Hi all, 

 

I've recently bought a (very high mileage) 2011 vRS and am sorting through the issues on it!

Loving the car, although I'm now getting down the list to the "weird issues" section 😁

 

Most have gone fine, but one that's got me stumped is a fault on the alternator/charge system.

 

On startup, the alternator doesn't switch on/start charging. Once it's been revved above ~1.5k it snaps on and works completely fine, but until that point the whole car is running off the battery.

I've attached a VCDS plot to show what's happening -- you can see the voltage dip as the starter motor goes, but then the ECU voltage doesn't rise until I tap the throttle to get the alternator working.

 

My suspicion is that the exciter wire that should provide the "bootstrap" current to the stator is broken somewhere. That'd explain the symptoms -- no output on startup, but once the engine is revved stray inductance means the stator gets just enough current to self-excite, then snaps on.

 

Does anyone have a wiring diagram that might show where I can trace this? Any idea if it just runs through the engine bay, or is it back in to the firewall via ECU/dash?

 

For any other troubleshooting:

• The battery light turns on when I put the ignition in the 1st position (which would normally indicate exciter current?), and then extinguishes once the engine has started. Picture attached.
• No particularly relevant DTCs, although I'll post another scan when I'm back to my tools after New Years. There's a few "sensor voltage low" ones, but I think they're because the alternator isn't running on startup.
• Engine is the 2.0 TSI CCZ.

 

Any insights appreciated!

 

If the car has A/C, then the blue excitation wire from pin 1 of the alternator signal connector goes to pin 2 of a 4-way connector pair at front left of engine compartment, from where it continues into the cabin, destined for pin 32 of the brown 52-way connector at the Onboard Power Supply Control Unit (also known as BCM).

When I talk about 'left of engine compartment', that's left as experienced sitting in the driver's seat facing forwards, so right side as you look into the engine bay from the front bumper.

 

I'd try to locate that four-way connector and check the wiring there; seems most likely problem area if the alternator connector itself is fully 'home'.

Fantastic, thanks for that! Yep, it's got AC fitted.

 

I'll have a poke around for that over the next few days. I'll double check the alternator connector itself too, looked seated when I checked it but will pop it off to look for any contact corrosion etc.

@rialobran is that pic of the cluster on first switch on? If so it says that the alternator control wires are OK because the battery light is on.

There's no direct electrical connection between the alternator excitation and the illumination of the cluster LED, so it may be model-dependent as to whether the light coming on means the wire is intact or not.

2 minutes ago, KenONeill said:

@rialobran is that pic of the cluster on first switch on? If so it says that the alternator control wires are OK because the battery light is on.

Yep, that's right.

 

Just now, Wino said:

There's no direct electrical connection between the alternator excitation and the illumination of the cluster LED, so it may be model-dependent as to whether the light coming on means the wire is intact or not.

That's good to have confirmed, I was wondering if that was the case! I'm used to it being in series in older cars/motorbikes, but I suspected that it might not be the case given all the clever ECU/diagnostics stuff in the Octavia.

The excitation current comes from the onboard supply control unit, via that skinny blue wire.  Approx 50mA only (from memory). 

i have the same problem with my 2015 Skoda superb 2.0tdi if i start the car without pressing the throttle i get only battery voltage give the throttle a slight tap i get 14.2 volts if i put on front and rear heated windows it will drop to 13.5v at idle scan says voltage regulator fault and scan also says brake regen isn't working  i was thinking its maybe been jumpstarted incorrectly at some time