Battery light on dash does not come on

[Tanksowner]

Hi all,

So I've had this problem for a while on my '85 300D. I'll do my best to provide as many details as I can.

When I turn the key to start the car, the warning lights come on as usual. I usually have the parking brake engaged. The battery light does not come on. After I start the car, the gas tank warning light, yellow brake pad warning light stay on until I either disengage the parking brake, or put my foot on the gas pedal.

Wondering what could be causing this? Want to try and nip it in the bud. Thanks!

[Mxfrank]

Test the voltage across your battery terminals. Once the car is running, it should be over 13-14 volts. I suspect you'll find that it's around 12V until you rev up the motor. The indicator lamp is more than an indicator...it's a critical component of the charging system. When the indicator bulb has blown or the circuit is open, the alternator isn't getting bootstrap current. It's possible that there's enough residual magnetism in the alternator rotor to get it charging when you rev up the engine, but it won't be as reliable as a working bootstrap circuit.

[Tanksowner]

Originally Posted by Mxfrank

Test the voltage across your battery terminals. Once the car is running, it should be over 13-14 volts. I suspect you'll find that it's around 12V until you rev up the motor. The indicator lamp is more than an indicator...it's a critical component of the charging system. When the indicator bulb has blown or the circuit is open, the alternator isn't getting bootstrap current. It's possible that there's enough residual magnetism in the alternator rotor to get it charging when you rev up the engine, but it won't be as reliable as a working bootstrap circuit.

I seem to be getting enough power in the car to use the climate control and the radio, and my headlights are plenty bright at night. Maybe the bulb just burned out? Agreed, 12V when you turn the key, 13-14 while running.

Can you explain the bootstrap current/circuit a bit more? The alternator was replaced back in 2010, but I've put close to 50K on the car since then. Could the voltage regulator need to be replaced again?

[Jeremy5848]

It's unlikely the regulator brushes have worn down in only 50,000 miles but not impossible. More likely the bulb in the instrument cluster is burned out.

The current through the bulb runs from the battery to and through the alternator's slip rings to the rotor (spinning part) and then back to ground (negative). That generates a magnetic field which, picked up by the stationary coils of the alternator and turned into DC by the diodes built into the alternator, charges the battery and powers the accessories of the car. Once the alternator is generating a current, the alternator light goes out because both sides of the bulb are now at positive voltages.

If the alternator light doesn't work, the bulb is most likely burned out. If so, however, the alternator won't generate electricity to keep the lights bright at night and all the rest. Something does not compute!

Jeremy

[optimusprime]

Could be that the speedo as been out before, and the bulbs holders have not been put in the right holes .Buy my guess is bulbs as failed ,

[funola]

The chance of the battery charge bulb burning out is very small since its on time so infrequent. It is more likely a dirty connector contacts at the alternator, or bad bulb contact in the cluster. I'd clean the connector first, if that does not fix it, then move on to the bulb.

[Tanksowner]

Originally Posted by optimusprime

Could be that the speedo as been out before, and the bulbs holders have not been put in the right holes .Buy my guess is bulbs as failed ,

We pulled the cluster several years back after I got the car to replace the odometer gear. I need to pull it again to change the dash light bulbs to brighter LED's, so I'll check the connection in there.

@Jeremy and funola, good advice and info. Appreciate it, and I'll look into both.

[Mxfrank]

Here's the way it works. First, imagine the alternator when it's running. An internal current supplied via the voltage regulator supplies the rotor, creating a magnetic field. In order to maintain system voltage, the regulator will modulate the magnetic field and thus the output voltage by varying the current supplied to the rotor.

When you start the car, the alternator isn't initially generating power, so there's no internal current for the rotor. To overcome this, the indicator light circuit is grounded through the rotor. This creates a small magnetic field to get things started. This is what's meant by bootstrapping the alternator. Once the alternator is running and providing it's own power, the indicator light gets 12v directly from the alternator, rather than reaching ground through the rotor. With 12v on both leads, the light goes out.

Since your bulb is out, the likely explanation is that either the bulb or some element of the "D" circuit is failing. (In addition to the bulb, there's wiring, connectors and the ignition switch.)

As to why things look better after you rev. The iron core and shaft of the rotor retain a tiny bit of magnetism after the car is shut down. If the alternator is spun up fast enough, it may be enough of a magnetic field to generate bootstrap current. This is how "one wire" alternators work. (The disadvantages of one-wires are that they need to be goosed to start, and they have no provision for an indicator light.)

The alternative explanation is that there's an internal short in the voltage regulator which is supplying some current to the rotor at all times.

I've installed a small voltmeter in my dash, it's a better way to know what's going on.

[optimusprime]

Can you let me know how you fitted the ammeter in to the charging circuit ?

[Mxfrank]

Voltmeter, not ammeter. Fitting an ammeter is a big deal, because all the current going to or from the battery has to flow through it. But a voltmeter can be wired to between 12v+ source and ground. Mine resides above the ashtray, and is wired into the lighter circuit.