If in the past you've made a few electronic kits - and so you know the difference between an IC and a resistor and you can solder - this little project is right up your alley. It uses a single LED to show the fuelling behaviour of the standard ECU - whether the engine is in closed loop, open loop, or the injectors are shut off. That's pretty useful information when:
- You're making interceptor-style modifications to the management (most interceptors can't change the mixtures when the engine's in closed loop)
- You want to drive for max economy (staying in closed loop will yield a heap more kilometres per litre)
- Or you simply want to see what the ECU is doing (it's amazing how much you can learn when you can see some of the decisions being made)
While in a way the Closed Loop Monitor works a bit like the famous 10-LED Mixture Meter ["Cheaply Monitoring Air/Fuel Ratios"], for monitoring open/closed loop this new device has two major benefits over the Mixture Meter: you only have to find a place on the dashboard for a single green LED (rather than try to fit in a row of ten LEDs), and it's very cheap to put together - including the pre-punched board, there are only five components, which should add up to a total of less than AUD$10!
How It Works
The Closed Loop Monitor watches the output of the oxygen sensor, which in most cars is a voltage that varies from about 0-1 volt. When the car is in open loop, the voltage out of the sensor is normally above 600mV (ie 0.6 volts) while in closed loop, the voltage rapidly jumps back and forth in the 400 - 600mV range. When the ECU switches off the injectors completely, the oxygen sensor output voltage drops to zero.
The Closed Loop Monitor constantly compares the oxygen sensor voltage with an internal reference voltage. This internal reference is able to be adjusted by means of the 25-turn pot that's mounted on the board. If the oxygen sensor voltage is below the reference voltage, the high intensity green LED lights up. If the oxygen sensor voltage is above the reference voltage, the high intensity green LED is off. (You can use any colour LED you like - we used green because a flashing green LED shows that all is well; a flashing red LED might be taken to mean something is wrong.) Flashing? So why is it flashing, then?
In use the pot is set so that the closed loop oxy sensor voltage swings up just past the switch-off point of the LED. This results in three LED behaviours:
- Flashing - ECU is in closed loop
- On continuously - ECU has injectors switched off
- Off continuously - ECU is in open loop
Making It
The Closed Loop Monitor doesn't use a printed circuit board - instead you can build it on a bare, pre-punched board or even simply wire the components one to the other and then cover the complete assembly in epoxy.
Required are:
- 1 x LM311 voltage comparator IC (Jaycar cat no ZL-3311)
- 1 x high intensity 5mm green LED (Jaycar cat no ZD-1794)
- 1 x 25-turn 100K pot (Jaycar cat no RT-4656)
- 1 x 470 ohm resistor (Jaycar cat no RR-0564)
- 1 section of pre-punched perf board (Jaycar cat no HP-9562)
Make sure that you take your time and carefully check each step in the build process against the circuit and/or photos - one wire in the wrong place and at best it won't work. At worse, it'll blow the chip or LED!
Most people will build the Closed Loop Monitor on pre-punched board, where all the wiring connections are made under the board. If that is the case, use this diagram, which shows the board from underneath.
This photo of the underside of the board shows how the prototype was wired. As you can see, occasionally I took different physical paths to make the layout easier - but the electrical paths that the wires take are all the same as in the diagram.
Looking down from the top, the device looks like this. Note the indent on the LM311 chip - this is a vital mark as it shows which end is which. On the diagram above this mark is shown as a black crescent at the end of the chip - often the chips will have this rather than the indented circle.
Instead of using a board, you can build the device wire-to-wire. In this case the soldering will be done looking down on the chip, so this diagram shows the view from above.
This is what the point-to-point wiring arrangement of the Closed Loop Monitor looks like.
In both approaches above the LED has been shown wired-in close to the chip, but in most cases you'll mount the LED on the end of a long pair of wires. Note that the LED's long lead is its positive connection.
Fitting It
Using a multimeter and/or the workshop manual, find the oxygen sensor signal wire (if the car has a sensor both before and after the cat, use the one before the cat), a 12 volt ignition-switched supply and an earth. On the oxygen sensor wire you should be able to read a fluctuating voltage - remember that the car has to be warm before this will happen. The connections can be made at either the oxy sensor or ECU end of things - we chose to do it close to the ECU.
Make the three connections and then start the car and let it warm up. Use a small screwdriver to turn the pot - at one end of its travel the LED should be on all of the time, and at the other end of its travel it should be off. (Note that the pot has 25 complete turns end to end, and normally you won't come up against a 'stop'.) Adjust the pot so that the LED flashes with the car idling, and then go for a drive. When you put your foot down, the LED should go off (in some cars it will just dim a lot), and when you back off sharply the LED should come on. At a constant cruise and at idle it should flash - not necessarily on/off evenly, but (and it will depend on how your car's ECU controls the closed loop mixtures) perhaps like this: quick on, off, quick on, off, long on, off - and so on.
When you're confident that the adjustment is right, you can find a home for the module. It's very small, so in our case we just wrapped it in tape (leaving the pot adjustment free) and then cable-tied it to the main ECU loom.
The LED can be positioned anywhere you like - here it's been installed unobtrusively beneath the navigation display. If you want the LED to be dimmer, fit a higher value resistor.
Conclusion
For the cost and its usefulness, you can't go wrong! Like many car instruments, its greatest usefulness will come with long-term watching. Then you'll see the distinct patterns that the car's ECU uses to control the injectors; any change from that pattern indicates a problem.
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Closed Loop?
So wtf is closed and open loop? 'Closed Loop' is the term given to the ECU behaviour when the oxygen sensor signal is being used to largely control how much fuel the injectors are adding to the intake. The ECU works in closed loop most of the time - when the car is warmed up and idling, in constant throttle cruise - and so on. The ECU watches the oxygen sensor output and if the mixtures are getting a bit rich, it leans them off. If the mixtures are getting a bit lean, it richens them up. This causes the mixtures to fluctuate rapidly around 14.7:1 air/fuel ratio - what's called the stoichiometric ratio. The ECU aims to keep the mixtures at this number because its there where the cat converter works best at cleaning up the exhaust.
However, an air/fuel of 14.7 doesn't give the best power, so when you put your foot down, the ECU forgets about closed loop and goes instead into an operating system called 'Open Loop'. This just means that it ignores the output of the oxy sensor, instead picking the right amounts of fuel from its internal memories. Typically, the air/fuel ratio outside of closed loop might jump to 13:1, then 12:1 and then even richer still at 10 or 11:1.
The final typical operating approach is when the injectors are stopped completely - yes, they're actually switched off sometimes even when the car is driving along! This happens on the over-run - you're travelling along at 100 km/h, reach an 80 km/h sign and lift your foot. The ECU will then turn off the injectors until either you reapply the accelerator or the engine speed drops to near idle revs.
Like all things, these ideas apply to most cars - not all. Some Porsches, for example, stay in closed loop all the time - even when the mixtures are richer than 14.7:1. In other words, the oxygen sensor (a special one) is used to give mixture feedback to the ECU is all operating conditions. Other cars have a 'lean cruise' system, where on the open highway the mixtures will gradually lean out to say 15 or 16:1, so saving fuel. But for most cars on the road, the above approach is used.
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Spikes and Voltage Variables
If this were an electronics magazine and not a car magazine, we'd have made the design a bit more complex. For a start, in this design the reference voltage will go up and down a bit as battery voltage changes. Also, the chip isn't protected against voltage spikes and other nasties. But in use the slightly varying battery voltage doesn't alter the reference voltage enough to make a major difference to the functioning of the LED, and in most cars the design will live a long and happy life.
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