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Discussion Starter #1
Our V10 has a serious voltage drop issue on start up. It will not start, until you use the emergency start. Batteries are fine. Any ideas?
 

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Define "batteries are fine"! How old are they? Have they been tested with a battery tester? Or did you just check their voltage? If the latter, then how - with the batteries disconnected, or while on circuit?
 

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In simple terms, the rear battery is the starter battery assisted by the front battery which runs the consumables.


If the rear battery is duff then the front battery will shut down various things so it has enough oomph to take over starting the car.


From what you describe, your rear battery is a dud.


There may be something else going on but I'd suggest you try another battery in the rear first to see if that resolves the issue.


Here's the VW Self Study Programme for the electrical system which explains everything in detail:


http://www.volkspage.net/technik/ssp/ssp/SSP_298_d1.pdf
 

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Cheers for your quick replies. I had the front battery tested at the shop, they confirmed that it tested fine. However, I didn't realise that the muppets didn't test it under load. I checked it myself this morning, turning the key on with the voltage meter, watching the voltage fall off. So I disconnected the battery and sure enough, it didn't hold 13 volt. So once I replaced it with another battery, problem fixed. Glad it appeared to be an easy solution in the end.
 

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Check the rear battery under load also.
 

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Cheers for your quick replies. I had the front battery tested at the shop, they confirmed that it tested fine. However, I didn't realise that the muppets didn't test it under load.
In short: they didn't test it at all. Whatever they did can not be considered "testing" the battery.
 

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Same problem this morning. Didn't drive my car for 9 days and now it won't start without the key in the ignition and switching to the other battery.

Front battery was replaced Q4 2017
Back battery was replaced Q4 2012 (I think we've found our culprit)

I'm about to measure the batteries with my multimeter... What readings should I be getting?

Thanks!
 

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What is important is the voltage drop during the starting sequence. If you log group 7 in address 09 central electrics you will see how both batteries show maximum voltage drop around 2:39 seconds into the aequence. With good batteries in cold weather I have recorded 9.9 front and 9.6 rear at that moment ans still started successfully. If you read below these numbers you need to replace one or both batteries. This is how I decide when to replace batteries.
 

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What is important is the voltage drop during the starting sequence. If you log group 7 in address 09 central electrics you will see how both batteries show maximum voltage drop around 2:39 seconds into the aequence. With good batteries in cold weather I have recorded 9.9 front and 9.6 rear at that moment ans still started successfully. If you read below these numbers you need to replace one or both batteries. This is how I decide when to replace batteries.
Thanks for the reply lgibson.

I measured the back battery in the meantime, 12.91V with the engine off, 13.91V with it running and 13.64V with it running while having the SATNAV and my high-beams on.

Sorry for being a newbie, but I'm not sure sure what "group 7 in address 09 central electrics" means?
 

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Sorry, didn't mean to hijack your thread @Xelam - I just noticed we had the exact same problem.

Do you also have your cruiser control turn off as soon as you set it?
 

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Engine on means little. Engine off (no load) looks OK.
Do you own VCDS? V10 owners need either deeeep pockets or VCDS (no ebay chinese clones).
O9 is VW’s address for the Controller Module for electrics. Each module displays Measured Value Blocks for various parameters, organized into Groups. In this case voltages are displayed in Group 7. VCDS has a logging feature.....very useful when numbers are changing too rapidly to see in this case.
 

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Only got a cheap Chinese knockoff VCDS. Will try to test front battery tomorrow to be able to exclude it from the possible cause.
 

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The best battery test is still on old style resistance load test.

To pass a load test, the battery must first be fully charged. In a true resistance load test, the battery actually runs a current thru a carbon pile based tester. This loads the battery down, the same way your starter, your glow plugs, your headlights etc do when the engine is started.

To pass the load test, the battery must maintain at least 9.6 volts for 15 seconds, when tested at one-half the CCA rating listed on the battery.

 

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V10 owners need either deeeep pockets or VCDS (no ebay chinese clones).
What's wrong with Chinese cables? I've had a clone cable for 5+ years and never had an issue.
 
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lgibson
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