"I haven't touched the red wire from the starter to the alternator."

Using a jumper cable temporarily between the two is a good way to test. If your charging voltage goes up with cable attached, that is your culprit.

Thanks for that tip.
Why didn't I think of that?

But it's still too darn cold today to be fussing with this.
Hopefully the cold snap will pass by the weekend, so I can try again...

Update from Jan 22nd.

It's a bitterly cold day here and I was expecting that the Bertone wouldn't start, as it was not able to start the engine last night after driving home with the battery warning light lit.

So I decided to drive the 240 to work (normally very reliable). Well the battery on the 240 wasn't able to crank my b23 engine over (it's an old battery and only had 550 cca when it was new, and that was a while ago).

Having nothing to lose, I opened the door of the Bertone and tried starting it. To my surprise the engine fired up first turn of the key, but the voltmeter wasn't registering anything, and the engine warning light was on.

Tried hooking up the jumper cables from the 780 to the 240, but I could hear the 780 laboring when I tried cranking the 240 and the 240 wouldn't start. I jumped back into the 780 and revved the engine up over 3000 rpm and the voltmeter needle jumped to straight up and the battery light went out. Now the engine was able to successfully jump the 240 and I was able to go to work.

I let the 780 run for about 15 minutes to build up a bit of a charge, and I'll check it tonight again to see if it starts.

I'm thinking the red wire between the starter and the alternator is flaky, and when I revved the engine I might have jogged it back into a temporary good (or at least better than bad) position.

If the car starts when I get home, I'll likely let it run for 15 minutes or so, pull the battery, bring it inside for the night, so I'll have battery power if the 240 needs it tomorrow. Then hopefully my Volvo guy can work me into his schedule to have the entire charging system checked out.

I'd love to resolve this one myself, but it's hard to do good diagnostic work and recheck all your connections when you're laying in snow and freezing various parts of your anatomy off...

Update from Jan 25.

Car will start, even though it's very cold here, but the battery idiot light will immediately come on, and won't go away even if I rev the car above 3000rpm, so there's no charging happening.

When I look at the alternator from above, I can see that the red wire end that comes from the starter and goes to a screw down connector on the alternator is glowing, and will occasionally spark if I touch it with a non-metal pole to move the wire.

Obviously not going to drive it anywhere in this condition. Hopefully it will warm up enough on the weekend to let me try changing this wire.

Does this sound like a symptom of a shorted alternator, or just a very bad connector wire between starter and alternator?

Still sounds like the alternator and maybe starter wiring should be replaced. Photos would help. Did you see my message about the size of jumper wires you made?
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I own a Volvo or Does Volvo Own Me? Try the easy to search Expanded Style FAQ Index http://40mph.com/Brickboard_700-900_FAQ_Expanded_Index_Version/

Update from Sunday, Jan 27th

It finally warmed up enough today to work outside without freezing off various appendages. Put the Bertone up on the ramp and proceeded to dig into the wiring.

Started with the red wire that goes from the starter to the alternator. It was completely fried with only a few strands on the connection, which accounts for the sparking and glowing I reported before. Cut that wire out and stripped the plastic covering. Everything inside was green and powdery, so that explains what that wire was having a difficult time carrying any current.

Replaced that wire with a brand new cable (6 gauge), and replaced the double positive battery cables (one was original, one was used but newer) with a brand new 6 gauge positive cable. Kept the negative dual cables in place.

Started the car and the voltmeter came up to straight up. So perhaps I've resolved this issue.

Just as an FYI for anyone else with a b280 engined car, getting under the car is the challenge. Even with ramps and jack stands the solenoid is right above the front crossmember, so it's almost impossible to be able to see where you're placing a wire, while you're laying on the ground. Or maybe I'm just not that agile... I think I'm going to have to figure out some sort of a ramp system that safely raises the car about 2 feet in the air if I want to do much other maintenance under the car.

Many thanks to all of you who helped me with your comments. Hopefully I'll be able to close this thread. But I'm going to give it a few days.

An alternator with bad diode(s) can put out the correct voltage but not enough amperage. After the connections are cleaned and you still have a headlight dimming moment, have the alternator tested under full load, it should be putting out its rated current. Yes it is miserable to get at on the bottom side of the engine, I would double check the replaced brush assembly which can be done in the car.

Update on the non-starting.

Let the car sit overnight. Didn't touch anything under the hood.
Came home from work tonight, and tried the car. Started first turn of the key.

Think this points towards the battery being the chief suspect.

2nd day after the no start.
Let the car sit all day. Came home after work, and it started up first time again.

Think this is pointing towards a battery replacement. I'll get it tested out this weekend.

Ok, Saturday morning. Came out to start the car. The car fired right up, but my voltmeter didn't move from the far left 10V position (not good).

Took the battery out of the car and took it to have it load tested. They charged it for an hour and then put it on the load tester. Now the battery tests fine with 12V and responds correctly as they increase the load on the load tester.

I'll redo the grounds with the Oxgard grease after I wipe off the dielectic grease as per the other comments in this thread.

Kind of think I now have some sort of a parasitic voltage draw in the charging circuit. I have a spare alternator that I could swap into the car, but I really don't want to have to do this in the freezing cold.

I've connected double sets of battery cables on the car. As of this moment I have only one set connected right now. Tomorrow I'll chop the battery connectors of all four cables, and then use a fresh set of dual lead connectors for both negative and positive cable sets.

Went to Home Depot to look for the Oxgard conductive paste, but they didn't have it. They said that Noalox is the same type of stuff, but when I look online for that, it says that it is designed for aluminum wiring.

Should I keep looking for Oxgard?

Ox-gard and Noalox sound like 2 different brands of the same product. They are designed to reduce the resistance between cheap aluminum house wiring from decades ago and copper connectors.
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1992 745 approaching 500k km

I'm not sure what brand Steve has in mind, but I have Oxgard and it is made for aluminum like the Noalox. They both can still be applied to copper and steel connections.
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Yes, those compounds help conductivity as well as keeping out moisture and oxygen to prevent oxide formation. Dielectric grease only does the latter and inhibits conductivity unless the components are in tight mechanical contact. Dielectric = insulator.
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1992 745 approaching 500k km

Parasitic draw testing is a good place to start and will be easier work in cold weather than doing an alternator swap, although you will still need to be outside!

12 V measurement must be with car off because I believe 14+ V is healthy voltage when idling...

I still recommend replacing the battery ground wire completely with a new wire. Off-the-shelf wire, pre-fab battery cables, at auto store will work fine.

Could be a parasitic draw or also a failure to charge:
http://www.brickboard.com/FAQ/700-900/ElectricalStarting.htm#NoJuicefromAlternatorNotCharging
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See the 700/900 "FAQ" at the menu bar top screen left side.

Dielectric grease is not helpful on grounds since it is essentially an insulator. The best grease to use is OxGard conductive grease, available in the electrical section of Home Depot, which is a protective but conductive material. You don't care about stray currents at the ground plane and you want the grease to both protect against corrosion and enhance conductivity.
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See the 700/900 "FAQ" at the menu bar top screen left side.

Any grease is better than no grease. and I think dielectric greas is less helpful, but not_useless for grounds because it is protecting it from water and potential oxidation, or rust.
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Try the easy to search Expanded Style FAQ Index. I own a Volvo or Does Volvo Own Me?

True, but for grounds only the conductive grease is best. But NOT for all the other connectors!

Well, it sounds like you didn't kill your battery just discharged it enough to not start. Curious, how old is your battery? Any parts store has battery testing equipment. Just make sure they do a "load test", not just "it's got 12v, it's good".
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Post Back. That's whats makes this forum work.

One of my rules of trouble shooting - everything breaks some time.

Perhaps your testing pushed your battery over the edge (cliff?).

A car that starts immediately with a jump, but won't start with its own battery either has a dead battery or a bad connection at one of its battery posts.

I think you should put the suspect battery in the 240. OR, clean your battery connections and try the 240 battery in your car.
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '72 Yamaha Rd400, '68 Honda 350-4, '12 XC70, the first 5 are mine, heh, heh, 525,000 miles put on 10 bricks James A Sousa

My 87 had a similar issue I found this weekend. The cable copper strands were corroded where clamped in the replacement battery post terminal. Connecting a jumper clamp across the terminal post clamp and the exposed copper going into the terminal clamp started it. I combed the copper cable strands out with a stainless steel brush to get rid of most of the corrosion, cleaned the terminals and clamps and it was good to go.
Checking the voltage to the starter 51 terminal will tell you if it is the starter solenoid if no click when key turned to start and 12 V is there, or the starter if click and voltage, but not turn, otherwise, follow the circuit back to the key switch.

Yep...battery swap is a sure test.

Sure, a battery could be drained with 30 on-offs. How old is the battery?

Besides battery age, both the positive and negative cables attached to the battery should be tested for voltage drop. This will determine how healthy they are, cables that *appear* okay can actually be degraded on the inside.

The car was built in 1988, so if the cables look original, I would replace them outright, get the charging system in top shape. The problem may not be fixed with a new a battery.
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I don't know about the v-6. But on mine, there is a big wire that connects to the engine. It is a bigger wire on back of the alternator. It is a vital connection. Once mine came loose and caused a parasitic draw. Take it loose and clean around it and reconnect tightly. Mine connects underneath the intake. Hope this helps.

Grounds should be good now. Ok, no or little charging. Connection to the alternator, bad job on the brushes, bad alternator, battery clamp corrosion, invisible corrosion at wires to the clamp, and yes, a bad battery.

Any Sears, Auto Zone, Advanced Auto, and a half dozen other places would check the charging system for free for the hopes of selling you a new battery. Forget Sears, can't trust them. Walmart should be OK too. They had to hook my car up to a charger load tester for 45 min to verify their battery went south before the 2 year warrantee was up.

Tom