MPG needle

[jhower08]

I was just wondering if the mpg needle reads off of the OBC? If not where does it get its reading?

[325iCabrio]

It doesn't read off the OBC. I'm pretty sure that all it does is measure the application of throttle and then gives a round-about estimate of mileage. So the more you toe into the throttle, the less MPG on the gauge.

[mtechnik]

I think it does more than that cause at time, my foot can be dead steady and the reading may vary by the incline of the road or wind conditions. I may be wrong here..

One reason for me to believe 325iCabrios answer is becuase with passengers you may notice the same indication while "technically" economy should suffer.

[325iCabrio]

I could be wrong, but the reason I think it's that simple is because I do a lot of highway driving, showing about 20-25 mpg on the needle, but the OBC says I'm getting an average of about 18 mpg (which sucks in-and-of itself).

[jlevie]

From the wiring schematic, the MPG gage signal is from the DME. I'd guess that it is derived from injector on time, like later BMW's.

[JGood]

The ECU has a harness that goes to the gauge cluster which has a harness that goes to the OBC. I'm not sure if the gauge cluster is a pass-through for the OBC or if it has an effect on the signal as it goes through. I assume it's just a pass-through and it pulls the instant MPG signal off of this.

The OBC only receives 3 things for MPG calculation: the speed pulses (from the differential sensor), the injector pulses (from the ECU) and the calibration factor (from the per-engine-type module).

So if the injectors are clogged, the O2 sensor will notice a lean mixture, the ECU opens the injectors longer, and the ECU will then tell the OBC/gauge cluster to report a higher fuel useage number (LESS MPG) even though you aren't necessarily getting worse MPG.

If you are going up a hill, and keep EXACT throttle input the same, your road speed will decrease, injector timing will remain constant, and you will get less MPG since you are traveling less distance with equivelant fuel use per second.

[jhower08]

The ECU has a harness that goes to the gauge cluster which has a harness that goes to the OBC. I'm not sure if the gauge cluster is a pass-through for the OBC or if it has an effect on the signal as it goes through. I assume it's just a pass-through and it pulls the instant MPG signal off of this.

The OBC only receives 3 things for MPG calculation: the speed pulses (from the differential sensor), the injector pulses (from the ECU) and the calibration factor (from the per-engine-type module).

So if the injectors are clogged, the O2 sensor will notice a lean mixture, the ECU opens the injectors longer, and the ECU will then tell the OBC/gauge cluster to report a higher fuel useage number (LESS MPG) even though you aren't necessarily getting worse MPG.

If you are going up a hill, and keep EXACT throttle input the same, your road speed will decrease, injector timing will remain constant, and you will get less MPG since you are traveling less distance with equivelant fuel use per second.

:jk: :up:

[JGood]

STFU or I'm taking your engine back out. Not only that but I'm puting an m10 in it's place. You think it's slow now? HA.

[Jscotty]

I believe that it reads the MPH directly off of the speedo and then calculates that against the RPM. When I was having problems with my speedo, the MPG needle would go to 0 every time the speedo stopped working. Also, I notice that when the car is coasting at a speed that is consistent with the engine the needle remains around 25 but as soon as the car is coasting faster than the engine, almost to suggest that the car is pushing the engine, then the needle goes to the extreme left past 40.

[ivo316]

I don't really know how it works, but in mine (i have an M10), when i bought it, it had lots of hoses disconnected, and the needle was not working, I replaced all the hoses and it came back to life.

It "feels" like a vacuum meter, when i coast at 30mph, mine says 7 liters every 100 km. if downhill it says 2-3 liters.

But it doesn't work when the car is not moving :p

so go figure how it works...

[Rocla]

I don't really know how it works, but in mine (i have an M10), when i bought it, it had lots of hoses disconnected, and the needle was not working, I replaced all the hoses and it came back to life.

It "feels" like a vacuum meter, when i coast at 30mph, mine says 7 liters every 100 km. if downhill it says 2-3 liters.

But it doesn't work when the car is not moving :p

so go figure how it works...

On m10 carburetor models it might vacuum operated, but on injection models ecu sends signal that correlates with injection time to the gauge cluster and cluster calculates mpg from vechile speed and injection time.

Gauge cluster sends the mpg reading to OBC.

[JGood]

I believe that it reads the MPH directly off of the speedo and then calculates that against the RPM. When I was having problems with my speedo, the MPG needle would go to 0 every time the speedo stopped working. Also, I notice that when the car is coasting at a speed that is consistent with the engine the needle remains around 25 but as soon as the car is coasting faster than the engine, almost to suggest that the car is pushing the engine, then the needle goes to the extreme left past 40.

The needle dumps to 0 when the speedo stops working because the speedometer is involved in the calculation/signal for the MPG needle, so it just stops operating when the speedo stops operating. It's the same circuitry, and likely whatever cut power to the speedo cut power to the MPG needle. If it is a bad differential mounted speed sensor causing the speedometer to cut out, same deal, it cuts the calculation because the computer thinks that the car is not moving.

It is not relative to engine speed, only injector pulses and road speed, both defined and sent to the cluster by the ECU, combined with the calculation factor plug. The reason your MPG needle max's out past 40 is because you actually are getting that many MPG. You are using as much fuel as you would use at idle, while coasting, but since you are coasting at 40,50,60 whatever mph, do the math - that is alot of MPG.

[Rocla]

injector pulses and road speed, both defined and sent to the cluster by the ECU.

Road speed is measured by cluster from diff speed sensor, and is sent from cluster to ECU for max. speed limitation, if present.

[JGood]

Road speed is measured by cluster from diff speed sensor, and is sent from cluster to ECU for max. speed limitation, if present.

Yeah your right, my bad. It's the green connector on the back of the cluster for the models that had it, I believe right next to the cruise control.

[jhower08]

STFU or I'm taking your engine back out. Not only that but I'm puting an m10 in it's place. You think it's slow now? HA.

hey thats not funny, my car is already teh slowness.