load signal alternator cable

[mountainmark]

Brilliant - that's so helpful, thank you

[mountainmark]

My car battery went completely flat on a short shopping trip a couple of days ago.  Which led me to this and related posts, as the battery warning light isn't coming on (and I think it used to). Consequently I was pretty confident it'd be the snapped blue wire problem. However, before I tried to access the onboard supply control unit, I tested the voltage on the (disconnected) alternator end of the blue wire, and it's at 11.7V with the ignition turned on, or 11.4V with the engine idling (and the voltage across the re-charged battery is ~12.5V without the engine running and ~12.1V with it running - even up to 3,000+revs). Can anyone suggest where I should be looking for a fault? Alternator bushes? Thanks!

 

(My Fabia is a 2004 1.4 Tdi 6Y2144  with an AMF engine)

[sepulchrave]

What's the voltage across the battery with the engine running?

[Breezy_Pete]

Has the car been unused for any amount of time prior to that shopping trip?

I'm wondering if the slip rings have maybe just tarnished a bit. 

Bosch or Valeo alternator?

[mountainmark]

Hey thanks for the help. It's a Bosch alternator, and the voltage across the battery when the engine is running is ~12.1V (even after sustained revving), which is somewhat lower than the 12.5V when the engine isn't running. It's fair to say the car hasn't been out much, but not drastically less than usual.

 

[Breezy_Pete]

Not sure whether there's sufficient access to remove the voltage regulator/brushpack with the alternator in situ, but if there is you could eyeball the sliprings and probably even get some very fine wet-n-dry on them if they do look grubby and tarnished?

I seem to remember that the Bosch alts are a bit friendlier to get the volt reg off compared to the Valeo ones.  

 

Do remove battery negative before anything else if you do get involved down there, the main +ve cable is fused, but it's a whopper of a strip-fuse so you could cause a big old spark otherwise.

[mountainmark]

Thanks again for the advice - I'll give that a go - tho' I may wait a day or two for the temperature to rise a bit!

 

[Breezy_Pete]

Still not very warm out, but I just tried something on my car to prove to myself that it would work as expected.

The blue wire is fed through a diode and 4 parallel 820R resistors (inside the Onboard Supply Control unit) from an ignition 12V supply . These resistors limit the current to the ballpark of 50mA into a short circuit.  

 

This means that you can test the excitation system, including the wire end-to-end, by shorting the relevant pin of the disconnected alternator connector to chassis through a current meter.  If the meter read in the 50s of mA dc you know that the full excitation is available, and if the alternator isn't starting up, it isn't because of the blue wire.    It's the pin nearest the rounded side of the connector.  Don't forget to switch on ignition just before testing. 

 

I suggest this as a possibly useful test because the wire could have a truly terrible, very high resistance connection or corroded section where  the weather has got in, and still be able to show a voltage close to 12V on a multimeter; because the input impedance of a meter is so huge.  

 

If your meter has a 200mA dc current range or similar (and you put the probes in the right holes, and set the dial to the right position) testing this current supplying ability is a much more thorough test of the wiring.

 

I wouldn't suggest anything similar with the DFM wire, not sure what would happen with that one and it's irrelevant to the starting up of the alternator anyhow.

 

 

[mountainmark]

Thanks Wino! My, apologies I hadn't seen your post until just now, when I came to add the following update.

I was able to access the regulator end of the alternator in situ, and had a cursory attempt at cleaning the slip rings - the resistance between them was ~2.7 Ohms, so I guess any tarnishing wasn't too bad. I've now replaced the regulator/brushes, and retested. The voltage across the battery terminals with the engine idling was still ~11.9V, (and the battery warning light still doesn't come on), but if I increase the revs to ~3,000rpm, then the alternator seems to kick in, as the voltage across the battery is now 14.5V (when idling afterwards). Am I right in thinking it is still worth testing the current flow to ground from the blue wire?

[Breezy_Pete]

Yes, I think that is the best next step. 

[mountainmark]

Wino, re-reading your comment about it being the pin nearest the rounded end... I'd been assuming  (Doh! ) that it was the other one, as that one had the voltage and the other (ie correct) one had only a minimal voltage! My mistake . I'll check the end to end continuity this week - thanks again for your helpful advice.

[mountainmark]

Just to conclude my tale... I was able to access the right hand connection box behind the windscreen wiper motor (as above) and used a soldering iron to melt some of the insulation off the blue wire, to confirm it was 11.5 volts there, and that also no onward connection from there to a similar spot just before the alternator connector. I soldered in a new wire between those two points. The battery warning light now working again, and there is 14.5V across the battery terminals with the engine running. Thanks again for your help Wino!

[thecuist1]

Hi all.

I thought it might be best to resurrect this thread as it has loads of pertinent information regarding my problem. Hopefully the super knowledgeable guys on this thread are still active and can give me some input. I have a Skoda Fabia 1.2HTP 04reg which came up with the battery warning light while driving,  this was intermittent for a day or 2 but then stayed on constantly. The alternator is def not charging the battery as battery voltage is only at 12v with engine running and alternator connected,  there is also no voltage output from the B+ terminal on the alternator.  The battery light is on when ignition is first switched on so after reading this thread I thought this precluded the common broken wires to alternator  but I checked them all anyway. I checked the alternator plug, the plug under the gearbox, removed the scuttle and checked continuity from the blue wire all the way to the alternator all seems good. This led me to believe that it was possibly a faulty regulator/brushes so I got a new regulator fitted it to the alternator but still no charging. Here is where it gets strange... I checked Voltage on blue wire while the alternator plug is off and I have around 11v with ignition on and engine off and with engine on. However, when I plug it back into the alternator with engine either off or on the voltage drops to around 2v only which doesn't seem to be enough to excite the coils. My next step was to cut the blue wire where it exits the bulkhead and run a wire directly to the connector on the back of the alternator thereby bypassing the whole blue wire run around the loom to eliminate shorts or voltage leakage but the voltage still drops to 2v when the alternator is connected.  I then decided to connect a wire directly from the positive terminal on the battery straight onto the blue wire  while the plug was connected to alternator (scraped off some insulation and used an alligator clip) and voila the alternator started charging and I had 14.5v at the battery, the battery light also went out. So to summarise: I believe I have eliminated a potential break in the blue wire ( I also checked the blue wire where it connects into the onboard control unit under the dash) I can get the alternator to work by powering the alternator straight from the battery, I also checked the alternator diodes using HatboyHarveys method and I have 0.950 one way and open circuit the other, (hatboy had 0.450 one way and open circuit the other) I'm left thinking that maybe my alternator is faulty with some weird voltage drain not related to voltage regulator, or possibly the onboard control unit is faulting in some way which is causing the voltage drop when the alternator plug is connected. Does anyone have any thought please? 

Thanks in advance

[Breezy_Pete]

Do you have a dc current range on your multimeter?

[thecuist1]

Hi Breezy, sadly not. I only have AC current measurement. I read the section regarding testing current draw on the blue wire. I can buy a cheap auto multimeter if you think its worth it. What were you thinking I should test?

 

[Breezy_Pete]

Yes that was going to be my suggestion. Precise measurement not needed so any meter at all will do 

[thecuist1]

OK, I have to go out and pick the missus up and I'll see if I can pick up a cheap meter with DC current testing on the way back, then I'll check the current draw on the blue wire to ground and get back to you. Thanks for your input. I'm called Pete too btw, grew up in Towcester. Northamptonshire. Chat soon. 

[thecuist1]

Couldnt find a meter locally so I'll pick one up tomorrow hopefully and give an update. 

 

[thecuist1]

Hi Pete

I purchased a DC meter and read 50ma on the blue wire with ignition on and wire disconnected testing to earth. Then I tested with the wires connected to alternator and I'm getting 41ma. (wires connected in series between the 2 sides of the plug under gearbox. This remains the same whether the B+ terminal is connected or not. What does this tell you if anything?

Thanks

Peter

[Breezy_Pete]

I would say that exonerates the supply and wiring side of it, leaving an oddity with the alternator itself. 

 

The extra current available when you connected direct from battery positive probably overcame the problem. Now that you have current meter, see what current you get if you measure that path instead (battery positive to pin where blue wire would go on alternator).

 

I'll just pop out and do the short-to-earth measurement on my Polo as a comparison.  Can't remember if I ever did that or reported the result above anywhere?

 

Edited 8 November, 2022 by Breezy_Pete

[thecuist1]

Thanks for replying so swiftly. I am reading 1.85A between battery and alternator terminal (thats with the gearbox connector unplugged) 

 

Edited 8 November, 2022 by thecuist1

[Breezy_Pete]

Yeah, I get 53.5mA blue wire to earth with ignition-on battery voltage showing at a rather low 12.0V.

 

 

[Breezy_Pete]

2 minutes ago, thecuist1 said:

Thanks for replying so swiftly. I am reading 1.85A between battery and alternator terminal (thats with the gearbox connector unplugged)

 

 

You mean the terminal/pin the blue wire normally goes to?

 

[thecuist1]

yes, I have removed some insulation on the blue wire just b4 it enters the alternator plug and have a crocodile lead on there. I am measuring 1.85amps between battery + and the blue wire while its connected to alternator (and disconnected under the gearbox, so essentially isolating the alternator from most of the loom. Its strange that you are reading more or less the same amps as me feeding your alternator though. That implies that my blue wire is working correctly.. However,  the alternator only worked when I supplied 1.85amps direct from the battery, so, the normal amps (in your case 53.3ma) works ok on your car but my amps were almost exactly the same at 50ma were not enough to excite the alternator. That seems to confirm an alternator fault in that mine requires 1.85amps to fire up..

 

 

Edited 8 November, 2022 by thecuist1

[Breezy_Pete]

6 minutes ago, thecuist1 said:

That implies that my blue wire is working correctly..

Yep, I think it is.  Theory (the origin and current limiting resistors in the onboard supply control unit) and practice seem to agree pretty well.  

50mA ought to be enough to kickstart it; but isn't on your alternator for some reason.  I can try the 'in series' (your 41mA) measurement or just check what volts I see at the blue wire with it connected later on, or tomorrow lunchtime, for further comparisons.

 

When the alternator is charging, the voltage at the blue wire follows that of the B+ output terminal, I seem to remember.

Edited 8 November, 2022 by Breezy_Pete