Free mpg gauge...sort of

[tasdrouille]

The 08 Elantra have a trip computer with mpg function available only in the limited model, which calls for a 5000$ premium over the GL model I bought.

I was going through the workshop manuals for my car when I found what's in the attached pic.

I can't wait to hook up my multimeter tonight and see what I get.

So if your car came with a trip computer as an option and you didn't get it, you should take a look at the ECU pinouts. You might get lucky.

[NoCO2]

Hm...neat, I don't think it was an option back in '96 though...sad

[Christ]

Dude, this is actually pretty common... they don't actually remove the feature, they remove access to it.

Changing most Honda ECU's from auto to manual is as simple as moving a few jumpers in the ECU itself... The Honda radio wiring schematic hasn't changed for years, if your Civic came w/o a tach, you can add it from another gauge cluster, or simply swap clusters, the foglight option already includes the wiring on all models, both for the switch and the lights themselves... etc.

My dad's '98 S10 didn't come with fog lights, but has the wiring for the switch in the OEM location, just plugged into a "blank" on the dimmer panel.

Only supplemental systems are ever deleted for feature changes... like one model of my car had power windows... but it was a supplemental wiring harness, so it was removed on models that weren't OE with it. Same with power locks and power folding mirrors.

Whenever you see a feature on a car that matches yours, and you want it, look on your car... that $5k option might only cost you a few hundred to install yourself. (parts and install time)

I used to have a '88.5 Escort GT... it had the vehicle maintenance indication system in it... my friend had a '89 GT as well, but it did not have that system... apparently it was an option or something... we removed the center console, and found that it already was prewired for it.. changed the console to mine, and put my monitor in, and it worked fine, without a single glitch... yet I'm sure it cost about $1000 extra back in the day to have something so "hi-tech" installed.

[whitewiz]

PWM = Pulse Width Modulation
can't read it with a multimeter

[NachtRitter]

Would it be possible to use a multimeter with the dwell function?

[whitewiz]

read with a frequency counter or an oscilliscope would give you relative differences

[hummingbird]

Folks,

For the first time in so many days, I can post something without some hesitation - I'm qualified to post

The frequency of the signal is 100 Hz - Do what you want, measure the frequency - It will be constant 100 Hz. peak voltage remains the same. What changes is the width of the pulses. If the output is low, the pulses turn skinny, but keep repeating at 100 times a sec - 100 Hz. Take the signal higher, the pulse width increases - pulses go fatter, ever at 100 Hz.

The most effective way to interpret this signal easily is through averaging it using a simple leaky capacitor - look into a digital electronics book for basics, or google. it would take a diode, at most two resistors and a capacitor. The result can then be seen across the capacitor by using a simple voltmeter. it will show something between 0.5V to the peak (12V in this case), depending on what the limiting pulse widths (duty cycles) in % are.

Have fun. If you want the component specs and circuit diagram (hardly anything there, but I would make one and check to be sure), just post the limiting duty cycles (would require some sleuthing / a waveform check in limiting conditions on an oscilloscope)

If, on the other hand, the multimeter handles the signals in an averaging manner (Inertia of the analog multimeter needle would mostly average out the pulses) it would give an average, proportional signal directly, in V. Tying it to the MPG rating is a different matter altogether, depends on what relation the pulse widths have to the exact MPG value at the instant.

If nothing, it would give a much needed feedback about instantaneous fuel consumption. Not bad for FREE!

[NachtRitter]

One of my cars (BMW 323) has an "analog" MPG gauge... a needle that moves along a dial to indicate the current mpg... sounds very similar to what was mentioned by hummingbird about the analog multimeter. This is a nice visual method for "driving for FE".

[Uncle Dave]

Takes me back to my Junk Box test equipment! On my VOM, (Volt Ohm Meter), I taped 50, 75, and 600 Ohm resistors. Quick and Dirty check that the scales were not damaged.

[oldbeaver]

Originally Posted by tasdrouille

The 08 Elantra have a trip computer with mpg function available only in the limited model, which calls for a 5000$ premium over the GL model I bought.

I was going through the workshop manuals for my car when I found what's in the attached pic.

I can't wait to hook up my multimeter tonight and see what I get.

So if your car came with a trip computer as an option and you didn't get it, you should take a look at the ECU pinouts. You might get lucky.

Very clever finding. Let us know how it works for you.

OldBeaver