Tip & How-To about GMC Jimmy

How to test an alternator

Diagnosing alternators are about the same on all vehicles. You will need a digital multimeter to do the tests. ( WalMart has cheap ones for around $9)
First, you need to make sure it isn't really a battery problem.
One at a time, take off each battery cable at the battery and thoroughly clean the terminals and cable ends and retighten securly. NOTE: take off negative (black) first, leave it off, then do the positive (red) cable. Re-attach positive and then negative last. This way you will avoid any contact spark on the positive side which could damage your computer or wiring.
Next, check the voltage on your battery. With multimeter on Volts DC 20 setting, you should get around 12.65 Volts DC.
NOTE: Always use red lead on meter to positive (red) cable / battery post and black lead to negative (black) cable.
If your battery was discharged some due to your alternator suspition, you can go to the next step and start / jump start the engine.
With engine running test voltage at the battery again. You should get 13.4 to 14.2 volts.
Wiggle your test leads, scratching at the metal battery / cable terminals to ensure a good metal contact.
If you get more than 14.2 volts DC, your voltage regulator in the alternator is bad and the alternator must be replaced.
If you get less than 13 volts DC, move your black test lead to an engine ground (metal bolt which holds the alternator on, or clean metal surface / bolt on engine itself) and see if your readings are the same. If you get a higher reading of 13.4 - 14.2 volts your negative connection (cable or attatch point on engine) is faulty.
If still the same (less than 13 Volts) Move your red test lead to the connection on the back of the alternator itself where the red wire connects and black lead to ground. If your voltage is higher in the 13.2-14 volt range, your positive (red) cable or connection in that circut is faulty.
If none of these test produces at least 13.2 volts DC, replace the alternator.
If you get good alternator output voltages, but your battery won't hold a charge, have your battery load tested (free at auto stores or WalMart) as it is probably bad.
Hope that helps!

Posted by on

Cars & Trucks Logo

Related Topics:

Related Questions:

1 Answer

my 96 volvo has problems starting after a cuple days after jumping it

Without some simple tests to provide another clue or two, this is impossible to sort out. You should consider buying a cheap multimeter. Just the cheapest one you can find will be a nice device to have around. If you have one, or get one, with the multimeter set to "DC Voltage", touch the black lead to the - post and the red lead to the + post of the battery. Remember your reading. Now crank the car. Repeat what you did a minute ago with the meter. The voltage should be considerably higher (like between 14 and 16.75, or even more for some cars). This means your alternator is working. That is the clue you wanted because now you can deduce that the battery needs to be replaced. If you read, say 11.5 volts with the vehicle not running and less than 12.5 with the vehicle running, you can strongly suspect that the alternator has failed. BUT -- first take the battery to the nearest auto store, or walmart and have them do a "dead cell" test on it (free). If the battery has a dead cell (highly unlikely with your current circumstances), replace and then retest. If the battery tests ok, then your alternator is not producing.

Feb 29, 2012 | Volvo Cars & Trucks

2 Answers

HoW Do I diagnose alternator problems in 1990 mercury sable

one is by disconnecting negative cable from battery for an instant with car running, if car stops running, your alternator is bad, other way is to read voltage at the battery with car running, what your reading is alternator voltage, should be between 12v and 14.9v any reading off means your alt. could be bad, but if your not shure, if you can, take the alternator out and have an autoparts tested for you they will tell you if is good or not. thanks

Jul 13, 2011 | 1990 Mercury Sable

1 Answer

truck won't start. I checked the battery and the battery is good. I had my wife give me a jump it started but later after letting it run for 10 mins, I turned it off and started it again.this time it started right away.I let it run another ten minutes then turned it off then started it again an hour later and it wouldn't start. I think it maybe the alternator.

I would suspect the alternator also, but I would check the recharge voltage before taking it off. You will require a digital multimeter. If you dont have one, you can get a cheap one at WalMart for about $12. With engine running, multimeter set to 20 volts DC, hold red lead to POS and black lead to NEG on battery. Your voltage reading should be 13.6-14.4 volts. Lower than 13 volts, you are not recharging correctly, but this could also be bad or corroded wiring, so check the voltage from the large wire (red lead) on back of alternator to ground. (Negative on battery and then to alternator to bracket bolt at alternator). If you get a good reading of 13.6-14.4 volts, could be a bad battery cable or corroded / loose connection in wiring. If your alternator checks OK, your battery may be failing. You can have it load tested free at Autozone and WalMart. Just because a battery reads 12+ volts, does not mean it can hold a charge with enough amperage to start your motor for an extended period of time. Running your battery very low or dead can cause it to fail like that. Also, batteries are usually due for replacement at 3-5 year intervals. They dont last forever.
Well, run some checks and let us know how you do. If you cant isolate the problem using those checks, reply and we can try a different route.
Thanks, Mike

Jun 24, 2011 | 2004 Chevrolet Colorado

Not finding what you are looking for?

3,118 people viewed this tip

Ask a Question

Usually answered in minutes!

Top GMC Experts


Level 3 Expert

78919 Answers

Colin Stickland
Colin Stickland

Level 3 Expert

22326 Answers

Ronny Bennett Sr.
Ronny Bennett Sr.

Level 3 Expert

6927 Answers

Are you a GMC Expert? Answer questions, earn points and help others

Answer questions