An alternator requires a +12 volt potential flowing through it before the car is started to pre-energize the electromagnetic field, which will then be used to generate the power to recharge the battery once the car is running. This pre-req is accomplished by the Battery Failure light on your dash board (which serves double duty to warn you of when the alternator fails, and also to energize the coils before the car starts). The current needs to flow for only a tiny fraction of a second, which is why you can turn you key in the ignition very quickly past postion 2 (run) to position 3 (start) and still have the alternator energized. (the bulb may not even illuminate, but enough current will pass through it)

Many alternators can self-energize themselves when brought up to a speed of 4,500 to 5,000 RPMs, assuming you have a wiring failure and it did not pre-energize. However I would not advise this as a working solution since reving your car up to 5,000 just after a cold start is not the healthiest thing for your engine.

Charging system failures are VERY common on the '80 through '87 Volvo 240/740/760 vehicles due to the biodegradeable wiring harness insulation that was used during those years. However, the newer vehicles seldom have this problem. For everybody who is reading this post that has an '80-'87 Volvo with a 4-cylinder engine, it would merit checking and/or replacing your Alternator D+ wire (small red wire on back of alternator, not the big one that goes to the battery), and your Oil Pressure Sensor wire as these two are commonly the first to fail. You need only to replace the wire section that runs from the Alternator or pressure sensor to the large square harness connection back at the firewall (or the strut tower on the 740/760 vehicles).

If you are experiencing this failure on a later model car, or you've already replaced this wire section on an early model car, check the integrity of the Battery Failure bulb in the instrument cluster. It may have a poor connection in it's socket, or a poor connection at the junction on the back of the instrument cluster. Also check and clean the D+ connection and the ground tether on the Alternator. If you are having trouble tracing the D+ wire or the Oil Pressure wire, consult the Volvo wiring diagrams for you particular year if needed.

God bless,
Fitz Fitzgerald.
--
'87 Blue 240 Wagon, 267k miles.
'88 Black 780, PRV-6, 149k miles.