How to check your ECU codes

All of the Supras have an on-board computer (in some cases two or more) which regulate various components of the engine - fuel injection, timing, fuel cut, and a variety of other things.  While these computers perform their functions well the majority of the time, components can and will wear out and fail, especially given the fact that Toyotas seem to run forever.  The on-board computer systems of the Supra series have a diagnostic function which allows someone to diagnose what's wrong by storing an error code in the memory when the computer senses something's wrong.  It is indicated that a code has been stored when the "Check Engine" light illuminates on the dashboard when the vehicle is running.  (It will always be on if you just turn the ignition to "On")  The light will remain lit for a brief time, then go out.  The diagnostic codes on the 1982-1984 Supras may be retrieved by following the process below, which includes jumpering a diagnostic connector then using an analog multimeter to interpret the readout from the computer.  The 1985-1986 MKII's allow the codes to be retrieved by jumpering the 2-wire diagnostic connector in the engine bay then just counting the blinks of the "Check Engine" light on the dashboard.

Stuff needed:

• 1982-1986 Toyota Celica Supra (I used a 1984, so yours may be different)
• Analog multimeter capable of detecting DC Volts from 0 to 5V (range is usually 0-15 VDC) (for 1982-1984 models only)
• Paper clips

1. To start off, the following initial conditions must be fulfilled:
• Battery voltage above 11 volts
• Throttle valve fully closed (throttle position sensor IDL points closed)
• Transmission in Park (Automatic) or Neutral (Standard) range
• Air conditioner OFF
• Engine at normal operating temperature.
2. Find the diagnostic connector set.  It should be hanging just in back of and below the coil pack.  There are three connectors in a group, all yellow colored.  One has one wire going to it, another has two, and the last one has three.  There should be little black rubber caps on them, but they're really easy to lose (I've lost one already).  Pull the caps off, and you will see this:


3. You won't need to use the single wire connector - I believe that one is used only with the Toyota EFI Checker (SST 09991-00100, if anyone wants to know).  You need to jumper the two contacts of the two wire connector.  I just use a bent-up paper clip to do the job (the pic isn't the greatest, I know):


4. Next, you need to put pieces of wire in the three-wire connector to be able to put multimeter leads in there.  I used paper clips again.  You need to put one in the upper left hole (VF), and one in the lower left hole (E1):


5. When you have the 2 wire connector jumpered and the 3 wire connector's leads put in, it should look like this:


6. Then you need to get out your analog multimeter and attach the positive lead to the top lead (Vf) on the 3-wire, and the negative lead to the bottom (E1) lead.  Set the multimeter to volts DC, with the maximum voltage of about 10-20V.  Turn on the ignition of the car (don't start it!), and the multimeter should start to fluctuate according to the codes that are stored in the computer - or lack thereof.

To check the codes in a 1985-1986 MKII, you only have to jumper the connector jumpered in step 3, turn the ignition on, and count the number of flashes of the "Check Engine" light on the dashboard.
 
7. To clear the codes when you're done retrieving them, you need to remove the "Stop" fuse located in the fuse block just in back of the battery in the engine compartment.  You should remove it for at least 30 seconds, more if it's cold outside.  I'd leave it out for 2 minutes or more just to be sure that the codes are erased.  You should remove the fuse after you remove the jumpers and connectors that you hooked up in the above steps.  If you make a repair which fixes the problem, the memory should be blank (see chart below).  If the computer detects the same problem, or another one, the code will be stored in the memory again.  If you fail to erase the codes after pulling them from the computer, the existing ones will remain there along with any new ones.