Running Codes from in-car

Running EEC IV Diagnostic Codes from the Comfort of the Driver's Seat
By Jeff Korn

We have all had it happen. You are on your way home from work, or out for a drive in the country, and you notice a slight stumble or miss, notice a rough idle when coming to a stop, or some other engine malfunction. Upon arriving home, you begin to diagnose the problem.  Even if you suspect the problem is those cheap parts store plug wires you recently put on, the first place to start any engine diagnosis is with an EEC IV diagnostic test, often referred to as running the codes.  Running the EEC error codes will lead you to the faulty sensor or activator in short order, but even if the error codes indicate the EEC system is in perfect operating order, You now know the problem is in some other engine system not directly controlled or monitored by the EEC system, like those cheap plug wires.  

So, how do you run the codes?  There are many approaches to this.  You can buy an expensive code scanner, such as the Auto X ray, or Autotap for $200 or more, you can buy a $30 code scanner that flashes out the two digit trouble codes on a light emitting diode, or, if you have a check engine light (87 and 88 model years), you can simply use a jumper wire in the EEC test connector, and watch the check engine light flash out the codes.  Any of these methods works, but I have found an easier and cheaper way to do it.  Depending on the contents of your junk box, the total cost will be between free and $5, and labor time will be an hour or two.  As an added plus, you will be able to put the EEC system in diagnostic mode, and run the codes without even getting out of your car.  This makes it easy to tap the brake petal, and goose the throttle when you are asked to do this at the dynamic response code issued during the engine running test. I have installed this system on all the EEC IV vehicles I have owned since 1986, and have been very happy with the ease and convenience it affords.  

PARTS REQUIRED

This depends on the model year of your of your car.  

If you have an 87-88 with a check engine light, all you will need is a SPST (single pole, single throw) switch, (i.e., and on/off switch), and some light gauge automotive grade wire.  

If you have an 83-86 without a check engine light, all you will need is a DPST (double pole, single throw) switch, (i.e., and on/off switch that will switch two circuits), a 12 Volt LED (light emitting diode) and some light gauge automotive grade wire.  

The switches and LED are available at Radio Shack for a around $2 each.  I prefer to use miniature switches, as they are easier to mount, and look good.  The switches only carry a few milliamps of current, so any switch will work for this application.  

BUILDING THE SYSTEM

You will have to tap into the EEC wiring harness at some point.  This can be done either at the EEC test connector under the hood, or at the EEC processor in the passenger side  kick panel.  I always tap into the wiring at the EEC test connector, but that requires running wires through the firewall.  Tapping into the wiring at the computer doesn’t require going through the firewall, but does require you to pull out the computer from the kick panel which, in my opinion, is a bigger hassle than running wires through the firewall. I run wiring through the firewall at the rubber boot that seals the steering shaft to the firewall.

   

I have done this on many cars for 25+ years, and never had a problem. Just poke a hole in the lower part of the rubber, and run the wires through, and seal it up with a dab of RTV silicone. Be sure to securely tie the wires off to the side inside the engine compartment so they are at least two inches from the steering shaft!  Since color codes may have changed through the years, I did not list wire colors, only the wire positions on the EEC test connector. If you are going to tap into the wiring at the computer, note the color codes at the EEC test connector, and find those wires at the computer.  

Find a place to mount the switch (87-88), or the switch and 12 Volt LED (83-86).  The panels on the left or right of the lower dash, where the light switch or ride control switch are make good mounting locations.  Drill holes, and mount the switch, or switch and LED.

Locate the EEC test connector, and the single connector STI (self test input) pigtail near it.  The connector is typically near the drivers side strut tower.  Refer to the figure shown above.  

If you have an 87-88 with a check engine light, simply tap into wires 1 and 2, and run these two wires to the dash mounted switch. You are done!  With the switch off, the car operates normally, and with the switch on, the EEC computer goes into diagnostic mode, and you can run the KOEO (key on, engine off) and KOER (key on engine running) tests, and watch the check engine light flash out the codes.  

If you have an 83-86 without a check engine light, simply tap into wires 1 and 2, and run these two wires to one pair of terminals on the dash mounted switch. Tap into wire 3, and run it to the negative terminal of the 12 Volt LED. The negative terminal of the Radio Shack 12 V LED is yellow.  Run the positive (red) lead of the LED to one of the remaining switch terminals, and run a wire from the remaining switch terminal to a 12 Volt source that is hot when the ignition key is in the run position. You are done!  With the switch off, the car operates normally, and with the switch on, the EEC computer goes into diagnostic mode, and you can run the KOEO (key on, engine off) and KOER (key on engine running) tests, and watch the LED flash out the codes.



Jeff Korn

Milwaukee WI

88 Turbo Coupe (lots of mods)

86 Thunderbird 5.0 (lots and lots of mods)

91 Escort (bone stock)

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