instrument cluster question- late dash

[rhett]

Originally Posted by 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...

Yep, looked at the CCT diags and Bell is spot on..for some reason Porsche looped the blue wire OP from the alternator through the alarm...or a jumper if not fitted, so there is a possible area for investigation.

However, there is also one more component that might be faulty, the 68R 5W resistor that shunts the bulb. If the bulb has gone and the resistor is faulty then the required voltage will not be present at the alternator. Might be an idea to check that CCT too. 68R across the resistor with no bulb...probably about 10R (ish) when a good bulb is in circuit.

[Guest765]

Well looks like I'm wrong on the bulb I guess. That's a very, very stupid way to wire it if that is the case. A true parallel circuit wouldn't care if the bulb existed at all, let alone was blown out.

[rhett]

Originally Posted by fwayfarer

Well looks like I'm wrong on the bulb I guess. That's a very, very stupid way to wire it if that is the case. A true parallel circuit wouldn't care if the bulb existed at all, let alone was blown out.

No, actually I think you might be right as a 'parallel' circuit is achieved with the shunt resistor across the bulb (#18 in diagram). If the bulb goes, current can still flow via the resistor, albeit the CCT resistance has increased. But, as I mentioned above, both the bulb and resistor would have to fail for the alternator not to charge.

See diagram:




What I find unusual is looping the CCT throught the alarm - not sure why they did that, anyone know?

[DP944]

Any ideas where this resistor is located? Im assuming its in the instrument cluster somewhere

[DP944]

Im going to check the resistance of pin 3 and pin 4 to the alt warning socket. Should have close to no resistance

[16vCorey]

I know it's been covered, but it won't charge if the bulb is blown. If you pull the cluster and the bulb is still good, you've probably got a broken wire somewhere. If all else fails you can just run a diode from the 12v lug to the exciter wire lug (blue wire) and it will charge, but you're light on the dash won't work, but it doesn't already.