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Immobilizers and Immobilizer Bypasses

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by: Kyle Duffy

Immobilizers have been around for years and are available in a few different designs but they all have one job; to stop people from borrowing your car without your permission. These electronic devices automatically shut down essential systems in your car to prevent the engine from starting. This includes shutting down devices such as the fuel pump, starter motor or fuel injectors. Should a thief try to steal your car, he won’t get far once these essential systems shut down. For those do-it-yourself mechanics that are used to carburetors, this is usually a nightmare of a system to work with, but it does its job. Intrigued? Here is a bit of knowledge on different immobilizers and theft deterrent technology.

Types of Immobilizers

1. General Motors (GM) PASS-Key I/II and VATS: The term Passkey and VATS are interchangeable terms in this article. The Passkey I was the first passive immobilizer. It was introduced back in 1986 on a corvette. Passkey I systems have a visible resistor “pellet” that is easily recognizable in comparison to other keys. Many people call the pellet a “chip” thinking that it’s some sophisticated theft system using advanced computer technology. However, this is not the case; it’s actually a very simple system. That visible resistor is not a computer chip; it is a little block of carbon sized to have a special resistance value. The decoder built into the car recognizes this resistance value and enables the fuel systems and engine cranking. Passkey II systems are relatively the same, the only difference is in regards to some of the hardware built into the car. The whole system is controlled by a black box called the TDM (Theft Deterrent Module). This list shows the resistance values of the different key blanks:

Key Blank Number Design Resistance Minimum Resistance Maximum Resistance
#1 402 368 438
#2 523 502 564
#3 681 650 728
#4 887 850 942
#5 1,130 1085 1195
#6 1,470 1411 1549
#7 1,870 1795 1965
#8 2,370 2275 2485
#9 3,010 2890 3150
#10 3,740 3590 3910
#11 4,750 4560 4960
#12 6,040 5798 6302
#13 7,500 7200 7820
#14 9,530 9149 9931
#15 11,800 11328 12292

2. General Motors (GM) PASS-Lock I/II: Passlock systems use a resister magnet just like the passkey system; however the magnet is located inside the ignition key cylinder and utilizes a Hall Effect switch. If the Hall switch is exposed to a magnetic flux greater than the operation threshold, a transistor is switched on. As the two magnets rotate past the Hall Effect switch, the magnet strength is compared to a stored value in the computer. If the value is correct, the vehicle will start. No resistor is built into the key, so what’s the point? This system was designed to disable the fuel injectors if your vehicle is started by any means other than a key rotation. The keys are no different than any other keys and do not have built-in carbon pellets.  Passlock was introduced on GM vehicles back in 1995 and is still used in vehicles up to 2007.

3. Transponder: Many modern automobiles, especially the more expensive models, have keys with transponders built into the plastic casing of the key head. The system has an antenna ring around the ignition key cylinder that sends a signal to the transponder built into the keys. The systems looks for the key to return the signal; if it does not receive a valid signal then fuel and cranking systems are disabled. Transponder keys have no battery; they are energized by the radio signal itself.  Transponder systems have been used since 1995 but are now becoming standard equipment in the vast majority of newer cars out on the market today.

Unfortunately, immobilizers can make installing remote start systems difficult. When winter is roaring, it’s nice to have a toasty car to step into and it’s better for the car to warm up before taking off. Most of us want the luxury of starting our vehicles from a distance to let them warm up or cool down. If your vehicle has an immobilizer, you will need an immobilizer bypass to enable a remote start device. An immobilizer bypass is a part specifically designed to temporarily bypass an immobilizer long enough to start the car from a remote. Many different manufacturers have bypass systems available for specific vehicles or even universal ones but there are too many to list here. Let’s take a look at the different solutions available (note: not all solutions are available for every vehicle).

Types of Immobilizer Bypasses

Bypassing GM Passkey I/II and Passlock I/II: Most of these systems can be bypassed using resistors and relays, but occasionally manufactures have bypasses available to simplify the process. Usually these manufacturer created bypasses are connected close to the remote start connections. Some vehicles have bypasses designed for them that will not work or could be incorrect. Make sure to do your research or give us a call to ensure that you select the right one for your vehicle.

Bypassing Transponder Based Systems: Generally there are three types of bypasses available for transponder based systems. Sometimes not all three versions are available for a specific vehicle. I will provide you with a bit of information about each kind, including the pros and cons.
1. Universal Bypass: These systems bypass the immobilizer during remote start by taking the signal from your key and sending it through a ring around the key cylinder into the factory antenna. The problem with this system is that it requires you to place your key in the box and mount it under the dash, so you effectively lose a key. They tend to be easy to connect, requiring few wire connections, but they can be sensitive to cold temperatures. Because it is universal, it will work with most vehicles so long as you give up a key.

 2. Vehicle Specific RF Bypass: The RF bypass works the same as the universal bypass except there is a chip that is programmed to the car. This means you do not have to sacrifice one of your expensive keys. These systems are not available for all vehicles unfortunately and they can be sensitive to colder temperatures. On the plus side they require few wiring connections and do not use an extra key.
3. Vehicle Specific Data Bypass: These data bypass modules have been around for about 6 years now and connect directly to the wires of your vehicle. When you activate remote start, it will send a data signal directly to the car’s computer enabling it to start. This system is reliable in all temperatures and will not use an extra key to function. Unfortunately they can be rare and they are not available for all vehicles. They also require more wire connections to the car.

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Written by Kyle Duffy