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Discussion Starter · #1 ·
OK, so Sunday morning my car was idling like crap so I plugged in my vag-com to try and see what was happening and noticed that the ECU voltage was around 11.3V (min spec is 11.5 I believe). After a while the idle speed went up to around 1100 (manual says this would be to try and increase charge rate) but there was no change to the voltage. Later, after revving the engine much faster it went to 13.8v and the idle settled down nicely.

So I did some tests with a multimeter at the battery terminals and found that the battery voltage is 12.6v (5 years old, green dot still showing). When starting the car it dipped to 10.3v (almost never have a problem starting) and the went back to around 12.5v. Only when the engine is revved to above about 2500rpm did charging appear to start with the voltage going up to 13.8v. It would stay there until the engine was switched off again.

I believe that it should be charging from idle? There is no battery warning light but I have seen it flicker recently and do get a warning tone sometimes shortly after starting but the message has always gone before I can look down and read it. There are no DTC codes for any component.

Anybody got any ideas? battery? alternator? wiring? What should I test next?

Thanks!
 

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Hi , Could be battery or alternator , as the battery is 5 years old it will prob be getting a bit tired

(even though showing green dot) Put the battery on a charger and see what amps it will take ,

a flat battery should charge at about 2amps dropping to 0.5amps as it reaches full charge .

If it doesn't show any amps through it , it has had it !

When on the car (engine running 2500rpm) it should show 14 - 15 volts .. 13.8 is a bit low .
 

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Check all the connections from the alternator to the fusebox on top of the battery,and if you can get to it that there is the same voltage coming out of the alternator as there is across the battery,at least 14.4 volts (the normal regulator setpoint)should be available there.

Chris.
 

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Discussion Starter · #4 ·
Hi,

Dealer had the car last week and used their 'automated tester'. It says the battery is providing about 80% of the cranking amps that would be expected from it's type (it is 5 years old) and the alternator is charging at the right voltage.

Of course this still doesn't explain the voltage being low when you start the car. I don't believe their test accounts for this. I even provided a graph (from vag-com) of voltage vs. rpm over time from an engine start, which clearly shows no charging until the rpm first gets over 2600rpm, seemingly no matter how long you wait.

Anyway, it seems my car has no changeable VRM, so it would mean a new alternator, which may behave exactly the same. At a quoted £450ish + labour to pull the front off the car it can keep the lumpy Sunday morning idle! Actually, I think it has always been like this but the ageing battery is showing it up more now, so I am researching a good choice of battery, especially with winter coming.

Any other suggestions welcome, or pointers to suitable (and perhaps lighter) batteries.
 

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The battery won't affect how it runs,as once the engine is running under it's own steam,all the power for everything comes from the alternator.If you get the revs to a point where the battery is charging,and then turn the engine off,what happens to the voltage across the battery terminals then?It should gradually drop down to about 12.7-12.8 volts and then stay there.You could try this again,get the battery charging,and then when you've stopped the engine,disconnect the battery as quickly as possible,and then take voltage readings again.I had a Fiesta that showed it was charging as it should do when running at 14.4 volts,but when stopped it was drawing 2 - 3 amps from the battery,meaning that after a few hours it was flat.
I'd get a new battery anyway just to be safe as winter's on its way,but still do these checks.

Chris.
 

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Discussion Starter · #6 ·
Just an update on my progress with this..

Every time now, when I start the car the alternator does not start charging. Sometimes this seems to be the cause of a lumpy idle (less often now I have a new battery so slightly higher voltage). Once the engine reaches about 2600rpm the lights brighten and everything is ok from then on.

Being that this is a v6, if I drive down the hill from home and into town through traffic, the engine might never get above 2000rpm unless I deliberately change up later.

Clearly this behaviour is not right, although not bad enough to stop me using the car. I have read up on the operation of alternators and discovered that a power supply from the battery is required to energise the magnetic field and make it start generating, but some alternators can self-energise once the speed is high enough. So it seems possible I have a fault with the wiring/connector to the alternator. Either that or the control circuit in it is knackered. I have had the front of the car rebuilt after a little accident (fans forward) so it is certainly possible the wiring has been damaged in some way. I'm not sure how I'm going to get near the thing though, without taking the front of the car off!
 

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Just an update on my progress with this..

Every time now, when I start the car the alternator does not start charging. Sometimes this seems to be the cause of a lumpy idle (less often now I have a new battery so slightly higher voltage). Once the engine reaches about 2600rpm the lights brighten and everything is ok from then on.

Being that this is a v6, if I drive down the hill from home and into town through traffic, the engine might never get above 2000rpm unless I deliberately change up later.

Clearly this behaviour is not right, although not bad enough to stop me using the car. I have read up on the operation of alternators and discovered that a power supply from the battery is required to energise the magnetic field and make it start generating, but some alternators can self-energise once the speed is high enough. So it seems possible I have a fault with the wiring/connector to the alternator. Either that or the control circuit in it is knackered. I have had the front of the car rebuilt after a little accident (fans forward) so it is certainly possible the wiring has been damaged in some way. I'm not sure how I'm going to get near the thing though, without taking the front of the car off!
I have seen this problem before on other cars, it is the current through the dashboard warning light that is used to start the alternator. If there is no current then the alternator will not start until 2000-3000 rpm.

Check that the alternator light in the dash comes on when the ignition is first turned on, if not check the bulb and associated wiring to alternator.
 

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I read on the Briskoda forum that there can be a fault in the wiring to
the alternator on VAG group cars. Apparently, the wiring can be
fatigued and fail due to movement of the engine. I can't remember
exactly whereabouts, but I think that somewhere around the gearbox area
was mentioned.
 

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It can be as simple as a bad earth to the alternator,try temporarily getting a jump lead on the case of the alternator if you can get to it,and than to the negative battery terminal,it worked on my Astra diesel and agave me just over 1 volt extra at the battery.

Chris.
 

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Discussion Starter · #10 ·
Right, I've been leaving this issue to one side for a while due to more pressing problems with the car (turned out to be a bad MAF connector in the end, that one can really lead you astray with intermittant problems!).

Now, the charging situation is as before, but the new piece of information is that when I turn on the ignition the battery/alternator light DOES NOT LIGHT UP.

From what I can gather this means the rotor is not energised and the alternator won't charge (however, it does do once past 2500rpm or so, and continues to when back below that until the engine is turned off, which is why it's not causing me major problems).

Because of the V6 intake, I just can't get my hand down to the connector on the alternator to test if it has +12v on it with the ignition on. Anyboy have an idea how I can do that? From underneath perhaps? Until I can test that I can't tell if the fault is in the wiring or in the alternator.

Thanks for any suggestions,

David.
 

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i know its an old post..but did it get resolved??? i have similiar issues...and this last post is the same as my 4mo...i do not have an alternator light showing on the dash board...in fact this whole post is identical to issues i am having at the mo

anyone offer any advice...my car is booked in next friday for a new alternator to be fitted (£285 all in)

will this cure the isses or will i justbe having a new bit for no reason??
 

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i know its an old post..but did it get resolved??? i have similiar issues...and this last post is the same as my 4mo...i do not have an alternator light showing on the dash board...in fact this whole post is identical to issues i am having at the mo

anyone offer any advice...my car is booked in next friday for a new alternator to be fitted (£285 all in)

will this cure the isses or will i justbe having a new bit for no reason??
HOW TO TEST
 

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Alternators charge at a different rate according to revs ( alternator phase )

Some are two phase , some are three .. on the 2.8 4motion i think it is three phase .

At certain revs the alternator will produce more amps .. it could be that the low phase of the alternator has burned out ( up to 2500 rpm ) but after that the second phase comes in and starts charging.

http://auto.howstuffworks.com/alternator.htm/printable
 

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i have tested the alternator as described above..and agreed with the testing methods and indeed have booked the car in for a new alternator.

i guess all im really questioning is the missing battery/alternator light on my dashboard....i dont recall having one since i bought the car.

should i have a light? will it come back when the car is repaired?

or is it easy to get the instrument consol out and replace a dodgy light

imagewerx...your advice has been/is invaluable thx....i went to a garage armed with some info. !! thx
 

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i have tested the alternator as described above..and agreed with the testing methods and indeed have booked the car in for a new alternator.

i guess all im really questioning is the missing battery/alternator light on my dashboard....i dont recall having one since i bought the car.

should i have a light? will it come back when the car is repaired?

or is it easy to get the instrument consol out and replace a dodgy light

imagewerx...your advice has been/is invaluable thx....i went to a garage armed with some info. !! thx
If the battery warning light doesn't work the alternator won't start charging at idle but will when you get to about 3K RPMs.It might not be the bulb though as it needs an earth through the alternator so could also be the wiring or the connector or the alternator itself. I'd day it's very unlikely it's the bulb as I've never know one to go wrong,but you can never rule it put completely.

Chris.
 

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I had a guy round the other day to diagnose the problem.He was convinced it was the alternator.

So i took it to a vw specialist,who changed the alternator, then told me it was NOT the problem..they traced the error to the connector above the alternator.which was stopping my battery light working and the alternator from charging below 2500 rpm.

Big thank you to IMAGEWERX for being absolutely correct on everything...even down to a loose wire !!!
 

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Hello i have the same problem and i just bought a new alternator.i have all of the above probems so all i need to find is the wire that is broke. did anyone find out what wire it was/ coming from and where it is.

Many thanks

Nick
 

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I had this exact problem on my V6 Bora, as did another V6 owner on the forum - see my other thread. THe problem in both cars was a broken wire at a connector close to the fan control module, somewhere down below the battery. It was a blue wire; its job is to initialise the alternator. If this is broken the alternator does a kind of fail-safe thing (forgive my lack of electrical knowledge) and kicks in when revved above 2.5K.

In my case all 3 wires in said connector were way too tight, causing the blue one to break over time. I lengthened all of them; job sorted. Will post up pics if this helps.

Don't replace your alternator until you've checked this!
 

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hi thanks for the help. i looked at all night for the wire and couldnt find what one. i am going mad. it cant be the bulb as its an led
 
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