instrument cluster question- late dash

[DP944]

Ok so my 85.5 944 is running really well.... As long as jumper cables are hooked up from my audi haha. swapped 3 different alternators...they just arent charging the battery....so

i need someone to go out to their car with a late dash and turn the key on w the engine off. what i need to know is whether there is a little red light illuminated at the bottom of the voltmeter that goes away when the engine is started




thanks. Im thinking that if there is supposed to be a bulb there, mine is blown, which is the root cause of my charging issue

[ernie9944]

Late dash 93-968,hope it does help.

[Gawernator]

Yes if you just turn the key but don't start the engine all the red warning lights should turn on.

[DP944]

im aware that all the warning lights should come on, just wasnt sure if there was one for the bottom section of the voltmeter. Thanks for the info and the pic

[Guest765]

That bulb has nothing whatsoever to do with charging the battery or not. You need to check continuity to the battery. That means you need to check both your engine block ground, and underneath the car ALL positive power goes through the starter connections before it makes it to the battery. If that connection is bad there you will never charge the battery. If you don't believe me on that, trace the wires and you will see positive power goes from battery to starter, then to the rest of the car. I believe it is a switched ground that operates the starter, same way the ignition coil is switched ground and not switched positive as you would expect it to be.

[DP944]

i think your wrong about that one. the bulb in question is part of the alternator exciter circuit. ive done a voltage drop with the engine running from b+ on the alternator to the b+ terminal of the battery and its within range, so i know there is not excessive resistance, there is definitely continuity. so unless i have 3 bad alternators, which ive swapped and flopped the voltage regulators around on, then it has something to do with the fact that my charging light is not lit, or something to do with that ckt

[jwade944]

You are correct. The bulb has to be in the circuit so that the battery voltage will excite the alternator. Once the alternator is producing, the battery and the alternator have the same voltage so no current flows and the light turns off.

[Guest765]

I call bs on that one, sorry, no dice. That bulb has nothing to do with charging your battery no way.

Also that's not a charging light, its a warning light for exactly the opposite. If you research the voltage and amperages involved (90-120 amps depending on your alternator) you'd realise that bulb would literally (yes literally) explode if it was part of the circuit.

Please, explain to me how people without having that specific gauge cluster whatsoever in their race cars get their battery to charge?

[Guest765]

And if that light is really needed then how did I drive my car for two years with a completely non functional volt meter? It just doesn't add up.

I'm more than glad to eat my words if someone has proof, I don't see it.

[jpk]

I'm not sure that light has anything to do with the voltmeter, other than sharing space. Here's the relevant page out of the 968 owner's manual. I don't think this is any different from the late 944. If it does not light up, you are supposed to contact your dealer

My understanding is that the bulb is part of the exciter circuit for the alternator; unlike a generator which is simply a DC device and will produce power whenever it is turned, an alternator needs to have an excitation current to induce a magnetic field. If the bulb is broken, then the alternator never establishes the excitation field and doesn't charge.



Furthurmore; from wikipedia on alternators:

"The field windings are supplied power from the battery via the ignition switch and regulator. A parallel circuit supplies the "charge" warning indicator and is earthed via the regulator.(which is why the indicator is on when the ignition is on but the engine is not running). Once the engine is running and the alternator is generating power, a diode feeds the field current from the alternator main output equalizing the voltage across the warning indicator which goes off. The wire supplying the field current is often referred to as the "exciter" wire. The drawback of this arrangement is that if the warning lamp burns out or the "exciter" wire is disconnected, no current reaches the field windings and the alternator will not generate power."

[Gawernator]

The voltmeter gauge and the warning light appear to me to be on a different circuit. My volt gauge didn't work when I got the car. I cleaned the connections on back and grounds and it worked. Yesterday it isn't working again :'( but the red warning light is still functional. When you turn the key. And it turns off when I start the car

[944 Ecology]

On the early dash cars (including the 924S cars), the alternator light was necessary to provide the exciter voltage to the alternator. If the bulb was burned out, the alternator would not charge.

On the later cars, the exciter voltage comes directly from the battery (by way of the ignition switch).

[DP944]

well im going to try to replace that bulb regardless....because it isnt turning on. while im in there im going to brighten up the instrument cluster with some leds and the foil tape modification. maybe i need an ignition switch. ill have my alternators bench tested for the hell of it as well

[DP944]

anyone had any experience running a jumper wire from the positive terminal on the battery to the d+ terminal on the alternator to replicate "12v from the ignition switch?" that way if the alternator does indeed start charging i know to hunt into the wiring to the ignition switch or replace the ignition switch.

[DP944]

ill need to look at some wiring diagrams. this problem is eating me up

[jpk]

Originally Posted by DP944

anyone had any experience running a jumper wire from the positive terminal on the battery to the d+ terminal on the alternator to replicate "12v from the ignition switch?" that way if the alternator does indeed start charging i know to hunt into the wiring to the ignition switch or replace the ignition switch.

That's a good idea; you can do it right from the back of the alternator, but run it through a light bulb. The output terminal on the alternator is basically the same as battery +; jump from there through a small 12v bulb to the D+ exciter terminal. The bulb should pretty quickly dim out and the alternator starts putting out power.
The other thing you could do is to just check for +12 at the D+ term on the alternator with the engine off/ignition switch on. I guess I'd check that first, and if you don't get the required excitation voltage with the engine off, start the engine and use the light bulb/jumper to start the field excitation.
Of course, you could also take the alternator out and have it bench tested.

[DP944]

ive tried 3 different alternators so far. ill have the two that i installed already bench tested. keep you posted.

[DP944]

ya not getting 12v at the blue wire with key on engine off. I am
However getting .22v at the d+ terminal regardless of ignition position. Strange. Dont have time to look into more tonight

[Guest765]

According to the Wikipedia posted above, unless they used the wrong terms, a parallel circuit is used ;

A parallel circuit supplies the "charge" warning indicator and is earthed via the regulator.(which is why the indicator is on when the ignition is on but the engine is not running).

According to that, if a series circuit was used, then the exciter would not be powered with a burned out bulb. A circuit in parallel supllies voltage regardless of the bulb.

From wiki;

In a series circuit, every device must function for the circuit to be complete. One bulb burning out in a series circuit breaks the circuit. In parallel circuits, each light has its own circuit, so all but one light could be burned out, and the last one will still function.

That doesn't mean porsche didn't wire it in series, but why would you when the bulb is powered off the rectifier according to wiki? I believe the wiki is meant to say

The drawback of this arrangement is that if the warning lamp burns out and the "exciter" wire is disconnected, no current...

Which would be logical when the (again, wiki)

The field windings are supplied power from the battery via the ignition switch and regulator. A parallel circuit supplies the "charge" warning indicator and is earthed via the regulator.

there is no reason the bulb would prevent the alternator energizing, again parallel vs series circuits. The bulb only prevents charging if it is in series with the enegizer. Q.E.D.

[bell]

Yes the bulb is required for the alternator to start it's charging state.....
also.....on 944s (late only iirc)) the alarm brain is in the passenger footwell, that circuit loops through it......
On cars without an alarm it still has the alarm harness which has a jumper plug in place of it.....
I have seen numerous people remove their alarms then have a no charge situation for this reason.
I have also done many conversions with gauges etc....and without that little bulb working the alternator will not charge.

Also the light and volt gauge run off of 2 different circuits...