I have an Octavia VRS Estate 2003 which has developed an intermittent electrical problem that I'm struggling to diagnose.

 

The car lost all power on the motorway just before Xmas, was towed to a  local garage who replaced the alternator and belt but then 2 days later the battery warning light comes on again, then the car loses power and stops.

I took it to another garage and they confirm that the alternator has been replaced and that it is working ok and also that the battery is in good condition holding a charge.

They are looking at the power cable that runs from alternator to battery as the possible cause of the fault as its getting hot when the engines running so perhaps not transferring the current properly.

 

 

Any advice on this much appreciated

 

Cheers

Ed

 

Edited 10 January, 20188 yr by eddie1

@eddie1 - My first thought was of the alternator control wires. If the battery warning light is showing it's unlikely to be that.

 

So next thoughts would be of the cited cable, the main engine earth, and the battery top fuse box. Is that fuse box getting hot when the engine's running?

Thanks Ken

 

the fuse box is not getting hot but the cable running from alternator to the battery is.

 

what is the cited cable please ?

Quote

Any advice on this much appreciated

 

Buy yourself a cheap cigar lighter type plug in digital voltmeter from Fleabay and leave it plugged in the cigar socket (unless you smoke!). When your alternator is charging you should get around 14.3 volts dropping to about 13V when the battery is full charged. If you see 12-12.5 volts whilst driving with lights and heated rear screen turned on, your alternator is not charging. When you get your fault, if you see zero volts, the display and dash goes out, you have lost your power source. If you get the ignition warning come on whilst there is a good voltage >12V still showing and the car stops, I would look at the ignition circuit wiring or fuses feeding those circuits.

 

Usually if the red charge warning comes on you have many miles of driving with loads like headlights switched off, before the battery eventually goes flat. If you are saying your light comes on and the engine then stops fairly quickly, that suggests a problem with the power source to your car and not the alternator - e.g battery ground, or battery positive or dirty terminal posts. You don't need a working alternator to drive a car some distance on a fully charged battery. Are you sure you haven't got a problem with the ignition switch or immobilizer?

 

If the large cable from the alternator to the battery is getting very hot without the engine running and the battery is getting discharged, you probably have a failed diode rectifier pack inside the alternator. If the wire is getting very hot only when the engine is running, your alternator is dumping a lot of power into the battery or whatever else is putting a load on it?

.

Edited 10 January, 20188 yr by voxmagna

Thanks v much  Voxmagna

 

to be clearer the red battery light comes on for some time before the power goes. But it comes on intermittently with weak red light that gets brighter. Then several yellow warning lights come on then the power dies.

 

the cable from alternator to battery is only hot when the cars engine is running 

 

2 garages have checked the alternator and tell me it’s working ok and that the battery is charging and holding charge 

O.K the scenario you describe isn't a sudden failure until all the other warning lights come on and I think your battery has then become fully discharged?

 

Quote

the cable from alternator to battery is only hot when the cars engine is running 

 

That suggests you problem might be a big discharge fault somewhere and the alternator is working hard to meet the fault load. One clue there is if the alternator body at the back is also getting hot or idle seems to labor?  To be absolutely sure you need to know which way the current is going down the wire - into a faulty alternator, or out of a good alternator into a faulty battery or near short circuit somewhere.

 

It sounds like your fault is intermittent because garages are testing your battery and alternator and finding nothing wrong? You describe the charge light as coming on weak. Because of the way they work, this can be caused by a voltage drop in the ground side of your wiring loom, it again suggests there is a short or partial short circuit somewhere taking a lot of current. The only high current parts not fused are the starter, solenoid wiring, battery wires and alternator which is where I would start looking for faults.

 

You are relying on garages telling you there is nothing wrong with the battery and alternator but you can get intermittent faults in both - o.k when they test, but fail some time after. How old is the battery? If It's more than 4 years old I'd suggest replacing it. You really need a digital voltmeter to go further.

The main cable from the fusebox to the alternator is a known failure point on the mk1 mainly due to a poor terminal attachment process on early cables that eventually builds up a large resistance in the cable hence the heat. What can make the condition worse is if the terminal nuts in the fuse box are loose or have heen loose at some point causing arcing across the connection and eventually eroding the plating on the terminal end and causing the above mentioned high resistance.

 

If you open the battery fusebox lid and you can move the terminal in the fusebox quite easily with not a lot of a feeling of it being restrained in the plastic fusebox (sounds complicated to explain, but you'll see what I mean with the lid open as the terminals are encased in the plastic fusebox) then the cable has already failed and your fusebox has partially started to melt in the past. It is actually part of the service schedule to regularly check all the fuse post nuts are correctly torqued to reduce the problem as much as possible.

 

Normally, when the fusebox totally fails at the cable, it will bring every warning lamp on in the dash, then cut the power dead shortly after.

 

Another good poit to check as Ken said are the earth cables under the battery tray, as they can get submerged in water and corrode, if the the tray legs get blocked with autumn leaf debris.

 

Voltmeter checks are a good call to be sure of the garages work, but in view of how often these cables fail, my money is on that at the moment. Most of the long term members on here who have run their vrs's for 10 or more years, have had this failure at about 8-10 years into the car's life.

 

This older thread from Bowders should fill in some of the gaps for you -

 

Edited 10 January, 20188 yr by kentphil1

Thanks again Voxmagna and Kentphil that’s some really useful  info you’ve given me there.

I’ll get back on it tomorrow and keep you posted.

Ed

3 hours ago, voxmagna said:

your alternator is charging you should get around 14.3 volts dropping to about 13V when the battery is full charged

 

I doubt the bit in bold is what you will actually see. The voltage regulator in the alternator will try to maintain 14.3-ish forever, in my experience, certainly in VAG cars of this era. How could it know what the battery's state of charge is, in order to back off the voltage setpoint?

 

1 hour ago, Wino said:

doubt the bit in bold is what you will actually see. The voltage regulator in the alternator will try to maintain 14.3-ish forever, in my experience, certainly in VAG cars of this era. How could it know what the battery's state of charge is, in order to back off the voltage setpoint?

 

Transistor voltage regulators in most alternators, particularly the older one's are very simple with only 1 transistor acting as a switch.  Battery terminal voltage rises with charge to the regulator cut off around 14.3 volts, depending on state of charge, condition of battery and temperature. The alternator regulator compares the battery terminal voltage with a reference zener diode.  This 'dead band' hysteresis up to 1 volt is where the regulator has insufficient gain or sensitivity to see the voltage drop for small differences below the cut off point and respond. Until battery voltage drops from its peak value, either with a load or by self discharge, the terminal voltage will not rise again to the 14.3 volt cutoff. The charge is continuously 'hunting' below and up to the full charge cut off point.

 

This backing off of voltage is not due to the battery, but the simple regulator design in early alternators using on/off switch regulation. This is desirable and helps it work like a crude taper charger. It's a bad thing to have the alternator sharply cutting in and out with high charging current if there is only a drop of 100mV. A small cyclic voltage lag helps give a smoother charge. Batteries working on chemistry do not have precise voltage values and alternators have voltage tolerances too. 14.3 volts for one battery may be 14 volts for another, it is not a precise science. Plug in a cheap cigar plug digital voltmeter and watch what happens in various driving phases - cold starts, low charge battery, warm Summers/cold Winters, long journeys with full charge capacity reached, loads on and off.

 

Modern pwm electronic fast chargers are more clever. They can set a higher cut off charge voltage corresponding to theoretical full charge point and taper the charge to get most capacity, then you might see the terminal voltage stay constant when fully charged, until they switch to trickle charge mode. Before the battery is fully charged they cannot maintain the cut off charge limit constant, because that would require a huge charging current outside the battery manufacturers spec. which is the same for the car alternator.

8 hours ago, voxmagna said:

Plug in a cheap cigar plug digital voltmeter and watch what happens in various driving phases - cold starts, low charge battery, warm Summers/cold Winters, long journeys with full charge capacity reached, loads on and off.

I have a non-cheap digital voltmeter permanently connected across my battery, in the middle of my dashboard. It has a quiescent current of only 0.5mA so not a significant drain.

 

When the engine is running it shows about 14.5V when the engine bay is cold, dropping down to 14.3 at operating temperature, and never drops below that unless I switch on virtually every electrical load I can, at low engine rpm. I can drive long distances with minimal loading, and it never falls in the "battery is full, take a break" way you suggest it might. Do you ever see this?

 

My understanding is that the alternator is (or tries to be) a constant voltage charging device. It will continue attempting to hold its output terminal voltage at the 14.3V (temperature dependent)  setpoint irrespective of what is connected to it (by PWMing the field current between 0 and 100%).

 

Resting battery voltage does indeed vary with state of charge (up to about 12.6-12.7V at 100%), but in an engine-running situation that voltage can't be seen because the battery is never resting.

 

Edited 11 January, 20188 yr by Wino

@voxmagna - What @Wino describes in the post before this is my experience of how an alternator works, based on having driven cars with OEM fit voltmeters, and played around to try and establish a permanent drain at idle (even with headlights, full speed fan and HRW on I could only just get a discharge reading).

Hi, I’ve been having this problem too and I want to replace the the cable that runs from the alternator to the batter fuse box. I believe VAG brought out a new one with a better connector. Does anyone know where I can get this and perhaps what the part number or proper description is?

It’s for a Skoda Octavia Estate 2003 1.9 diesel.

Thanks in anticipation.

Hi, welcome to Briskoda.

 

The cable will be dealer only, as far as I know (at least I have never found one outside the dealer network up to now).

 

If you confirm your 3 letter engine code I will endeavour to find the number for you, (it will be something like AGR, ASZ, etc, but you get the basic idea).

 

I've not heard that the cable has been modified myself, but the amount of time that the part has been in production, it is highly possible that a revision has taken place.