I tried a search and came across several topics on the subject. Most were relating to the older style alternators.
So as times move on, many will now be converting to the later 1.9 IDIs and TDIs which came with newer serpentine belt style Bosch and Valeo alternators.
Today I fired up my M-TDI.
I have connected the red / black tach signal wire from the W terminal which is on the two pin connector to the alternator shared with the blue alt warning light / exciter wire.
On start up the Passat B3 TDI tacho just pinned itself full way round at 6k even though the engine was ticking over. With the red / black wire disconnected the tacho sits at zero.
So is this an issue with the W terminal? I have so many alternators here I cannot be sure if it was a gasser or diesel one I used. Both have the same two pin connector but possibly the W terminal is not configured correctly if I have used a gasser alternator.
The cluster was originally from a TDI so I hope the signal for the tach stepper motor is compatible with the W terminal and not normally wired through the ECU to provide a signal as that has now been discarded.
Is there a way to measure continuity or resistance to check the W terminal is as it should be?
If you have access to an oscilloscope you should see a square wave from the W terminal that looks like this:
The frequency of the square wave should change as you rev the throttle.
Because it's a square wave a multimeter set to "AC Volts" will read it as well:
Because the voltmeter is only reading the amplitude of the square wave the meter's reading will *not* change as you rev the throttle.
Might have the wires reversed? Alt may be trying to charge your tach... Does the alt (battery) light go out, indicating it's going into charge mode?
I'm trying to troubleshoot my tach right now, same issue sort of, newer 140A TDI alternator on an AAZ in a '86 Syncro. I have the right diesel tach, but it no worky. I have to shoot the wires for continuity next week.
Thanks for that information Vincent. I will check for signal next time I have it running.
The charge warning light behaves as it should so the wiring is correct. If I have the tach signal wire disconnected from the W terminal I get no tach reading, if I connect the W terminal wire the tach needle goes to the redline even at tickover.
I will update my findings when I sort the issue out.
one of my cars has a little black thing attached to my w terminal one doesn't what could that be? why would one car both 91's have it and not the other. one is an eco the other an n/a.
I think the black box on your alternator, the one with one pigtail, is a capacitor, i.e. a radio frequency suppressor, so you can't pick up alternator noise (interference) on your radio. I thought it went to a secondary B+ terminal though.