Reading and Writing Immobilizer EEPROMs in Circuit

Sometimes when working with immobilizer EEPROMs, the ideal way of programming the EEPROM is by doing it while it is in the circuit, as removing it, programming, and resoldering it back to the circuit, add a bit more of effort to complete the job.

Doing it “in-circuit” can be accomplished in many ECMs (not all) by using the SOIC-8 clip, connecting it over the EEPROM chip while it is in the circuit, but sometimes, some inconveniences are present. Some of those inconveniences are; the need of cleaning the EEPROM leads from any protecting coating the ECM might have, like silicon or soldering flux, or the fact that the EEPROM in question might have a too low profile, making it impossible to have enough grip on the chip body to keep the clip in place. Also, the situation that we will cover here; the ECM circuit is sensitive to voltages that the EEPROM programmer injects to it while reading or writing the EEPROM chip. In this latter case, the problem is that the microcontroller unit (MCU) of the ECM, receives enough current from the EPROM programmer that it actually attempts to startup(run). This situation will vary from ECM to ECM and from programmer to programmer, but if it tries to start up, the data being sent from the programmer to the EEPROM gets corrupted. To be on the safe side, we have a simple solution here.

The solution for this situation is to disable the crystal oscillator of the MCU in the ECM circuit to prevent it from running. That is accomplished by simply jumping the crystal by running either a 0.1uf capacitor or a small jumper wire across it (in parallel with the crystal oscillator). In the picture below, it is shown how a 0.1uf capacitor was temporarily soldered across the crystal terminals to disable it.


In the event that you don’t have a SOIC-8 SMD clip to read and write the immobilizer EEPROM or you simply don’t like that method for being somewhat unreliable, below is pictured another approach for doing it. For this, you will need to construct the harness or buy it already assembled, as it is already available to be ordered online. It is made of 8 micro clips and an 8 connections dual pin header (4 + 4) for connecting it to the programmer. Once constructed, you will be able to use it over and over. It will have even more of a lifespan than the SOIC clip itself. If you decide to construct it yourself, be as neat as possible and use high-quality materials so you end up making a durable tool and don’t just make a prototype like the one shown here :-)




There are online stores that sell some of the equipment used on this job. For example, you may get the SOIC-8 SMD clip complete with its harness and ready to use from stores over the Internet. You will find examples below.

On a final note, please be always careful when working with static sensitive electronics. Always use an ESD (Electro Static Discharge) protection to avoid unrepairable damage to the circuits and/or devices.


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18 Responses

  1. Calvin says:

    When i open my car speedometer i didnt see 93c56 chip, i can only see a chip with c56 d910 writing on it
    If you interested in this car chip i can send you the picture
    Please tell me how do i read and write data to this chip

  2. Calvin says:

    I cant.clip using the pomona clip
    Because the chip too small
    8pin chip but smaller.than the pomona.clip
    Please help how to program this chip

  3. zilog357 says:

    Hello Calvin. When that happens, the solutions are two; take out the chip (desolder) and grab it with the clip, or use 8 wires with micro-pincers at one end and a DIL-9 terminal at the other. The solution is here in the blog, but looks like images are not showing for some reason after an update of the software. We are working to have the website back to normal, so you may visit us at a later time to see if the contents is restored.

  4. J haro says:

    Hello I have one question what is safer reding and writing the EEPROM un circuit or resoldering the chip thanks.

  5. zilog357 says:

    Hello Josue. Obviously it is safer to desolder the EEPROM, but the in-circuit approach is tried first as a way of saving time and sometimes, saving the circuit if you don’t have the needed tools to work with surface-mount devices. Depending on the ECM circuit -AND- on the device programmer, you will be able to do it in circuit in some models, but there will be others that no matter what you try, the EEPROM will need to be out of circuit to be read correctly.

  6. Xonx Xenox says:

    I have a 2002 Toyota Sienna XLE of which I damaged the chip in the key.

    If I erase all the keys stored in IC900 in the ECU (89661-08070), will the first blank key inserted into the ignition be programmed and paired with the ECU?

  7. zilog357 says:

    Hello. You will need a virgin file for that model. The virgin file does exactly that, it erases all the key codes in that chip, but also leaves a pattern for the ECM to detect it as “no keys and ready to accept new ones”.

    Erasing all the keys alone will not do the trick as there are other variables in that chip, like a key counter.

    On the other hand, if the chip is not full, a maximum of 8 keys or so are allowed depending on the model, there is a process for registering new keys. It is there for the Rav4 and Corolla and I am positive it is there for the Sienna, but I will need to look it up to check the process. In the mentioned models it is about a sequence of depressing the accelerator and the brake pedals in a certain pattern and timing.

  8. Xonx Xenox says:

    I have a FMI question, when keys are “Programmed” is information actually stored or changed on the rfid chip in the key? Or is it more of a pairing?

  9. Xonx Xenox says:

    Is the immobilizer eeprom (IC900) in the in the ECU (89661-08070) only for key information?

    Aside from the ignition systems needed to “immobilize” does it effect other systems?

  10. zilog357 says:

    At least, in these Toyota models and all makes that use the same RFID chip type, the information is stored in the ECM or in the Immobox if present. The key is only read.

  11. zilog357 says:

    I would have to check because on some models, it is only for storing the key codes and keeping a key count, and other things for managing the immobilizer. But in some other models, if it is an automatic vehicle, it does the same in the upper half of the memory and in the lower half, it stores the automatic transmission codes for the shift points.

  12. Xonx Xenox says:

    You are my Main Man Zilog, I am going to give you “My First Born Male Child” for your help.

    There is only one caveat, “SHE” is a 38 year old and gets out of a supermax prison for the criminally insane.

    A top rated MM artist, ex special force captain, top rated sniper and hand gun tactical course.

    Her parole officer will deliver her to your door in a straight jacket, chained to a hand truck, wearing a steel face grill, he will be accompanied by four guards, two with taser prods and two with fire arms.
    Hope you know how to make lemonade.

  13. Xonx Xenox says:

    It’s me again Zilog,
    I am hearing two different things. I suspect that both are true but are dependent on the year of the vehicle.

    One say that I must get a Virgin File from somewhere and upload it to the eeprom and another say that I must reset the reset the eeprom to a virgin state by zeroie the values of certain data in the eeprom.

    I have a 2002 Toyota Sienna XLE
    ECU: 89661-08070
    IC 900: 93C56

    Which is the correct method for my ECU?

  14. Xonx Xenox says:

    Sorry, I asked the same question twice.

    I gather that virgin files are bin files of a few lines of code?

    As with everything in the western world, someone will sell you anything if they can get away with it, can I find safe virgin files to download free of charge?

  15. zilog357 says:

    Both, getting a virgin file or resetting the eeprom to a virgin state will do the same. Any of those two procedures will leave the same binary file in the eeprom.

  16. zilog357 says:

    Yes, in these models the virgin files are from 256 bytes (93C56 eeprom) or 512 bytes (93C66, 25040 eeproms). Yes, you may get it free of charge in some forums where members share things. You just have to ask them in a forum thread.

    I can not promise anything, but if you can read the eeprom of your ECM and email to [email protected], I can check by comparison if I have the proper virgin file for that model. If I have it, I will email it to you at no cost. You would need to write it in the eeprom (IC900) with an eeprom programmer, using some sort of interface to connect the small device to the eeprom programmer. The most commonly used is the SOIC-8 eeprom clip.

    About the forums if you wish to look further information, I can recommend you two of them:


  17. Xonx Xenox says:

    Zilog, I am getting a little bit exasperated. I got a CH341A mini programmer and am having a time trying to get it to work on a Windows 10 laptop.

    It seemed to be for XP. Also GitHub say that the software for the CH341A is dangerous.

    I need to find programm software that works well with Windows 10 and/or get a better eeprom programmer.

    Do you have any ideas?

  18. zilog357 says:

    I don’t use Win 10, I’m still in Win 7 because of that, the possibility of any of the many software I am using may not work on Win 10.
    Does Win 10 have the option to run a program in compatibility mode, like emulating Win XP or Win 7? If yes, have you tried? Also running it as administrator (admin rights).

    About the programmer, I only experience with EETools, specifically the ChipMax model. This is the one I’ve been using since 2002. It is so good that it had never broken and accepts all the devices I work with and does online updates of the devices it accepts. The bad thing is that it is a little bit pricey.

    The only thing I would try without changing the programmer you have is booting your computer in Windows XP and trying it. For that, I use a bootable disc intended for troubleshooting computers that do not boot. It uses a “mini WIndows XP” version. The disk is called the Hiren CD. It is free to download, but I no longer have the resource. You would need to look in google where to download it and burn it in a CD or DVD with Ultra ISO or Power ISO, or similar and then boot your PC with the burned disk. The CD will use RAM of your PC as a virtual drive, but the hard disk or any other mass storage device attached to the PC will be listed too. Just make sure you left your physical drive alone and use a pen drive for trying the programmer’s software.


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