With engine running how low does alternator voltage have to be for the alternator light (dashboard idiot light) to turn red? I have a feeling that no matter how low the voltage is, light will only turn on if the engine stops, or the alternator belt breaks.

Bottom line is the idiot light does not warn of all failures. A voltmeter won't either, unless you scan it as often as you do the speedometer. The lamp will glow visibly, not brightly, with as little as 1.8V difference between D+ and B+.
--
Art Benstein near Baltimore

Before I got married, I had six theories about bringing up children.
Now I have six children and no theories. (John Wilmot)

Hi Art,

Thanks for the voltage value. You're right that the idiot light doesn't meant that. I wish to add here that even in newer modern day cars the human touch and logic is still needed in troubleshooting issues. I've aquired an OBD II tester for wife's Hyundai thinking it would simplify repairs but no. I could now read the error codes as opposed to counting LED blinks in OBD I but approach to repairs still almost the same.

A modern digital gadget like battery monitor (many of these on eBays) could be plugged into cigarette lighter socket to monitor data streams, but then who's gonna pay attention to the road ahead?

Amarin.

"A modern digital gadget like battery monitor (many of these on eBays) could be plugged into cigarette lighter socket to monitor data streams, but then who's gonna pay attention to the road ahead?"

Yes! Or rather, because we DO pay attention to the road over the instrumentation, we need idiot lights to grab that attention when necessary.

I'm not sure about 940 dash, but the 240 dash, since 88, has a "service reminder" light that comes on briefly after starting the car when it hasn't been reset in 5000 miles. Meant as a reminder to get your oil changed. A return spring and poorly built means of resetting intended for your dealership professional fails on most instrument panels long before the cars reach the age of yours and mine.

I'm testing a re-purpose of that SERVICE light. It has a 555 timer on its circuit board to limit the illumination time to the first few minutes of driving, so with a few trace cuts and a bit of rewiring, I gave it the job of flashing the warning light when the battery voltage falls below a charging level (13V).

While a hack like this isn't for all of us, the same idea needs to be somehow added to our old cars for those of us who do not scan the voltmeter as regularly as they might scan the speed meter when driving. The 240's blower motor on high totally masks the sound of a slipping alternator belt, and that doesn't put the battery warning light on until it is too late.


--
Art Benstein near Baltimore

In a triode, no one can hear you screen.

Dear Art Benstein,

Hope you're well. The 940 instrument clusters have a service reminder warning light, which works as does that on 240s.

To re-set the service warning light, a rubber plug is removed from the cluster's "faceplate". A 1/8"-thick rod can be used to depress a button on the left-side of the speedometer.

Hope this helps.

Best wishes for the Holidays and a Good, Healthy 2020!

Yours faithfully,

Spook

Hello,

The basic wiring scheme of the alt light is that one end is connected to B+ batt terminal while the other end to D terminal at alt. It won't light up when both ends have equal voltage ie. when engine is running with good working alt. In the event engine stops or belt breaks, no voltage is produced at D terminal so current would flow from B+ terminal and lights it up. In the event of worn alt brushes, the alt produces faltering voltage at D terminal then depends on how much current flows it would light up dimly or flickers. The alt light itself being an incandescent bulb requires a minimal voltage for it to start glowing and lights up. If you could set a bench test circuit with variable voltage from 0 to 12 volts you might know at what minimal voltage it would light up. The 900 series normally doesn't have a dash voltmeter to tell what voltage that is.

Amarin.

Hi Boris,

The Volvo 700-900 Series FAQ hosted right here at the mighty and powerful brickboard helps you.

You can also search for the issue using the search feature:

https://www.brickboard.com/FAQ/700-900/ElectricalStarting.htm

Find gems in the article like:

"Exciter Current Through the Warning Lamp on the Panel. The alternator (charging) warning light ..."

Also see about too short of alternator brushes and other alternator failures. In short, you alternator is not charging or there is a fault in the D+ excitor circuit wire between the instrument cluster and the alternator (smallest red wire on a spade quick connect terminal at alt rear.

Maybe read here:

https://www.brickboard.com/FAQ/700-900/ElectricalInstruments.htm

Alt at start should produce 14.5 or so Volts DC at the battery posts. With warmed engine (and alt) maybe 13.8.

Art Benstein's image of the back of the alt on 240. Though the connections are the same in appearance more or less on your 1992 940:



As you have the Nippon Denso or maybe Bosch the back of the alt is somewhat different.

If you need to buy a replacement alt, get OEM new (may need to swap pulley from old alt to new), or rebuilt bu the manufacturer (Nippon Denso or Bosch).

Questions?

Hope that halps.

Ceramic Christmas Tree Boy with gallon bottle eggnog bottles as ornaments.
--
Jonathan Harshman Winters III: The Mightiest, Greatest, & Most Powerful North American Comedian & Comedic Actor in Perpetuity