Nissan and Infinity have had transponder key immobilizer systems since 1999. The 1999 to 2004 systems are typically structured around the engine control module (ECM) and immobilizer control (Immo) units directly communicating with each other over a dedicated single communication wire. Some 2004 and forward systems are body control module (BCM) to Immo control unit-based. We will focus on the ECM-based systems in this article, and cover the BCM-based systems in a future article.
The factory troubleshooting for security system ECM DTC P1612 "Communication Chain of Immobilizer System" consists of the typical, "Check power, ground and harness integrity between the ECM and Immo unit. Then change the Immo unit and recheck. If that did not fix the problem, then change the ECM."
Throwing parts at a problem certainly is to be avoided in the independent repair world. So exactly how do we decide whether the communication problem is the ECM or the Immo unit?
Let's start with an example of how a normally functioning system communicates. The waveform that occurs when the ECM and Immo unit are communicating properly on their dedicated single communication line and the vehicle starts is shown in Figure 1. We can see in the waveform that there is an 11.5 volt bias on the communication line that gets pulled to ground when communication takes place.
What we do not know by looking at this waveform are two key pieces of information that we must ask ourselves when diagnosing any single wire communication line: Which module(s) creates a bias voltage, and which module(s) pulls a bias voltage to ground? On this system, both modules have a bias voltage output and both can pull a bias to ground.
Here is how we know — and how we can use this information to make a diagnosis. We cut the communication line to watch the communication activity from the ECM separate from the Immo unit. There typically is a connector in this line that you can pull the terminals out of if you prefer. We then attached channel one of an oscilloscope to the ECM side of the harness, and channel two to the Immo unit side of the harness shown in Figure 2. Then we inserted the key and turned it to the ON position.
In Figure 3, we see a 6-volt bias provided by the ECM, as well as the ECM pulsing this 6-volt bias to ground in an effort to communicate with the Immo unit. The Immo unit is providing a 12-volt bias to the same wire, but is not pulsing this voltage to ground in an attempt to communicate.
We know the ECM can both physically bias and communicate on this line. We do not know whether the Immo unit will speak only once it is spoken to by the ECM, so the fact that the Immo unit will not pulse its bias to ground does not tell the whole story on its own. However, using a process of elimination, we see that the ECM can do what is needed to communicate a successful vehicle start. This leaves the only questionable half of the equation as our bad part. Indeed, a new immobilizer control unit fixed our vehicle.
Note: If the immobilizer control unit, ECM or new keys are installed, you must program the new component to the others using a security system-capable scan tool. If you do not have this, a mobile automotive locksmith or mobile diagnostic professional can come to your shop and perform the programming part of the repair for you.
Jim Garrido of "Have Scanner Will Travel" is an on-site mobile diagnostics expert for hire. Jim services independent repair shops in central North Carolina. He also teaches diagnostic classes regionally for CARQUEST Technical Institute.