You should have done your measurements with the connector disconnected, than you would have seen your Voltage! Great work! Nice to see you work on European Cars!😎👌

Same symptoms you'd get if your battery light bulb is burned out, or if the brushes inside the alternator are almost ready to go. One more thing. Your charging bulb is in parallel with a small resistor in most cars (located in the cluster), so in case it burns out, the alternator will still charge right from the get go. Here you had a broken wire, so the alternator was totally separated from the cluster (from the bulb and the resistor as well), hence no charging at all at idle.

Nice diagnosis. I'm still trying to work out that light circuit function but do appreciate you taking the time to explain it. The comments are very helpful as well, so it's a good discussion. Thanks!

good catch... basic stuff that the first guy (alternator installer) should have found.

Wow, I have seen this charge issue on a 1.2 TDI Polo once, would not begin charging after raising the rpm above 1500 one time. after that it would charge fine. Customer didn't see a problem in it as he drove like that for months already. Can't tell if the battery light didn't work though. Nice diagnosis! Will keep that connector in mind.

Great video, just spent an hour thinking about the exact same symptoms on my wife’s car, no charge of battery on initial start up, Rev the engine and all fine, not a problem when you know this happens but couldn’t think why it’s happened in the first place, thanks for the vid saved us a lot of time.

I've personally seen on a few imports, the alternator default strategy is to charge once it hits 2000+ rpm. Even if there is no field control or no wake up signal present. If an alternator will charge off idle ok and continue charging until the key is cycled its a red flag for no control/ excitement. Usually wiring related. But I have seen bad voltage regs too

Interesting. As other viewers mentioned, a lot of manufacturers started adding a resistor in parallel with the instrument cluster warning bulb. The idea was that the bulb and resistor in a parallel circuit have a lower resistance, so excitation current is greater to set up the field in the alternator. If the bulb burned out, since they were incandescent bulbs, the resistor would still pass current to allow the alternator to continue charging. I suppose, since newer vehicles use LEDs for instrument cluster warning lights, which, in theory, should never "burn out", the resistor is no longer used as a fail-safe in a parallel circuit. The old General Motors 10SI alternator, for example, has the parallel resistor internal inside the alternator case, so even an external bulb circuit problem wouldn't affect alternator output.

I had to laugh when you showed the Pico and said CHECK IT OUT, SWEEEET. Awesome glad to see you got yourself a Pico, Love the vids, Thanks for uploading.

By far one of my favorite videos on alt. Diagnosis congrats on the pico it is sweeet

Somebody got an early christmas gift... Great video lime always Tom

Pico 4425 is definitely SWEET. I love that thing. The only thing that they could make better is if the next automotive version has 500 volt inputs so no need for attenuators at all.

The current from the globe flows through the rotor and back to ground through the regulator. This small rotor current flow is enough to generate a magnetic field. As the alternator increases in speed the D+ diodes provides power to the rotor and 12v. on the D+ circuit. When there is no initial current flow on the rotor from the globe, there is no initial magnetic field on the rotor and it is necessary a lot more alternator rpm before some voltage is generated in the D+ diodes

Pico, NICE !!!!!!!! Good call and glad you went back to figure out how the system worked !

Hey Tom, thanks for sharing. Good explanation of charging system. Lamp circuit current in fact travels through field coil to the ground which can be disconnected by a voltage regulator (it's job in fact) by means of a transistor. We can check for 12 volts unplugging the connector at the alternator. Also check the bulb circuit by grounding L circuit at the connector to the alternator. Also check alternator by providing through a testlight current to the L terminal of the alternator itself ;) The question: Why it begins to charge at higher RPM? )))

This issue coupled with a poor battery creates HAVOC! Start car let idle, any heavy load device won't work (rear defrost, slow windows harsh trans shifts, heavy power steering the list is long). The fun part is they are all intermittent as soon as the regulator is manually (default) energized all the problems go away. I found it by plugging a cigarette lighter digital volt meter in and noticed 10.8V at idle with a host of issues (intermittent) raise RPM and they all go away. Great find and interesting to see how the grounding of the battery bulb through alternator at rest works.

You just opened my mind up on something I would of never figured out I'm going to check step by step what you showed and hopefully I finally fix my issue on my 08 2.5 jetta thank's a million.

You're going to love the Pico Tom...good investment and the detail is great. Great video, diagnosis & repair Tom!!

Great Catch on that harness Tom.. Looks like someone used a yard dart to back probe those wires some time before and got the green crusty disease..Dang! And Thanks to 5Dale65..saved me a bunch of typing. I was about to say the same thing about the resistor and the burnt bulb too. Good call.! Typically a 510 ohm resistor. At least Ford does anyways. Cheers (:

In the old days (when cars had only an ammeter on the dash) the ammeter was not sensitive enough. You could drive a battery flat and not really notice (driving only in daytime and no accessories). Car makers wanted a idiot-proof light that would tell the operator when there was no charge. The ammeters disappeared from the dash; the light remains, and you might have a voltmeter but the light is in every car/motorcycle/lawnmower/tractor. And, the light came after customer issues using the ammeters, so is designed to be more idiot-proof than the ammeter.

Thank you so much. You just helped me resolve this exact issue on my wifes 2010 beetle. Ironically that same connector have me issues a few years ago with the AC compressor not kicking on. I am south east of Pittsburgh, small world.

haha as soon as you said battery light doesn't come + charging system will kick on after revving over 2500RPM I knew it was that dash light wire. Had a 92 VW Jetta with that very same issue. swapped alternators thinking it was a faulty unit and it still didn't work. Someone on a forum mentioned if the dash light didn't come on to put a meter on the system and rev it to 3k and if it started charging then it was this single pin wire coming out the alternator that goes through a diode to the light on the dash. Sure enough found a corroded wire around the battery. Nice fix dude!

excellent video Tom great news on the Pico love your videos one thing brought to mind was using an led or low amp test light hooked to bat pos and the other end at alt term from cluster that was broken and see if light comes on KOEO and goes out KOER and always charges 14 volts scannerdanner had a mazda miata with cluster fuse blown that caused similar thing thanks again

Old VW's ands some old Chevy's used this same system with the added fun that if the bulb burned out your alternator would not start to charge. One some cars they finally got smart and put a small resistor in parallel with the bulb so the alternator always got the excitation current. If you had never worked on one of the old VW's this would have been hard to find. Good job.

You miss one thing. The alternator was excited from the residual magnetism. I've been wanting to listen to this speech for sometime today I have a Volkswagen beetle 2001 diesel died last night and I'm jump starting it today and I'm hoping that I can stick to magnets on the outside of the alternator and tickle the charge up a lot earlier than it did before and if the lightbulb goes out the lightbulb goes out it's just going to be a lot quicker to jump up and charged

If you want to see voltage on the bulb wire with just the key on you have to disconnect the alternator cause the alternator in grounding your voltage

Thanx for the video..........Saved me a ton of work swapping out a good alternator for a new $200.00 one and would have still had broken wires and no charging .....Soldered additional wire pieces in and Bam 13.9 @ idle ......... Thanx Again

Interesting case. Thanks for sharing.

also, you should write corrections on the screen(mr. scandan does that)..like, when u say: "..there should be 12V that comes from the cluster.."..written: "no, i m wrong, there should be 0V key on, but the bulb should be lit. i will explain later in the video"..man it s your reputation..after all you are mr. scandan for the euros..the world is watchin! :)

always check voltage/gnd on each side of a broken wire! ;)..bcus, wires are mostly insulated, not exposed..so, when opportunity presents itself, why not? :)

Good video...paul danner would be proud..lol Initialy i would have suspected a blown bulb in the dash..

Exellent find tom,well done.. Ste fro uk

Very good vid. Great job! More 😊please😊

"control module", so it probably senses rpm or gets signal from pcm saying engine is running, then excites the wire to start charging. many cars wont excite it for so many seconds after engine start. it would be interesting to watch that wire on a scope from initial key turn to running :)

Greaat and clear explanation on how the system works. excelent video!

Good job Tom! Thanks for the video! 👍

Hi . Good diagnostics All Bosch alternator got a fail-safe build in to charge when rotor speed reaches 5000 rpm. First charge fault check is the charging charge lamp. no lamp open circuit, a normal fault is worn brushes. Charge circuit turn a transistor on and residual power from B+ to D+ on reg to Positive brush, rotor, neg brush to earth

no tom that's not it, when you turn the key on engine not running the voltage regulator sends a ground to turn on the bulb when the engine starts the voltage regulator takes the ground away and the bulb goes out your only going to have 12 volts there when the engine is running, if the voltage regulator quits while the engine is running that ground will be applied and turn the bulb on, you way over complicated that.

From my knowledge the battery light only turns on after the voltage drops to around 5 or 6 volts. And reving it up to 2500rpm ussually means that u have a intermitten alternator

Sweet pico bro. A game changer for sure. Good video. We need more euro stuff. Keep it coming.

GREAT VIDEO 💪 💪 💪

Hi Tommy. If you ever get a chance to diagnose a charging system on a newer VW with two wire connector, one wire from ECM and other wire from Vehicle Electrical System Control Module, would greatly appreciate it. Can't find any diagnostic info on what should be expected on the ECM wire nor VESCM wire. Would like to know who supplies what and type of voltage to expect on these wires when unplugged KOEO. Also if the voltage regulator terminals should have a voltage on them when the connector is unplugged. Also what the signal should be when plugged in and charging. Thank you Tommy. I appreciate your videos.

Wouldnt you see 12 v unplugged?

Where is the fuse for alternator? What is the number for fuse ?

that's why I like volt meters in the dash :)

Attack of the green crusties!

Nice job

Nice. How many people would have put an alternator on this first?

Another nice video

Nice diag!!

what if you have a flashing battery light

Was that connector at the alternator or where? I have an 06 that the battery doesn't charge either, ever...

Nice video

burned out light bulb on alternator,or bad wire. ., (no start voltage to regulator.) after hitting the gas the remanent magnetism kickes in and overide the light. after that the alternator kicks in. , so loose wire or burned out light.

Where is the location of that 4 pin connector? Did you access it from under the vehicle?

Tommy, concerning the W terminal, does the ECM supply the voltage to the regulator and the regulator creates the pulse to ground for the feedback monitor signal?

what is this howling sound from the engine? I have exactly the same, I just can't find the reason, howls loudly, especially when the engine warms up

what kind of voltage should you have coming to the alternator from the each of the 2 wires and which goes to ecm and which one goes to the instrument cluster

Guess it is self exciting if you get enough rpm

My friend bttele battery keep drain. He changes almost 10 batteries within 3 years.

Nice diagram

I have a somewhat weird issue I replaced the voltage regulator and car still won't charge if I apply direct battery power to the wire coming from the computer bypassing the computer it will charge goes up to 15 volts but as soon as I put a load like the ac or lights and the power at the battery drops it goes back to 12. Some volts if I kill the loads it jumps right back to 15 v could this be a bad regulator? Thanks in advance

Hi there i have a voltwagen cc 2.0t 2010 luxury i have a code p0658 actuator power supply A and P2257 and p 0491 u0101 i check all the selonoid in the tcm module and they 're okay

could you named this 4 wires to me thanx in advance

So what is the wire between the ECM and the alternator ?

Stupid putting the buld in series. They should put a resistor in parallel next to buld just in case the buld goes out.

After reading some of the comments posted,,,It's a wonder why people post how to videos..........Way to many critics out there. #1 Cochran.....Pgh Guy...Me to !

Bump the idle up and ship it! Haha 😂

Here’s what I know2011 Jetta 2.5 alt not charge charges on a bench test put a new one in it also test good on bench test main fuse was blown previous owner put a jumper wire across fuse wire melted we replaced the fuse still not charging the number 2 wires that go to the alternator the number 1 wire is giving me 12 volts number 2 is giving me 11 volts neither will light a test light the battery light blinks as does my meter goes 0 to 12 when car is running

Bro it is self excited after 2500 prm I am sure you had don it for a 5 minutes ,video absolutly for auidience

000979133EA 2 SHOULD BE OK.FIX ALL 4 OF THEM......

Typical fault. broken excite circuit.

You're going to love the Pico Tom...good investment and the detail is great. Great video, diagnosis & repair Tom!!