85 gt losing power

1985 mustang gt 5.0. here is the story. wife called said it wouldnt start. i get there and everything had power and battery was reading 12.34 volts and dropping slowly. would not turn over so i gave her a jump and there she was. we get a mile or so down the road and it dies. had to jump again and again made about another mile before dying. put the car on acharger and charged fine holding a good charge. hooked up terminals to battery and with my voltimeter noticed the volts dropping. drops rapidly while running. checked fuses, alt. is putting out 19+ volts while started, but the volts in the battery drop. checked ground from battery(good), positive is good. the only time i can get the starter to engage is on a jump. where is all those volts from my alt. going.

If your alternator has 19V at its output terminal then there's a good chance that it's ok, and for it to have that voltage, the field coil must be receiving current via the voltage regulator so chances are that the field coil and regulator are both ok too. My best guess would be that you have an open-circuit somewhere between the alternator and the battery, so I'd inspect the connectors and wiring very closely. If visual inspection didn't reveal anything amiss then I'd switch the meter to the resistance range and start doing continuity checks on the wiring, looking for an open-circuit somewhere. I'm not familiar with your vehicle and I don't have a wiring diagram for it so I can't be any more specific than that, unfortunately.

If you suspect that you have an excessive current drain somewhere - which I doubt, because that wouldn't explain the high voltage at the alternator but low voltage at battery - but anyway if you suspect it then the easiest way to check for that would be just to recharge the battery, put it back in the vehicle and then leave it parked overnight - you should have around 12.6V the next morning, prior to starting the engine.

You can also check for excessive current drain using either a 12V test lamp or a multimeter set to its current range, so let me know if you want details of how to do that.

When faced with potential battery / alternator / charging system problems, I always check three voltages (measured at the battery terminals), which give a quick indication as to the state of the system. The three voltages are:

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Voltage after sitting parked overnight, but before starting engine - should be around 12.6V (indicates that there is no excessive current drain with the ignition in the Off position, and that the battery is capable of holding at least some level of charge).

Voltage with engine idling - should be towards the high end of thirteen-point-something volts - maybe as high as 14V (indicates that the alternator is producing output current, and that that current is reaching the battery. Also indicates that the rectifier is functioning, converting AC to DC).

Voltage when revving engine slightly - should climb to around 14.2V and then hold steady at that voltage regardless of further increase in engine speed. (Indicates that the voltage regulator is functioning, and clamping the upper voltage threshold at the correct level).

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For the moment, I reckon you probably have a simple open-circuit, but let us know if you find anything else which may be of relevance.

P.S. I know the 19V at the alternator output seems way too high, but the voltage regulator will be looking for a reference voltage on its sensor terminal, which I reckon it cannot see because of the open-circuit. Therefore it is increasing the current to the field coil in order to increase the alternator output in order to get that voltage at its sensor terminal. That's what I think is happening, and I reckon it will all settle down again when you find the open-circuit. Good luck.

thanks ive got a ford guru coming over later to help out. ill let you know what we come up with

turned out to be bad voltage regulator. went ahead and changed out the alt to.

That's good, thanks for letting us know. Here's hoping you get many trouble-free miles from the new components.