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Discussion Starter #1
Hello all,

About 2 months ago I bought a 2013 981 base Cayman equipped with Adaptive Cruise Control. This functioned fine during the test drives I did, and for the first two weeks all was fine. After that, the ACC started disabling itself after a few minutes of driving. Every now and then it re-enables itself and then the system seems to work just fine for again just a few minutes. However, 95% of the time the warning message about the ACC system being disabled, is present.

Additionally, the horn in my car is not functioning.

Could both issues be related?

I have not consulted with any professionals yet and I thought getting a bit more insights myself before spending my hard-earned euro's on repairing these issues. I did search this and other forums: the issue does pop up in quite a few places, but I did not find any conclusive root cause in any of those threads. And of course, my sensor is free of debris and dirt.

Thanks in advance for your replies!
 

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Welcome, sorry to hear about the troubles with ACC, these new systems can be complex but the cause of the problem may not be.

As for the horn not working now as well could point to some electrical gremlin, did 'anything' get changed when this all started, anything at all?
 

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State of Battery? Original or was it replaced? Seven year old battery is a suspect. Porsche has embedded gremlins if you do not replace the battery with their overpriced battery and dealer programming.
 

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Agree. ACC is almost certainly the battery. As the battery ages, the ACC doesn't detect enough charge and disables itself. This is especially the case if it has been sitting overnight or a couple days. Once the battery charge is high enough, it will start working again.

I don't think the horn is related, but who knows. Did you check the fuse first? You should be able to test the horn itself with some wire connected to the battery. If not the fuse or the horn, then the horn relay would be the next suspect item.
 

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I hope the OP did not buy a car that spent some time underwater where multiple wire harnesses are corroded out.

Maybe, the horn relay or wiring may be shorted enough to pull down the battery and messing with ACC. I bought an old Chevy years ago where a very unskilled shade tree mechanic tried to install a car alarm that had a similar result.
 

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Discussion Starter #6
Thanks all for the suggestions!

Recently I did a long roadtrip with the car, and the time and intervals the adaptive cruise control is available, do not seem to vary depending on the battery state. I typically only drive the car in the weekend, so it does discharge somewhat during the week. However, this roadtrip (1000+ km in 2 days) showed that even when the battery is completely charged, the ACC does not work.

For the horn I'll go and do some tests. Check for the relay being activated, and I'll check the fuse as well.

I'll keep you posted!
 

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"Almost certainly the battery" is German for "I have no idea what the problem is, and neither does the dealer, or probably, for that matter, the factory, thanks to their use of overengineered technological solutions which the company outsourced from third party-suppliers without insisting on sufficient self-diagnostic capabilities."
 

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Thanks all for the suggestions!

Recently I did a long roadtrip with the car, and the time and intervals the adaptive cruise control is available, do not seem to vary depending on the battery state. I typically only drive the car in the weekend, so it does discharge somewhat during the week. However, this roadtrip (1000+ km in 2 days) showed that even when the battery is completely charged, the ACC does not work.

For the horn I'll go and do some tests. Check for the relay being activated, and I'll check the fuse as well.

I'll keep you posted!
A “completely charged” 7-year old battery is still a problem. As others have asked, how old is the battery?
 

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Discussion Starter #9
Just checked the battery: it's an original Porsche part. Hard to determine whether it's still the original one. I didn't see any date on the thing.

I also did some voltage measurements. Just opening the trunk, after a week of standstill, it was at about 12,3 V. Putting the key in the slot it dropped to 12,2 V. Turning on the ignition (PCM+audio on, headlights on) the voltage dropped to 11,9 V. With the engine running, the voltage went to 14,4 V.

Are these normal values?

I also checked for the horn: fuse ok, no audible relay noise when pressing center of the steering wheel. Strange ...
 

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Just checked the battery: it's an original Porsche part. Hard to determine whether it's still the original one. I didn't see any date on the thing.

I also did some voltage measurements. Just opening the trunk, after a week of standstill, it was at about 12,3 V. Putting the key in the slot it dropped to 12,2 V. Turning on the ignition (PCM+audio on, headlights on) the voltage dropped to 11,9 V. With the engine running, the voltage went to 14,4 V.

Are these normal values?

I also checked for the horn: fuse ok, no audible relay noise when pressing center of the steering wheel. Strange ...
I’ve got a 1-year old original Porsche battery in my car. I’ll check these values for you today.

Edit: I can’t tell from your profile where you are but I’m on Vancouver Island and have an OEM battery you can have for free (4 y-o). Not worth shipping it if you’re not nearby tho...
 

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Just checked the battery: it's an original Porsche part. Hard to determine whether it's still the original one. I didn't see any date on the thing.

I also did some voltage measurements. Just opening the trunk, after a week of standstill, it was at about 12,3 V. Putting the key in the slot it dropped to 12,2 V. Turning on the ignition (PCM+audio on, headlights on) the voltage dropped to 11,9 V. With the engine running, the voltage went to 14,4 V.

Are these normal values?

I also checked for the horn: fuse ok, no audible relay noise when pressing center of the steering wheel. Strange ...
Those voltages seem quite low, see the attached chart...

265305
 

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Discussion Starter #12
Edit: I can’t tell from your profile where you are but I’m on Vancouver Island and have an OEM battery you can have for free (4 y-o). Not worth shipping it if you’re not nearby tho...
Thanks for the offer! I'm living in Belgium, though ...

In the mean time I found this thread: "Who has done a battery replacement in a 981?" . Apparently it is not without risk replacing a battery yourself. I guess I'll contact some official dealers and Porsche specialists in my area and ask for their advice and price for replacing the battery.
 

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Just checked the battery: it's an original Porsche part. Hard to determine whether it's still the original one. I didn't see any date on the thing.

I also did some voltage measurements. Just opening the trunk, after a week of standstill, it was at about 12,3 V. Putting the key in the slot it dropped to 12,2 V. Turning on the ignition (PCM+audio on, headlights on) the voltage dropped to 11,9 V. With the engine running, the voltage went to 14,4 V.

Are these normal values?

I also checked for the horn: fuse ok, no audible relay noise when pressing center of the steering wheel. Strange ...
Trunk open: 12.44v
Key in: 12.33v
Car acc on: 12.16v

The car has sat for a couple of weeks now (at least). I’ve put the battery maintainer on it for tonight and I will give you full values tomorrow (we can see if they line up with the figures from Westcoaster).
 

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Thanks for the offer! I'm living in Belgium, though ...

In the mean time I found this thread: "Who has done a battery replacement in a 981?" . Apparently it is not without risk replacing a battery yourself. I guess I'll contact some official dealers and Porsche specialists in my area and ask for their advice and price for replacing the battery.
It is a lot of money to replace the battery at Porsche. You may want to consider getting a fresh compatible battery from somewhere, plugging it in to see if your troubles go away and if they do then paying Porsche to use PWIS to re-code the ECU to the new battery. Having the coding missing is not going to prevent your troubleshooting.
 

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Trunk open: 12.44v
Key in: 12.33v
Car acc on: 12.16v

The car has sat for a couple of weeks now (at least). I’ve put the battery maintainer on it for tonight and I will give you full values tomorrow (we can see if they line up with the figures from Westcoaster).
Trunk (actually frunk) open: 12.78
Key in: 12.63
Acc on: 12.45

Also my original numbers may be suspect as I discovered this morning that I didn’t have the leads properly seated on my multimeter (so they may be slightly low).

Hope this helps.
 

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Thanks for the offer! I'm living in Belgium, though ...

In the mean time I found this thread: "Who has done a battery replacement in a 981?" . Apparently it is not without risk replacing a battery yourself. I guess I'll contact some official dealers and Porsche specialists in my area and ask for their advice and price for replacing the battery.
In the US, you can take your battery to an auto parts retailer and they will load test the battery for no charge. From your voltage measurements, it appears your battery is on borrowed time. A VW Passat battery will work in a 981 but you may get phantom fault messages that will come go (air bag usually). Thanks to the Porsche engineers that designed the computer and software systems. I got rid of my Cayman and got a reliable Chevrolet.
 
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