Blue wire on alternator?

[stormforge]

Can someone give me a more detailed explanation of the light-gauge blue wire on the alternator? Previous threads and Bentley are a little confusing/unhelpful...

Is the blue wire only a connection for the warning light on the dash, or does it also provide excitation voltage for the field coil? What voltage should I see on the blue wire with:

A. Ignition off
B. Ignition on
C. Engine running

Thanks!

-Bill
'89 Syncro

[Christopher Schimke]

The blue wire should have 12 volts going through it when the alternator is spinning and should provide a ground path when the alternator is not spinning. It feeds the alternator light and the oxygen sensor mileage counter.
_________________
"Sometimes you have to build a box to think outside of." - Bruce (not Springsteen)

*Custom wheel hardware for Audi/VW, Porsche and Mercedes wheels - Urethane Suspension Bushings*
T3Technique.com or contact me at [email protected]

[stormforge]

Thanks Chris! I seem to have the strange low-rpm alternator bug that has been alluded to on several other threads:

1. Fire up the engine, alternator light is ON
2. Check voltage at alternator = 12v = no charge
3. Rev up engine briefly
4. Check voltage again -- 14V & charging fine

Seems to be good now even when RPMs drop back to a low idle...

Alternator is almost new
Battery is almost new and in good shape
Cleaned up my grounds recently
Alternator belt might be a little loose

Any idea what's happening here? I suppose the alternator belt might have been quietly and discretely (I was staring right at it) slipping and then it engaged more securely when I revved the engine... Or is this something amiss with the field coil supply or the voltage regulator? Is this scary, or something I shouldn't worry about?

Thanks!
-Bill
'89 Syncro

[Franklinstower]

[jptech]

The wire in European DIN terms is D+/61. It is the ground side of the dash charge indicator lamp. When the ignition is switched on, run bus power is supplied to one side of the lamp. the other side of the lamp is the blue D+/61 circuit that provides a ground thru the Alt brushes. This does two things. It completes the charge lamp circuit allowing it to light during KOEO and provides full voltage to the field rotor. When the engine starts, D+/61 is flooded with Alt output voltage. With 12V on both sides of the charge lamp, current cannot flow and the charge lamp goes out. The regulator takes control of the field rotor power & Alt output. If the Alt fails to produce power the blue D+/61 circuit remains a ground, current flows and the lamp remains on during KOER.

John

[stormforge]

Thanks John -- that helps. When I was seeing no alternator power I measured about 2.5 volts on the blue wire. Any idea what that's telling me?

Cheers,
-Bill
'89 Syncro

[josh_w]

I'd heard the blue wire was an anti-theft bit- if you hotwire the car instead of turning the ignition switch, the blue wire doesn't get connected, the alternator won't charge the battery, and eventually you'll be walking.

Anybody else heard this?

[tencentlife]

[jptech]

Hi Bill,

Its pretty much all or nothing; KOEO - D+/61 = ~0 volts & KOER it = B+ volts. You may be seeing AC leakage due to diode damage. Switch your voltmeter from DC to AC and check the big red B+ & small blue D+ terminals to ground KOER. There should be NO AC voltage. (< 0.2 VAC)

John

[n2wheelies]

thanks guys for a great post.. started here, followed the steps and in the end one of the brushes barely touched the commutator anymore.. problem solved.. ordered a regulator for $60 off ebay today and even made a video... you guys rock!
_________________
'87 Vanagon Weekender 'ELNINYO'
past vw's...
'09 EOS 'petal'
'78 scirocco
'66 T2 panel dead tour vehicle, 'mr bus'
'67 13 window
'95 eurocamper 'charlie' (phish tour vehicle)
'03 Mk4 TDI 'steve'
'04 W8 Phaeton 'david'

[jlrftype7]

[Butcher]

Most every alternator I know of if you rev the engine high enough [4k] the alternator will start charging. No blue wire needed.

I've personally had this problem but was too busy/lazy to fix it. Drove the car for weeks before I handled it.

If the alt light does glow, it's because the voltage is not the same across the bulb. Simple voltage drop test will determine where the voltage loss is.

Alternators have been around for a while and circuit 61 was not meant for theft.

[crazyvwvanman]

In VW boxer engines as well as many VW inline engines the alternator warning light also acts as an impending engine damage early warning light.

The pulley(s) and belt(s) that spin the alternator also spin the coolant pump.
Whatever causes the alternator light to come on may also mean the coolant pump isn't turning.
The driver should immediately check the belt(s) if the alternator light comes on when it shouldn't, to verify whether or not the coolant pump is affected.

Mark

[jlrftype7]

[DanHoug]

[jlrftype7]