Thread: Anybody have tachometer or speedometer?

Originally Posted by Bugbyte

I'll only partially hijack this thread with the actual circuit as this is mildly relevant. The diagram shows at the bottom the LED emitter and detector. The rest of the circuit is the actual counting stuff.

Don't they have magnetic sensors you could put on an axle that could feed counts into the FB?

You invented factory VSS....

Seems crazy to drill holes in axles when any car with abs should have individual wheel speed sensors and even cars without have a trans vss.

Originally Posted by ACCLR8N

Yes, but I'm trying to use the factory VSS. I have a frequency to voltage converter to provide 0-5V signal for the FB. I design door latches for a living. Producing the circuit above is over my head.

My speed sensor circuit works briefly, then measures at the FB 1.5-2V that won't go away until I shut everything off and start over.

it actually looks pretty simple, its hard to read so im not sure if i could pull if off just by going with the pic, but it doesnt seem that bad at all...

do you have a diagram of your circuit?

No. I've purchased the Hz to V board from the FB store. The VSS is a 17 pulse per driveshaft revolution out put as an AC sine wave, factory GM '95 LT1 M6 transmission.

Originally Posted by ACCLR8N

No. I've purchased the Hz to V board from the FB store. The VSS is a 17 pulse per driveshaft revolution out put as an AC sine wave, factory GM '95 LT1 M6 transmission.

ok i wasnt aware there was a kit. why are your cooling fans causing weird readings? as if the voltage drop in the system throws off the sensor?

Turns out it wasn't the fans so much as the check engine light which was on at the same time. Residual voltage was carrying around from my check engine indicator on the FB and bumping up my tach and speedo values. Replaced check engine voltage divider with a relay that made car PCM happy. Speedometer still unstable with high spikes. Tach I think is figured out.

Still like to hear from somebody else that got to this to work in a car.

I'm using the outputs on a FB to drive three 7-segment LED displays, projecting the speed on my windscreen, but I don't suppose that what you're after ;P (My input is from a GPS receiver)

It's pretty cool none-the-less! Post up some pics (maybe in a new thread so we don't hijack this one)

A bit late on this but FYI - some vehicles use a reed switch as a speed sensor. (Isuzu - Impulse/Piazza late 1980s, and their Rodeos/Jackaroos (Pick Ups & Troopers) around the same time.)

I didn't believe it at first - I don't consider reed switches to be robust, but it sits on the plastic speedo casing and reacts to the spinning speedo magnet.
The magnet is probably at least 8 pole. So 8 or 16 or more closures per cable rev, ~680 revs per km (~1,000 per mile) and they last for several 100,000 kms/miles.


I originally did what BugBite did - added an IR chopper system.
These days I'd use a Hall sensor.

Otherwise a reluctor sensor to sense the spinning prob shaft should be ok (get one from an old distributor). I used one to sense the universal joint spider. (Better than mounting magnets on the shaft.)


And FYI - I recently found the PCB layout for my electronic tacho or speedo - a digital display surrounded by a LED bargraph (about 90mm/.5" diameter). The funny thing is it looks MUCH simpler than SiliconChip's recent design that uses a PIC!

My final solution was tapping the VSS before the PCM and adding jumpers before my sensitive inputs on the FB. The frequency to voltage board didn't like the "cleaned" signal the PCM was outputting. The jumper pulls the FB capacitor to down to zero before measuring the F2V boards output.