71 Mustang Charging System

Hi,

I've recently bought a mustang that looks like it had a new altenator and battery. The ammeter shows between 11 and 12 volts. The car finally died and of course the battery was dead.

We tested and recharged the battery, and swapped out the old alternator and regulator with one that has the two together. Unfortunately, this didn't fix the problem as the battery drained again. I swapped batteries and am still having the same problems.

One strange thing that may be related. The tach reads about 1200 rpm with the ignition switch off, but when I turn it on (without the engine running) it drops to 0 rpm where it should be. Any good ideas?

Thanks in advance,

Ted

Reply to
tedalmeida
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I don't have a ready answer, but the tachs in those cars actually measure current going through them to the coil, unlike aftermarket units which count "pulses" directly off the coil.

If the tach is reading something with everything shut off, I suspect that some wire, probably in the ignition harness, has "melted" together with the ignition feed so that it is getting voltage all the time, thereby draining the battery. A symptom might be a hot coil that should be cool from not being driven in a bit.

I've seen such melted wiring over the years, caused by a variety of things. Inspect the wiring leading to the ignition switch. I know this goes into the steering column, but you may see something below that. Pull the primary coil wire off (the +) side and check it for voltage with a VOM, and see if the tach drops. Report back here and we'll proceed.

Reply to
CobraJet

CobraJet,

Thanks for helping out. I disconnected the battery and noticed that the aftermarket tach is still reading 1200 rmp, so I'm sure there's a minor issue with the tach.

But I'm still stuck with the alternator issue. Do you think the new one went bad already?

Reply to
tedalmeida

Sorry, I thought you had a factory tach.

Assuming that the new battery has not been damaged by all this (each individual cell tests good, and holds a charge sitting on the bench for a couple days), you would first check the alternator with a VOM for

13.5 to 14 volts at idle. If you get less than that, it cannot possibly charge the battery. If the reading off the back of the alternator (line leading to starter solenoid) is right, but the reading on the positive battery terminal is lower, then you have corrosion somewhere in between, or an old wire barely hanging on to its connector. Likewise, as Kate mentioned, *all* the connections in the charging circuit must be inspected and cleaned, especially the ground line from the battery to the block.

The "ammeter" you referred to; is this in the dash or handheld?

Reply to
CobraJet

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