I have a 4Runner, 1993, V6. It's not my main car so I have about 150K miles on it. I don't drive it that often so it sits in my driveway. But I drive it once a week to recharge the battery and keep the fluids going.

Yesterday (Fri), I tried to start the car but it didn't turn. So I thought the battery had drained. I found this strange since I had driven the car 3 days before (Tues).

I jump-started the 4Runner and was able to drive off. It was about 5PM so I had the interior lights on but I noticed that the dashboard light was dim when the 4Runner was idling but got brighter as the car was driving and the RPM was higher. Perhaps it was the alternator?

I pull in to a store to run an errand. Again, the 4Runner didn't start. But I had a jumper starter so I was able to restart the 4Runner and drive home.

Today, I did some diagnostic testing on the 4Runner.

Using a multimeter, I checked the voltage of the battery. It was 11.77V. This is a bit low but I check my cars once a week and I know that the 4Runner battery is powerful enough to start the car at 11.9V or higher.

I checked the alternator. No obvious defects. Pulley looked fine.

Had to jump start the 4Runner again but the engine started up fine. Pulley on alternator works fine.

I checked to voltage on the battery while the engine was running and it was only 13.35 to 13.37V. That's sort of low. Should be closer to 14V.

Then I checked the voltage between the negative port of the battery and the post on the alternator. It was only 13.35V when the engine was idling. I pulled on the throttle latch and revved the RPM up. But the voltage only went up to 13.65V when the engine RPM was increased.

Is this voltage too low? What should the voltage be from the alternator when the engine is idled and what should it be when the throttle is revved up?

Trying to diagnose where the problem is. Would appreciate if anyone can share what the voltage should be from the alternator that is working fine.

Thanks.

 

A battery must be Load Tested to determine its condition.
Merely determining its static voltage is not sufficient to prove it remains serviceable
13.65v is a little low but should be sufficient to charge a battery in good condition.

 

I can do a load test on the battery.

But for the alternator, what would be the acceptable range for the volts coming out from the positive post of the alternator for a 4Runner alternator? Should it be 14.00V or greater?

 

The spec voltage is 13.9-15.1v at 25°C, 13.5-14.3v at 115°C. http://personal.utulsa.edu/~nathan-b.../2onvehicl.pdf So you're a tad low, but not enough to make a difference. Your battery is almost certainly toast, and it's pulling down on the alternator. You can surface-charge your battery to 11.77v (temperature dependent, but still pretty low), and there just aren't enough electrons in there for the party.

Almost any auto parts store will test your battery for free. They do it so they can sell you a new battery, so you might want to find out prices first (if Walmart is 1/2 the O'Reilly's price, you might even skip the free test).

 

Scope, thanks for the reply and information. Even if the problem is the battery, from what you provided me, I can see that the alternator is not providing the power as specified. I might have to replace the alternator in the future.

Thanks again.

 

So I load-tested the battery. It's not what I expected.

1) I have one of those battery rechargers. You plug one end into the wall outlet in the garage, then connect the other end (two cables) to the battery terminals. This recharges any lead battery.

It took almost a day but I recharged the battery of my 4Runner.

After recharge, the battery showed 12.41V.

2) Reinstalled/reconnected the battery.

3) Connected a multimeter to both terminals on the battery. Hit the Min/Max button so the multimeter will record the min and max voltage.

4) Start the car with the ignition. Let the car idle after it started. This will drain the battery to start the engine, then recharge when the alternator starts recharging it.

5) The multimeter showed that the min. voltage was 10.87V. It's higher than 9.6V so the battery can hold a charge.

6) While 4Runner was idling, moved the multimeter's positive cable to the alternator's post. Multimeter's negative cable still on negative terminal of the battery. Again, multimeter shows 13.35V when engine is idling. When I turned the throttle latch to increase the RPM, it went up to 13.71V.

7) This was strange. I thought it was the battery. So I turned the car off and tried it again. Waited about 5 minutes.

8) Same setup as before, cleared the voltage records from the multimeter and set it to record min/max. With the engine/alternator off, it read 12.19V.

9) Started the car up again. Car started fine. Multimeter's min voltage for the 2nd startup was 10.78V. Again, higher than 9.6V. Battery seems fine.

Is my problem still with the battery? Or is the problem the the alternator because it is under 13.5V in output when it's running? Does anyone know if I'm missing something as I try to narrow down my problem?

 

Check for parasitic load that might discharge the battery while the truck sits idle for more prolonged time.

 

How old is the battery? Also not sure if you can do a true load test with a multimeter? I may also add when a battery is bad, going bad etc they do things, strange things.

 

This is what I was thinking. I'll do some research. For now, workaround is driving around with a jump-start kit.

 

Battery is 4 years old, based on the receipt. The alternator was replaced 9 years ago (2016).

How can I do a true load test? I did it the way some of the auto mechanics showed on their YouTube channels.

 

Not sure, I searched on the net and got conflicting articles, some have steps with the multimeter and some say it just doesn’t show what an actual battery tester can duplicate load and test the voltage drop. But usually 4-5 years is a normal batteries lifespan unless you have a sealed gel cell or lithium which I have had last ten years.

I just know when batteries are going south whatever readings you get don’t make sense