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Clough · Feb 12, 2010, 01:13 PM

Hi, All!

Long story short...

Got stranded at a filling station last night. '91 Nissan King Cab truck woudn't start. Did finally get it to start.

Drove to an auto parts store. Thought that I might need a new battery. Worker there checked the battery. Battery was drained. He then checked the alternator after jump starting my truck. Said that it was only putting out 30 amps when it should have been putting out 90 amps.

Ended up getting a new battery because the battery was so old. Probably need to get a new or used alternator.

At 30 amps, is the alternator charging up the battery at all?

Thanks!

smoothy · Feb 12, 2010, 01:22 PM

IF it has enough charging voltage... and IF the current loads are LESS than 30 amps total it should charge... how fast depends on the differences between the output and the load. BUT with that said... you aren't always operating the engine at the point that the alternator has max output... its not a flat line across all RPM's, but an uphill slope where you would likely has a load that excedes the charging current at lower RPM's which will drain the battery. I'll bet this is what's happening in your case. 30 amps isn't much... not if you have the lights and or stero running. Wiithout sitting down and doing some math... I'll guess that's insufficient for most situations.

You should replace that alternator based on those test results

TxGreaseMonkey · Feb 12, 2010, 01:23 PM

This link below should help:

https://www.askmehelpdesk.com/cars-t...ml#post1973613

If the regulated battery voltage is between 14 and 15 volts, the alternator should be fine.

parttime · Feb 12, 2010, 01:27 PM

Clough, besides letting you down last night, how'ya like that nissan, I'm on my second, and just love them. I had over 250,000 when I sold the 88 and I'm driving a 97 now with a little over 200,000 miles. Good luck with yours

parttime · Feb 12, 2010, 01:32 PM

Forgot to say, only problem I've had with mine was the starter, just put one on the 97 and think I put two on the 88. So don't overlook that.

TxGreaseMonkey · Feb 12, 2010, 02:38 PM

At 30 amps, is the alternator charging up the battery at all?

Yes. The alternator is likely only putting out 30 amps, because that's all the loading requirements are. Continue testing the regulated battery voltage, but start loading the system down. Have another person turn on the head lights (10 amps), A/C (30 amps), rear defroster (15 amps), and so forth. While this is being done, the regulated battery voltage should start to drop towards 12.5 volts. This gives you a better picture of how the charging system is supposed to function under load. As the loading progresses, it will give you an idea of how much amperage your alternator can produce. Remember, alternators are designed to produce maximum amperage for only brief periods.

Clough · Feb 12, 2010, 05:53 PM

Thanks for all of the replies!

The guy at the auto parts store tested the alternator with just me starting the engine. Nothing else was turned on.

So, if I understand correctely, the alternator only puts out the amps that it needs to put out, based upon the load requiements that are happening?

Thanks!

Clough · Feb 12, 2010, 05:58 PM

Originally Posted by parttime

clough, besides letting you down last night, how'ya like that nissan, I'm on my second, and just love them. I had over 250,000 when I sold the 88 and I'm driving a 97 now with a little over 200,000 miles. good luck with yours

Hi, parttime!

I love the truck, but not in the wintertime when there's so much snow here! It's really light weight, and my tires are not in the best of shape.

Thanks!

TxGreaseMonkey · Feb 12, 2010, 06:00 PM

Exactly. In practice, here's a little more on how the charging system works:

The alternator FR (Field Response) signal communicates to the ECM how "hard" the alternator is working to meet the electrical demands of the car, including the battery and any loads which aren't monitored by the ELD. This square-wave signal varies in pulse width, according to the load on the alternator. The ECM places, approximately, 5 reference volts on the wire. The voltage regulator will drop this signal to approximately 1.2 volts, in proportion to alternator load. The ECM compares the electrical load (ELD) signal with the FR (Charging Rate) signal from the alternator and uses that information to set the idle speed and turn the alternator on and off. This helps fuel economy. Higher alternator amperage comes at a cost.

If you had an ammeter installed in your car, the needle would be at "0," if everything was normal. If the alternator wasn't able to balance everything out, the scale might show a discharge of -5, -10, or -20 amps. Conversely, if the alternator was charging a lot, the needle might show +5, +10, or +20 amps. Alternator amperage output is a function of load; i.e. it would be foolish to have it provide maximum amperage all the time. It wouldn't last long. Most people don't think about this, but an alternator's rpm is probably 6 times engine rpm. Finally, ammeters show the charging system's status (flow) real time, unlike voltmeters (pressure).

Clough · Feb 12, 2010, 06:16 PM

You're getting over my head here, TxGreaseMonkey!

What do the acronyms stand for that you haven't already explained, please?

Thanks!

TxGreaseMonkey · Feb 12, 2010, 06:34 PM

ECM--Engine Control Module (or computer)
ELD--Electrical Load
FR--Field Response (charging rate)

TxGreaseMonkey · Feb 12, 2010, 07:04 PM

Glad you asked your question. Many others will learn a lot about alternators from it.

Let me know the results of your tests. I bet the alternator is fine and the problem resided only with the battery.

Clough · Feb 12, 2010, 07:11 PM

Originally Posted by TxGreaseMonkey

Let me know the results of your tests. I bet the alternator is fine and the problem resided only with the battery.

Do you really think so? The battery was really old, though...

This truck doesn't have all that many miles on it, although it's a 1991.

The people who I purchased it from, only used it for gardening stuff and to haul the dogs around.

Thanks!

TxGreaseMonkey · Feb 12, 2010, 07:26 PM

Yes.

Clough · Feb 14, 2010, 11:50 PM

Well, tonight the truck "died". Won't start at all. Barely got it across one of the bridges over the Mississippi. Managed to park it in the parking lot of a Quick Mart type of store.

Need to now have it towed to a service station and see what's going on.

The symptoms were, losing power, decrease of any lighting, hesitation and what I would guess I'd call lack of any acceleration power when pressing on the accelerator pedal.

I now have none of my own transportation, so this is definitely an issue with which I need to deal!

Just got a couple of new tires for the truck as well as the new battery.

Money is incredibly tight!

Advice, please?

Thanks!

TxGreaseMonkey · Feb 15, 2010, 07:32 AM

Is your truck a 2WD and do you have the 4-cylinder engine?

Perform tests in the link below:

https://www.askmehelpdesk.com/cars-t...ml#post2123675

What were the results of the regulated battery voltage test the other day? Do you believe the alternator is at the heart of your problem?

Clough · Feb 15, 2010, 10:55 AM

It's a two wheel drive. It's also a four-cylinder engine.

The results of the battery test was that it was drained. Yes, I believe the alternator is at the heart of the problem.

I did just have it towed to a service station, though. No other choices for what to do.

I'll let you know the outcome.

Thanks!

smoothy · Feb 15, 2010, 11:15 AM

You may want to have the mechanic check to see what the parasitic power drain level is... basically with everything turned off. You will laways have some on any newer vehicle... whats key is how much.

It IS possible for the battery to drain overnight even with a perfectly good alternator... just that the alternator is the most likely point of failure.

Clough · Feb 15, 2010, 11:22 AM

Originally Posted by TxGreaseMonkey

Is your truck a 2WD and do you have the 4-cylinder engine?

Perform tests in the link below:

https://www.askmehelpdesk.com/cars-t...ml#post2123675

What were the results of the regulated battery voltage test the other day? Do you believe the alternator is at the heart of your problem?

By the way, there's some great information on the thread for which you provided a link! :)

Thanks!

Clough · Feb 15, 2010, 11:26 AM

Originally Posted by smoothy

You may want to have the mechanic check to see what the parasitic power drain level is... basically with everything turned off. You will always have some on any newer vehicle... what's key is how much.

It IS possible for the battery to drain overnight even with a perfectly good alternator... just that the alternator is the most likely point of failure.

parasitic power drain level

What is that, please?

Thanks!