Transponder key and immobilizer theory

Post by **Clemens** » 24 Aug 2018, 14:17

Because Misha pointed out the the EU 850ies got transponder keys in 96 this leaves me to deal with a subject i never wanted to wlaste my time with:

I have a 2 tone gray and black key where the transponder is probably hidden inside and a remote control which did lock and sometimes unlock the doors. When it didn't unlock the blinkers would go on and I couldn't start. After this happening to me at night once I jumpered the relais as described in this forum and lived happily ever after till now. I think I could modify the immobilizer/remote relais to still have the remote but not the immobilizer anymore.

But now it seems I also have the transponder key and ring. Now, is there a way to get rid of this feature because our cars aren't getting younger and i'd like to be able to delete it if it ever starts making problems. I think on the 96 and 97 cars the transponder number is fixed and the ring and transponder have to match the number in some control module. Now, where is that data hidden? In the remote/immobilizer relais. Or in the ECU?

Reason for asking is also because I plan on getting a VA5T M4.4 ECU and I'd hate to pay a few hundred bucks and then finding out I can't use another ECU because the transponder doesn't work.

Any insight on how the immobilizer, remote and transponder key work would be highly appreciated.

Thanks

Post by **jimmy57** » 24 Aug 2018, 15:37

If the replacement ECM does not have the Immo feature enabled then you are good. The Immo is a communication that acknowledges the code of the Immo control unit inside car is the approved one and the Immo then communicates that the key is an approved key (Read by the antenna send and receive activity when you first switch ignnition to position II). If the ECM does not have the security feature enabled then it does not look for external approvals to operate the engine.

Post by **Clemens** » 24 Aug 2018, 16:25

ECM meaning ECU?

Post by **abscate** » 24 Aug 2018, 18:40

Yes, the are the same

ECM is the correct name per Volvo.

Post by **Clemens** » 25 Aug 2018, 01:06

Thanks. So if the ECM looks for no signal the transponder ring signal is set but noone cares?

What about relay 210/211. Is it only necessary for the remote? There should not be any connection between keyless entry (and the immobilizer function in relay 210/211) and the transponder immobilizer i guess. In theory both have to be be good to allow start, but they don't interfere. Is that correct?

Post by **misha** » 25 Aug 2018, 02:56

Chip in the key in for cars from '96-'99 communicates ONLY with immo control module under the dash via antenna ring around ignition lock.
'96-'99 car ecu just need a "green light" from immobiliser control module.
'99 and later have coded ecu along with immo box and it's totally different protection system.

Jimmy 57 covered everything.

Post by **Clemens** » 25 Aug 2018, 03:14

So the immo unit could be removed and if necessary a + 12V signal set to to fake the OK code from the immobilizer bpx under the dash? Sorry, I'm really slow on this

Post by **misha** » 25 Aug 2018, 03:55

I don't know details but skilled tuner/flasher will know how to fool it...but...either way...Immo unit MUST be in place and connected!
"Green light" is encrypted...so i don't know details.

850forum · Post by **850forum** » 17 Sep 2021, 03:24

"When it didn't unlock the blinkers would go on and I couldn't start."

IIRC you have to then use a key in a door lock, right?
Been a long time since I did something like reach in the open window to open a locked door & set it off...

"After this happening to me at night once I jumpered the relais as described in this forum"

Do you recall the name of that post?