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Moderator Emeritus
January 26, 2004
Reaction score
City, State
Mechanicsville, Virginia
Year, Model & Trim Level
2004 Acura TL
First off, I will like to reference back to a few things. First thing is a link to another Thread regarding facts about the EEC-V system (click here to read). There you will see the basics of EEC-V (pretty short). In the thread are links to other threads regarding trouble codes (DTCs), how to retrieve DTCs, and learn about other sensors, actuators, inputs, outputs, and more.

Second thing before we begin: EEC-IV was used until 1994-1995 (depending on vehicle) while EEC-V is from 1995 to today. I will get into how to determine which you have later down the road.

In this thread you will learn about the suddle differences between the EEC systems and hopefully will help you better understand the differences between the two (benefits and disadvantages). So let's start:

First off ; The actual computer:


This is what does all the "thinking" for what happens next. The computers in vehicles have a set way to run and will do this on cold-start (engine cold). During this phase the computer is in OPEN LOOP (to read on closed loop and open look click here) and is considered to be reading basically off of just the Engine Coolant Temperature (ECT) to set the fuel trims (how much fuel is added/taken away at each cylinder). Once the exhaust/catalyst has reached its operating temperature of 500-600 degrees F, the computer will flip over to CLOSED LOOP which will begin the computers outputs to be set by the inputs. Meaning, that the computer will simply read the inputs from the sensors (cam, crank, H02S, MAFs, MAPS, etc.) and check that with the computers "programmed" information, and then act accordingly for specific objects (fuel injector pulse width time, spark, etc.). EEC-IV and EEC-V are very similar in the way they run things on Cold-start. Now, remember; during OPEN LOOP the engine is firing (spark) and injecting (fuel) via cam and crank sensors, but you don't get the optimum fuel consumption until CLOSED LOOP!

EEC - What is that?

EEC simply stands for Electronic Engine Control. This is manufacturer specific (Ford only), so don't go talking to GM and Chrysler guys and talking about how their EEC-V computer should be doing this and that. For them it's just a PCM or ECU/ECM or whatever other words you want to use. EEC is pronounced like this: "EEK". After you say that - just add a number after it (either four/IV or five/V). There have been 8 different EEC versions since Ford introduced the system back in '78, which of course are all numbered (EEC-I, EEC-II, etc.) but with a few different versions of each. Earlier versions were used mainly for just fuel injection.


EEC-IV is the simplier of the two systems. A lot of Ford fanatics say that an EEC-IV computer can pretty much run any engine as long as it's hooked up properly and programmed correctly. Most emission related and Government regulated requirements are deleted from the IV system, it can get the job done and is just as good as EEC-IV when it comes to the vital controls/sensors. The ROM (where all the vital information is/ long-term memory) can stay alive for 20 years without having current/voltage! That's great for you junkyard junkies who go scrapping for Computers to run your engines! IV operates at 15MHz (15,000,000 taks/second) and can do a great job of controlling an engine.

EEC-IV is considered the "best factory" engine control computer to use on your "project". However, you need to get in with the "computer geeks" of cars and sit down with them and talk about what you want to build. They will be able to help you out in a huge way that no write up or informational lesson will give you!


IV and V are very very much alike. However, there are always upgrades and better things in a revised system: which include - faster, more capabilities, ease of alteration (flash programming) to the PROM, and better diagnostic use. As stated above, IV operates at a very quick speed of 15MHz, while V is passing by at 18,000,000 taks a second (18MHz). The internal memory is 4 times greater than IV. New systems were brought into the automotive field (air bags, tracation control, ABS, air suspension, etc.) - so the PCM need to be able to run all of these things - which a IV will not. On V, you can hook up a scan tool and pull of live streaming data that the computer sees - on IV you can no real live data (except maybe a few things). EEC-V has a mandated connector (for diagnostics atleast) that everyone must use. IV has manufacturer specific connectors which variey from year to year and vehicle to vehicle.

Connector views:


That is a picture (via FFI.com - listed below), that shows the connector view AT the actual computer of EEC-IV. Please realize that this is NOT the scan tool hook up! This is where the huge harness plugs into the computer. IV uses 60 pins to do all of its functions.


Above is a picture of the EEC-V connector, which uses 104 pins to control everything.

Diagnostic Ports/Data Link Connectors (DLCs)

EEC-IV : Under Hood


EEC-V : In Vehicle (must be within reach from driver seat)


Note: EEC-IV is OBD-I while EEC-V is OBD-II

Well, I hope that was enough information for one thing, but I hope you learned a little more than what you knew before. Computers aren't always the easiest thing to understand, but once you do - it keeps making more and more sense. Hopefully we'll get some other post from other smart guys on this site. :thumbsup:


Contributors: FordFuelInjection.com (for pictures).


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