Wereis the voltage regulator on a 1991 dodge ram pickup truck

have the shop check it and necessary have them replace it.

The ignition module is part of the engine computer.
The pickup module in the distributor could be the problem. Could be a faulty coil too.

When you replaced the alternator, did you also replace the voltage regulator? I have read that on this truck, the voltage regulator is internal to the ECU and is located on the firewall near the ignition coil. You may be able to install a separate, dedicated voltage regulator available at your local auto parts store. Please note that you may have to rewire from the ECU connector to the stand-alone voltage regulator. Good luck.

Should be a medium size metal box about the size of a smart phone mounted on the firewall with one harness plug. Usually the box is silver.

not like the old days it does not have an external regulator-it is integral with the alternator now, use a digital voltage meter to read the output at the rear of the alternator (large post)-is should be about 14.2 volts.

you need to test the alternator output in the car with the engine running it should read around 14.2 vdc. if the out put is lower the voltage regulator is part of the engine control computer. and will need to be tested. if the voltage is correct. then you will need to do a battery draw test hear is a viedo on that.

It sounds to me like the voltage regulator is trying to decide what to do after the voltage draw from starting.
I would not be to concerned unless it stayed low.

The voltage rate should be in the are of 13.8-14.8, so it may be a bad voltage regulator, bad connection or a bad gauge. I would take it to an autoparts store to have it checked, and they can tell you if it is the alternator or voltage regulator. If it is neither of the two, the good thing is that the electrical system is working, and the bad thing is that you may need another gauge.

on back of alternater

Replace the voltage regulator.