I'll check that and get back to you, thanks. Hadn't thought to measure that. The D+ wire, disconnected from it's post, reads the same voltage as the battery - and the B+ post of the alt. when engine is running.

 

dytha99 - Well, the harness has NOT been replaced - but the circuit is accessible without the harness;the D+ wire from the relay box on the firewall can be accessed - so I rewired from there to the D+ - no change; I also replaced the hot wire from the starter to alt - which disappears into the harness, no change, so I just went back to the original wire. I'm TOLD the ECU and Throttle PS are NOT involved in the circuit - so the original doesn't seem to be the problem. That biodegradable insulation only seems to rot when exposed outside the harness and I've replaced the D+ wire end that did have rotted insulation- - so it seems the one clue is the D+ post, its wire connected - engine running - alt spinning - is always 1.5V's. and should be 7-9 V's. Going to measure D+ wire, disconnected, engine running, which may give a clue.

 

not with engine running

measure the D+ wire voltage to ground with just ignition on. with the D+ wire disconnected, and the ignition on, the battery light should NOT be lit, and if you ground the D+ wire, the Batt (Alt?) light should come on brighter than normal. key off, and the wire should be 0V

 

OK I'll get those figures.

 

Battery at 12.75Volts - unhooked to batt. cables
12.52 Hooked to batt. cables
Key position II - unhooked D+ wire 12.35
B+ on alt. 12.43

Engine/alt on B+ 12.18 (low?) - D+ 12.11

Just for the hell of it I put voltmeter on AC and got 26 volts at B+ post of spinning alternator. Probably doesn't mean anything.

I forgot to look at batt light with unconnected D+ but in the past the light is ON at Key II and then then off when engine started/ that light will go on if I ground the unconnected D+ wire - engine on.

 

I'm having a hard time understanding what you did but what I think is you measured 12.75v at the battery terminals but only 12.11v at D+ with the wire disconnected from alternator,
That said if you have that much of a different between the two points you have a bad connection in that circuit, even with that voltage drop across the "Battery" light you should maybe be reading 12.70v on the end of D+ wire and the negative terminal of the battery.
I would re-visit checking all the "grounds" for good connections !

 

the circuit for the D+ wire is

battery + -> positive terminal -> ignition switch -> dashboard battery light -> D+ wire

on a 240, the positive terminal is on the left fender, its under a slim black plastic cover, and if you open that up, its just a brass strip with a whole bunch of red wires connected to it, one of which comes directly from the battery...


that high of an AC voltage at the alternator *might* suggest bad diode array.... maybe you should stop buying junker rebuilt alternators at bigbox auto parts stores.... the ONLY thing I buy at those stores is wipers, oil, and coolant. ok, and window washer fluid and light bulbs. If I needed an alternator, I'd either get it from a dealer, or from IPDusa or FCPeuro, and I'd pick one of those vendors premium brands.

 

Madman - I had the same thought.

 

Gonna recheck the grounds. But that AC read is weird. Could it possibly be that I got 5 bad alternators from the big box stores? Yikes!

 

I just want to add to this, I know it's an old thread but you are not on your own as I too am having all of the exactly the same problems as you with my 1991 Volvo 240. I have so far purchased 3 brand new and working Volvo alternators, but when wired up to my 240 they don't charge the battery, or work for a very short time and then stop charging the battery never to charge it again, but when taken off of my Volvo 240, all of the alternators are still working and charging - just not on my 240? It's a massively frustrating quirk with the Volvo 240 that suggests that there is something inside an original Volvo alternator that the Volvo 240 wiring needs, but I can't for the life of me work out what it is. The only solution is to recondition an old one, but I want an upgraded one for mine as I'm running a lot of amp. I wish I knew the solution (bulbs behind the dash are all on, revving the nuts off of it doesn't trigger any exciter wire) If anyone can help please. Thanks

 

the alternator needs a good ground, it needs D+ from the dashboard 'battery' light, and it needs a solid connection to the battery +, and it needs to be spun.