Author Topic: Tach problems finding a better way of doing a tach pickup (Read 4307 times)

scopefrfd · « **on:** January 17, 2007, 05:49:36 pm »

I have a tdi engine and a tdi CTN tranny in a 92 MK2...I installed a MK2 factory turbo diesel tach, but the tach is off by a mile. For example at 5000rpm on the tach, the speed is 180km/hr. 5000rpm- 180km= 111.6mph. A tdi CTN tdi tranny, with 195/50/15's. I punched those numbers into a gear calculator. At 5000rpm I should be doing 141.68mph. The 112 mph
calculated to approx 3900 rpm...tach is off by atleast 1100rpm!!

I haven't had time to swap out the speedo to a US spec mph. I did some measurements on the pulleys of a 1.6 V-belt setup and the setup that's in my car.

The 1.6 v-belt is 152mm OD on the crank pulley and 72mm on the alt pulley. This gives a ratio of 2.11. The setup on my ALH tdi is crank 145mm OD, ABA gas alt has 55mm OD, that gives a 2.64 ratio.

I can see why the tach is off....which if you do the calc..it's an approx 25% faster ratio which at 5000rpm would translate to approx 1250 rpm...so I think the tach is working correctly..now it's trying to find the right serp pulley ratio.

The tdi alt pulley with the 1-way clutch is 61mm od. That would bring the ratio to 2.37. It looks like I need a larger alt pulley or a smaller crank pulley...or install one of the dakota signal adapters or try and use a different pickup signal...from either the crank sensor or the #3 injector.

Has anyone else converted to a tdi and still using the MK2 tach?

wolfsburgnut · « **Reply #1 on:** January 17, 2007, 07:22:28 pm »

Before you go changing pulleys, there has to be a way to change the tach itself. I remember seeing on here how to convert a gas tach to diesel, by just changing the resistor values. Maybe you could talk to the person who came up with the resistor values to see how they figured it out.

Peter

scopefrfd · « **Reply #2 on:** January 17, 2007, 07:25:52 pm »

I was hoping there was some adjustment screw that allowed a range of adjustment....when it comes to tachs and electrical stuff...I get a little frustrated.

dubCanuck1 · « **Reply #3 on:** January 18, 2007, 03:26:56 am »

You mean this one?

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

scopefrfd · « **Reply #4 on:** January 18, 2007, 08:22:37 am »

not rally that's for converting a gas tach to diesel...I'm looking to adjust the diesel tach or use a different pickup that is more accurate like a crank sensor or the #3 injector

fatmobile · « **Reply #5 on:** January 19, 2007, 04:15:43 am »

The gasser to diesel tach conversion shows alot of pictures of different tach circuitry.
All of the tachs have small potentiometers that allow them to be adjusted.
Hard to adjust while it's in the cluster... I drilled a hole through the front of my gasser tach cluster so I could adjust it.

scopefrfd · « **Reply #6 on:** January 19, 2007, 04:30:55 am »

WOW...that's brilliant! Can you post a pic of where you drilled the tach...what did you use to adjust the tach? A tiny flat head?

fatmobile · « **Reply #7 on:** January 21, 2007, 09:50:59 pm »

I ended up melting a hole through the front of the cluster. Through the front clear plastic cover, on the '91 Golf. Drilling would have cracked the plastic. I don't think I have a picture but I drilled a hole through the tach faceplate then installed it in place and lined the hole up. It takes a fairly long, skinny screwdriver to do the trick.

The A1 Gti cluster has it's variable pot facing toward the right side of the cluster so I could probably drill a hole through the side and fit a small flat blade screwdriver though it. I don't think it would work easily with the instrument cluster bolted down.