No Charging Light on 89 930, But Charging

[73ess]

The charging light in my 89 930 is not turning on when I turn the ignition key to the on position. But I'm measuring 12.99 voits at the battery terminals with the engine running and headlights on. So looks like I'm getting some charging? I thought there should be no charging if the alternator/charging light doesn't turn on? Haven't checked the bulb yet.

[DRACO A5OG]

Actually, you should be getting 13.8-14.3 at initial start.

Maintain at 1800 RPMs see if you get a higher reading if not then there is an issue with the charging system.

Yes, you should be getting the alt light in on position prior to start.

[73ess]

Originally Posted by patz

No light when turning to on position, no charge.

That's what I thought as well. But my multimeter measures some charging at 12.99v with headlights on and engine at 950rpm idle.

[mboxler]

I thought that when the engine is running, voltage on either side of the bulb should be equal, therefore the bulb should be off. When not running, there is 12 volts on one side of the bulb, 0 volts on the other side, and the bulb lights.

[wwest]

Some models had a (failsafe?[***]) resistor in parallel with bulb.....

Some alternators will "self-boot" due to even the tiniest level of magnetism remaining in the rotor post engine shut down.

Not likly to see "resting" voltage after starter crank and with headlights on.

*** incandescent bulbs are not exactly long term reliable.

[wwest]

Originally Posted by mboxler

I thought that when the engine is running, voltage on either side of the bulb should be equal, therefore the bulb should be off. When not running, there is 12 volts on one side of the bulb, 0 volts on the other side, and the bulb lights.

"..0 volts..." Depends on the downstream resistance (VR, brushes, rotor winding) to ground. But otherwise your statement is correct.

[73ess]

Ok. I guess I'll measure voltage at 1800 rpm and see what I get. Took a peak at the rear of the gauge and looks like the charging bulb is in place. But not seeing any glow from the rear. I do see a the glow from the oil pressure light. This is with ignition switch to on, but engine off.

[wrxnofx]

I had the same situation on my '87 when I got it. No alternator light illuminated with key in position 2, but the alternator charged the battery just fine while the car was running.

The bulb was burned out. When I replaced it, it now lights up properly at position 2 until the car is started.

P/N is either 900 631 102 90 (2 W bulb) or 900 631 103 90 (4 W bulb). I can't remember which it is, but hopefully someone will chime in. Edit: I'm pretty sure it's the 4W bulb, whereas the 2W bulbs are for illumination of the gauges themselves.

If you've never removed the gauge before, it can be a bear. My suggestion is to NOT pry the gauge from the front, as that just dents the slightly squishy stuff behind the vinyl surrounding the gauges (ask me how I know). Rather, open the driver's door and front boot, stick your left arm into the boot, under the cowl, and push on the back of the gauge. With your right hand, pull on the front of the gauge. If you don't have gargantuan long arms like me, then use a broom handle or something else long enough to push from behind.

[73ess]

Originally Posted by wrxnofx

I had the same situation on my '87 when I got it. No alternator light illuminated with key in position 2, but the alternator charged the battery just fine while the car was running.

The bulb was burned out. When I replaced it, it now lights up properly at position 2 until the car is started.

P/N is either 900 631 102 90 (2 W bulb) or 900 631 103 90 (4 W bulb). I can't remember which it is, but hopefully someone will chime in. Edit: I'm pretty sure it's the 4W bulb, whereas the 2W bulbs are for illumination of the gauges themselves.

If you've never removed the gauge before, it can be a bear. My suggestion is to NOT pry the gauge from the front, as that just dents the slightly squishy stuff behind the vinyl surrounding the gauges (ask me how I know). Rather, open the driver's door and front boot, stick your left arm into the boot, under the cowl, and push on the back of the gauge. With your right hand, pull on the front of the gauge. If you don't have gargantuan long arms like me, then use a broom handle or something else long enough to push from behind.

Aha! So the system can be still charging with a burned-out lightbulb! A few people told me that a burned-out lightbulb will disable the charging circuit. I guess they were wrong!

[mboxler]

Originally Posted by 73ess

Aha! So the system can be still charging with a burned-out lightbulb! A few people told me that a burned-out lightbulb will disable the charging circuit. I guess they were wrong!

The light bulb really has nothing to do with charging. With the ignition on and the engine not running, the bulbs internal resistance causes current to flow through it AND the alternator field winding to ground. The alternator needs this current to start up. A 40 ohm resistor (?) will do the same job, but you will get no indication if something is wrong. Once the engine is running, the bulb's only purpose is to warn you that there is a voltage difference between D+ (alternator) and B+ (battery).

At least I think that's what's happening.

[wwest]

Originally Posted by mboxler

The light bulb really has nothing to do with charging. With the ignition on and the engine not running, the bulbs internal resistance causes current to flow through it AND the alternator field winding to ground. The alternator needs this current to start up. A 40 ohm resistor (?) will do the same job, but you will get no indication if something is wrong. Once the engine is running, the bulb's only purpose is to warn you that there is a voltage difference between D+ (alternator) and B+ (battery).

At least I think that's what's happening.

Yes, the bulb circuit's purpose is PRIMARILY to "kickstart" the rotor field winding.

Since you cannot always rely on the alternator "start-up" without the bulb/circuit it should be in place.

[darrin]

also, given the load the headlights place on the system, 12.9 with headlights on isn't too far off. What reading do you get with engine running and lights (and other stuff) OFF? Should be upper 13-14.1 range if all's well

[73ess]

I'm reading around 13.8 with the headlights off. So I guess I'm ok, even with the light bulb not functional. I'm going to pull the gauge to check it. Will let you all know what I find. mboxler, your explanation makes sense.