By Matt Dixon and Omar Trinidad, Automotive Technology Department,
Southern Illinois University Carbondale
It won’t crank. It won’t start. The radio doesn’t work. The gauges drop out. I can’t get module response with the scan tool.
These are issues that many of us deal with every day. It’s a safe bet that on any troubleshooting procedure, among the first of your checks are powers and grounds. Checking the switched ignition feed to various modules sounds pretty straightforward, however a curve ball may be headed your way. In some cases, these might include finding modules designed without an ignition feed or, in other cases, discovering that the ignition feed does not originate from the ignition switch.
The automotive ignition switch, formerly a straightforward device used to connect battery voltage to various circuits and prevent theft of the vehicle, has recently been switched up. This article is going to provide some examples of the obvious and not so obvious ignition switch changes so that when one of these vehicles gets thrown your way, you won’t be striking out.
The ignition switch has evolved slowly over the years. Some milestones include switch integration with the steering lock, transmission shift interlock and electronic theft transponders. Despite such added functionality, the electrical portion of the switch had remained fairly conventional. Now, some new methods are being used to power up the vehicle. Several late-model vehicles are equipped with push-button start and/or factory remote start.
Before you think the whole world has gone mad, remember that when you turn on your computer, you push a button. That button, much like the one on your digital camera, is not a conventional switch, but merely a request input. Instead of turning a keyed lock cylinder to log onto a computer, the operator enters a log in and password.
Vehicle ignition key transponders have performed a similar function during the last decade. New ignition switches take vehicle power up to the next level, making it more like booting up a computer. These changes present challenges and opportunities for the vehicle service industry.
The first example on the mound, a 2008 Dodge Avenger, is used because the switch appears pretty “normal,” yet it is slightly different. It is also available with a factory remote start system. Some new Chrysler models no longer have a metal key blade or ignition lock cylinder, at least providing a visual hint of change. The Avenger incorporates keyless entry fobs into the key head, but retains a key blade and lock cylinder.
No visual clues of different operation are provided. The conventional portion of the ignition switch connects battery voltage in the start and run positions to only the WCM (wireless control module) and the TIPM (totally integrated power module). There is no connection from the switch to any other position including accessory.
There is no direct connection from the ignition switch to the PCM (powertrain control module) or any other modules.
So how does this vehicle ever start? The other portion of the switch uses what Chrysler refers to as resistive multiplexing (MUX) and acts as a position sensor. Using a MUX portion of the ignition switch type is not new it has been around since the 2001 model year Chrysler minivans it’s just that the MUX portion has become even more important. Understanding the MUX side of the switch is pretty easy if you understand a coolant temperature sensor circuit.
This system uses just two wires, a switch position voltage signal and a ground. The 5-volt switch position voltage passes over a fixed resistor inside the WCM and is sent to the ignition switch. Instead of a thermistor, as in the ECT sensor, the switch detent positions provide resistance paths to ground. Depending on the resistance at each switch detent, a different voltage value is monitored and interpreted by the WCM.
The WCM is on the CAN IHS (controller area network, interior high speed). Instead of the ignition switch providing switched voltage, the WCM “wakes up” most modules via a CAN message. The radio, for example, has no ignition voltage feed, only a battery one. Once the WCM broadcasts an ignition run or accessory position message, the radio can be operated. This method allows for an accessory delay function in the off position without the use of a relay.
WCM responsibilities also include receiving and processing theft transponder key data (performed by the sentry key module on earlier era vehicles). CAN IHS can become active with or without the ignition on. For example, a keyless entry fob request will cause the WCM to wake up and start communications. The CAN C network only becomes active with the ignition on.
The TIPM provides the ignition voltage to the PCM, not the ignition switch. That is a pretty important statement to catch because diagnosis of no cranks, no starts and PCM communication problems is now changed. The TIPM also provides voltage to the starter solenoid. Similar to other manufacturers’ offerings, Chrysler provides a feature known as “TIP start.”
Without having to keep the key held in the start position, the starter will crank the engine until the engine starts or for nearly 15 seconds. Many expected checks are performed before the starter is ever engaged. The PCM monitors engine rpm, the transmission range/clutch switch, electronic throttle and theft status. If all checks pass, the PCM grounds the C3 circuit, fed voltage by the TIPM. The TIPM then provides voltage directly to the starter solenoid on the T750 circuit. There is no external starter relay.
If remote start is requested, then additional switches and sensors are used to monitor hood and door position, fuel level, brake application and more. The bottom line is that successful CAN communication between the TIPM, WCM and PCM is required to power and start this vehicle.
This is a step-by-step run down of starting the Avenger with the key:
Note: Some of these steps are virtually simultaneous.
1. Key inserted into ignition lock cylinder and turned to start (then left in run), CAN networks become active.
2. The WCM communicates with the key transponder and confirms valid key code data.
3. Ignition switch provides switched battery voltage to the WCM and the TIPM on the F20 circuit.
4. The WCM monitors ignition switch position by a voltage change on the G20 MUX circuit.
5. The WCM broadcasts ignition switch position
message and “ok to start/theft” over CAN IHS to TIPM and other modules, accessories can operate.
6. The TIPM provides PCM ignition feed at up to three terminals depending on broadcasted ignition switch position.
7. TIPM provides 12 volts on starter request circuit C3.
8. The PCM grounds the control of the automatic shutdown relay (will keep on for about 2 seconds without rpm signal).
9. The PCM monitors transmission position/clutch switch, electronic throttle control and theft status over CAN C bus.
10. If the PCM starter inhibits pass, the PCM grounds circuit C3.
11. The TIPM provides voltage to the starter
solenoid on T750 circuit, starter solenoid engages, starter cranks engine.
12. Engine starts, the PCM detects start to run transfer rpm, removes ground on C3 circuit.
13. TIPM removes starter solenoid voltage on T750 circuit.
Starting the Avenger with the remote-start button:
1. Remote start fob button is pressed twice.
2. WCM receives and validates signal, CAN networks become active.
3. WCM sends remote start request out over CAN IHS, only limited accessories are allowed operation.
4. TIPM provides PCM ignition feed.
5. TIPM provides 12 volts on starter request circuit C3.
6. The PCM grounds the control of the automatic shutdown relay (will keep on for about 2 seconds without rpm signal).
7. CCN (cabin compartment node) reports on door lock, door ajar, hood ajar and fuel level status via CAN IHS.
8. PCM monitors transmission position/clutch switch, electronic throttle control, MIL status and brake switch.
9. If PCM starter inhibits pass, PCM grounds circuit C3.
10. The TIPM provides voltage to the starter solenoid on T750 circuit, starter solenoid engages, starter cranks engine.
11. Engine starts, PCM detects start to run transfer rpm, removes ground on C3 circuit.
12. TIPM removes starter solenoid voltage on T750 circuit.
13. PCM will allow engine operation for 15 minutes, will shut off engine if door, hood or brake switch status changes.
To continue into normal operation without shutting off the vehicle:
14. Operator can enter running vehicle with valid unlock fob command and insert key into ignition and turn to run.
15. Ignition switch provides switched battery voltage to the WCM and the TIPM on the F20 circuit.
16. The WCM monitors ignition switch position by a voltage change on the G20 MUX circuit.
17. The WCM sends ignition switch position message and “ok to start/theft” over CAN IHS to TIPM and other modules, vehicle and accessories can operate normally.
Next up on the mound is a 2006 Toyota Avalon. The Avalon has an available engine start/stop button also known as a power switch on the dash that replaces the keyed ignition switch function.
The power switch button contains an LED that can illuminate amber or green depending on status. A handheld “smart key fob” transmitter has to be in range to successfully operate the button. The smart key fob transmitter contains a battery, but can provide limited functions to start the vehicle even if its battery is dead by holding it up to the power switch button. To start the vehicle, the operator has to press the brake pedal and the power switch.
The Avalon relies on a long chain of wiring, relays, switches and several modules to operate the system. When the power switch button is pressed, the signal circuit voltage sent from the power source control (PSC) ECU is pulled low. Before providing a crank request, the PSC ECU directly monitors the brake switch and monitors vehicle speed sent from the ECM. Behind the scenes, the smart key ECU (sometimes referred to as the certification ECU) powers up key oscillators, which communicate with the smart key fob transmitter and confirm the identification code.
The code information is passed over a LIN bus to the immobilizer ECU. The correct code information allows the steering lock ECU to unlock the steering wheel, the PSC ECU to unlock the transmission selector and send a start request signal to the ECM. The immobilizer also sends communication to the ECM over dedicated circuits.
The PSC ECU, located behind the glove box, pays a very important role in powering the vehicle.
This module effectively provides switch positions of accessory, run and crank by utilizing relays. When the power switch is pressed once without touching the brake, the accessory mode is entered. The PSC ECU latches on an accessory relay by providing voltage to the control side of the relay. Fused battery voltage is then provided to vehicle accessories including the radio.
A second press of the start/stop switch without touching the brake will place the vehicle in the run position. The PSC ECU latches on the ignition 1, ignition 2 and start cut out relays by providing voltage to the control side of each relay. The ECM receives fused ignition voltage from the ignition relay 1. The smart key ECU and steering lock ECU receive fused ignition from ignition relay 2. All links in this chain have to connect to get this vehicle to start.
To start this vehicle, the PSC ECU sends the ECM a 12-volt start switch request voltage on a circuit known as “STSW.” The ECM monitors engine rpm and will allow the starter to engage between 2 and 25 seconds, depending on coolant temperature, or until the engine fires. The ECM will not engage the starter on an engine already running. If satisfied with conditions including a message from the immobilizer ECU, the PCM honors the start request by sending an output voltage pulse on a circuit known as “STAR.”
This circuit feeds the switch side of the starter cut out relay. The cut out relay is activated by the PSC ECU on the control side in turn connects the switched voltage through a junction connector, through the park neutral position switch and then on to the control side of the starter relay. A branch of this switched voltage provides feedback to the PCM on a circuit known as “STA.”
If the feedback provides switched voltage to the ECM, it will continue to provide voltage output on STAR. Both the feed and output on the switched side of the cut out relay are connected to and monitored by the PSC ECU. Once the starter relay is activated, fused battery voltage is sent to the starter solenoid and the engine cranks.
When the ECM is providing the output necessary to engage the starter, it also provides a signal on a circuit known as “ACCR” to the PSC ECU. When the PSC ECU receives the signal it deactivates the accessory relay to prioritize available current in starting the engine. Shutting off the vehicle is also just a push of the button. If the vehicle is moving, the operator will have to press for several seconds, otherwise just for an instant.
The power switch signals the PSC ECU, which in turn deactivates the ignition 1 and ignition 2 relays. Without ignition power the ECM shuts down spark and fuel to the engine.
2006 Avalon System Operation
I. If the driver depresses the button once and releases it without the brake pedal depressed:
A. The power switch will send a signal to the power source control (PSC) ECU.
B. The PSC ECU will then turn on the IG2 relay and the start cut relay.
C. The IG2 relay will then turn on the smart key ECU and the steering lock ECU
D. The smart key ECU powers the oscillators to contact the key fob (smart key), if it is in range.
E. The key fob senses the request signal and sends a code signal back containing the security code.
F. The key antenna receives the security code and sends it to the door control receiver.
G. The smart key ECU verifies the code and sends a signal, through the LIN bus, to inform the immobilizer code ECU, power source ECU and the steering lock ECU that the correct key is in range.
H. The steering lock ECU will unlock the steering wheel and the PSC ECU will turn on the accessories.
II. If the driver depresses the button with the brake pedal depressed:
A. The power source ECU will detect the brake switch input and request the engine control module to power the starter relay.
B. If the transmission is in park or neutral, voltage from the engine control module can power the starter relay through the start cut relay and the P/N safety switch, and thus powering the S-terminal on the starter.
C. Simultaneously, the immobilizer code ECU will inform the engine control module that the driver has the correct key, allowing the engine to run.
Well, hopefully this glimpse of new ignition switches and their strategies will better prepare you for any curve ball they may throw you. Both examples used quite a few middlemen between the switch and the starter. The Dodge relied on CAN bus networks between a few modules and the Toyota utilized several ECUs and relays.
In both examples, any break in the chain can lead to undesirable operation including a no crank. Stay tuned for possible future articles where we examine some diagnostic strategy on these vehicles.