OK so 89 F150 5.0 that had been sitting for a while and the battery was too low to start. But I messed up and put jumper cables on backwards at first, was only connected for a few second like that. I fixed the cables, started it up, but when I disconnected the cables it died soon after. So I jumped it again, and come to find that when I disconnect the cables the voltage dropped fast. So I assumed I had maybe killed the alternator or it wasn't working in the first place, I got some electrical gremlins with this rig.


Anyway, I only needed to move it so I left it hooked up a bit and moved it. When I was finishing I disconnected the cables and watched the voltage drop to around 11.3 in a about a minute and shut it off for the day to deal with later. However when I shut it off the voltage started to climb quickly. And within a minute was back to 12.


That makes no sense to me, unless for some reason the alternator was actively drawing significant juice off the battery and when I shut it off it stopped doing so allowing the battery to recover? How this is possible I have little idea, could the field have been drawing that much juice to have that much effect? BTW I had no accessories on in the truck, no fan, no lights, etc. Given how an alternator works even if the regulator was f'd or even reversed the diodes wouldn't allow it to reverse polarity. UNLESS the diodes reversed, if the diodes reversed even if all else was working fine it would have the exact opposite effect as desired.


Ever heard of anything like this, is it even possible?

 

Usually when you reverse the cables, you take out some fusible links. If you have other problems going on, pulling the battery cable off with the engine running is very risky. You can blow other things up in a hurry by doing that, I am surprised it's still running.

The only thing that controls the voltage when other things go wrong is the battery. By taking the battery off, things can go wild and you can blow up anything electronic. I would not recommend doing that from now on.

You have fuel injection, so that takes power to run, so that is one load that was on the battery that would pull it down, but when you turned it off would let the battery relax and the voltage would climb back up.

From now on if you want to check to see if it's charging, get a voltmeter and check the voltage on the battery with the truck off. Then check the voltage on the battery with the truck running. If it's charging, the voltage will be higher on the battery with the truck running.

Take your meter and put the black lead on the neg of the battery, and put the + lead on the large output terminal on the back of the alt. It depends on what model alt you have as to where the output wire(s) are connected. You should get 12v on that spot. If you don't, then the fusible link in that wire is burnt and needs to be replaced. If you get 12v, take the alternator to the store and let them check it.

If you have any other problems with things not working electrically after the cable mix-up, check all those fusible links over at the starter solenoid. These are colored rubber things in the wire and are made to burn out when bad thing happen.

 

Whoah, I didn't disconnect the battery, I disconnected the jumper cables.


Voltage at the alternator was same as battery.


It wouldn't run but for a minute or two with jumper cables disconnected before the voltage got too low for it to even keep running.


I've never seen a battery recover almost a full volt within a minute.

 

the reason it will not stay running is because the battery is low. so once the jumper cables are disconnected it can not supply enough current to keep the truck running.
a battery can show 12 volts on a meter, but not have the amps to keep the engine running.
fully charge the battery and get the alternator tested, and it should run on it's own power.

 

Sorry, I saw "cables" and thought you were talking about the battery cables. Double check that you have battery voltage on the back of the alternator. If you don't, then that is probably why it's not charging the battery, there is a fusible link in that wire. If that's good, then more than likely your alternator is shot but they should be able to check to make sure.

 

Well that's the question, was the battery that low, it clearly wasn't that low if it recovered almost a volt in a minute.


I ain't gunna have some minimum wage monkey "test" my alt, I know how.


I've seen a lot, I've never seen a battery recover voltage that fast on it's own. In particular when there wasn't a large draw(like starting) shortly before hand.


"a battery can show 12 volts on a meter, but not have the amps to keep the engine running" that's not really true unless the amp draw is unusually high. Plenty of times I've had to limp with no working alternator, ya can go some time, and if you're still showing 12V you're golden. Vehicle electronics are designed to run at a low(as low as 10) battery voltage, they must to reliably start.


Anyway, I'll work it out, maybe this weekend.

 

Put the battery on a charger and charge it thoroughly to 100%, at least overnight. This saves tremendous wear and tear on the alternator stator, diodes, regulator etc. Ever hear the belts groan after an engine is jump started? Jump starting should be reserved for emergencies if possible.

Now a dead battery, especially if it's getting a little long in the tooth, might be defective from sitting dead. They don't like to be drawn down very far. It should be tested, after recharging. It pays to do this. The idea here is to prevent a defective battery from damaging the alternator. At the same time, test the alternator under load for proper charging and regulation. The idea there is to prevent a defective alternator from damaging a good battery.

 

i have seen batteries that show 12 volts static show 9 under load. then go rite back to 12 volts when the load is removed.
i have also seen batteries show 13.6 volts but only put out less than 100 amps because the battery had no storage capacity.
so even though the battery had proper voltage, it did not have the power to turn over the starter.