Q for those running EFI and electronic speedo...

[Cobra Dude]

If you're using Fuel Injection AND an electronic speedometer... what are you doing about VSS (Vehicle Speed Sensor) signal?

[ The VSS is important for idle and shift management (IAB control), so I don't want to start a debate on why it is or is not necessary. ]

On a Mustang T-5, the VSS is physically connected to the speedo cable on the tranny. When using an elec. speedo, there is a special (speed) sensor that replaces the cable in the transmission, so the VSS can't fit there.

Would love to hear how any of you have solved this.

The folks at USI (United speedometer & Instrument, from whom I bought all my VDO gauges/sensors) tell me that the computer wants 8 pulses per rotation (ppr), but mine is 16 ppr. Not sure if actual ppr amount is important for non-cruise-control perhaps the computer just needs to know if the vehicle is moving or stopped for IAB management???

Only thoughts I have to far is...
(1) Use the regular VSS with a piece of a speedo cable into a custom "box" that will let me attach the elec speedo sensor.
(2) Use the elec sensor and tap off the signal wire for the computer. Obviously this means figure out signal levels from sensor and for computer and building a custom circuit for this.
(3) Get a mechanical speedo -- nope! (Good reasons).

I'm sure this has to be a common problem. Help, please!

Cheers,
-Neil.

[Wade Chamberlain]

Most folks ditch the VSS without problems, but it looks like you've ruled out that option. The beauty of the electronic speedo is the ease of calibration (as you know). This also allows you to use speed sensors w/ differing numbers of pulses per revolution. For example, the Autometer speedo's can be calibrated for anywhere from 500 to 200,000 pulses per mile. I *think* your VDO can go from 500 to 399,999 pulses per mile. You should be able to tap off the VSS signal going to the 'puter and use that signal, even though it's only 8 pulses per rev.

Most mechanical speedo cables run around 1000 revs/mile, so if you're anywhere close to factory gearing in your car, you'd see about 8000 pulses/mile w/ the factory VSS sensor. Even if you're not close to factory gearing, you should still be well within the range of acceptable input for your speedo.

[Cobra Dude]

Wade, thanks for the info. Yes, it should be 8 pulses when the speedo sender I have is 16 pulses, so I can easily reprogram for that. I did an experiment with the VSS in a drill and the voltage rose proportional to the speed of the drill. At max drill speed, I got 10 volts. With your info, top speed on my speedo is 160mph, so that's 160,000 pulses per hour, 44.4 pulses per sec. That seems fairly slow, and that's at 160mph! So my concern now is that at low speeds, there won't be enough signal voltage to get the speedo to notice it. As I accelerate from a start, the speedo may read 0, then at a certain speed, it would suddenly jump to show the speed I'm at.

But I'm guessing here. An oscilloscope would be soooo handy right now.

I may have to go build some isolation and buffering electronics, but I've forgotten all that stuff by now.

[Cobra Dude]

Well, this problem is solved now. I ended up building a little circuit with a comparator to clean up the pulses, and amplify them to 5V, no matter how low the signal voltage. Seems to work well with vref set to 0.7V. At lower voltages, the needle oscillates as the car starts moving (noise in sensor/line I guess), and at higher voltages, once the car starts moving, the speedo doesn't, then there is a big jump to the actual speed (like say 20mph). But 0.7V works well. I just needed to recalibrate the speedo for the lower pulses.






Thanks for the help,
-Neil.

[stuarth]

neil,
Is your sender of the open collector (effectively a switch to ground) variety, my sender is, so requires a pull-up resistor (1-10K should be OK) to 5V to work properly. If this is the case you could use the signal directly or if you're still worried use a comparator with hysteresis (or a schmidt trigger) - email me if you need further details.

Stuart

[Cobra Dude]

My sender is the actual Ford VSS (I eliminated the VDO sender). It's a magnetic pulse unit, and yes I did need some hysterisis on the comparator. Works well.