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ON 1985-1992 TOYOTA CRESSIDA/SUPRA
Posted 4/16/2001
By Paul Kujawa
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“To diagnose the circuit, it's important to know what's going on in the system.”
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Vehicles:
1985-1992 Toyota Cressida/Supra. Vehicles equipped with factory-installed theft deterrent system.
Engine:
7MG-E, 7MG-TE
System:
Body/Chassis/ Electrical
Symptom:
A no-crank situation can sometimes be confusing, especially on vehicles with security systems. To diagnose the circuit, it's important to know what's going on in the system. Most manufacturers don't include wiring diagrams for security systems, especially if they were installed at the dealership or port-installed, which is the case with most Toyotas.
What you need to remember is that the theft deterrent computer sends out voltage to the switches (key unlock switch, door lock switch) and the voltages are grounded by the switches. The computer is located by the steering column and is labeled “Theft Deterrent ECU.”
If the vehicle will not crank, try to reset the theft deterrent system as follows:
1) Close all the doors and lock them with the key from the driver's door.
2) Install the key into the driver's door lock cylinder and turn it to lock. Hold in lock position for one second. Repeat this step (the system needs to see two lock signals).
3) Go to the passenger side and unlock the front passenger door, using the key in the door lock cylinder. Hold in unlock position for two seconds.
4) Try to start the vehicle with the key. If it starts, the problem is with the driver's side key switch or wires to the switch.
If vehicle still will not crank:
1) Check voltage on pin 10 (blue/white tracer wire) of the 20 pin connector on the theft deterrent computer (control unit is looking for reference voltage to go to ground - a drop from 12 volts to 0.0 volts - when operating switches).
2) Connect (back probe) the voltmeter positive lead to pin 10 and negative lead to battery negative. There should be 12v from the theft deterrent computer on this wire with key off, engine off, and doors locked.
If there is voltage:
1) Install the key into the driver's side door lock cylinder.
2) Turn it to the unlock position.
3) If voltage on pin 10 does not go to ground when turning the key, this could mean open contacts in the switch or a broken wire between the theft deterrent computer and the switch, or a poor switch ground connection.
If there is no voltage, it could mean that the wire is shorted to ground, there is no power to the theft deterrent computer, or that the theft deterrent computer is faulty.
Check the door lock cylinder and switches as follows:
1) With the switch in neutral position, place the voltmeter across two wires at the switch - positive lead to blue/white on the driver's side or green/white on the passenger's side, and negative lead to white/black wire at each switch.
2) Turning the switch to the unlock position should ground the 12v signal. If the voltmeter still indicates key battery voltage, then the switch is not grounding the voltage from the theft deterrent computer.
3) If the voltage goes to ground as required by the theft deterrent computer to tell it that the doors have been unlocked by the key switch, but it will still not crank the engine over, manually ground the starter relay wire at the theft deterrent computer by taking a jumper wire from battery negative and touching it into the red wire pin 18. Turn the ignition key to crank the engine. If it now cranks over, you can be assured that the theft deterrent computer is causing the no-crank situation.
4) If you have the correct ground signals from the key switches in the doors but there is still no crank - check for battery voltage to the theft deterrent computer on pin 6 (blue/red wire) and pin 17 (solid red wire). Then check the grounds on pin 7 (black/white wire). If all check OK, then the theft deterrent computer is faulty.
Many customers want to bypass the theft deterrent computer. This is possible by taking a red wire from pin 18 and connecting it to a good ground. Then disconnect the theft deterrent computer. When disabling the theft system, make sure you fully explain to the customer that the system will not work, and always write on the repair order that the customer requested the system be disabled.
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