In march tuck wouldn't start mechanic replaced alternator truck ran good, battery light stayed on had test done at O'reillies Last week drove to Port Aurthur & on the way back gauges stopped working off & on several times then finally started working again till last Tuesday Coming back from Port Author gauges failed & truck died got jumped off and again e everything working.

Have u ever went on the internet about your car.there may have been a recall on your alternator or other people with the same trouble your having.I had a mercury that I to replace the alternator three times.Come to find out there was a recall on my alternator because of how It sat in the engine.the wires running to it kept getting burned out by the powersteering belt.Just a thought when u keep replacing the same part.

Usually a sign of something draining the battery.
The machine test probably showed the battery could hold a charge and the alternator was charging the battery.
You may need to have someone check for a power drain.
When the key switch is off, there may be a weak short to ground or a relay or component that is using power.

check the connection on your battery terminal ends. if they are loose or corroded that could explain the intermittent no spark. also could be a short somewhere but id check the terminals first. I had that problem once and the car did all sorts of weird things. airbag light, check engine light, randomly wouldnt start until i would mess with the battery.

you can drive even if your alternator is not charging but your problem is you have a posetive wire touching the body sumwhere trace your wires properly n u will find the problem

The alternator light comes on when the field voltage drops below a certain voltage. There is a possibility the connector to the alternator is corroded, as this is commmon, and may be a source for a false indication of trouble. There is also the possibility the regulator is failing. If the voltage was below 13.8 volts when he measured the voltage, he was only reading battery voltage. if he measured between 14.2 and 14.8 volts the alternator was putting out power.

An indirect way of checking is turn on the headlights, high beam when it is idling at night. turn on the heater so you can see the lights dim slightly, and then step on the accelerator to bring engine speed up to about 1200 RPM- the lights should get brighter. If they do not, the alternator is not putting out current like it should be.

Load testing would only measure the battery, it would not load the engine at all if the alternator is not putting out power. If the engine bogged down when alternator is load tested, the alternator is putting out power. The alternator is converting engine power to voltage and current to charge the battery, hence the load on the engine.

If the alternator has 75000 miles on it, replacing it is a pretty good idea as the brushes are quite worn at this point. That should remedy the situation.

It would also probably be a good idea to replace the starter solenoid. Turning the key 2 or 3 times before the starter engages is never a good sign.

The only way the heater might enter into your situation is if something is not shutting off when the key is in the off position or removed. Those phantom loads will find an electrical path to ground through the fan motor. If you have installed anything aftermarket that can be powered up without the key needing to be in the ignition is your culprit. The pahnatom loads like that can discharge a battery completely in a matter of just a few days.

Could be lose wire on alternator if light come on check belts

That would make sense, because when he jumped your car if he left it hooked up for awhile. Then he charged your battery up, so it started the next day. Then you drove to his shop and back it ran the battery down. It's easy enough to test and change alternators you could take it to autozone or O'Reillys if there is one close they can test your alternator. If you have more questions about this or if you would like to try and change it yourself just reply to this post and I will walk you through it.

Why would you drive a week with the check light on &
all those problems?

Why would you drive the car at all, without taking it back to
where the work was done

We have had On-Board-Diagnostics on all vehicles for 31 years,
that's why the check light is on

Surely you have heard of checking for codes when you get
a check light on your dash

Until you do that, you got nowhere to start

You do realize the Alternator has nothing to do with starting
a car,& wouldn't every fail at the same time as the starter

Before messing with the alternator, make ABSOLUTELY sure that the alternator is plugged in completely and fully, carefully. Sometimes, the medium voltage wire required to energize the windings likes to slip (on some 5/3 wire alternators), and with a 1-wire alternator, there's not really much to lose in checking tightness. If it still has issues, first get a multimeter, and check to make sure the battery, while off, is between 13.1-14.8V, and while running is more towards the 13.5-15.5V mark. This will indicate, if not charging, that it's not actively discharging.