My 20-year old son has been "borrowing" my '86 for the last year or 2. The car wouldn't start and a jump got it going. Had him take it to where I get my alternator/starters rebuilt, they checked it for free and said it's the alternator or wiring, that the battery was at 12v, not 14 or something. Visually the wiring looks OK around the alternator, I think I repaired it a few years ago. I bought a multimeter last night, have been stalling it off for 20 years (had ALL my tools stolen by a herion addict 20 yrs ago). So do I just test the hot wire to the alternator to ground? What should it read? Maybe I should have just got a continuity tester. Thanks.

First check that the alt light (and other dash lights) is on when key is in accessory position II but engine not started. If it isn't then check to make sure that the exciter wire (thin red) is properly connected to the alt. As well as the short stiff blue grounding wire from alt case to alt bracket.

If light is on, then when engine is started it should go off. (If it remains on and bright then exciter wire may be shorting to ground.)

Now check voltage at alt. If 14V then may be thick blue grounding wire from engine block to battery. If much less then alt problem (eg. regulator brushes)
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1980 245 Canadian B21A with SU carb and M46 trans in Brampton, Ont.

Here's the basics to look for .... Battery should be 12v when engine off.
Battery should read about 14v when engine is running. Headlights should be normal brightness with engine running. Do not remove the battery cables with the engine running. Battery cable connectors are Tres importante! Both ends.
In doubt ... swap in a known good battery and test again.

Here is something that helped me when I was troubleshooting my son's alternator. His problem was a faulty ground wire.

http://www.excelauto.com/online/Tech.d/altvdt.html

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1980 240 328k 20 years.

One other thing to suspect is the wire harness going under the timing belt cover. The '86 was in the years of the biodegradable wire insulation so there can be miscellaneous shorts in the harness.

There's a multi pin connector on the firewall with four wires x 2. One goes to the small (red??) wire terminal on back of alternator, around 2 o'clock. That's the exciter wire. If the harness under the t-belt is damaged you can run a dedicated wire from the firewall connector along the fender and to the alternator. Just make darn sure you have the right wire.

Another possibility is the voltage regulator assembly. Inserted in back of alt, held in by a couple screws. Can be replaced with alt still in car, cost about $30. Look in the FAQ here, it probably gives the details you need. Easier and lower cost than complete alternator replacement.
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Sven: '89 245 NA, 951 ECU, expanded air dam to 7" from ground, forward belly pan reaches oem belly pan, open-front airbox, E-fan, 205/65-15's, IPD sways, E-Codes, amber front corner reflectors, quad horns, tach, small clock. Wifemobile '89 245 NA stock.