Auto Repair question - alternator

[jgofnj 5]

Just curious if this scenario make sense. 

 

Last week the car "died" - and was told I needed a new battery.  The mechanic said he checked everything else out including the alternator, which he said is perfectly fine.  1 week later, the car dies again, wouldn't start, take it back, this time he said the alternator is not working and needs to be replaced. This is all fine, the car is 10 years old, don't recall having the alternator replaced before.  

 

My question is this, is it realistic to believe the alternator was tested and perfectly fine last week, and now after they put in a brand new battery, all of a sudden the alternator needs to be replaced?

 

Just wondering.

 

 

 

[lunker 274]

In my experience (with older cars) no. He took the easy route and got burned.

[T Bill 649]

Could be the regulator is flaky. Most new cars the regulator is built into the alternator.  I had a problem with the alternator once, tested OK, replaced battery and then same thing battery dead again. Turned out one of the brushes in the alternator wore out. Spring to hold it in place was now making contact with armature. When the spring wore down, contact became flaky until spring worn out. So sometimes it tested good, other times dead.

[67gtonut 847]

A lot of techs wil do a simple alternator output test after getting car started..... and deem the alternator worthy.....    what they fail to do is recheck it after about 20+ minutes of running.....  which can change the output of alternator.

[jgofnj 5]

A lot of techs wil do a simple alternator output test after getting car started..... and deem the alternator worthy.....    what they fail to do is recheck it after about 20+ minutes of running.....  which can change the output of alternator.

That makes sense, good point.

[jgofnj 5]

In my experience (with older cars) no. He took the easy route and got burned.

When you say got burned, do you mean I probably never needed a new battery, or that never properly tested the alternator the first time ?

[67gtonut 847]

How old was the battery anyway?

 

Low voltage is the devil in a car's electrical system......   

[1LtCAP 4,262]

Just curious if this scenario make sense. 

 

Last week the car "died" - and was told I needed a new battery.  The mechanic said he checked everything else out including the alternator, which he said is perfectly fine.  1 week later, the car dies again, wouldn't start, take it back, this time he said the alternator is not working and needs to be replaced. This is all fine, the car is 10 years old, don't recall having the alternator replaced before.  

 

My question is this, is it realistic to believe the alternator was tested and perfectly fine last week, and now after they put in a brand new battery, all of a sudden the alternator needs to be replaced?

 

Just wondering.

your car will not die on the road from a bad battery. the battery in your car exists for only a few reasons.....to start the car, to maintain power for computer memory, and for backup in case the alternator goes bad.

 in my opinion, the alternator was on its way out the first time.

[beltfedmx5 0]

One other option is your battery and alt are fine but you have something drawing the power and killing it over night. For example an interior light or bad trunk or hood.contacts causing a light to stay on 24/7.

[Catch22LR 0]

your car will not die on the road from a bad battery. the battery in your car exists for only a few reasons.....to start the car, to maintain power for computer memory, and for backup in case the alternator goes bad.

in my opinion, the alternator was on its way out the first time.

Is that still true in newer cars where the fuel injection and spark plug timing is controlled by computers? I mean, will they not stop and be unable to start if the battery dies while driving?

[twentynine30s 1]

Is that still true in newer cars where the fuel injection and spark plug timing is controlled by computers? I mean, will they not stop and be unable to start if the battery dies while driving?

Yes and No, the alternator will keep everything running.

 

You can start your car, remove your battery and drive it. It should stay running until you shut it off.. This is as long as your alternator is working properly. This is also true if you have a dead battery and a manual transmission car... push car, pop clutch and you're good to go.

[Porthole 15]

Unless your "mechanic" did a proper load test on both the battery and the alternator, he was just guessing.

If your alternator has diodes a common problem quick checks don't reveal is a bad diode will still output 13+ volts but the alternator will not have the charging amp capacity.

 

But, you could have both, a bad battery and a bad alternator, or one could have caused the other to fail.

 

Yes and No, the alternator will keep everything running.

 

You can start your car, remove your battery and drive it. It should stay running until you shut it off.. This is as long as your alternator is working properly. This is also true if you have a dead battery and a manual transmission car... push car, pop clutch and you're good to go.

your car will not die on the road from a bad battery. the battery in your car exists for only a few reasons.....to start the car, to maintain power for computer memory, and for backup in case the alternator goes bad.

 in my opinion, the alternator was on its way out the first time.

 

Modern vehicles that have a battery that is bad could very well not run with just an alternator supplying power.

If the car needs 30-40 amps to run and the alternator is supplying current just under the load required, the car will stop.

 

Many of the modern electronics run at 5 volts. There are literally dozens of computers and sensors running that all require 5 volts. That requires a source sufficient enough to step down the voltage to a reliable and steady 5 volts.

[TheLugNutZ 21]

Yes and No, the alternator will keep everything running. You can start your car, remove your battery and drive it. It should stay running until you shut it off.. This is as long as your alternator is working properly. This is also true if you have a dead battery and a manual transmission car... push car, pop clutch and you're good to go.

That's us how I would suggest testing the alt.

[tony357 386]

That's us how I would suggest testing the alt.

Thats a bad idea, disconeting the battery with the engine running can cause a spike and damage electronic components.. 

[jgofnj 5]

Thank you all for your great responses.  Learned a lot. 

[CMJeepster 2,777]

I did the same thing on my Explorer. Battery first as it was easier. Wound up doing the alternator 2 hours later... Ugh...

[TheLugNutZ 21]

Thats a bad idea, disconeting the battery with the engine running can cause a spike and damage electronic components.. 

 

DId realize that.  Been doing it for years with no issues... Guess I got lucky

[lunker 274]

When you say got burned, do you mean I probably never needed a new battery, or that never properly tested the alternator the first time ?

I just meant that probably 90% of the time the problem is the battery. It can become a "go to" answer if the mechanic isn't conscientious about checking the alternator, regulator, etc.

[1LtCAP 4,262]

Is that still true in newer cars where the fuel injection and spark plug timing is controlled by computers? I mean, will they not stop and be unable to start if the battery dies while driving?

batteries don't just die suddenly. if you shut your car down, and everything was runnign normally, then suddenly, it doesn't crank...then you were missing the warning signs(slow crank mostly).

even on newer cars, the alternator runs everything. the battery does nothing when the car's running, and all systems are functioning properly. BUT.....unlike old non-computer controlled cars, DO NOT DISCONNECT THE BATTERY WITH THE CAR RUNNING!! doing so, can spike one of the computers on the car.

[1LtCAP 4,262]

Yes and No, the alternator will keep everything running.

 

You can start your car, remove your battery and drive it. It should stay running until you shut it off.. This is as long as your alternator is working properly. This is also true if you have a dead battery and a manual transmission car... push car, pop clutch and you're good to go.

no no no NO!!! DO NOT REMOVE BATTERY WITH CAR RUNNING!!  you can spike one of the cars computers.

 

 also, some systems will not fire the injectors, or coil packs if the system goes below 9V. in this instance, you will not be able to pop start the car.

[1LtCAP 4,262]

proper testing of the alternator requires a load tester. simply putting a dvom across the battery terminals, and seeing your 13.2v to 14v doesn't tell you diddly. you need to load the alternator, and verify its output under load. most modern cars have amp output ratings of 80amps up to about 125amps. some smaller cheaper cars may only have a 60-70amp alternator. when loaded, your output should be within 10% or less of the rated output. \

 

 also, when checking output, you must verivy battery voltage at the alternator. you must check voltage drop between bat+ and the alternator terminal. you must check voltage drop between the alternator case, and bat-. and alternator and chassis ground. none should be above .1v. if any drops are significantly higher, then you need to check for corrosion between terminals, and mountings.

[Porthole 15]

batteries don't just die suddenly. if you shut your car down, and everything was runnign normally, then suddenly, it doesn't crank...then you were missing the warning signs(slow crank mostly).

even on newer cars, the alternator runs everything. the battery does nothing when the car's running, and all systems are functioning properly. BUT.....unlike old non-computer controlled cars, DO NOT DISCONNECT THE BATTERY WITH THE CAR RUNNING!! doing so, can spike one of the computers on the car.

 

This is not correct.

Batteries can give up the ghost without much warning if they get shorted or "open" internally.

 

Long before computer controlled cars is was bad to disconnect a battery while the vehicle was running. It was bad when they were equipped with generators, mechanically regulated alternators as it is to the modern day vehicles.

 

The battery is a key component in all vehicles, serves a load and voltage stabilizer.

A fully charged wet cell battery should read 12.6 volts at rest, no load.

[RobertJames 14]

Unless your "mechanic" did a proper load test on both the battery and the alternator, he was just guessing.

If your alternator has diodes a common problem quick checks don't reveal is a bad diode will still output 13+ volts but the alternator will not have the charging amp capacity.

 

But, you could have both, a bad battery and a bad alternator, or one could have caused the other to fail.

 

 

Modern vehicles that have a battery that is bad could very well not run with just an alternator supplying power.

If the car needs 30-40 amps to run and the alternator is supplying current just under the load required, the car will stop.

 

Many of the modern electronics run at 5 volts. There are literally dozens of computers and sensors running that all require 5 volts. That requires a source sufficient enough to step down the voltage to a reliable and steady 5 volts.

Old cars with generators would remain running once started without a battery.  An alternator requires 12v+ from the battery to energize the field to create and maintain a charge rate of ~14vdc.  If the alternator fails, once the battery voltage drops below about 9vdc, there is insufficient voltage for the coil(s) to fire the spark plugs.  Your cars engine will sputter and stall and will not re-fire.

[Porthole 15]

Automotive alternators have field current supplied by the battery, but once the alternator is up to speed field current is also supplied by the diode group. So a car "can" continue to run without a battery, once started.

The DC output is not steady and continuous, more of a very quick pulse. The battery helps even things out, and for electronics that need very clean power, filters are added.

 

A typical automotive wet cell battery is considered 100% discharged at 11.8 volts, not 9V. I have never seen a car that can run with a dead battery and a non working alternator.

[Murphy4570 15]

Be aware that a bad battery can also cause the alternator to fry itself. Not common, but it can happen. When you run a car with a dead battery, the alternator is constantly at full field, trying to charge it. Running at full tilt like that can burn it out.

[tony357 386]

Be aware that a bad battery can also cause the alternator to fry itself. Not common, but it can happen. When you run a car with a dead battery, the alternator is constantly at full field, trying to charge it. Running at full tilt like that can burn it out.

This is the main cause of alternators going bad..

[Porthole 15]

This is the main cause of alternators going bad..

 

Heat, then component failure.

 

Batteries go bad all the time, alternators do not. Actually, for what they do, and the conditions they operate in, alternators are very reliable.

[intercooler 41]

Did they do a parasitic draw test, this is especially relavant if the car has an aftermarket radio, alarm etc. also, alot of OEm radios like to fail and cause the battery to drain . Others like puddle lights, fans etc can do it too