older Dodge pickup

My sons Dodge pickup that i had posted about be fore has a problem I can't help him with.
It had sat for several years and we put a new heavy duty battery in it, started it and drove it about 20 miles home. After several small issues his daughter was driving it while her care is in body shop and yesterday it wouldn't start when she got in it to leave school activities.
It is a 225 slant six and was running great. 1984 is the model year . Model 100 2w drive. Only less than 200 miles since he got it going.
The truck dies if you disconnect the battery. He got a good spark to the battery when he disconnected to see if it would run off the alternator. He took the alternator to have it checked and it checked fine at auto store. He cleaned the ground to the frame and all connections are clean. I'm stumped. It does start when it's jumped but will only start once, then its the same as before.
Any ideas as to what to look for.
Thanks, Jim

Edited by D17JIM2 - 15 May 2018 at 6:59am

No turbo on 225 1984 slant six gasser.

I`m not familiar with a flux capacitor.  I have owned a number of these old Dodge pickups and cars.   The first thing I would buy and have a spare in the glove box is what I called a resistor,  an oblong white ceramic box that was about 2 inches long and mounted on the firewall.  (engine side)  Eventually it would fail and the engine would not start.   The newer ones had a double resister that would allow the engine to start,  but then stall.  (Similar to the internal resister in a coil??) I would try that and if that is the problem,  buy an extra one for your glove box,  just to be safe.

Edited by weiner - 15 May 2018 at 11:54am

It starts if jumped. Dies if you disconnect the battery.
Will not start after you shut it off. Meter on truck dash says it's charging.
Altternator was taken off and tested at Auto Zone and showed it was good.
Battery has maybe 200 miles on it and is heavy duty battery.
If resistor was bad would it start if jumped.

Thanks guys for a couple of things to try.
We put a resistor on the heater and had to get one on line . Nobody had them for this old a truck. Over $20.
Some one told me that this problem is probably the regulator. It's mounted in the fire wall and has avtriangle plug. He's replaced many back in the day.
Thanks again, Jim

My son used O'Reilly so that works ! Thanks

It was the Voltage regulator. An old mechanic told us. He also said to keep a resistor. The VR was real easy to replace.
He charged the AC and it seems to hold . Amazing !

Thanks again.

A car or truck WON'T RUN with the battery disconnected.  They would with a generator, but not an alternator.
Battery was failing because of a faulty VR.  But yes, keep an extra resistor in the glove box.  Had one fail on me years ago.  Drove it home and it wouldn't start the next morning.  Jumping it wouldn't work either.