Author Topic: Alternater only works above 4000 RPM (Read 9104 times)

burn_your_money · « **on:** November 10, 2006, 06:04:28 pm »

I just got my car safetied so it is still new to me. It's a 92 1.6 TD with a tach. My alternator does not work at all below 4k. Once I rev it past 4K my tach will bounce up (from 0) and the battery light will go out. I replaced the brushes in it so that isn't the problem. Could it be a poor connection? I tried swapping the alternator with one of my spares but the mounting points were differant

Jetta Fan · « **Reply #1 on:** November 10, 2006, 06:07:13 pm »

Could be a bad connection. Might also be a loose belt....check the tension on that too.

burn_your_money · « **Reply #2 on:** November 10, 2006, 06:16:47 pm »

Oh the belt is quite loose, I can spin it 360 degrees. I hate mk2s, my mk1 was so much easier to work on. It doesn't squeel though.
Also, prior to me removing the alt the belt was tight and it still didn't work

Jetta Fan · « **Reply #3 on:** November 10, 2006, 06:24:48 pm »

Thought about it a little more...if it works over 4K but not under, I would say it's an internal problem. Does the alternator put out any juice? Or is it just that the tach isn't woking and the light is on?

It might be whatever they use to pick up the signal for the tach/light inside that is faulty. Or if the alternator isn't putting out any juice, it's time for a rebuild. Any decent alternator/starter shop can test it for you, as will Canadian Tire and I believe NAPA. Most places don't charge much for this and it might save you some headaches.

burn_your_money · « **Reply #4 on:** November 10, 2006, 06:29:00 pm »

The alternator is putting out nothing. No power, no tach signal.
I remeber when I bought the car the guy said that the alternator had to "warm up" before there would be a tach reading. He reved the engine a bit and then the tach started working...

If I could afford to have this alternator rebuilt I defintly would, however I'm strapped for cash so I'd like to either fix it myself or make one of my other alternators work, whichever is easiest

Jetta Fan · « **Reply #5 on:** November 10, 2006, 06:34:35 pm »

I can relate to the strapped for cash scenario.

I don't usually get into rebuilding mine....a friend of mine does it (very reasonable I might add) so I just take them to him.

I'm trying to remember when I had mine rebuilt off my MK2 what else he replaced besides the brushes, but I'm drawing a blank as it's been a few years.

Just a wag, but it might be the windings....not sure.

Doug · « **Reply #6 on:** November 10, 2006, 06:50:52 pm »

Seeing as you have checked the obvious things like a loose belt then your alternator more than likely has a failed diode on the rectification side of the output. A 3 phase alternator in an automotive application usually has a 6 diode full wave rectifier which renders the AC into DC current. Loss of one diode on the bridge will result in low output at moderate speed and no output at low speed. The rectifier/regulator on the VW is replaceable as a module for a modest price. Then again maybe it is the brushes? or maybe a poor connection internally...

Jetta Fan · « **Reply #7 on:** November 10, 2006, 06:54:50 pm »

Thanks for jogging my memory Doug..it was the rectifier he replaced on mine (along with the brushes).

burn_your_money · « **Reply #8 on:** November 10, 2006, 06:59:51 pm »

I already replaced the brushes. Do you have a picture of this part? I have spare alternators that will gladly donate thier internals to the cause

Jetta Fan · « **Reply #9 on:** November 10, 2006, 07:03:26 pm »

I tried to google it but only found one for a Bosch alt on a Porsche.

burn_your_money · « **Reply #10 on:** November 10, 2006, 07:05:40 pm »

I'll have a look in my mk1 bentley

Jetta Fan · « **Reply #11 on:** November 10, 2006, 07:06:49 pm »

That should give you an idea what it looks like.

Doug · « **Reply #12 on:** November 10, 2006, 07:09:18 pm »

Seems to me it bolts on the back side of the alternator (Bosch) having a few cooling fins on it and is shaped like an old style hearing aid as in slightly eliptical?? I am sure if you go to the book they will show you a picture. Last time I priced one it was about 30 bucks or so. My idea on the subject is that when the regulator/rectifier goes the rest of the alternator is not far behind unless it is not old. If it is old, then spring for the rebuilt as you get the new reg, brushes and bearings plus a five year guarantee at CTC.

Jetta Fan · « **Reply #13 on:** November 10, 2006, 07:33:28 pm »

This link is the best I could find showing a regulator and rectifier. Not sur eif they are exactly like yours or not. I'm on dial up, so searching is not easy.

http://www.wagneralt.com/catalog/flyer/Monthly%20Ad%20NOV%202005%20web.pdf

Hillshy · « **Reply #14 on:** November 11, 2006, 03:03:25 am »

i agree with the diode pack theory, but usually if a diode goes down this is seen by the charge light staying lit but very dim. you can also change the hand of an alternator. you mentioned you had another one spare?

is it the same type with the W terminal?

would it fit if you changed the alternator hand?

in other words remove the allen screws that hold the two body halfs together, and rotate 1 half until the hand is swapped then replace screws, i've done the same thing for my bus, i have 90amp passat alt, instead of the 55amp standard jobbie.

or failing that replacement of the diode and retif pack which is a soldering job but not really hard.

hillshy