Breaking The Code On TCCS Systems

Toyota's digital TCCS reduces processing time and improves self-diagnostics.


Toyota introduced its Computer Control System (TCCS) way back in 1985. The system has gone through several evolutions since, and here we take a closer look.

Analog TCCS

Pulling codes from the analog TCC system varies according to the vehicle. MR2s, Celicas, Camrys and trucks were some of the first to use the rectangular diagnostic connector found in most models today. The connector is usually gray, but it can be black. You'll find it in various spots around the engine.

It usually has the word DIAGNOSIS on top of the cover. Pop it open, jump terminals T and E, turn on the ignition and count the blinks in an analog fashion. The TCCS can set up to 13 codes, while EFI can only set 7. Here too, a code one means everything is okay.

Digital TCCS

Toyota's most recent offering in engine controls is known as a digital TCCS. Basically an upgrade of the earlier analog system, it uses a digital computer to reduce processing time and improve self-diagnostics.

Digital TCCSs first found a home in the 1983-84 Cressida and Supra. Then, the system was added to some 1986 Celicas, the 1986 Van and Supra, and virtually everything else since. Pre-OBDII TCCSs were used up through the 1993 model year.

Codes are displayed in a flash-pause-flash format, where the first group of flashes on the CHECK ENGINE light represent the first digit of the code, and the second group of flashes represent the second digit.

To proceed, find the same yellow diagnostic connector as used on the earlier system, near the distributor. Then find a second, 4-pin, yellow connector on a bracket on the left inner fender by the battery.

Pull the connector out of its rubber boot and hold the connector so the pair of terminals that form a straight line are horizontal and on top. The other pair of terminals will be vertical and side-by-side. Connect the positive lead from your analog voltmeter to the horizontal terminal on your left and the negative lead to the vertical terminal on your left.

Set your meter to the DC volts position and connect your jumper wire across the diagnostic connector (terminals T and E1). Turn on the ignition and watch the needle carefully. If no codes are present, the needle will fluctuate between 5 volts and 2.5 volts about every half-second. This will continue as long as the ignition is on.

The needle behaves differently when there are codes in memory. At first, the needle will show 5 volts for 2 seconds, followed by a 2.5-volt reading for another 2 seconds. At this point the 2.5-volt reading becomes the reference point for both digits of the code. Needle movement in the positive direction represents the trouble code's first digit, and movement in the negative direction represents the trouble code's second digit.

For example, 2 upscale swings followed by a single downscale swing means a code 21. A 2-second pause will follow each code, then a 5-volt reading for 2 seconds to mark the end of the code sequence. The display sequence starts at the lowest code and goes to the highest.

As long as the ignition is left on and the diagnostic connector is jumped, the sequence repeats over and over again. You may want to watch the sequence several times to make sure you've marked down and read all codes correctly.

The 17-pin, rectangular diagnostic connector is used on all late-model Toyotas. Connect your jumper wire across terminals TE1 (T on some models) and E1 to activate the code display. Turn on the ignition and read the codes out on the CHECK ENGINE light.

If the light blinks on and off every 0.25 second, the ECU has no codes in memory. If the ECU has a code in memory, the light will flash every 0.5 seconds. These codes also appear in 2 digits, with a group of 0.5-second flashes separated by a 1.5-second pause, then another group of 0.5-second flashes. The codes will display from highest to lowest with a 2.5-second pause in between each of them. A 4.5-second pause indicates the code sequence is starting over.

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