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Batt or Alt. ?


maymay's Avatar
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09-08-03, 02:06 PM   #1  
Batt or Alt. ?

Mazda 626 1999: I may have a bad battery or bad alt. I understand that there may be a simple test to determine which one may be bad. The test is, with the engine running, unhook the battery, if the engine continues to run, it would be a bad battery, if the engine quits running, then it would be the alt. Is there any truth to this and do I have the proccedure right. Thank you in advance

 
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michael van's Avatar
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09-08-03, 02:28 PM   #2  
I would not do that

I would not do that on a newer car you could get away with that in the 60s.Todays cars are computers and doing so could damage it.
Go get a multimeter from Radio shack for 9 bucks get a digital one.
Start car and check for voltage at batt terminals.If the alt is charging you will get about 13.5 volts.Use the DC scale on your meter.Turn on lights and check again should not drop more then half a volt.
If your battery is more then 4 years old it is on its last leg.

 
billys68ss's Avatar
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09-08-03, 04:05 PM   #3  
Also you can put the positive (red) leg of the DVOM on the big lug on the alternator and the negative (black) leg on a known good ground, (which would be a non painted metal surface) and you should get between 13.8 and 14.2 volts. As Michael said DO NOT remove the battery cable while the engine is running as you can send spikes to your computer and other electronic components and cause you unnecessary problems.
Hope this helps,
Billy

 
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09-09-03, 06:40 AM   #4  
Joe_F
Original poster:

Whoever told you that obviously shouldn't be working on cars. That is NEVER a valid test, and on most cars can wipe out the alternator diodes (and a whole lot more) in a jiffy.

As noted, an inexpensive DVOM will do the trick and tell you where the problem is.

 
maymay's Avatar
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09-09-03, 06:15 PM   #5  
Thank you all for your expertise, I followed your advise and it was the battery. I replaced it and all is well with that problem. But, perhaps you could help me with another problem with the same car without opening a new post. When we apply the brakes, the vehicle shutters like the dickens. It lessens as the speed drops. We took it in to have the rotors turned, but the problem still exists. We do not have anti lock brakes. Can you make a suggestion? Thank you in advance

 
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09-09-03, 06:54 PM   #6  
about the only thing it can be is warped rotors are out of round drums if applicable depending if it has rear disc or drum brakes.

 
billys68ss's Avatar
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09-09-03, 07:28 PM   #7  
On a 1999 model I am sure you have Antilock brakes. But you probably have a warped rotor. There is a possibility of ABS issues that can cause the type of pulsation you describe, but most likely its in the rotors. Get the suspension checked out real good. Also If you had the rotors turned and it is still doing it then you might want to replace the rotors. Check the hub bearings if equipped. Also check the tire balance. Rotate the front tires to the rear and see if the problem goes away.
Hope this helps ya,
Billy

 
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09-10-03, 06:27 AM   #8  
Joe_F
Billy is right on the money.

 
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