spedometer question

[1987HondaAccord]

ok, so i'm taking a class called "electronics for scientists, and I was thinking it would be really cool to make an instantaneous miles per gallon meter. I would need to measure the fuel flow going into the engine, as well as the amount returned. I would also need to measure the speed of the vehicle.

I was thinking I'd have to install fuel flow meters, but i wasn't sure how to get the signal from the spedometer. Is the signal for the spedometer just a voltage? or is it something more complicated to work with?

any help would be great! thanks

[POS carb]

it is a mechanical hookup to the tranny, not electrical, however there's probably an electrical signal present in the circuitry. I just threw away my old one yesterday w/out looking tho, sux 4 u lol
I dunno check the shop manual schematics and keep in mind it's printed for foreign version. There's probably some kind of pulser in there, check the cruise control system circuits.

[1987HondaAccord]

so its probably a pulse signal? so its gonna look something like a sign wave that speeds up or slows down depending on how fast the car is going?

[SiR]

Thats really cool. The only way without computers to compute is getting a gauge to read manifold pressure. That how cars without computers like older and vintage BMWs get instantaneous miles per gallon gauge. You wouls have to right in "MPG" numbers instead in inches and calibrate it the first time.

[FyreDaug]

I think this is alot of work, most of which you would be unaware of. It'll be really cool if you pull it off though. Keep us posted.

[1987HondaAccord]

Thats really cool. The only way without computers to compute is getting a gauge to read manifold pressure. That how cars without computers like older and vintage BMWs get instantaneous miles per gallon gauge. You wouls have to right in "MPG" numbers instead in inches and calibrate it the first time.

wait, what? manifold pressure? could you explain how that works a little more?

[hot-87-hatch]

I just found this out tonight. The gauge is driven mechanically by the transmission yes but there is a signal from the gauge to the ecu. It goes to the ecu pin location b18. Another alternative is to install a vehicle speed sensor from another car on the tranny and use the voltage to operate your system.

Manifold vacuum is the negative pressure on the intake side of your engine cause by the downward moving pistons drawing air into the engine. You could use a gauge that reads manifold vacuum and with math you could tranlsate that to mpg. You would have to know the displacement of the engine, the volumetric efficiency, the air to fuel ratio, etc. This wouldn't be submittable in an electronics class though.

You could relatively similar information by using the throttle position sensor. It outputs a certain voltage for a coresponding position of the throttle. At least then you wouldn't have to translate a mechanical principle like manifold vacuum into a signal usable by an electronic device.

If you had a data aquisition system, you could take period readings of all the conditions that affect MPG. There are sytems out there that measure air fuel ratio for instance.

As I am typing this I have an idea. You could read off the fuel injectors. Since you not calculating actual engine efficiency, you just want to know how many miles you are going for a gallon of fuel. The injectors are on or off. If you have injectors that flow 240cc of fuel per hour (if they were constantly spraying) you could calculate how much fuel is flowing through them by measuring how long they are actually open in a given time interval.
I'm just rambling, though. I hope something in here has given you an idea. I read your question about intake vacuum and felt you could use a little more information. Sorry if I'm boring you. Great topic though.

[1987HondaAccord]

the information is great! what kind of signal is it going to the ecu? is it like a pulse that is proportional to the spedometer? or is it a constant voltage that increases with speed? or is it a current?

thanks for your help!