Dies after jump start cable disconnect

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Hello,
I'm sure this has been questioned hundreds of times but I can't pin-point the answer. Thanks in advance for your advice!
We've got this Explorer that was starting fine until today. Rapid
clicks when the key is turned. She was left sitting for 10 hours and started right up but then, I tried to lower the power window and she died.
So I jump started her and she fired right up. When I disconnected the cable, she died.
I guess if I leave the good car running and connected a while, the dead battery can charge up a bit more and hopefully she'll keep running?
What do you think is the root of this issue? Old battery, alternator? Battery cables are pretty corroded. I'm gonna clean and re-attach those thoroughly tomorry.
Thanks in advance for your advice!
Ford Explorer 1991 90,000
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snipped-for-privacy@gmail.com wrote:

Sounds like the alternator is toast. Once started, it should run. If you charge up the battery it will only run for a while. Could be that both are bad but I'd look at the alternator first.
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snipped-for-privacy@gmail.com wrote:

Almost certainly the alternator, but could also be corroded connectors. A voltmeter of some kind should be in every automotive tool box, even the Radio Shack 22-112 for about $6.
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snipped-for-privacy@gmail.com wrote:

I think you have more than one problem.
Your symptoms... esp if it starts after 'waiting' point to multiple corrosion/poor connections.
Go through your entire underhood power wiring connections, battery terminals first. Then do a tug/wiggle test on all battery traced wiring.
Get the cheap voltmeter as said elsewhere, and look for over 13.5 volts at battery posts once car has been running a while - this should increase to 14.3 once battery is charged.
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Backyard Mechanic wrote:

On initial startup charging, it should be 14~14.5 system volts, with a tapering down charge to 13.5 volts or so. A fully charged battery (ignition off, overnight) is about 12.7 volts.
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I dont think so. Unless that stupid PCM regulation does something different than the classic charging paradigm.
On a discharged, though healthy, battery Regulator Current limiting reduces voltage.. once Current limiting drops out, voltage regulation takes over.
A full battery under charge, about 14.5.. as you say 12.5 after overnight key off
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Backyard Mechanic wrote:

??? It's done it the way I described on every vehicle I've ever owned for the past 50-some years; the startup charging (amps or volts, however you care to measure) flow is high, then tapers down as the battery voltage increases.
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Measure it again... apparently, like me, you're getting senile! But I remember THAT!
;)
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Backyard Mechanic wrote:

But I have voltmeter gauges on all three of my vehicles, and the gauges all behave the same way; the initial system voltage is 14.2~14.5 volts after startup, and then tapers down to 13.5~13.8 volts after driving for a little while. Maybe we're not talking about the same thing?
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what backyard is talking about is a volt meter you hook up to the battery. the voltmeter in the dash is not very accurate
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Tom wrote:

There's a small amount of "line loss" amounting to 0.1~0.2 volts less than reading directly from the battery posts, but my VM gauges are all aftermarket dash-mounted digital gauges, and they all show the same thing; the voltage goes high immediately after startup, then tapers DOWN as the battery is recharged from the depletion of the battery charge for the startup. I've really never seen one start with low voltage and then climb as the battery becomes fully charged. If it behaved the way others have described, I'd suspect a runaway alternator/bad VR.
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I think that hes talking about is the alternator should be putting out 14.x volts when the alt is running ok, and the battery after its sat for some time should only be running slightly over 12, and i've seen mine run 11.9. If after boosting the battery is still only running 11.x or 12.x after running for a bit, i usually conclude the alternator has failed. Anyway, I think i'm in the ballpark here, it hasn't failed me yet, unless i've replaced many a good alternator with a new one hahaa.
Don't look at the meter on the dash, that is about as useful as a two legged table :)

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wrote:

The charging AMPERAGE starts high, but the VOLTAGE is low. The output current is NOT regulated by the regulator or PCM. It is limitted by a combination of the rotor field strength, stator resistance and the load resistance. As the battery gains charge, it's effective resistance goes up, reducing the amount of current flow. The charge voltage therefore goes up, untill the regulator starts cutting the field strength to reduce the charge. Untill the charge voltage gets up into the 13.8 range somewhere, the regulator is doing virtually nothing. On a "dead" battery, a 160 amp alternator may be able to force 160 amps into the battery at something like 11 volts. As the battery charge come up, the current quickly drops off to, say, 100 amps and 12.2 volts. As the state of charge comes up, the charging rate drops to 60 amps and say 12.6 volts, then to 40 amps and 13.0, 20 and 13.4, 15 and 13.8, and levels off at something like 5 and 14.4. 2.2 volts per cell is considered full charge, and it takes roughly a 10% voltage "overhead" to charge a typical lead acid battery, so 1.1X13.2.52 is the typical charge cutoff voltage of a typical 12 volt system voltage regulator.
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Thanks for the voltage information! My dash guage indicates ~ 12-14 volts.

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Thanks for all the answers! I suspect the battery. Looking at in the light, I see that it is leaking battery juice from near one of the posts.
I want to get the alternator tested but, I can't figure out how to remove the drive belt on this vehicle. None of the accessories pivot and I can't figure out how the idler pulley can be used to release belt tension. Any ideas?

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snipped-for-privacy@gmail.com wrote:

Clean it up, replace the battery. Put the rest on hold until you can see the results. Should be able to take it someplace for a test without removing it (providing you even need to).
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F. H. wrote:

if its leaking you can most certainly smell it...
if its leaking, theres nothing you can do, unless you're better with krazy glue than i am hahaa
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Thanks F.H.
This is exactly what I did. I cleaned all the corrosion off the cables and replaced the battery. It started and ran fine then but the voltage was low. Prolly about 10 volts and was not climbing. I took the alternator in and had it checked out. It was bad. Replaced the alternator and now no problems at all. New battery, new alternator and clean cables. Voltage somewhere between 12 and 14 volts according to the stock dash guage.
Great advice!
BTW, I took the time to take the alternator out of the vehicle to have it tested. Did not even know that this can be done in the vehicle. I know now. Thanks!
F. H. wrote:

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Thanks all for your help! Job done with the help of everyone here. Replaced Alternator and Battery and now she's good to go!!
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Just curious, what age was the original battery that you replaced? It may have died as a result of the alternator slowly dying and not being able to properly maintain a charge to the battery. Thus, as in your case and many others, you end up replacing both the battery and the alternator instead of just one failing component.
Sharky
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