DIY Car Alternator Test (Voltage & Amperes) | Multimeter Edition

these are a few symptoms of a possibly bad alternator charging system warning or battery light doesn't go off after the engine starts you hear a growling rumbling or a squealing noise headlights are dim or the other electrical components are malfunctioning the battery keeps dying or is always low on water your engine doesn't crank or crank slowly you have a dimly glowing battery light let's check a few things before we start testing make sure that the battery isn't bad a lot of times it's just a bad battery and not the alternator for that check out our video on how to test a car battery your batteries should also be fully charged quickly check the wiring for kinks or loose connections if they're loose corroded or sulfated clean and tightened up more often than not that will fix any starting issues you're having some stealer ships will tell you that it's a bad battery or an alternator issue when it's only a bad terminal to post connection can't stress the importance of clean connections enough visually inspect the fuse if it's covered you can check it with the help of a test light the test light can also help check for power in the clip check for a worn out or broken drive belt also adjust the tension if the belt is loose slippage causes friction and thus generates heat which will glaze the belt and further decrease its gripping ability tighten any loose alternator mounting bolts and brackets vibrations from that will lead to alternator damage while the engine is running never disconnect a battery cable to test the alternator the resulting high voltage spike may damage the alternator or other electrical components don't wear loose clothing that could get caught in moving parts or jewelry that might bridge an electrical connection try not to run these tests in extreme temperatures charging voltages will vary and you might not get accurate readings to detect a problem a dimly glowing battery light means that there's a bad diode that's because the current flowing through the light circuit from the positive side is not being offset fully from the negative side with the alternator charging the diodes supply voltage to ground side of the warning light the light will get brighter if multiple diodes fail at the same time as more diodes fail the feedback current from them will reduce the alternators ability to generate power to test the amount of alternating current being released into the system set your multimeter to AC volts now connect the red probe to the alternators beep post and the black lead to the alternator housing alternators produce AC which is converted into DC by the rectifier with the help of diodes diodes are one-way gates and allow current to flow through them in only one direction also known as their forward direction with a faulty rectifier the alternator releases an excess amount of alternating current into the electrical system that can trip error codes and cause problems a shorted out rectifier may also drain your charged battery overnight because of its direct connection with the battery start your car and connect your multimeter probes to the battery red lead to the positive side and black to the ground disregard that initial spike in voltage let it taper off and level out the charging voltage varies depending on the overall battery health its current state of charge the temperature and the amount of load on the electrical system voltage should read between thirteen point eight to fourteen point two volts at idle with all accessories off for most alternator's charging properly check the owner's manual for manufacturer's specifications the acceptable range of charging voltage in general as thirteen point eight to fourteen point seven volts raised rpms a few times there shouldn't be any significant rise in the charging voltage if it crosses fifteen volts and continues to rise you have a bad voltage regulator most modern vehicles regulate voltage output through PCM the powertrain control module now let's test the output voltage at the alternator itself this is what the positive post on the alternator looks like touch your red lead to the batter beep plus post on the alternator and the black to the alternator housing the readings should roughly be the same as the battery if there's a difference there's resistance in the circuit you'll need to check for bad positive or ground connections causing that we will also run a few voltage drop tests on both the supply in the ground side later in this video voltage is important but so are amps without those amps the voltage level won't mean anything your battery has an amperage rating and so does your alternator newer vehicles have higher output alternators compared to older vehicles current output is easy to check if you have the right clamp meter that measures DC amps simply clamp it around the wire that connects to the B+ post of the alternator while it's charging compare the reading with your manufacturer's specifications that should give you an idea of its current performance at idle without any load the amount of amps produced will be the least when a load is applied and the engine speed increases current output increases to maximum output is achieved at around 2500 rpms now let's check how much of these amps produced are being sent to the battery the difference between both the values is what is being consumed by other electrical components like the fuel pump etc you can see that the amps being sent to the battery are fairly low as compared to the alternators current output that's because the battery is almost full and it will only accept a saturating charge this number would have been a lot higher if it were discharged let's put a load on the alternator in increments and see its reaction in terms of amperage output as we turn off the accessories the output comes back down your alternator may need to be repaired or replaced if it fails to keep up with the increased electrical demand but before that let's make sure that there is no abnormal resistance within the car's electrical circuit voltage drop tests are a good way to find problems in a circuit poor wiring connections on the battery ground straps or anywhere else in the charging circuit will cause resistance that will result in voltage drop and a reduced current flow the alternator will be forced to keep up causing stress and unnecessary wear with the engine running and a load applied on the system touch one lead from your voltmeter to the beep post on the alternator and the other one to the battery positive post any reading over 0.4 volts means there's excessive resistance in the circuit if the meter reads above 0.4 volts check the wiring connections this includes the battery post in the terminal the alternator beep post in the cable next check the fuse box connections clean and tighten if necessary sometimes the fuse itself might be putting up some resistance check that too if there is no significant drop between all these points your cable might have gone bad supply-side cable runs from the beat post to the fuse box then from the fuse box the cable goes on to the battery to check which part of the cable is faulty check for voltage drop between the B post and the fuse box input connection then check from the fuse box output to the battery positive terminal if you find the source of that drop here replacing that part of the cable will most likely solve the charging issue remember voltage drops on the positive side can cause under charging now let's test the negative side again with the charging system loaded touch one lead from the multimeter to the negative battery post and the other one to the alternator housing voltage drop should be less than 0.2 volts if not inspection and cleaning of ground connections is in order voltage drop on the negative side can cause overcharging it fools the regulator into thinking that the battery is low check each connection to find the source of that resistance ground goes from the battery to the chassis the whole chassis then becomes a giant ground for the electrical components to share from the chassis it goes on to the engine ground straps like these should not be loose or dirty and finally to the alternator from the engine on which it is mounted begin by testing the connection between the negative battery post in the terminal next check between the post and the chassis then from the chassis to the engine finally between the engine and the alternator housing the largest reading that you get between any two points is where the problem lies fix it retest for voltage drop to make sure that there aren't any more bad connections contributing to the problem if you don't have the equipment or these tests seem too difficult to perform on your own have your alternator bench tested at some qualified place nearby thoroughly tested before throwing it out and also test the replacement beforehand to save yourself from the added hassle later on in case your alternator needs to be replaced make sure you get one with a similar pulley and an amperage rating that matches your needs if you're running a lot of power-hungry accessories consider upgrading to a higher output alternator always replace the drive belt after 80 thousand kilometers or 50 thousand miles if the battery is discharged or dead always use a battery charger to charge it first alternators are not designed to charge a rundown battery they are just there to maintain the charge never charge a frozen battery let it thaw first only then can you charge and test if it functions properly perform a parasitic draw test if both the battery and the charging system are functioning properly but the battery keeps running down try not to idle for too long in hot temperatures with radio lights and a/c running that will be all for today also check out our other videos on how to properly test and replace a battery if this helped you in any way like share comment and don't forget to subscribe until next time goodbye

Channel: YouTilitarian

Views: 80,810

Keywords: YouTilitarian, how to properly test an alternator, alternator test, diagnose, alternator, generator, test alternator with a multimeter, multimeter, voltmeter, ammeter, dead battery, car won't start, battery down, faulty alternator, not charging, regulator, voltage, bad battery, battery dead, discharged, battery check light, lights dim, cranks slowly, voltage drop, rectifier, amperage output, diagnosing alternator problem, amps, DIY Car Alternator Test, volts, ampere, bad alternator symptoms

Id: A3XcIXdH_Cg

Length: 12min 44sec (764 seconds)

Published: Wed Jul 08 2020