Differences between 3Plug and 4Plug ECU's

Gannon · Post by **Gannon** » 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

BrennyV · Post by **BrennyV** » Sat Sep 16, 2006 1:02 pm

Could have just said ones got 3, Ones got 4. They are diffrent!

artrixx · Post by **artrixx** » Sat Sep 16, 2006 8:46 pm

which is the better system?

Gannon · Post by **Gannon** » Wed Sep 20, 2006 5:59 pm

There always seems to be confusion between the two systems. So i thought i would write up the differences so Ausubaru users can easily identify what system and sensors their car has.

The later 4 plug ecu is the better one. It has computer controlled ignition timing and a faster CPU.

Gannon

Suby Wan Kenobi · Post by **Suby Wan Kenobi** » Wed Sep 20, 2006 6:52 pm

The 3 plug setups dont use points although you can if you have too. They where an electronic setup with a larger module and another separate loom coming from the dizzy to control knock.

steptoe · Post by **steptoe** » Thu Sep 21, 2006 10:57 am

Thanks for taking the time, Gannon.

Motorvated me to look and rip mine out

March, 1987 3 plug and yep flapper style, ECU part number 22611 AA002

got any links to the wiring diagram ?

you guys call a pin out don't you ?

why so bloody many wires ?

Gannon · Post by **Gannon** » Thu Sep 21, 2006 5:45 pm

Thanks for the info Suby Wan.

I did a bit of research and made corrections.

Jono,.. i think i have a diagram for the 3 plug ecu, i only have a 4 wire diagram. But there was one floating around the board here somewhere. Someone is bound to have it.
The "pinout" is a diagram of what the pins of the ecu connect to.
Dont get me started on how many wires.

Gannon

steptoe · Post by **steptoe** » Thu Sep 21, 2006 11:31 pm

ta, I'll search sometime, then make a request if no luck.

Craigus · Post by **Craigus** » Mon Sep 25, 2006 6:32 pm

got any links to the wiring diagram ?

Gannon · Post by **Gannon** » Mon Sep 25, 2006 9:37 pm

Thanks Craigus

Suby Wan Kenobi · Post by **Suby Wan Kenobi** » Tue Sep 26, 2006 12:05 am

Most of the wires in the loom you dont need just to run the engine, although the 4 plug ECUs have near double that of a 3 plug.

sinads1 · Post by **sinads1** » Fri Jan 12, 2007 4:56 pm

Hi Craigus

I am also about to undertake the conversion to the RX motor etc.

Unfortunatley I cannot read the PDF either, no matter how big I enlarge it or print it. It must distort when crossing the Nullabor!!!!!

Any chance of faxing it to me of posting a photocopy?

I am more than happy to cover any costs and a bit extra for a beer to say thanks.

Thanks heaps for posting the diagram.

Cheers

Simon
Simon@wapda.com

Radeon · Post by **Radeon** » Wed Jan 24, 2007 8:23 pm

Does anyone have diagrams for a NON-turbo 4 pin ecu EFI motor from an 89 L series?

Gannon · Post by **Gannon** » Sat Feb 03, 2007 5:00 pm

Radeon, you have a PM

The turbo and NA wiring is the same, except the turbo has a WGDS (wastegate duty solenoid) and a knock sensor

ea82T · Post by **ea82T** » Wed Nov 21, 2007 7:52 pm

I have a 1983 MY Subaru wagon which i am going to put a ea82 turbo motor into. I have every thing for the conversion except for the air flow meter, the engine uses the 3 plug computer with the part number 22611AA003. I have been in contact with Subaru and they gave me 2 part numbers 22680AA110 & 22680AA120 for air flow meters to suit that part number on the ecu.
Now i am confused, am i able to use the hot wire afm instead of the flapper style afm?

Gannon · Post by **Gannon** » Wed Nov 21, 2007 9:27 pm

ea82T wrote:Now i am confused, am i able to use the hot wire afm instead of the flapper style afm?

No.

There is no way that you will get a Hotwire Maf to work on a car designed for a flapper style maf. Or vice versa.

There were more flapper maf EA82Ts in Australia than hotwire types. You shouldnt have too much trouble finding a Flapper for your engine.

Try wrecking yards and the "Wanted" section of this forum before buying you MAF from subaru. It'll cost you lots less

ea82T · Post by **ea82T** » Thu Nov 22, 2007 5:47 pm

thanks for that info

discopotato03 · Post by **discopotato03** » Fri Feb 22, 2008 8:38 pm

Just to add to the confusion the SPFI USDM uses an optical CAS/distributor and a hotwire MAF (mass airflow meter) .

Cheers A .

Gannon · Post by **Gannon** » Sat Feb 23, 2008 8:59 am

discopotato03 wrote:Just to add to the confusion the SPFI USDM uses an optical CAS/distributor and a hotwire MAF (mass airflow meter) .

Cheers A .

Yes but only the 87 models on. The 85-86 SPFI had the normal hall effect sensor dizzy and a flapper MAF like the 85-86 turbos

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