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Sun Oct 17, 2010 3:55 pm

I made a TDI conversion on my 1994 Eurovan 1,9TD. (VW T4) Everything is working but the rev counter is no accurate. Since my rev counter was getting the signal from the W terminal on the alternator I add a W terminal on the TDI alternator. Since the pulley of the alternator or the puller of the crankshaft has a different diameter, the rev counter reads less rmp than it should. (rev counter 2000 RPM engine 2500 RPM)
The rev counter is the one on the picture. Does anyone know which resistance do I have to change?
I also don't know if the ECU signal would work on this rev counter.

João

Sun Oct 17, 2010 8:01 pm

I take it that there is no potentiometer screw to adjust the needle reading?

If not, look at this thread: http://www.vwdiesel.net/forum/index.php?PHPSESSID=24da21ca352b8b5a46d8066591899dcd&topic=12589.0

That mod is going in the wrong direction for you (that tach was overreading, not underreading), but it might be a start to figure out which resistor you should change to get a different reading.

In that tach, "2,22" on the face, if you switched the 13k resistor for an 8.2k resistor, you'd drop about 1000 rpms on the needle.

If you have a 13k resistor inside your tach like the 2,22 has, then try replacing it with an adjustable resistor, or a decade resistance box, and play around with it. Increase the resistance up to (?)around 17k, and see if the needle sits where it needs to be.

Mon Oct 18, 2010 12:43 am

Witless thank you for your help. The tach doesn't have a potentiometer screw to adjust :cry:

The tach is a "2,99". Where is the resistor supposed to be on the circuit? Is it before the IC (frequency/voltage) or after the signal from the IC to drop the output voltage?

João

Mon Oct 18, 2010 8:16 am

If you have the tach working on the W terminal, you might be able to use one of these. http://www.dakotadigital.com/index.cfm/page/ptype=...prd128.htm

Mon Oct 18, 2010 9:03 am

That could be a solution, but it’s very expensive. For that money I can buy a cluster from a VW Passat 35i that fits directly and I can connect the signal from the ECU.

João

Mon Oct 18, 2010 11:42 am

Joaot4,

Here is the schematic for the older VW diesel tach, "2,22". You can see how everything connects to the 8 pin IC.

The 13k resistor that was swapped for an adjustable resistor (+/- 8.2k) is shown at position R6. The problem was that the 22k variable resistor (with pot screw) could not be adjusted low enough. When the pot screw bottomed out, the tach still read too high. Fixed by swapping that 13k resistor with a variable resistor set to about 8.2k.

I recommend that you look at these two threads at the VW GTD forum:

http://www.vwdiesel.net/forum/index.php?topic=3096.0

and

http://www.vwdiesel.net/forum/index.php?topic=3741.0

and maybe PM fatmobile and regcheeseman, who know more about VW diesel tachometer circuitry than most VDO engineers...

Good luck.

Mon Oct 18, 2010 12:02 pm

thank you :D
João