W terminal issues/tach issues

[burn_your_money]

I'm working on a 89 NA Jetta. It has a freshly rebuilt alternator on it. The tach only works above 2400 RPM. Belt is new and tensioned properly, it isn't slipping.

I tried measuring for AC volts but I only got a reading of .2 at idle and it went up to about .8 when I reved the engine up. I was under the impression that I should have saw a steady 8v.

Any thoughts?

[Vincent Waldon]

Do a voltage drop test. 

All kidding aside, what's the output on the charging side like?  Steady 13.5V ish, or does it suddenly take a jump over 2400 RPM as well??

[rallydiesel]

My tach only works below about 1500 rpm. Might be the cluster?

[burn_your_money]

Might be the cluster?

2 points for you. Somethings the matter in there. I swapped in the one from my Golf and it worked like a charm.

Do a voltage drop test. 

2 points to you as well. I have to rev my engine up a bit to get the alt light to go out and the tach to kick in.

[macka]

Vince 4
BYM 0

 

[Soot Sandwich]

My tach only works up to 3k rpm past that it just jumps around.  Are these issues related?

I also have to rev up to 2500rpms to get the batt light to come off is this normal?

[burn_your_money]

I also have to rev up to 2500rpms to get the batt light to come off is this normal?

It's normal but not correct.

Search in the FAQ for voltage drop test.

I'm not sure about the above 3k thing.

BTW how did Vince get 4 points already?

[Vincent Waldon]

Two extra points for "handsomeness" ??!!!

[VW Smokr]

I also have to rev up to 2500rpms to get the batt light to come off is this normal?

It's normal but not correct... "

Damn straight... even true on a couple of my air-cooled VWs!

On the 90A Bosch re-manufactured Quantum alternator in my 1980 Dasher diesel, once the engine's blipped to 1200rpm, alt light goes off,  voltmeter reads 14.6v, and the tach is perfect... from idle to redline... until the engine's shut down & re-started. Then do it all over... varooom, blip to 1200rpm, life is good... have just gotten used to it! The original 45A Bosch did the same! Puzzling.

J.R.
SoCal

[truckinwagen]

all of my vw's required this, the RPM that it occurs at depends on the pulley size ratios, but they all needed a blip of the throttle.

all except for my old rabbit that I replaced the LED with a small incandescent bulb, and then it would go out as soon as the car was started.

[gilligan5000]

and you guys all have the excitor wire hooked up?  I always just attach it 12v constant...and I don't have to blip anymore.

[theman53]

Does it still charge if the light is on? Every VW I had even my gasser GTI does this. I usually just give it a rev, but in the winter I didn't as the oil pressure is usually pegged at 100psi...just wondering.

[Vincent Waldon]

Nope, it doesn't start charging until the red light goes out.

The red light really just shows the difference between the alternator and the battery... when there's a potential difference the light is on, when there is no appreciable potential difference the light is off.  When the alternator is not is not charging it's also not putting out much of a W signal and so the tach is generally off/erratic as well.

[GTD.]

My battery light stays lit all the time and the rev counter/tach doesnt work either as the VW dash doesnt draw enough current 

The VW cluster uses a LED and 3 resistors in parallell, but if I add a side repeater bulb between the D+ terminal on the alternator and the battery + post it lights up then when I start the car the alt kicks in immediatly and the Tach/Rev Counter works.

Looks like I need to add another resistor in line on the exciter circuit in the clocks.

[Vincent Waldon]

The cluster's circuit, as designed, should work just fine... sounds like you've got a cluster problem?