Differences between 3Plug and 4Plug ECU's
Posted: Sat Sep 16, 2006 7:58 am
I just thought that i would point out a few differences between the 2 types of ECU that was used by early Subarus
The first EFI system for Subarus was known as the 3 plug ECU.
***Key Points***
1. Model Years 1984-1986 - in 84 it was used with the EA81T, in 85 & 86 it was used with the EA82 L series Sedan, Wagon and Vortex.
2. Has 3 Plugs at the ECU
3. Flapper style AFM (Air flow Meter) - A mechanical flap connected to a resistive potentiometer to measure volume of air intake
4. Used Vaccum (and boost on turbo models) switches to inform ECU of engine load ect. Under boost, the ECU went into open loop mode, where it ignored some input sensors and controlled fuel injection soley from the fuel map.
5. Had a Breakerless Disty (Magnetic Reluctor & Hall Effect Sensor) with Vaccum and Centrifugal Advance. Ignition Timing was not controlled by the ECU. Turbo models had 2 extra wires that went to the Knock Control unit.
6. Turbo models had a seperate Knock Controller to retard the timing during pinging.
7. For NA modles, the Throttle switch contacts at both Idle and WOT (Wide Open Throttle) positions. The Throttle switch of the turbo models contacted at Idle and Half throttle.
The second EFI system was known as the 4 Plug ECU.
***Key Points***
1. Model years 1988-1994 L series Sedan, Wagon and Vortex. (1987 was a grey area with some cars using the 3 Plug system, others using the 4 Plug system)
2. Has 4 PLugs at the ECU
3. Hotwire AFM - Calculated intake air volume by measuring the temperature drop across a heated wire.
4. No Boost or Vac Switches
5. Had an Optical Disty (Disc with 360 slots, LED/Photodiode Sensor) which utilised a Crank Angle Sensor. The ECU uses input signals from the engine sensors and ECU ignition map to calculate the optimum ignition timimg.
6. Knock Controller was incorporated inside the turbo ECU
7. Has a TPS (Throttle Position Sensor) and a Throttle Switch. The TPS is a potentiometer to tell the ECU the precise throttle opening. The Throttle switch contacted at idle only
8. Some turbo models had a WGDS (Wastegate Duty Soleniod) An altitude compensator built into the ECU to raise boost pressure slighty when ambient air density was low.
9. Fuel cut is experiecned at higher than factory boost levels, but is actually a result of the amount air measured by the AFM , not boost pressure like it it commonly mistaken
If you have any corrections, please dont hesitate to post
Gannon
The first EFI system for Subarus was known as the 3 plug ECU.
***Key Points***
1. Model Years 1984-1986 - in 84 it was used with the EA81T, in 85 & 86 it was used with the EA82 L series Sedan, Wagon and Vortex.
2. Has 3 Plugs at the ECU
3. Flapper style AFM (Air flow Meter) - A mechanical flap connected to a resistive potentiometer to measure volume of air intake
4. Used Vaccum (and boost on turbo models) switches to inform ECU of engine load ect. Under boost, the ECU went into open loop mode, where it ignored some input sensors and controlled fuel injection soley from the fuel map.
5. Had a Breakerless Disty (Magnetic Reluctor & Hall Effect Sensor) with Vaccum and Centrifugal Advance. Ignition Timing was not controlled by the ECU. Turbo models had 2 extra wires that went to the Knock Control unit.
6. Turbo models had a seperate Knock Controller to retard the timing during pinging.
7. For NA modles, the Throttle switch contacts at both Idle and WOT (Wide Open Throttle) positions. The Throttle switch of the turbo models contacted at Idle and Half throttle.
The second EFI system was known as the 4 Plug ECU.
***Key Points***
1. Model years 1988-1994 L series Sedan, Wagon and Vortex. (1987 was a grey area with some cars using the 3 Plug system, others using the 4 Plug system)
2. Has 4 PLugs at the ECU
3. Hotwire AFM - Calculated intake air volume by measuring the temperature drop across a heated wire.
4. No Boost or Vac Switches
5. Had an Optical Disty (Disc with 360 slots, LED/Photodiode Sensor) which utilised a Crank Angle Sensor. The ECU uses input signals from the engine sensors and ECU ignition map to calculate the optimum ignition timimg.
6. Knock Controller was incorporated inside the turbo ECU
7. Has a TPS (Throttle Position Sensor) and a Throttle Switch. The TPS is a potentiometer to tell the ECU the precise throttle opening. The Throttle switch contacted at idle only
8. Some turbo models had a WGDS (Wastegate Duty Soleniod) An altitude compensator built into the ECU to raise boost pressure slighty when ambient air density was low.
9. Fuel cut is experiecned at higher than factory boost levels, but is actually a result of the amount air measured by the AFM , not boost pressure like it it commonly mistaken
If you have any corrections, please dont hesitate to post
Gannon
