If your battery is not fully charged, it will still be drawing current when the alternator is capable of charging it; that is when its output voltage exceeds the battery terminal voltage.

You say your B+ terminal voltage is still only 13.2 referred to battery negative terminal even with minimum load and cool alternator frame? I'd suspect an open diode -- something you're not going to prove to yourself or anyone else without an oscilloscope or bench test rig. Well, you could, if you went so far as Phil did to unsolder the diodes and test them individually.






--
Art Benstein near Baltimore

Anybody who doesn't know what soap tastes like never washed a dog.
-Franklin P. Jones

Hi Jeff,

I’m glad you were following the other thread and you didn’t do what you proposed with the B+ cable.
I will lend you some more advice as I know you are getting 🤢 over this. (:)


The 12.59 is a good enough charge for the most part but 12.65 is better after a long rest period of no charging. Like if it the car set for a few days.
So the 12.59 does reflect that you are taking a lot of short drives and the battery is not getting or staying topped off.
Battery age can affect this some but three years should not unless the battery has had a history of setting dormant a lot.
That alone, helps take life out of a battery.

I personally do not care to let any battery set longer than ten days without maintenance of a float charge.
A float charge is 13.2 volts after it’s completely topped up. Current rate is only a few hundred milliamperes maximum.

This float of voltage just overcomes the internal discharge rate of sitting active is the electrolyte solution.
The acid is always eating on the lead plates transferring ions. This is more so below its own equilibrium.
A discharge lowers the voltage a few hundreds or even tenths of a volt the regulator will adjust current output accordingly.

The battery is a big absorber or reservoir to the entire system. It takes what’s left over in the system in its own rate. That’s why there is the 14.2 limit in place. Most all alternators can make 16+ volts
A direct to B+ to D+ crossover can cause full output.
Be glad you didn’t take the absorber out of the circuit.

I have seen people disregard that rule and can or did get away with it. Or, I think they did.
But then, they were troubleshooting their charging system for a reason. Duh?

A weakness can develop with an output diode.
Like I said when talking about these large diodes they are a brute force current handlers that are not great for rapid response in dissipating its saturation threshold.
This happens during the breaks under a load that can cause a radical surge.

I believe B.B. and Dave Stevens have said that losing one of the three can cause under charging.
This is where having the alternator spun up on a testing machine is recommended. It looks for the three outputs to accumulate to bring the current up to a specification while loaded to some percentage of its output. This is why you want a reputable shop to test it.

I have a Motorhome with a Lester dual alternator. It’s two separate outputs built into one unit.
One circuit for coach batteries and one circuit to the vehicles engine battery system.
I had it spun and was told it was $600 new or rebuilt was about $300. He didn’t stock them.
I took it apart and found nothing wrong but took the diodes in because one appeared open to me.
My auto electric guy tested it with a ohmmeter like I did.
But he was suspicious that another had a slightly different reading.
Going with both our experiences, I bought two diodes and pressed them into the heat sink plate my self.
The diodes from him were $10 each! That was about half a dozen years plus ago.

So it’s possible you have a weak diode pack.
Spinning it or repairing it yourself, is the only way to truly blame the alternator itself.


Now to throw a wrench in all this.😳
If you had an older battery or its an abused one it can refuse to take on current to a full charge or won’t seem to get there.
In these cases a battery’s cells or one cell is short in depth or capacity.
Its voltage will be low or superficial with voltages across the whole battery fool the charging systems. It will think it full up and drop off it charge.
In flooded batteries where you have caps you can read each cell by touching into the liquid over to a post with a voltage meter. Those days are pretty much over with sealed tops.

The battery never gets back to new and will display lower voltages after sitting and right after getting used for a bit.
It will show a lower voltage more quickly and for a longer period of time.

A load test of a battery and it’s recovery time has to up to a good, weak or bad battery specification.
I have a tester that simulates a engine cranking for 30 seconds and the voltage must return to a certain level in 30 seconds to be good. Longer times use a grading scale or advise charging and retest.
This is how I learned the characteristics of batteries over the years.

I have used a couple batteries for up to sixteen years but I think they are not making batteries like they use too. I was told by a Costco employee to not expect them to last that long anymore a few years ago.
Johnston Controls made their own lead plates but I think they sold out to Exide.
Exide couldn’t beat them but big enough to buy out the competition.

Just like GoodYear GatorBack and Continental.
Where Continental discontinued the Elite “V” belts with their slant cog design. They left the GatorBack name on the table. This about five years ago. I Don’t know about the New Jersey jobs?
But purchased the rights to the proprietary polymer rubber composition is to be used in their other belt lines. GoodYear has lousy distribution issues with the chain and on-line stores.
IMHO, They make good belts but not as quiet, for as long, as a GoodYear GatorBack was more connected to industrial backgrounds.

I’m thinking Continental is not interested in supporting V belt driven components.
Only serpentine ones from looking into their catalogs.


Let’s us know as the saga continues.

Phil

Hi Art.

I've never bumped into one of those external battery temp sensors. They would have been used to keep the charging voltage going too high so as not to cook a battery and shorten its life. Those may well have been in the days before sealed maintenanzce free batteries became the norm. The regulators with internal temp sensors would have taken longer to notice a hot engine compartment, especially if located on the intake side of the engine.

They're not shown in the aftermarket 240 wiring diagrams I have, nor in the charging circuit sections of the later 700/900 green manuals I have. However, in the master wiring sheet at the back of the 1989 740 green manual I have, it is shown as an optional component living under the battery (comp 1 at coord B1 on the sheet).

These would have had external spade connectors on the regulator and I don't ever recall seeing that kind of thing in North American Volvo parts listings.

I do recall seeing mention of these external temp sensors in some 240 alternator charging circuit diagrams and discussions in other forums. It was a button shaped thing under the battery tray, early/mid 1980s as I recall.

I just found a 2019 thread over on Tbricks that mentioned them (referencing a TWC hybrid regulator) and you remarked not having seen one. It's mentioned for 1985-1986 240s and that the regulators were troublesome. Someone incorrectly mentioned that the battery temp sensor was for the ECU (which we know the LH ECUs don't use, only the block temp sensor, and they have no control over the charging circuit).

I now see in Volvo parts schematics mention of an alternator temp sender in some of the old 240 55A Bosch alternators in the carb'd B200/B230 engines (1979-on), so that would be non-North American market, which makes sense. It's not shown as a separate part, just noted in the alternator description that there's a sender, so those alts may have been those using an external regulator with a temp signal from the alternator?

I do see mention of a battery temp sensor used in some of the later Volvo FWD/AWD models. I'm not sure if that's directly connected to the alternator or ECU.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now