# Diagnose battery vs. alternator by removing terminals?



## DJ in WA (Jan 28, 2005)

We have two vans at work that wouldn't start today. My fellow workers like to try their diagnostics and decided the Dodge has a bad battery because after jumping it, they removed the ground terminal and it kept running. Date on battery is Sep '06.

The Ford, after jumping, would die after removing the terminal. So they are saying the alternator is the problem. Date on battery is Aug '08.

If this is correct, please explain.

We took the Dodge into a mechanic a few weeks ago after occasionally not starting, and after testing some things, he said it was corroded terminals - cleaned them and charged $170, so we're a bit leary of doing that again. Will probably just get a battery and try it.


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## agmantoo (May 23, 2003)

In older vehicles that predate the use of electronics to control the functions on the engine it was acceptable to disconnect the alternator to see if the vehicle would continue to run to isolate whether the problem was battery or alternator. With the introduction of electronics to control the engine that disconnect practice can create more problems than it can solve. Get a new battery and temporarily put it in the Ford and go to one of the big auto discount stores. They will diagnose the problem. With the Ford fixed put the new battery in the Dodge and take it to the discount store for a verification the alternator is functioning properly. You should then have both vehicles roadworthy and having bought no more than a battery or two and an alternator which you can install yourself.


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## farminghandyman (Mar 4, 2005)

on batteries keep the terminals clean a dirty terminal can and will cause battery problems and charging problems and starting problems, also check the ground where the cable bolts to the block or the frame of the auto. many times just a simple cleaning will do wonders, also wash off a dirty battery, dirt and moisture will conduct electricity, (even more so if there is a little battery acid in it) and can short or drain a good battery.


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## ace admirer (Oct 5, 2005)

i would not disconnect the battery from a modern running auto...for reasons stated above...the voltage can go high and destroy some electronics.....why not just drive the thing to advanced auto or such and let them put it on their machine...just make sure the guy knows how to run the test machine or the results may be mis-read.


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## HermitJohn (May 10, 2002)

ace admirer said:


> ...just make sure the guy knows how to run the test machine or the results may be mis-read.


How exactly do you do that? Diploma with Phd in testing batteries on the wall?


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## Beeman (Dec 29, 2002)

This is all good if it were 1960 but even then they couldn't tell if it was the alt./gen or the regulator. Basic 12v auto starting and charging system can usually be accurately diagnosed with a $29.95 digital volt ohm meter and a little knowledge.


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## foxtrapper (Dec 23, 2003)

Generically, I agree with your fellow workers. The Dodge kept running because the alternator was producing enough juice. The Ford died because the alternator was not producing enough juice. Both may still have bad batteries.

Disconnecting the battery is a pretty good way to accidently ruin the alternator, but that's a seperate discussion.


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## Wis Bang (Feb 20, 2009)

Most computers will shut down if the battery voltage falls below a set voltage. If the battery terminals were corroded and you still have a problem, the battery is probably not retaining enough voltage, are the terminals still clean?

GM's 'side terminal' bolts used to get a thin layer of corrosion that was hard to see and stopped 'em dead!


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## cfabe (Feb 27, 2005)

I second the suggestion to either take the vehicles in to have them tested or take out the batteries and have them tested. Most auto parts stores can test either for free. Sep 06 battery is old enough it could be bad if it was abused or a cheapie to begin with.


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## HermitJohn (May 10, 2002)

I wouldnt play games with all electronics on todays cars by removing battery cables with car running. Pre computer cars, you start car, turn on headlights to put load on alternator, then could disconnect battery to test alternator by seeing if it stayed running. This was the "no tool" shade tree method. Modern car, if you suspect alternator or battery, best advice is to remove it and take it to parts store in a functional vehicle. Dont want to play games starting dead car in some parking lot or stalled along the road.

Quick modern shade tree method of checking alternator on any vehicle is first stick voltmeter on it with car running if you get 14 to 15 volt, its probably good. Nice just to install a voltmeter gauge on dash. Now for further test, on back of alternator take piece steel, nail, bolt, screwdriver, whatever. Hold it in vicinity of rear shaft bearing (middle of the back of the alternator). If good, you will feel a very strong magnetic pull. Better yet is to have a ammeter connected in the charging circuit and turn on headlights, heater fan, etc. Should take the load and stay above zero. If its in discharge side even a bit, then alternator isnt putting out enough current even if it has correct voltage. Alas modern cars have such high electrical loads old fashioned dash ammeter probably not good idea as all current for accessory load has to pass through it. Need one of those you hold over wire, inductive ammeter think they call them. Used to have one and it was handy though at times would give confused readings as it was also picking up current travelling in different wire.


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## Wis Bang (Feb 20, 2009)

My '99 Grand Am shuts down when the battery voltage goes under 11 volts. Most test procedures fault trees start w/ charge the battery and then begin test...

W/ the car it was a bad alternator, AAA gave the wife a jumpstart at teh supermarket and she barely got home b/4 it died again. I have a load tester & a charger for the batteries so it was E Z to eliminate the battery...


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## Rocky Fields (Jan 24, 2007)

Hey.

A fully charged battery is about 12.5 volts. With a fully charged battery in the circuit and the vehicle running, voltmeter will show about 14.1-14.2 volts across the battery terminals if alternator is working. Borrow a voltmeter.

RF


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## DJ in WA (Jan 28, 2005)

Thanks for the replies, I couldn't log on here last night to respond. I took a quick peek at this yesterday morning before heading out and shared the consensus with the guys. One was a bit peeved that I was questioning his diagnosis - I said he could still be right about the alternator, but the removing-terminal-test on modern vehicles is the problem.

Anyway, while there are some good ideas here on further messing with them, we need to focus on our real jobs. And when I/we end up wrong on something, the man with the money gets upset, so I'm just letting him tell us what to do. Which is now to go buy a couple of batteries and put them in. Which I did yesterday. They ran fine afterward, but I'm assuming they could be living off a fresh charged battery. Sent the Dodge on a 40 mile run with several stops, and did okay. Will keep the Ford close for awhile to see if fails.

If further problems, we'll just have to get them into a professional. Don't know what they missed last time on the Dodge, unless some kind of intermittent problem hard to diagnose - wiring, connections? Or can batteries test okay and still be bad somehow? I did have NAPA test the Dodge battery since was one of theirs and they said it was still good, but with just a low charge. Anyway, we'll see.


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