Battery drain and rev counter fault - Alfa Romeo Forum
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(Post Link) post #1 of 9 Old 29-09-10 Thread Starter
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Battery drain and rev counter fault

Hi all, first post on this forum. Recently bought my second 156 V6 and it has a charging problem. The car can sit for days or even weeks without draining the battery so I am fairly convinced it doesn't have a short anywhere, but when you run the engine it will go for about an hour or so then die with a flat battery. The previous owner sold the car with this fault claiming he has had the alternator checked and it came back all clear but the symptoms point directly to that being the fault.
The alternator has excitation voltage going to it when the ignition is turned on and the charging output terminal is definately connected to the battery positive terminal.
There is a lot of talk of a rev counter fault causing battery drain when the car is parked, but like I say I don't have that problem. The only thing that makes me wonder about the rev counter is the battery warning light never comes on, even when you first key on when all the other warning lights come on, so I am wondering if there is some link there.
I've monitored the battery voltage with the engine running and it sits around 12v and drops slowly as you turn on lights and wipers etc. Revving the engine doesn't help matters.
Like I say, all the symptoms point to the alternator being faulty except that the previous owner insists he had it tested and has sold the car for a song based on the fault beating him.
At some point I saw some photos of the rev counter PCB with the faulty chip arrowed, but I can't find them now, does anyone know which chip is the culprit? There are no obviously dead ones on my board.
Sorry for the lengthy post and hopefully someone can help.
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Voltage across battery with engine running should be arount 14.4v.
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Certainly sounds like it's not charging.

Pull the main beam on with the engine running. If the revs don't drop when you do this, the alternator's not working (or not connected).
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Normally the excitation circuit goes through the charge light, if it does not light up, the circuit to the alternator could be broken. Try to excite the alternator externally from 12V+ on battery via a 12v 5W globe, and see if the voltage picks up on the battery to around 14 V, then you know the excitation circuit in the car is broken.
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(Post Link) post #5 of 9 Old 18-10-10 Thread Starter
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OK, got an alternator off a 30K 156 v6 and all is good now. That was a bit of a marathon! All the symptoms pointed to it but the previous owner insisting he had got it checked was giving me doubts.

So as a result of this, I have a bag of chips available. The silicon sort rather than the potato sort. They are the VND810MSP that has been found to cause the battery drain issue some people are falling foul of. If anyone wants one, send me a message.
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Now was it just a loose 10mm nut then?
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(Post Link) post #7 of 9 Old 19-10-10 Thread Starter
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Nothing was loose, checked all that during the diagnosis stage. I could get the old alternator bench tested, but to be honest now it's working I have moved on to the challenge of getting it MOT'd.
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Hi! I ´ve just disassembled my rev counter and found a cooked VND810MSP, however I can´t find this IC in my country and the ones I´ve found in internet are sold by 400+ items, I just need one. Can someone tell me if
VND810sp would work as well. I have checked the datasheet on both IC and are similar but not identical. Just some differences with the interrupting current graph and the ON state resistance. If someone knows please help. Or can anyone tell me where to find a VND810msp through internet . Thanks
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Send a PM to Leewurf - he may still have a few, else if you do a search on "Rev Counter Issues" or "VND810" there are a number of threads that may assist.
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battery , counter , drain , fault , rev

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