MPGuino Fuel Mileage Meter  


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My new MPGuino MPG readout module works!

I can't use the new  OBD 2 computer interface to get the miles per gallon, since the RV has OBD 1 interface. The old interface to the Engine Control Module (ECM) module was limited and didn't have those signals. So someone invented the MPGuino. It is based on an Arduino computer module with interface circuits to pick up VSS (speedometer) signals and fuel injector signals.

MPGuino Fuel Mileage Display

Installing an MPGuino

It is an easy hook up since I bought the completely built module. Get it here. Only four connections. Sounds easy, the always on 12 volt power and ground are easy to find, and the injector wire I traced out quickly, I really had fun finding VSS signal. I looked all over the Internet for information on a 1992 GM P30 diagram of the ECM module and related modules.


I resorted to finding a used GM manual.  "Electrical Diagram Manual Light Duty Forward Control Chassis; Value van and Motor Home Chassis "  I found one on an EBay store. If you need it, just do a search for the above document name.  It is worth it to get connections right. If it is wrong some damage could occur or the MPGuino won't work.

Most likely your vehicle probably has different wiring, it is essential to do it right, but other than finding the VSS wire it is easy. Your local friendly mechanic can help you find the wires to use. They may be willing to hook it up for you.

I settled on the speedometer signal from the ECM module. It was a LT BLU/BLK wire that went to the instrument cluster plug.

If you get the wrong wire the only problem would be that the MPGuino won't work. It circuits are protected by 10,000 ohm resistors so if you hook it up wrong you can't blow the MPGuino or the vehicle wiring.

Instrument cluster 1993 Chevy P30

Instrument cluster 1993 Chevy P30
Look at the light blue/black stripe wire on the upper side of the connector. That's the VSS wire to the speedometer.
This was the hardest wire to find. The injector wire was easy. I tapped into the wire with a red common snap on connector.

I picked off the injector signal in front of the Throttle Body. With the JayCar pulse and Hold module you have to cut the B+_ injector wire and add a .1 5 watt resistor.

I like those wire tap connectors. Just put the connector on the main wire and push your  wire on the side. and crimp the connector with pliers and fold cover over. This makes a very good connection.

That made the connection easy and reliable. They make all kinds of connectors like that to help us make it easy.

MPGuino Calibration.

I'm in the process of calibrating the MPGuino. I took some time to get acquainted with the setup of the parameters. Most people put the MPGuino in a tiny car and mod the car for the best fuel mileage possible. Like 100 miles per gallon (mpg) or even better.

Here is what I came up with. There are 11 parameters in the MPGuino data bank you can change.

1  VSS Pulses / mile                                                                   7848     Pulses per mile from the speedometer.   *

2  Micro/sec/gallon                                                                     250,000,000   Time the injector is open.    * 

3  Pulses/2 rev                                                                            2    Not used in calculations

4  Timeout                                                                                42000000 Time allowed for backlight display.

5  Tank Gallons X1000                                                              75000  Presently not used  I have a 75 gallon tank

6 Injector Delay                                                                          500  msec                *

7  Weight                                                                                    14,000   weight of vehicle, not presently used

8  Scratch Pad ODO                                                                   Miles on Odometer, not presently used

9  VSS Delay                                                                               6 msec Time the VSS signal to change. *

10  Inj Trigger                                                                              0 for count up 1 for count down. If the display reads backwards change this number.

11 Metric                                                                                     0 for English measure, 1 if metric.

* The website for these values shows how to calculate your values for your RV.   The MPGuino Calibration Page  More info

Back to the MPGuino. Now that the fuel mileage monitor is installed I did a mpg test. I filled up the motor home and and drove it 40 round trip miles up the US31 expressway. I got 13.08 average mpg and the MPGuino said 13.65 mpg. The calibration is real close. I need to run that again, with attention to the MPGuino setup. Then I can change parameters to match my actual results.
My latest fuel test was on 4-25-2011, and I got 11.65 mpg and the MPGuino said 12.25 mpg. I recalibrated the MPGuino and that will make the results spot on.

I found out that my perception of what was good driving was wrong. Here is what I found out when I took off. The slow driving mpg is absolutely awful. It was 3-4 mpg at 10 miles per hour  (mph). As I stepped on the throttle the speed picked up and the fuel mileage got better. At 15 mph it was creeping up to 10 mpg. At 30 mph it was about 12, and at 45 mph it was 14-15 mpg.

The thing I get from this is to accelerate somewhat briskly, since good fuel mileage comes with more speed and when the transmission up shifts. On each up shift my fuel mileage got better. While watching the MPGuino I noticed the speed where I lost fuel mileage occurred at 54 mph. At 54 mpg and greater air friction took over and started to kill fuel mileage.  So slow down! The gas you save will be a lot. Actually, the best speed was 50 mph.

It was very pronounced as well, like hitting a brick wall. Driving at 70 mph is definitely a fuel mileage killer. I haven't checked that out yet since I was more worried about seeing what my absolute fuel mileage would be. I remember when I first got the motor home I went to Dayton, OH and I got 9 mpg driving 60-70 mph.

Something I noticed was that when I drove over on an overpass I could feel the throttle kick in to keep my speed constant. I was on cruise control. I thought that the fuel mileage would drop down, but it stayed the same. Apparently the motor is on its most efficient speed and this helped keep the fuel mileage constant. I really like finding that out. I don't have to drop out of cruise control for overpasses.

I would think when the grade was too steep and the transmission would shift to a lower gear the fuel mileage would drop. This didn't happen on my fuel check run so I don't know for sure if that is true. I'll keep you posted.

I have been trying several different things to enhance the fuel mileage. When I reach 50 mph I level off and engage the cruise control. Let the speed settle and note the fuel mileage in Big Instant mode. Next press the cruise control increase speed button once and note the fuel mileage. Keep increasing the speed and when the mileage starts to go down you are driving just over the fastest speed for the best fuel mileage. Press the decelerate button once to get back to the most efficient speed. This was you can be driving at the highest speed and getting the best mileage. I found out that you may get a 3-4 mph gain which is very good.


MPGuino LM339 comparator circuit.

I built a circuit to make a better injector signal for the MPGuino to process.

I did find out the comparator circuit didn't work for me.

I now use a JayCar Pulse and Hold module. See more below.

Pictures added 10/19/2011

The Jaycar signal processor circuit. I screwed it to the base of the mount.

The .1 5 W resistor module is above with yellow, orange signal wires and silver jumper wires to the injector. I hooked the injector wires with butt splices.
I held it with a tie wrap strap.



Checking out the comparator circuit.

Injector signals from the cold side of the injector. Oscilloscope trace, at idle, Top trace injector, bottom output pulse.  Top trace injector at 2000 rpm, , bottom trace, output pulse.

This squares up the pulse. Notice the second trace double positive pulse shows a much wider gap. I think this is the actual pulse. The comparator squares off the whole pulse width.

This leaves me with what I started with.  I need a circuit to just measure the width of the first two pulses.

I confirmed the bad results yesterday (9-7-2011) when I made a test run and it was showing 50 mpg! Since the circuit fooled the MPGuino about how much fuel it was using,
I unplugged the comparator. Back to the design desk.

I disconnected the comparator circuit and bought a JayCar Peak Hold Injector circuit. This is a really neat module. It lists at Australian $19.95 and $10.00 shipping to the USA. I eagerly opened the package after it arrived (it took 2 weeks) and looked it over. It is a kit. You need to assemble it. The kit has all the parts you need. It has a nice manual that lists each part and where it goes. The circuit boards are clearly marked where each part goes. Use a quality soldering iron. I liked the solder that came with it as it worked well. I resorted to using my Digital Multimeter to sort out the resistors.  The color codes are hard to figure out. I have been doing electronics repair for 35 years and I can read resistors easily by looking at them. Not this time! The Multimeter made the job easy.

Just follow the directions in the kit and you will have it done in 2 hours or less. You have to cut the B+ lead going to the injector. Follow their instructions. Once the wire is cut, ( If you need to extend the wires.) use butt splice connectors to attach 6 inch wires to the injector wires. Next crimp in spade lugs to hook up the .1 ohm  resistor.  Attach the resistor to the wires. Make up two more wires. I had to make up longer wires than the red and black wires provided. Use a color code to keep them separated. attach spade lugs to one end and crimp them to the .1 ohm resistor. Run those up to the JayCar module. Strip and connect them to the INJ posts. The + wire goes to the +12 INJ post. Hook up the JayCar module to the MPGuino. Start the vehicle and put it in drive and see if any thing comes up on the display BIG INSTANT . If 999 appears, the variable resistor needs adjusting. I have an oscilloscope to adjust it and you don't have to drive . Just adjust the variable resistor  It will work somewhere in the range of the variable resistor. Roll the car forward and back to see if the MPGuino is working. The fuel mileage will appear if the adjustment is right. If the numbers appear to work wrong (up mpg instead of down mpg) you have to go into the MPGuino setup  and set the Injector Trigger from 0 to 1

Follow the MPGuino calibration and you will have a working MPG meter for a Pulse and Hold Injector.

I can assemble the kit for you, ready to install for $45.00 USD  Order JayCar module.   Allow 3-4 weeks for delivery. Make payments at  send money to



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