DIY 4A-GE Smallport EFI Wiring Pt.2: EFI Wiring Breakdown

On this second part of my 4A-GE Smallport EFI wiring DIY posts, i'll breakdown the wiring of each pins in the ECU and show you how EFI parts interconnect to each other to form a complete EFI system.

Below is the pin-out diagram to smallport ECU P/N 89661-12590.


We'll start from the upper leftmost of the pin and work our way to the right.

E01, E1 and E02
E01 together with E1 and E02 will form the main ground connection for the ECU. It's good idea to attach these pins to a point with good grounding like the engine block or a centralized ground system. Also it's a good idea to use slightly thicker cable for these pins as they are the only ground output for the ECU, beside the ECU casing itself. Twist the cables on these pins together and hook it to the best ground contact you can find.
Now that we're done with pins E01, E1 and E02, i'll shade these pins so that we know they have been discussed. The same work pattern will be used throughout this post.



#10 and #20
Pin #10 controls injectors number 3 and 4. Which ones are number 3 and 4? Don't quote me on this but they should be the two closest to the firewall if you place your 4A-GE longitudinally. This also tells you that injectors number 3 and 4 will always inject gasoline at the same time and the same amount. The same goes with pin #20 except it controls injectors 1 and 2.
On the diagram above, you can see that all injectors are connected to the +12v of the battery via 30A fuse once you put the key on the ignition position. This means the ECU tells the injector to inject the fuel by grounding them (not by supplying +12 volts) to form a closed-loop. Take notice also that since there are no resistor packs placed in series to the injectors, our injectors are called high-impedance injectors. You can read more about injector impedance here. Thick cables should also be used to wire the injectors since they'll flow quite amount of current through the cables.



VISC, +B and +B1
VISC controls a Vacuum Switching Valve VSV that when the valve is open, bleeds small amount of air to the intake plenum which allows the ECU to increase the engine RPM. This is useful when you need to increase the idle RPM like when you turn on the Air Conditioner. When the A/C clutch is engaged, it adds load to the engine which would lower the idle RPM. To avoid engine stalling, we should increase the idle RPM and this is what VISC pin do.

The idle-up VSV itself is located in the airbox and connected to the vacuum port located above the throttle body through a rubber hose.
That particular VSV is designed to sit in the air filter box and would not work without it, quoted from here. So what if you're like me, you get your engine without the air filter box? On the link i just gave you, there's a workaround to get the VSV working on different air filter box. However, since i am going to use an open air filter, this is how i am going to do it. There's a similar functioning VSV in a car A/C system. I don't know what it's called but maybe it's just plain Vacuum Switching Valve. Try asking your local car A/C parts seller. It has two air ports, in and out and of course an electrical connector. Using that, this is what i'll do.
Hope that explains my plan. Of course my workaround is based on the assumption that the VSV on ISC is the on-off type, meaning that it can only has two conditions: prevents and allows air and can't regulate the amount of air flowing. Below you can see how the VSV is wired to VISC pin.
The VSV will receive +12 volts from the EFI Main Relay once we put the key to the ignition position. The other connector to the VSV is connected to VISC pin which tells you that the ECU controls the VSV the same way it controls fuel injectors: by grounding it to create a closed-loop. Take notice that EFI Main Relay also supplies voltage to pin +B and +B1 when the ignition is on. It also connects to one of the pins on the diagnostic connector. This way if you want to check whether the ECU is getting power, you can tap to the diagnostic on the +B pin, useful when troubleshooting engine problem.



IGT, G2, G1, NE, G- and IGF
It's impossible to discuss IGT pin only without bringing G2, G1, NE, G- and IGF to the the topic because they all form the ignition system. 4A-GE uses the so-called VAST or Variable Advance Spark Timing ignition system. I will only give you a diagram of the system. If you wish to know more about VAST, here's a good reading. I strongly suggest you read it since it'll give you a good basic understanding of how our 4A-GE ignition system works.
Here's how those pins are wired.
NE, G2, G1 and G- should use screened cable to avoid interference with other equipment. The screen should be grounded. Notice that the same wire that goes to the tach is also connected to the IG- on the diagnostic connector. This allows you to tap into the ignition and see its performance.


ECT, L1, L2 and L3
These pins are supposed to be connected to the ECT computer which controls Electronically Controlled Transmission on some vehicles. Since obviously my T50 gearbox has no such feature, we'll leave these pins unused. Hopefully nothing wrong will happen if i leave them unused.

I'll mark these pins and any unused pins differently just to differentiate them to the pins that we have discussed and must be used.


STA and FC
STA is used as an input to the ECU which informs the ECU when the starter motor is cranking the engine. This allows the ECU to increase the fuel injector timing which would ease engine start-ups.

FC controls the Circuit Opening Relay which basically controls the fuel pump. FC is used as a safety measure. For example, on accidents which caused the engine to die, the ECU would detect that the engine has died since there's no signal from the ignition system. The ECU then turns off the Circuit Opening Relay which would disable the fuel pump. This would hopefully prevent fuel leak and minimize possibility of fire hazard.

Below is how STA and FC are connected to the EFI system.
One thing to note about the diagram above is how the cold start injector is wired. You can see that it's not controlled by the ECU! Hence, now you know that you can disconnect the CSI safely without fear of generating any error code.

If you want, you can ditch the circuit opening relay and control the fuel pump manually via switch for more race-y look. That way, FC will be left disconnected. Just remember that you still need to connect the STA pin to the starter motor. Not doing so may result in difficult start ups.



NSW
I believe it stands for Neutral Start Switch. This pin is used to monitor whether or not the transmission is in neutral position. I think (which means i could be wrong) this is used only for A/T. If the A/T is not neutral, the ECU will refuse to start the engine. However, since i am not sure, initially, i will connect this NSW pin to +B (supplying +12V to the pin when the key is in ignition). If it cranks and starts normally, i will try to disconnect it and try starting again. If it now fails to start then i know i have to keep the NSW pin connected to +B. Other than safe start-up purpose, i believe it somehow also allows better performance in gear-shifting, by slightly reducing the fuel supply during shifts.



FPU
There are times when you need to increase the pressure in the fuel rail like perhaps during high boost on forced induction where extra fuel supply is necessary. This is achieved via FPU. This pin is supposed to connect to a solenoid which controls certain type of fuel pressure regulator. Since my 4A-GE has no such feature on the FPR, i'll leave FPU disconnected.



VF and T
These are used for diagnostic purpose. Below table taken from here saves me few joules of typing energy.
T terminal ON means that T is jumpered to E1 on the diagnostic connector.




E21 and E2
These pins serve as common ground for the MAP sensor, Water Temp sensor, Intake Air Temp sensor and Throttle Position sensor. The current flowing through these pins are small so there's no need for thick cables. Inside the ECU box, these pins are routed and connected directly to E1, E01 and E02 and thus they will be connected to the engine block ground eventually.




OX and KNK
OX will be connected to the Oxygen sensor which is attached to the exhaust pipe. Oxygen sensors are used to provide air/fuel ratio or AFR feedback information to the ECU. This info is used to adjust the AFR to stoichiometry during warm idle and cruise operating conditions. The stoichiometric AFR delivers one pound of fuel for 14.7 pounds of air entering the intake manifold and results in the most efficient combustion and catalyst operation. When the electronic control system is using the info from the O2 sensor, the system is said to be in closed loop mode.

KNK pin will be connected to the Knock sensor. Knock sensor is a piezoelectric device mounted to the cylinder block which generates a voltage whenever it is exposed to vibration. When engine detonation or knocking occurs, vibration of the cylinder block causes the sensor to generate a voltage signal. When the ECU senses this voltage, it will retard the ignition timing until the knocking stops. Below diagram shows you how it works..


This is how the O2 and Knock sensor are wired. Keep in mind that since the voltage generated by these sensors are very small, we need to use screened cable to safely send the signals to the ECU, avoiding any interference from other engine parts which would cause wrong reading by the ECU.



THW
THW is used to connect the Water Temp Sensor. This sensor monitors engine coolant temperature by means of an internally mounted thermistor. The thermistor has a negative temperature coefficient (NTC), so its resistance value decreases as coolant temperature rises as shown on below table.

This sensor is required because fuel vaporization is less efficient when the engine is cold. The THW signal is used by the ECU to determine how much fuel enrichment correction is necessary to provide good cold engine performance. Below is how the THW sensor wired.

Note that since thermistors are non-polar, you don't have to worry about determining which pin on the sensor supposed to connect to THW and which one should go to ground. Choose any of the two pins on the sensor to connect to THW and the other pin should go to ground.



VCC, VTA and IDL
These pins go to Throttle Position Sensor or TPS. VCC outputs a stabilized 5 volt to the TPS. This voltage will be varied by the TPS depending on the position of the throttle, starting from 0 volt on closed throttle and linearly increasing to the voltage given by VCC (in this case is 5 volts) on wide open throttle (WOT). The varied voltage is sent by TPS via VTA pin. The IDL pin is used to monitor whether the throttle is in completely closed (engine idle). Here's a summary of what i've been saying.

The ECU monitors the voltage sent to VTA and IDL to calculate the required fuel to inject.Here's how TPS is wired.



THA
THA connects to Intake Air Temperature sensor (IAT sensor). The IAT sensor monitors the temperature of air entering the intake manifold by means of thermistor, similar to that used in Water Temp sensor and has the exact same resistance vs temperature characteristic to THW sensor. Hence, you can directly replace THA sensor with THW sensor. IAT sensor is necessary because the pressure and density of air changes with temperature (remember P.V = N.R.T back in high school?). Because air is denser when cold, the ECU factors intake air temperature into the fuel correction program. More fuel will be injected to the chamber when intake air temperature is colder.

The THA is wired the same way as THW (yeah, i am too lazy to put the wiring diagram..)



PIM
Yet another pin used to monitor the amount of air coming into the combustion chamber. This one connects to MAP sensor which basically monitors the air pressure inside the intake plenum. MAP sensor measures intake air volume by monitoring changes in manifold absolute pressure, a function of engine load. The MAP sensor varies the 5 volt reference voltage given by the ECU from VCC pin (the same pin used by TPS) and outputs the pressure-dependent voltage to PIM pin. Here's a chart to show you the correlation between manifold absolute pressure and PIM voltage output.

Here's how PIM is wired.



ACT
ACT supposed to connect the AC amplifier. Since i don't have any AC amplifier on my Charmant, i'll leave this pin disconnected.



ELS2 and A/C
This pin signals the ECU when significant electrical load has been placed on the charging system, requiring increased idle to spin the alternator faster and create more charging current. Significant electrical load can be activated head lights, rear window defogger, etc. Basically anything you think would require significant power consumption from the battery. AC compressor will also give significant load to the charging system but don't connect it to this pin, connect it to the A/C pin instead. Below is how A/C and ELS2 wired.

Notice there's an array of diodes coming to ELS2 just before each load is connected? I doubt you'd ask why it's there if you're actually reading this DIY so i won't explain the purpose of the array. Comment if you still need some explanation.



ELS1
I am still confused about this particular pin. Some say this is the STOP pin that you connect to the stop light switch. If this is actually the STOP pin, this is used to monitor whether the pedal brake is engaged. If the pedal brake is depressed, the ECU monitors this via ELS1 and applies deceleration fuel cut which would increase fuel efficiency significantly, especially on stop-n-go situations not unlike in traffic jams. If this is not the STOP pin, then this is just another ELS pin just like ELS2 where you connect any significant electric load. Personally, i'd connect this pin to the stop light. I'd get better fuel efficiency if indeed the ELS1 is actually STOP pin but if i am wrong, i think the brake booster would benefit from the increased idle speed. Do correct me if i am wrong.



SPD
This pin supposed to connect to the instrument panel which allows ECU to monitor the vehicle speed. In JDM cars, ECU would limit the car to 180 km/h and SPD pin is how the ECU knows the car speed. No such speed monitoring feature on my Charmant instrument panel, of course. Hence i'll leave this pin disconnected.



CCO and EGW
CCO and EGW i believe have something to do with Exhaust Gas Temperature sensor. Since i don't think my engine require such sensor, i'll also leave these pins disconnected.



BATT
BATT must be connected to "always live" wire which means it should always be connected to the +12v from battery regardless the key position or engine status. The ECU needs to keep some values in its lookup table which is stored in a volatile memory. Should the ECU loses power supply from BATT, it'll also lose the stored lookup value, requiring the ECU to repeat the learning process of the engine's environmental condition and come up with new values to the fuel and spark timing correction table.



W
Finally we're at the last pin.. pin W. Pin W is connected to the engine check light or also called the warning light. The ECU informs of detected errors in the EFI system via this pin. Should there be any error, the warning light will lit. The warning light is placed on the instrument panel but since my Charmant don't have such lamp, i'll just place the warning light somewhere on the dashboard where it is visible. I'll share about the error code sent by the warning lamp next time. For now, here's how pin W is wired.

Don't use high wattage lamp on as the Check Engine light since the ECU directly controls this lamp, not via relay. This is to avoid busted driver transistor inside the ECU.



Phew! Finally i've finished this writeup. Took me nearly a week to finish it but it's all worth it cause now at least i have some ideas on what to watch for on the EFI wiring i am about to do. Next post on this DIY will either be on the actual wiring work or about the pinouts on the EFI parts like TPS, MAP sensor etc.

With writeup this long, mistakes are bound to be found. Hence, if you find one, please do share with us by commenting. Thank you guys!