Author Topic: Wildly swinging RPMs!!! (Read 5506 times)

Zu! · « **on:** March 14, 2007, 07:27:05 am »

This scared the daylights out of me!!

Here I was minding my own business cruising on the highway at about 90 kmh when my tachometer drops to zero, flies up to 4000, then drops to idle and then up to the actual RPM (about 2,000) :shock:

I noticed in the morning starts sometimes that the tachometer shows zero, even after the car has started...going up to idle after a few blips of the throttle.

Any ideas? Thanks!

Zu!

QuickTD · « **Reply #1 on:** March 14, 2007, 07:40:03 am »

The alternator provides the signal to the tach. If the wiring to the alternator "w" terminal is faulty or the alternator itself is on its way out, the tach will be erratic.

Vincent Waldon · « **Reply #2 on:** March 14, 2007, 08:39:42 am »

And most earlier VW alternators don't start to self-excite (generate on their own) until that first blip of the throttle to over 2000 or so... you will notice that the red alternator light stays on as well until this happens.

No generation = no W signal from the alternator to the tach = erratic tach readings.

Erratic signals on the highway could be a very loose belt but most likely, as QuickTD says, faulty wiring between the W-terminal and the dash or an alternator that is on its way out.

My experience with the latter is that the little red light lies like a rug... the light may stay off but the alternator has long since stopped giving out any real charge. A battery shop can test your entire charging system in about 5 minutes.

Vince

Zu! · « **Reply #3 on:** March 14, 2007, 09:21:43 am »

Phew! Geez, thanks guys. I'll check out the wiring and then the charging...

burn_your_money · « **Reply #4 on:** March 14, 2007, 12:11:30 pm »

Also, I've heard someone mention something about the grounds being bad in the cluster area and then the power gets routed through the tach, which can cause it to go nuts. This will ruin the tach (from what I read)
I forget who posted that info unfortunately.

Zu! · « **Reply #5 on:** May 03, 2007, 09:03:50 am »

Sorry for posting so late...the problem has been figured out. First the alternator belt started squeeling really loudly. At one point, it sounded like a jet taking off! Anyway, got that fixed by simply making the belt tighter and voila! No more swinging tach. Thanks for all the suggestions guys.

jtanguay · « **Reply #6 on:** May 03, 2007, 02:01:54 pm »

i had the same problem as you... loose belt! pretty much any kind of electrical draw on the system (low battery) would make the squeal and drop rpm's to about 300 on the tach. very annoying