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Discussion Starter · #1 ·
Bizarre issue, but when I started the car this morning, I noticed that the clock in the sports chrono was intermittently going between dark and bright, with no regularity. Almost like a battery issue.

Anyone seen this before? Do I need a new battery?

H
 

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If the car started normally, it seems hard to imagine it not being able to run a clock light, especially with the vehicle running. What is the battery voltage doing while you are driving around?
 

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If the car started normally, it seems hard to imagine it not being able to run a clock light, especially with the vehicle running. What is the battery voltage doing while you are driving around?
I think he believes the sports chrono may have a separate battery that needs replacing like most quartz clocks do. I wouldn't believe he would think this is an indication of the car battery health.

On a side note the intermittent switching suggests a loose wire or connection.
 

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There's probably some night-dimming system in there.... maybe you were in a threshold condition. I don't suppose the headlights were going on and off at the same time... I hope it's an easy/cheap fix.
 

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Discussion Starter · #6 ·
See this all the time. It is an auto-dimming "feature" as trickybit suggests.


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Is it because, with garage door open, car can't decide whether it should be dimmed or not? So it just freaks out and fluctuates between both?
 

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I have same issue when cloudy morning and head light is still on. I took a video to show dealer, but I think it is normal.
 

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Is it because, with garage door open, car can't decide whether it should be dimmed or not? So it just freaks out and fluctuates between both?
Yes, is the short answer.

The longer explanation is about poor electronics design. Light "pressure" is measured by a photocell (a device that changes resistance depending on the photons striking it) or a photovoltaic cell (which generates electricity when struck with photons). In either case a circuit measures the amount of light striking it.

The poor yet simple circuit design we have in our cars makes decisions about things like the brightness of the sport chrono clock on the basis of a set amount of light hitting the sensor. When the value is exceeded the clock is brightened. When not exceeded it is dimmed. Now what happens if you are right at that amount?

In my case when I drove down a tree-lined street the clock would dim for each tree I passed under and would brighten up otherwise. In your case you are right at that value with your garage door open and small changes like passing clouds or reflections from outside are triggering the change).

This is a really stupid design, but I suspect they did it for safety reasons (and because it was cheap).

One solution would be to introduce a delay before the circuit made a decision about whether to brighten the clock or turn on the headlights. But this would result in a safety issue when you entered a tunnel. For a brief period of time (whatever the delay is) you wouldn't have headlights and no doubt there would be an accident and Porsche paying for a NHTSA recall.

The right solution is an overlap in the values necessary to trigger the change. For instance if the range of values produced by the photocell is 0 (absolute darkness) to 5 (Death Valley), and the current trigger value is 2.5, then you design a circuit to brighten at 3.0 and dim at 2.0.

Porsche didn't do that as far as I can tell.

(Disclaimer: I am neither an electrical nor automotive engineer)
 

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Yes, is the short answer.

The longer explanation is about poor electronics design. Light "pressure" is measured by a photocell (a device that changes resistance depending on the photons striking it) or a photovoltaic cell (which generates electricity when struck with photons). In either case a circuit measures the amount of light striking it.

The poor yet simple circuit design we have in our cars makes decisions about things like the brightness of the sport chrono clock on the basis of a set amount of light hitting the sensor. When the value is exceeded the clock is brightened. When not exceeded it is dimmed. Now what happens if you are right at that amount?

In my case when I drove down a tree-lined street the clock would dim for each tree I passed under and would brighten up otherwise. In your case you are right at that value with your garage door open and small changes like passing clouds or reflections from outside are triggering the change).

This is a really stupid design, but I suspect they did it for safety reasons (and because it was cheap).

One solution would be to introduce a delay before the circuit made a decision about whether to brighten the clock or turn on the headlights. But this would result in a safety issue when you entered a tunnel. For a brief period of time (whatever the delay is) you wouldn't have headlights and no doubt there would be an accident and Porsche paying for a NHTSA recall.

The right solution is an overlap in the values necessary to trigger the change. For instance if the range of values produced by the photocell is 0 (absolute darkness) to 5 (Death Valley), and the current trigger value is 2.5, then you design a circuit to brighten at 3.0 and dim at 2.0.

Porsche didn't do that as far as I can tell.

(Disclaimer: I am neither an electrical nor automotive engineer)
For all intents and purposes My car does have a delay. I notice it because when passing under an underpass my headlights do not come on. Same thing when driving on heavy tree lined roads. From what I can gather the response time is adjusted based on how far down or up the scale the photcell is.

For example. While driving on a tree lined street that is still lit it takes a few miles before my headlights auto turn on. When I enter a tunnel they turn on instantly. They do not turn on when passing under wide underpasses, which in my opinion simulate a tunnel.

So I do believe they did make use of what is known as a "dead-band" but more related to the intensity, or lack of, the light.

another feature where they used a dead-band is the wing on your car. It raises after 75 mph and retracts below 40 ish.

I would have to believe that the lighting and dimming is not as rapid as the original post makes it seem.
 

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For all intents and purposes My car does have a delay. I notice it because when passing under an underpass my headlights do not come on. Same thing when driving on heavy tree lined roads. From what I can gather the response time is adjusted based on how far down or up the scale the photcell is.

For example. While driving on a tree lined street that is still lit it takes a few miles before my headlights auto turn on. When I enter a tunnel they turn on instantly. They do not turn on when passing under wide underpasses, which in my opinion simulate a tunnel.

So I do believe they did make use of what is known as a "dead-band" but more related to the intensity, or lack of, the light.

another feature where they used a dead-band is the wing on your car. It raises after 75 mph and retracts below 40 ish.

I would have to believe that the lighting and dimming is not as rapid as the original post makes it seem.
Have you noticed consistent behavior between your headlights and sport chrono clock? Perhaps the headlights have a "dead-band" and the clock/dash lights do not?
 

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As fond as I am of my excellent 981, I would view with healthy skepticism the resume of a software developer/designer from Porsche whose code did not run within the engine. The entertainment system is laughable. I mean, has unbounded quirky charm. I don't know if they screw stuff up on purpose to enhance the "it's for driving!" brand proposition, or if they have a serious attrition problem because they don't value software development highly, or if their schedules are unrealistic. But if you shipped that entertainment system as an iPhone app you'd get about 1.5 stars max.

The "dead band" has a name, and some nifty graphs and everything: https://en.wikipedia.org/wiki/Hysteresis. The light sensor sounds like it's not as smart as your $29.99 thermostat in your house.
 
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Have you noticed consistent behavior between your headlights and sport chrono clock? Perhaps the headlights have a "dead-band" and the clock/dash lights do not?
Yes my headlights and the lighting for my instrument cluster and sports chrono are all linked. They behave as you would expect as soon as the headlights come on they come on and the same with them going off.
 

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I have seen a similar issue when driving with dash ligjting but not with headlights. At my common speed under street lights on my commute home, I often see the dash lights toggle between daylight brightness and night brightness. Since I have like.my dash very dim at night, it is a bit annoying. Both poor design and another endearing Porsche quirk.
 
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