With a planned change in tire size, the speedometer will no longer be accurate. It's really more of a guide as it is. I know there are converter bits that can be attached to the cable to gear up and down, but I'm not really sure how well those work. There's the option of doing surgery on the speedo, like this page here suggests, but that's a little scary. Still, if I want the original speedo to show the right speed after putting larger tires on, is does read like a viable alternative.
I could figure out a means of getting the speed from a Hall-effect sender. These are basically a magnet on the rotating wheel and a magnet sensor picking up the magnetic field as it passes. This signal is effectively a square wave that an electronic speedo can interpret. OR, the computer could know how fast I'm going, if I route the signal to the ECU. But, I'd still see the old needle on the original dash, showing the wrong speed. I like the idea of being able to keep the original dash operational so I may need to do something to the original speedo no matter what I do about the ECU.
I am currently unable to determine how much fuel is in the tank. I tried swapping out the fuel gauge when I thought I had confirmed the sender was good, but that didn't work. So, I have to consider that the fuel sender has failed. If replaced with a ALH-ranging sender, the computer would know the fuel level. But, that would also mean that the fuel gauge on the original instrument panel wouldn't work. So, maybe I could figure out a way of installing an original sensor in the tank and then splicing a second signal from that, but shift the signal in the second signal to match the modern sensor.
Original VW Bus sensor TDI
10ohms (full) 35ohms
75ohms (empty) 285ohms
If the bus range is 65ohms and the TDI computer expects a 250ohm range. For each ohm change in the sender, I'd need .26 ohms of change. Plus, the floor resistance would need to be increased by 25ohms. I haven't done electrical work like this since high school, but I'm not sure how this would work. In the table below, I've split the target ohm values into 13 5ohm increments from the original bus side. The diff column represents the difference between what the sender would provide and what the TDI gauge would expect. The step increase is the additional amount of resistance needed from the more-full to less-full increment. Nothing is simple.
Maybe, I could get a converter like the Fuel Gauge Wizard. These are designed to meet this problem for any gauge/sender pair which provides less resistance the fuller the tank and is empty at 500ohms or less. I could splice it in to the original wires so the stock signal is untouched while the new signal goes to the ECU.... hmm... Then, the stock gauge could still read while also informing the ECU to support the digital gauge.
Back to Dashing
If I had solved the fuel and the speed, only the turn signals would remain from the original dash that I'd need to retain. I wonder if there's a way of telling the computer that the turn signal is on....
Continuing down this mental thread, the space available for dash concepts: 13 or 14" across. 5" high. So, fitting a Jetta IV instrument panel (even if I wanted to) wouldn't fit. The one that came from the donor Beetle definitely wouldn't fit. Maybe a tablet could. A typical 7" Android is 19.2cm x 12cm -or- 7.559055" x 4.72441"
Perhaps, if oriented such that the thick "bottom" (or the far right end in the picture here) were set on the outer edges, we could support two screens for a dash.
Seeing that there really isn't an iOS comparable, I looked around for a cheap tablet. $40 gets you a bluetooth enabled, 4GB tablet (link). I couldn't use two of them without modifications to either the vent / heat controls or something else more drastic. Still, its an interesting mental exercise. Maybe it's worth only doing one, covering the blank spot where a tachometer should have been installed stock from the factory (Really VW?, Really?) and most of the speedometer, leaving the cluster with the fuel gauge, turn signals and idiot lights still visible. If the tablet can be easily removed and installed, I could pull the tablet out of the way at will, leaving the stock dash in place. I got one of these tablets just for laughs and the low price comes from the weak battery. Still, this is an interesting idea.
So, assuming the tablet isn't unattractive, how to get the engine computer to tell the tablet what's up? There are a surprising number of tools out there for this, actually. I went and bought this one from scantool. It came with free software for the tablet which I played around with a little bit.
For now, this stuff is sitting in a heap while I consider how I want to address the fuel tank level sender. Ultimately, the sender needs to be replaced, and it makes the most sense to just replace it with an original ohm-range sender. If I want to go further with the ECU stuff, I can do the fuel gauge wizard, build a hall-effect speed sensor and really jump into the electronic dash all while keeping the original functionality.
That's it for this week. Thanks, as always, for following along. With the holidays upon us, and family descending upon us, I may not have much time to post. I am taking some time off, so I'll have some time to generate content of course. I just may not get to telling the stories until some time in January. I appreciate your following, rare comments and more regular emailed thoughts and ideas. Please keep sharing.
Last, if you're in the Portland area and need a place to fix your daily, I'd be hapy to help. You can even use my driveway. Hapy Holidays and Hapy New Year-