The TEMS Bible: by Drake69
The TEMS system (short for Toyota Electronically Modulated Suspension) is an advanced suspension system found on many of the 3rd generation MKIII Supras and the Soarers of that era as well as newer Toyota vehicles like the Sequoia and Land Cruiser, and was and still is an impressive feature. The TEMS system uses a series of actuators controlled by variously placed sensors and a TEMS computer to improve the suspension of the car in the more aggressive profiles, such as cornering, acceleration, and deceleration, by increasing or decreasing the damper for each shock. This does not mean that the suspension can be “lifted” like other electronically controlled systems for ride height like the TEMS on the Soarers can, but it does allow the handling of the vehicle to be more flexible in tight situations. Softer settings are for general cruising, harder settings for performance.
From the Toyota Supra Reference Manual (TSRM), here is a link to troubleshooting TEMS problems…
Some important items to note: When the TEMS system encounters an error, either due to a loose connection, frayed wiring, etc…, the TEMS indicator located on the dashboard will flash all three of its lights, indicating trouble in the TEMS computer. When driving, if the TEMS system is flashing an error and a turn is made that would normally engage TEMS, the indicator will go to one flashing LED for the direction of the turn, such as right side for right-hand turns, and left for left. There are a number of faults that can cause this error, so follow the above link to properly troubleshoot the problem. Also, to ensure the TEMS system is working as expected, turn the ignition key to “Acc” or “On” without starting the vehicle, make sure there is no sound inside the cab, and cycle through the two options on the controller. An audible “click” can be heard when switching modes. Note that any changes made to your TEMS system should be done with the “sport” button engaged as recommended by Toyota prior to work.
Components of the TEMS system are:
1: Four actuators with covers located at the four shock points (also known as strut towers) inside the vehicle. These actuators control the dampening pressure to the shocks at each strut point. Two are found in the engine bay above and behind the wheelwells, and the other two are found underneath the plastic trim by the rear speakers in the cargo area.
2: Four sensors, listed below, in various locations in the vehicle.
3: The TEMS computer, located in the passenger-side cargo bay area beneath the trim, towards the back corner.
4: The TEMS controller, located in the center console, with two buttons that adjust shock firmness, marked “Normal” and “Sport”.
5: A wire junction between the TEMS computer and controller, located in the passenger-side kicker panel which is above and behind the ECT (Electronically Controlled Transmission) ECU.
6: The TEMS LED panel, integrated into the dashboard, that displays the current TEMS-level setting. Any errors to the TEMS system will cause these indicators to flash.
What’s surprising about this system is that it was not FIRST used on Supras, rather the system was initially designed for the Z10 Series Soarers from 1981 – 1985, as well as the Cressida models of those matching years. In the middle part of 1986 when the first MKIIIs rolled onto the market, Toyota included the TEMS system in the Sport editions of the Supra along with an improved limited-slip differential. As there were many changes in the packages offered for these cars, it is just as common to find one in an N/A (naturally aspirated) as it is for the Turbo version, with the Turbo version more likely to have it available since it became a factory option for Turbos in later years.
This system allowed the driver to select between “Normal” (soft) and “Sport” (medium) modes via a 2-button switch, along with a computer-selected “Firm” (hard) mode for more aggressive driving. Depending on freeway speeds, the TEMS computer would set the shocks to either soft or medium for smooth driving at highway speeds, followed by hard for increased cornering, hard braking, and sudden acceleration/deceleration. Hard steering input such as those in twisted curves would often trip the sensor as well, stiffening the ride to accommodate the change in driving stance. However, if the “Sport” button was already engaged by the driver, the system would only select between medium and hard settings as based on driving conditions, allowing for faster response.
The four sensors which assist in the various settings are:
1: Throttle Position Sensor (TPS): for anti-squat
2: Brake sensor: for anti-dive (nose-forward during hard braking)
3: Steering-angle sensor: for anti-roll (side-to-side)
4: Speed sensor: for anti-squat and “Firm” setting
In 1989, several changes were made to the Supra to improve various options, and the TEMS system was no exception. Toyota reduced the size of the actuators to each strut and increased response times to the computer so that handling became more efficient. As a drawback to this, European and Japanese-owned Supras who swapped out their older actuators for the newer ones noticed no appreciable changes in suspension with the U.S. the exception to the rule. As a result, U.S. owners of the older model MKIII’s need only to replace their actuators with the ‘89+ ones to improve their drivability, whereas other models required the TEMS computer from newer model cars to be swapped out as well. There is no mention of the sensors needing to be swapped out, and as such have not been included here.
Some aftermarket items were made available for TEMS, not the least of which is the HKS TEMS Controller, or EAC-T. This rare item, no longer in production, is a 4-button switch as opposed to the 2-button affair found on the center console of most Supras, and allows for the setting of the suspension to full “Firm” mode, a setting only reserved for the TEMS computer itself. Still a highly sought after item fetching anywhere from $300 ~ $600 on eBay and various forums, its usefulness has been overshadowed by a wiring mod in the fuse relay box that allows for direct control of the “Firm” setting, and when turned off will resume normal operation. The link below is a document describing how to do this mod:
Another aftermarket item and one woefully needed is Tokico Illumina II shocks which replace the factory shocks with a lesser expensive alternative to OEMs. As of this writing, only the Illumina II’s are a direct replacement for the TEMS shocks, and as such, most owners opt for removing the TEMS system altogether and replace it with static shocks or manually adjustable ones (some even decide to get the newer electronic shocks instead for more control and more options available, but this is very costly!). Some coils may also need to be replaced to adjust for ride height and tire fitment inside the wheelwells, but more research is needed before it can be included here as compatability may also be an issue.
A third aftermarket item is the Blitz controller, which is very similar to the HKS controller as it performs the same function. As this is even rarer than the HKS, no discernable info is available at this time as it might have been more readily available to the European and Japanese market than here. Most pictures of the Blitz controller show only two buttons as opposed to the standard four of the HKS, but due to lack of info this will not be included in this faq.
The HKS TEMS Controller – EAC-T:
HKS USA was kind enough to make a controller for the TEMS system to give owners more control over their suspension by allowing for a third button press that engaged the computer-only “Firm” setting for the dampers. This allowed for even greater control and performance over the normal 2-button setting, but HKS stopped selling the system sometime around 1999. As a result, demand for this controller still runs high with used items selling for $300+ on the forums and as much as $500 ~ $600+ on eBay. Often, these controllers will change hands multiple times as is common in the used market.
Due to their used nature, damaged HKS controllers can have:
1: Hacked-into harnesses
2: Pulled/frayed wires
3: Unresponsive buttons
4: Lighted buttons no longer lit
5: Missing instructions
Out of all these, about 99% of the EAC-Ts being sold are missing the actual instructions for proper installation, and as such this can cause the unit to not work as designed. Fortunately, the instructions are fairly simple and will be posted here for clarity.
Here in this picture you can see the front face of the EAC-T. The last 2 buttons are ones most people are familiar with, since they are already featured on all TEMS systems currently installed in Supras, “Normal At Soft” and “Sport At Medium”. The button to the far left is an “On/Off” button of sorts that allows you to “Manually” set the EAC-T to the level you want, or go back to the “Automatic” settings as part of normal driving. The “Hard” button is the “firm” setting once reserved by the TEMS computer and is now under direct control by the driver.
A variation of the front faceplate shows a small diagram connecting the various buttons together for operation.
Here is a lighted view of the EAC-T, behind the shifter.
As of this writing, there are two different models of the EAC-T (aside from the varying faceplates) that have one very important difference. To display the differences, here are two pictures to compare.
HKS with the wiring loom and ground plug visible.
HKS with the wiring loom (circled in BLUE), ground plug, AND an additional plugin for power, identified by the RED circle (black/white wire).
It is surmised that the older HKS units had the additional wire as needed to supply power to the EAC-T, and the newer HKS units removed this wire because the loom in the kicker panel supplied this same power (this may ALSO be model specific by year of car, as owners of the newer units in Supras after ’89 did not need to supply power to the new units because it derived power from the loom.). What is important is that power HAS to be supplied to the EAC-T for it to work as designed, as without power it will cause an error condition in the TEMS computer when switched from “Auto” to “Manual”, forcing the three LED lights on the indicator panel to flash. Also note that the ground plug is consistant in both types of EAC-Ts, and as such have to be grounded to the frame to function as well. Since the ground plug is located at the end of the loom in the kicker panel, two ground points, the screw that attaches the ECT ECU to the frame and the screw that reattaches the kicker panel, can be used to ground the unit.