Testing Alternators

Thread Starter

john2k

Joined Nov 14, 2019
84
My car has quite some high mileage and I've read in places that the alternators are known to fail on them. The car is 12+ years old with 190k miles (not km) So i can either chnage the alternator out which isn't cheap or keep with it until it fails. Right now I haven't had any issues with it. Car can sit in the freezing cold for a week and starts up fine and then an 30min to 45 min drive seems to charge it enough for it to start up again properly. I hear that some auto parts stores can test alterntors. Is there any special way to test if an alternator is still functioning normally or if its on its way out? Can I not just use a multimeter? I read that measuring voltage on the battery terminals with car off should read about 12 point something and with car on and accessories like AC and headlights turned off it should read over 14 volts. Is this a good test?
 

SamR

Joined Mar 19, 2019
1,825
You will quickly know when it fails. Leave it alone until it does as it may never fail as long as you own the car. Failed alternators usually do so in one of 2 modes, burned-up bearing or blown diode. Diodes can be replaced cheaply (if you can find a shop that does it), bearings not so easy and usually a rebuilt replacement needed. Sounds like a battery issue since it charges back up which means the alternator is working. For alternators, it is an all or none situation.
 

shortbus

Joined Sep 30, 2009
7,461
Like Sam said. You can get it tested at a place like Autozone and it could fail on the way home. If it's not broke don't fix it. But do open the hood and look at the belt, you will replace many more belts than alternators in your life time.
 

Thread Starter

john2k

Joined Nov 14, 2019
84
You will quickly know when it fails. Leave it alone until it does as it may never fail as long as you own the car. Failed alternators usually do so in one of 2 modes, burned-up bearing or blown diode. Diodes can be replaced cheaply (if you can find a shop that does it), bearings not so easy and usually a rebuilt replacement needed. Sounds like a battery issue since it charges back up which means the alternator is working. For alternators, it is an all or none situation.
Thanks Sam. I'm not having any issues at all with it and battery is new. All is working fine. But since i've read of the alternators failing on several forums, it got me thinking if I can test it to make sure mine isn't on its way out or anything.
 

Thread Starter

john2k

Joined Nov 14, 2019
84
Like Sam said. You can get it tested at a place like Autozone and it could fail on the way home. If it's not broke don't fix it. But do open the hood and look at the belt, you will replace many more belts than alternators in your life time.
I've checked the belt and had changed it about 5 years ago and done about 65k on the belt.
 

Thread Starter

john2k

Joined Nov 14, 2019
84
I've came across this really cheap piece of kit pictured below that seems to allow battery condition and alternator condition reporting. I wonder how it works compared to say using a multimeter to measure voltage with car off and on.

 

SamR

Joined Mar 19, 2019
1,825
The usual belt issue is too loose and slipping. If the belt is fairly taut on pulleys it's good. If it shows extreme wear or missing chunks replace it.
 

SamR

Joined Mar 19, 2019
1,825
Most good parts stores will test the battery for free (hoping to sell you a new one). I don't trust most of those testers as they are typically unreliable. IE won't find a dead cell. Most good brand batteries will last the advertised number of months and not much more. Batteries are listed for the typical model of car and do not include extras such as winches and hi-power amplifiers. Those may require a better battery than listed for the car. And possibly a higher amp alternator. Most alternators are ~30-40A and can go up to 100-200A.
 

twister007

Joined Feb 29, 2012
43
Large trucking companies do preventive maintenance. If their truck breaks down on the road, they are at the mercy of tow trucks, repair shops, and late delivery charges. For this reason they don't wait for things to fail. They know that a alternator, water pump, starter or other items will start to fail after a certain number of miles or hours, so they change them ahead of time.
 

shortbus

Joined Sep 30, 2009
7,461
Large trucking companies do preventive maintenance. If their truck breaks down on the road, they are at the mercy of tow trucks, repair shops, and late delivery charges. For this reason they don't wait for things to fail. They know that a alternator, water pump, starter or other items will start to fail after a certain number of miles or hours, so they change them ahead of time.
None in this area do that. The drivers are usually smart enough to see the warning signs and tell the mechanics. do you realize how expensive it would be to change out alternators and water pumps just by mileage?
 

Thread Starter

john2k

Joined Nov 14, 2019
84
Another question: As i understand there is some sort of rectifying diode(s) inside alternators that prevent voltage spikes. Apparently voltage spikes can happen if these diodes fail. But if voltage spikes happen when a diode inside a alternator fails then doesn't that fry the ECU's or do the ECU's all have diodes to handle voltage spike too?
 

ronsimpson

Joined Oct 7, 2019
447
The drivers are usually smart enough to see the warning signs and tell the mechanics.
Many cars only have a "stupid light". When the battery voltage gets below 11.5V the light comes on but it is very late by then. I have trucks that have meters. (battery current or voltage) By watching the meters I know the conditions of the battery/alternator.
 

shortbus

Joined Sep 30, 2009
7,461
I have trucks that have meters. (battery current or voltage) By watching the meters I know the conditions of the battery/alternator.
My reply was to a member that claims fleet companies change alternators and other expensive parts on a mileage basis not a need basis. I know of nobody doing such a thing. Your watching gauges is what is common, and only replacing things when needed.
 
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