Hi I'm looking for some help as I have run out of ideas.

It's my wife's car 2009 2.0TDi Laurin and Klement Octavia hatch.

Started having problems just over a month ago had to jump start my 1.9TDi Fabia using her car, later that week her battery ran flat. So, changed out battery, it was the original, the new one then ran flat.  I then checked out alternator, by using a meter inside car connected to battery terminals by cable. I took car out for a test run and the strange thing was sometimes I was getting a charging voltage well over 13v then at other times it would go to the battery voltage 12.70v and then start dropping as it was draining the charge, then it woul go back up over 13v.

Decided it must be the alternator, so I got an exchange Bosch one and fitted it yesterday. Got it all together and started the engine, 13.5v at the battery so I thought it was solved. Went out for a drive yesterday and checked the battery when we came back and the voltage was down. Started the car and checked the voltage, not charging. Trickle charged battery overnight and then this morning checked wiring with meter from alternator back to engine fuse box and battery, all OK. Checked engine fuse box, all fuses and relay out, fuses all OK, don't know how to check the relay. Started engine and got 13.5v, so just to make sure I connected meter inside again and took it out onto the motorway for a test drive, same again sitting well over 13v then dropped off to battery voltage, so I came off at the next junction relay out and back in, fuses out and turned 180 degrees and back in, voltage up at 13.5 and then dropped back again. Did it one more time and it just did exactly the same thing.

So as said above, I'm now totally confused and have ran out of ideas. What does the relay in the engine fuse box do? It is AE TL1 (7MO 951 253A 12v 70A 692.60), could it be causing the problem?

 

I would be extremely grateful for any help in solving this.   

 

Dougie

 

 

Hi Dougie, 

Sorry I cant help but having some charding/alternator issues myself and interested to know if you got to the bottom of this at all?       I'm seeing 13.5v from my alternator however I'd have expected more, any car I've had before would give 14v+

 

Cheers! 

A lot of modern cars have the alternator output controlled by the ECU. Try switching on as much electrical load as possible with engine running then testing voltage. If the battery at off with no load is over 12.6v then the alternator would seem to be doing the job.

Both your Octavia and Fabia will have what we we in the Trade call a "smart alternator", basically they charge the battery on demand, this is ECU controlled. 

 

If the battery voltage drops below a certain limit the ecu will tell the alternator to charge the battery untill an upper limit is reached, it will also trigger the alternator into charging as you turn more items on in the car.

 

Quite clever but also expensive things.

23 hours ago, Muzza80 said:

Hi Dougie, 

Sorry I cant help but having some charding/alternator issues myself and interested to know if you got to the bottom of this at all?       I'm seeing 13.5v from my alternator however I'd have expected more, any car I've had before would give 14v+

 

Cheers! 

 

I too would expect something over 14V, preferably about 14.3V (unless a lot of loads are turned on and the engine is at low rpm).  Cleaning up the earth connections between engine and chassis/battery negative can often improve this situation. The main engine earth tends to connect to a point on the starter motor fixings at one end, and the nearest point on the chassis leg on that side, with a very chunky wire. 

 

Not sure that any of the cars referred to in this thread by you or the OP have anything 'special' in terms of alternator control.  As far as I can see they do all have a 'DFM' connection to their ECUs, but this isn't a controlling signal, it's an information signal telling the ECU how hard the alternator is having to work moment by moment.  All the control is done within the alternator by its voltage regulator module, which adjusts the field current up and down as necessary to attempt to keep the output voltage at about 14.3V whatever the electrical load on the system is.  It can't always succeed at this, if the engine (and so alternator) revs are low, or the electrical demand is high (lots of consumers turned on).  It always tries its best to maintain this voltage though.  The only impacts the engine ECU can have is to tell other systems via CAN comms that the alternator is overwhelmed and to inhibit non-vital stuff like seat heaters if necessary to maintain an adequate system voltage. It can also use the info to stabilise or increase the idle rpm a little.

 

If you can access the area safely, try measuring the voltage the alternator is putting out right at its output terminal, relative to a negative connection on its body (without much in the way of cabin loads turned on). If that's more like 14.3, but the voltage across the battery terminals is much lower, you can be fairly confident that there is a poor connection between alternator and battery, either in the positive feed or the earth.

 

 

 

 

Edited 26 September, 20178 yr by Wino