RPM signal for mk2 GTD rev counter?

[TurboJ]

So I have a GTD gauge pod for my '91 mk2 Jetta with CE2 electrics. It has a factory rev counter gauge in it.

The rpm signal for the rev counter is taken off the alternator's W-pin as most of you will know.
Now, there is different information concerning the working principle of the W-pin sender.
Some say it works according to the turning speed of the alternator, others (including a VW agent) say
it works according to 'loading pulse' or whatever.

The question is, will my rev counter read correctly after fitting different belt wheels on the engine?

I have converted to a serpentine belt setup and Aki made me a custom billet lower wheel to replace the
cracked and troublesome OE iron/rubber job. Because of this I have to use a different belt wheel on the
alternator as well. Both wheels have a different diameter to the original V-belt wheels.

What is the truth behind the workings of the W-pin sender?

If my setup will indeed yield wrong rpm readings, can I fix this somehow, with a resistor mod or something? 

[RabbitJockey]

that is a very interesting topic, i never thought that maybe it is detecting the pulse of each combustion rather than the pulse from each time 1 of the windings is passed by a magnet. regardless you will be able to adjust it, there is a potentiometer on the circuit board of the tachometer, so long as you have another accurate way to measure rpm to calibrate it to.

[TurboJ]

Can you tell me where this potentiometer is located?

I can get a true rpm reading at a dyno shop..

[RabbitJockey]

i have not ever taken a mk2 cluster apart, but i know they are very similar, you will need to remove the tachometer from the rest of the instrument cluster.  when you look into the side of the tachometer you will see the circuit board and the pot will be in there, there should only be one pot so that makes it easy.  and you shouldn't have to disassemble it any further to adjust it.

[burn_your_money]

I think this thread will help
http://www.vwdiesel.net/forum/index.php?topic=12589.0

[TurboJ]

Thank you!
 I believe it will. I'll look into this, then!

[R.O.R-2.0]

changing pulley size WILL change the accuracy of the alternator...

[TurboJ]

Hey, since I'm going to have to alter the resistance in there, can I switch to a gasser tacho at the same time?
I mean, will a resistor mod alone make that work on my car as well?

I take it that you can use a gasser tacho on a diesel gauge pod...?

[R.O.R-2.0]

Hey, since I'm going to have to alter the resistance in there, can I switch to a gasser tacho at the same time?
I mean, will a resistor mod alone make that work on my car as well?

I take it that you can use a gasser tacho on a diesel gauge pod...?

the tacho units swap over, yes..

the clusters themselves are the same..

the foil circuit and tacho unit are what is different.

[TurboJ]

the foil circuit and tacho unit are what is different.

Umm...? So what do I need to change if I just put a gasser tacho inside the GTD gauge pod? Other than the resistor, of course..

[RabbitJockey]

Check the FAQ it goes over how to change over the gas tachs.  It also has the td schematics that I have used to convert a gasser tach that wasn't covered, basically you just make the gas tach circuit match the diesel one by putting matching components in, but really that'd bE dumb u already have the diesel circuit.  So you could just swap face plates which is probably want you meant anyway, but yeah switch face plates and calibrate to that.  You'd just have to experiment with how much resistance is needed

[TurboJ]

Coming back to this subject; I have the option to use a gasser rev counter face on the GTD rev counter unit, and adjusting/changing the aforementioned resistor should help calibrate the new rev counter combination.

I have still some unresolved issues about this, however.

So, I can fix the rev counter calibration to meet my differently-sized pulley wheels, but can I at the same time also calibrate to a different gauge face?

The gasser gauge face has a different reading pattern, i.e. the numbers 1.... 2... 3... and so on, are at different distance from each other on the different face plates, diesel and gasser respectively.
So, using a different face plate on the diesel rev counter body, I might be able to adjust, say, 2000 rpm on the face to correspond to 2000 true rpm, but how would I accomplish a setting that would read true at every rpm point? Or maybe I would need to use the gasser rev counter circuit/body as well?

My goal is to make an OE-looking gauge pod that will support my 6300 rpm redline, and also the increased top speed ( I will change the speedo too).

Any ideas?

[RabbitJockey]

Coming back to this subject; I have the option to use a gasser rev counter face on the GTD rev counter unit, and adjusting/changing the aforementioned resistor should help calibrate the new rev counter combination.

I have still some unresolved issues about this, however.

So, I can fix the rev counter calibration to meet my differently-sized pulley wheels, but can I at the same time also calibrate to a different gauge face?

The gasser gauge face has a different reading pattern, i.e. the numbers 1.... 2... 3... and so on, are at different distance from each other on the different face plates, diesel and gasser respectively.
So, using a different face plate on the diesel rev counter body, I might be able to adjust, say, 2000 rpm on the face to correspond to 2000 true rpm, but how would I accomplish a setting that would read true at every rpm point? Or maybe I would need to use the gasser rev counter circuit/body as well?

My goal is to make an OE-looking gauge pod that will support my 6300 rpm redline, and also the increased top speed ( I will change the speedo too).

Any ideas?

i think you will have great luck just swapping the gauge faces and then adjusting the gauge to what u are certain is another rpm such as 4000, and the whole gauge should be linear.  but if it is not you can adjust the "zero" point to make everything linear.  zero being the needle rest and span being a higher rpm 3-4k, I have never had to adjust a diesel tach for zero and span, usually just span gets u right on, or close enough for me to where i have calculated what rpm i should be at for different speeds and they always match up.  I work with medical equipment and i calibrated alot of gauges and oxygen blenders that all must be adjusted in the zero-span style, i think if i was doing a vw tach this way i would find true 4000 by driving or with an external tach, then adjust the potentiometer until the cars tach reads 4000, now find true 1000, move the needle on the shaft to adjust the 1000, then go back to 4000 and adjust the pot, then back to 1000 and adjust the needle.  you have to keep going back and forth until they are both dead on accurate, and the needle rests at zero.  the needle may end up with preload on it so that when u lift on it a bit it will swing past the stop pin thats fine.  you can definitely get your tachometer dead on accurate.

[92EcoDiesel Jetta]

Do not swap gauge faces if the to another tach if spacings between ticks are different. It will never read correct. I'd suggest getting the tach to work first 'on the bench" before spending a lot of time to make it look "factory". The last thing you want is have a tach that looks factory but does not work. Get a photo tach on Ebay to calibrate yours. They're cheap and accurate.

[RabbitJockey]

Do not swap gauge faces if the to another tach if spacings between ticks are different. It will never read correct. I'd suggest getting the tach to work first 'on the bench" before spending a lot of time to make it look "factory". The last thing you want is have a tach that looks factory but does not work. Get a photo tach on Ebay to calibrate yours. They're cheap and accurate.

I had thought this too but I have never seen a mkii tach with the tick spacing was inconsistent like say on the same face the space between 1-2k is 1/2" the space between 2-3k is 3/4" and space between 3-4k I'd 1" and so on if that's what u meant.  As long as the spacing is consistent on the new tach, the spacing doesn't have to match the diesel on it just has to match itself, then it can be calibrated how I said I have never done it but I am quite confident it will work, and it is atleast worth a try since as long as he already has the parts there is nothing to lose but time