13.5 volts at the battery terminals will generally not charge a flooded lead acid battery to 100% - in this century. You want the charge voltage to be around 14.2 - 14.5 volts at fast idle.
Repeat the voltage test at the alternator B+ terminal. That will be the terminal on the back of the alternator which has the large wires (should be red insulation) that go to the +12v distribution block over on the fender. If you have > 14 volts at the alternator and 13.5 volts at the battery terminal you have a problem with the wiring between the battery and alternator. Inspect carefully. If you have 13.5 volts at the B+ terminal, then you have an issue with alternator and / or your voltage regulator.
Assuming the problem is alternator / regulator related, fist check the condition of the wiring between the alternator and the voltage regulator including the grounding of the alternator and the regulator. Next, your 140 will have a charge indicator light. With the ignition on; but, the engine not running does the charge indicator light illuminate? If it does not illuminate that indicates a failed indicator light bulb or a problem in that wiring. The alternator need the tiny current that flows through the charge indicator light bulb to flash the field at start up. Without field flashing the alternator will not generate significant voltage under load or take a long time to start generating voltage.
If the wiring and the charge indicator light bulb function, then you have an internal alternator / regulator problem. The electrical section of the 1800E service manual (if you don't have the 140 service manual) describes both the SEV Motorola and the Bosch alternator if you need to do repairs.
https://volvo1800pictures.com/docume...1800E_1971.pdf
There is a simple test that you can do which which will help to determine whether the problem is in the alternator or the voltage regulator. It is called a field flashing test. Set up to measure the vehicle voltage. With the engine running apply +12v to the DF terminal on the alternator. You should see the vehicle voltage rise to somewhere close to 15 volts. If the filed flash test generates around 15 volts then you alternator is likely OK and you have a voltage regulator issue. If the field flash test fails, then you have an alternator problem.
If you do a search on Google using the term alternator field flashing test you should be able to find some descriptions of the test.
My 142 has the Bosch alternator / regulator. As I recall you can adjust the output voltage with the Bosch regulator. I don't know about the Motorola alternator. I still have an original Bosch 35 amp alternator; but, my regulator packed it in and I have replaced it with one of Dave Barton's adjustable solid state voltage regulators which does a good job of keeping ht evoltage around 14.3 volts.