The best part is that it works I just did mine tonight in 5 minutes. Cost a total of $5.76
A guy from my local car forum Customobsessions.com figured this one out.
Originally Posted by gdt[size=18]The Official : “Get Your MK2 VR Tach to Work Without an Expensive Black Box” thread[/size]
First off, I’m not an electronics expert so if I call something by a wrong name don’t shoot me!!!
In fact, I look forward to any experts out there to comment on what I have come up with, that means you Dewie!!! :lol:
I have decided to Nerd out a little bit on the background and my theory for this, if your not interested in reading that and just want the DIY, go down to the second post :lol:
For the rest of you, keep reading.
You built your mk2 VR and you feel pretty proud of yourself, now if you could just get the Tach to work!!! I know you can put in a mk3 dash and cluster or hack the mk3 cluster up to fit into the mk2 dash, but some of us want to keep the mk2 look!!
The problem is how the different generation of cars get the signal to the tachometer.
On MK2 cars, the tachometer gets it’s signal from a wire connected to the negative terminal on the coil, (this is for gas cars, Diesels are another story and won’t be covered here). Normally, this is 0 volts, as the engine runs and each time the a cylinder fires, this changes to 12v then back to 0 volts again, these events are called pulses. These pulses are considered to be an “analog” signal.
On a 4 cylinder engine, every 180 degrees of crankshaft rotation a cylinder fires therefore there would be two cylinders firing per revolution. I made a diagram that illustrates this.
Therefore, for each revolution of the engine, 2 of these pulses would occur. The circuitry on the MK2 Tachometer is built to convert these two pulses into rpm. As the engine speeds the up, the frequency these pulses increase and the Tachometer reading goes up!!
On car’s that came with a VR6 (Corrado, Passat, Mk3s) the signal for the tachometer comes from the ecu, this signal is considered to be a “digital” signal. The circuitry inside the tachometer in these cars coverts this digital signal to rpms….So even when you have everything hooked up properly the MK2 tachometer cannot process the signal coming from the vr6 ecu. There is a way to convert this digital signal and there are many commercially available units to do this, but even if you buy one of those digital to analog converters you still have another problem!!
Remember that on a 4 cylinder engine a cylinder fires every 180 degrees of crankshaft rotation or two cylinders per revolution. On a 6 cylinder engine a cylinder fires every 120 degrees of crankshaft rotation or 3 times per revolution. Therefore the signal from a 6 cylinder would have one extra pulse per revolution, the reading you would see on the tachometer would 1.333 time the actual rpm!!! Here is another illustration to show the firing events on a 6 cyl.:
The convertor box has to covert the digital signal to analog and reduce the pulses by 1/3….They are commercially available and are expensive , last time I checked , $249.
This will only work with coil pack VR6’s , it will not work with Dizzy Vr’s , sorry Honie… :cry:
On a coilpack Vr6 motor the coils fire in pairs in what’s known as a wasted spark ignition system. There are 3 coils, each coil has 2 spark plug wires attached to them. The paired cylinders are 1/6, 2/5, 3/4. So, when cylinder 1 fires so does cylinder 6 even though cylinder 6 is not on it’s compression stroke, the spark is wasted , therefore, wasted spark…
Here is the schematic of the mk3 vr6 coil pack:
The coil pack is supplied with 12v and is attached to ground (0v), each coil has a wire attached to the ecu. The ecu switches a ground to the each coil when it wants that particular cylinder to fire. These ecu connection points on the coilpack are essentially the same thing as the ground terminal on a coil in a dizzy set up.
My reasoning was this; For each revolution of the engine, each coil pack would fire once for 3 pulses per revolution. What if you only tapped into 2 of the coils, and attach those two wires together? I think, it would produce 2 pulses per revolution, there might be slight error due to the 2 pulses being closer together than the 2 on a 4 cyl, but I though it was worth a try!!! In order to prevent back feeding to the other coil I decided to use a rectifying diode to isolate the two legs. On the internet guys were building similar circuit to get the rpm signal to a megasquirt ecu and they use 1N4004 diodes, So I chose to do the same….
Once they are tied together it could be fed to the appropriate spot in the Cluster harness (pin U1/6 in CE2 talk). Here is a drawing of the circuit:
I tried it out and it appears to be working. To check my work I attached vag com and watched the rpm signal in one of the measuring blocks and compared it to the value on the tachometer. I tried 1000, 1500, 2500, 3000, 4000, and 5000 rpm and was satisfied with the correlation.
So now down to the business of the DIY….
Originally Posted by gdt[size=18]DIY BEGINS HERE[/size]
You will need:
2 x 1N4004 rectifying Diodes.
I bought mine from the Source, The package i bought had different types, so make sure you use the 1n4004 ones!!
Some 22 or 18 awg wire
Heat shrink tubing
Male and female blade connectors
Soldering iron and solder
Wire strippers, and crimpers.
Something to heat up the heat shrink tubing.
8mm socket and ratchet
Here is the circuit you will be building:
Please note that Diodes have polarity which is important later on, here is a pic to show you what I mean.
1.Heat up your soldering iron
2.Take your wire and cut 4 pieces about 2 to 3 inches long and strip both ends of them.
3.Take one of your diodes and twist one end of one of the wires around it like so:
4.Now solder that bitch
5.Repeat Step 3 and 4 for the other end of the diode. You will end up with something like this:
6.Repeat Step 3, 4 and 5 for another diode and the other two pieces of wire.
7.Install some heat shrink tubing over each diode making a note on which end the little stripe on the diode is!!!
8.Twist the two ends of the wires together. Make sure the ends you twist together have the stripe on the diode closest to them. You will end up with something like this:
Please note I left one of diodes not covered for illustration purposes!!
9.Now attach 4 feet or so of wire two those twisted together ends. You can use the soldering method, I like butt connectors, many don’t. Just ensure you cover the connection up to make it weather tight!!
10.Time to get at the car… Locate your coil pack and unplug it!
11.Remove the cover over the terminals, it will just pry off!
12.Loosen the two 8mm nuts in the middle
13.Put one wire under each nut and tighten them down.
14.Cut the end of the cover to make room for the wire to come out the side, like so:
15.Reinstall the cover
16.Route the wire in a safe manner to the firewall and find a spot to fish it through, here is where I went (orange wire):
17.Time to go inside the car:
We want to attach this wire to the harness going to the instrument cluster. If we consult this page:
It says it’s pin T28/10 which is goes to pin U1/6 at the fusebox, it’s a green wire on a blue plug.
18.Drop you fuse box down, if you need help figuring out which plug is U2, this will help:
19. I found the wire and used a male and female blade connector to make the connection:
20.Put your fuse box back up
21.Plug you coil pack connector back in
22.Start it up !!!
Here is a comparison with vag com at idle:
And at about 3400 rpm…
Original thread and discussion: