the chassis. Test any fused-wire in the + path for unexplained resistance.
  • Something is causing a big current drain. Measure the voltage between the alternator output and the battery positive. Engine idling. This should measure less than 0.2 volts. Does it?
  • The alternator belt is slipping or the alternator pulley is the wrong diameter.

It may not be too late, recharge the battery, then check the voltage output, should be about 13.5 volts in 90 F weather.

If you don’t have a battery charger you can buy one for about $40.

BTW, the 13.2 volt reading you observed suggests that the alternator is working but the load is higher than its output.

George your presumptions I believe are correct. The voltage drop from the Battery Pos terminal to the alt pos terminal was under .2 volts as you suggested. Does that suggest there is not a large current drain?
The belt seems good and the pulley diameter looks correct.

I am possibly looking at a faulty alternator, for what ever reason. I measured the 13 volts with in seconds of starting the car with a jump box. How long does it take for a diode to over heat. A few seconds or a minute or longer?
Unless I stated otherwise all of the voltage readings are with all accessories off. Tester , if 13.2 volts is ok would that suggest the alternator is under sized for the truck? The only load is running the engine and charging the battery. I guess the battery load is pulling down the voltage perhaps and after it gets fully charged it should go up…Is that what you are saying? I did read in that link that the PCM controls alternator output voltage. Is this another area to look at and how is that tested?

I still want to check all engine grounds…for anyone familiar with this Truck can you direct me to where the ground straps may be?
I can get the battery trickle charged but not for a few days and I know she is going to want to drive it tomorrow. They do what they want…I have no control.

It sounds more like a faulty alternator or a ground problem. Less likely, a faulty battery. The battery and alternator can each be tested independently at many parts stores. Ground problems can be detected by measuring the voltage at places where the voltage should be more or less the same, but when you measure it, the voltage is different. Like the battery + post to the battery - post measure 12.6 volts, but battery + post to chassis measure 12.2 volts. That’s a problem. Or the post to post measures different from the post connector to post connector. Same thing with the alternator.

The less than 0.2 volt voltage drop between the alternator and battery indicates the wiring is probably ok there, and there isn’t an unusually large current draw the alternator is trying to keep up with. sounds like you are Sherlock-ing the problem. The game is afoot!

Nevada’s idea above to just recharge the battery with a battery charger and re-take the voltage measurements makes a lot of sense. At the battery posts after the car has sit overnight with a fully charged battery, and then after you first start it up. when I do that w/my Corolla I get 12.6 volts w/engine not running, and 13.5-14.5 volts right after starting it. Usually it is around 14 volts. That second voltage measurement depends on how discharged the battery is, what the ambient temperature is, and how long since I started the engine. The voltage drops quick rapidly after starting the engine.

No, it indicates that you did not recharge the battery.

If I set our Snap-On charger to “auto” to recharge a battery while replacing an alternator the battery may draw up to 80 amps, that is the capacity of the charger. That alternator can’t charge at that rate for very long. There is no reason to look for cables that have fallen off, recharge the battery.

All I can suggest is that she go to a real shop instead of a parts retail store. They’ll have the expertise and equipment that AutoZone lacks to do a proper diagnosis and repair.

She did come back for me to charge the battery…When she arrived the voltage at idle was 13.5 volts. When I turned the car off the voltage was 13 but that is common to have a residual excess voltage… That is a better sign than if it was 12.0.

It is better after diriving it for 40 miles or more and as some of you have suggested it may be acceptable output for this make, model and conditions. It is about 80 degrees.
The battery charger said it was at 75%. I am not confident it has an accurate meter but nonetheless I charged it to 100%.

In any event I connected it back up and she went on her way. Because the car is garaged elsewhere I wont be able to get a reading on the battery after being off all night. That is good advice and would be my next step to confirm the battery’s health.

I did the best I could for her considering the circumstances. Thanks for all the help

I have the same concerns about Diagnostics at AZ. I would rather remove the items and take them to a REAL shop as you say to get diagnosed.