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Strange things going on with my e24:

First the OBC decides to quit - no apparent power, nothing on it lights up; no display, no LEDs, nothing.
Next the alarm "chirps" randomly - one time it chirped and killed the engine while idling, but I was able to restart it.

Is there some anti-theft module that I am unaware of? Or are these functions handled by the ECU? The car seems to run fine otherwise, but I am concerned about getting stranded if the alarm takes over and won't allow the engine to start or run.

Any ideas from the long time (experienced) guys?

Thanks in advance.

I've not experienced alarm chirping before but the OBC is an antitheft device. I'm not sure if your 83 OBC has the security feature

Losing power to the OBC may have caused a reset but I would think it would default to operational status until you enter a security code. Wish I could help more

Ralph

Hi Ralph,
Thanks for your reply. My '83 OBC does have the ability to set and require a code before it will start, but I am not sure if the antitheft and alarm are part of it. You are correct that when the OBC failed it did default to being able to start the car. Other than replacing the OBC, I'm not sure where to go with this.

New wrinkle - car was sitting in the driveway not running and unlocked and "chirped" once all by itself. This is getting weird.

smtguy wrote: ↑Tue Sep 27, 2022 2:02 am Hi Ralph,
Thanks for your reply. My '83 OBC does have the ability to set and require a code before it will start, but I am not sure if the antitheft and alarm are part of it. You are correct that when the OBC failed it did default to being able to start the car. Other than replacing the OBC, I'm not sure where to go with this.

Not sure if it helps at all, but I'm pretty sure the sounder for the OBC system is somewhere near the base of the windscreen on the left hand side of the car (RHD car in my case). And is totally separate from my alarm/immobiliser system that was added shortly after the car was first put on the road. Can you hear where the chirp is coming from?

Hi Phil,
I can hear the siren horn (under the hood) and what sounds like a chime which sounds like it is coming from inside the car. They both are short and both seemingly go off together. Completely random and will happen with the engine running or not.
Strangest thing I have ever seen.

I've had several BMWs (including E24s) which gave a 'beep, beep, beep' sound when the ignition was turned off. Could never work out what this was, as the majority of my cars have never made this sound?

I replied to your other thread about integrations between Motronic and the OBC.

However, to answer your question in this thread:

Next the alarm "chirps" randomly - one time it chirped and killed the engine while idling, but I was able to restart it.

Is there some anti-theft module that I am unaware of? Or are these functions handled by the ECU? The car seems to run fine otherwise, but I am concerned about getting stranded if the alarm takes over and won't allow the engine to start or run.

Yes, there is both a Horn Output and an Anti-theft function (Security Code) built into the OBC.

The power flows from the ignition switch, through the OBC Immobilizer Relay and energizes the Main relay.
The output of the Main relay in-turn energizes the fuel pump relay.

The OBC relay is know to fail and will prevent the car from starting.

To bypass the OBC immobilizer, either:
A) Jumper (bridge) the two green wires (pins 3 & 4) in connector C2 at the OBC relay box
B) Jumper the Main Relay pins 30 to 87 & 87b with a Y-shaped wire.

The OBC Relay box is located under the LHS of the dash:
* RHD Cars, above the Glove Box
* LHD Cars, above the Driver's knee panel

Hi Tm,
Thank you for your reply. With my car, the engine was only shut off once (at idle) after the "beep" which I assume is what you indicated is supposed to happen due to the immobilizer function. Other times though it never prevented the engine from starting or running, but I can expect to hear the short beep at some point; and the strange thing is that just sitting in the driveway off with the key out of the ignition it will "beep" randomly at any given time. My only clue to when this started is there seems to be no power at all to the 12 button OBC, it seems completely non-functional. I have checked the fuses and they are good. Could this be the faulty immobilizer or OBC relay you mentioned? Would a bad OBC relay cause the OBC to not have any power?

I very much appreciate the help!

You're going to have to start digging into the circuit diagrams sorry mate.
Start with Power delivery first, then work your way through the other signal wires.

Have a look at the ETMs for your year model.

Sources here: viewtopic.php?p=234785#p234785

TimU wrote: ↑Mon Oct 03, 2022 11:56 pm I

The OBC relay is know to fail and will prevent the car from starting.

To bypass the OBC immobilizer, either:
A) Jumper (bridge) the two green wires (pins 3 & 4) in connector C2 at the OBC relay box
B) Jumper the Main Relay pins 30 to 87 & 87b with a Y-shaped wire.

The OBC Relay box is located under the LHS of the dash:
* RHD Cars, above the Glove Box
* LHD Cars, above the Driver's knee panel

Do this!!!! The early OBC is, as Tim says so clearly, well known for immobiliser problems - jump these wires as a matter of course. It's a five-minute job.

I knew mine was playing up when the horn-that-wasn't-the-horn went off and the car wouldn't start! (Meaning the horn on the side of the blower motor cage, now removed).

Good luck with the other problems, but do this for sure.

Cheers,

Rob

TimU wrote: ↑Mon Oct 03, 2022 11:56 pm To bypass the OBC immobilizer, either:
A) Jumper (bridge) the two green wires (pins 3 & 4) in connector C2 at the OBC relay box
B) Jumper the Main Relay pins 30 to 87 & 87b with a Y-shaped wire.

Just to add:

The OBC jumper mod can be made permanently, without affecting the other functions of the OBC.
The whole C2 plug can be left unplugged, and you will only lose the useless immobiliser and warning horn functions.

However, the main relay jumper wire should be used as a limited test / troubleshooting method only, and should be removed as soon as you can fix the root cause of whatever problem you have.
Leaving the jumper in-place will drain your battery.

Thank you Tim - I will give it a try when i get back in town!

Gentlemen,
Where can I locate C2? I don't see it on the electrical schematic and don't know what to look for under the drivers side (left) kneepad. Also where is the OBC relay?
The weirdness continues with random "beeps" and now the OBC display came on with some of the LEDs lighting for 30 seconds then went off again. This while driving.

Thanks!

The "OBC relay" is the black box highlighted below.
C2 is the white plug with 2 green wires in Pins 3 & 4.

This is for a Late Model OBC from my 1986 Right-Hand Drive car.
I think there may be different coloured wires for earlier models.
The ETM for your year model will tell you the correct colours.

Thanks Tim, but I think my car is old enough not to have any of what I am seeing in your picture. That is probably why I couldn't find C2 in any of the pages of the ETM for an '83.

Your year's ETM shows the diagram below.
I've marked the connections that bypass the OBC security lockout feature.
It appears to be Green to Green/Red in your year model.
I couldn't tell you what the OBC Relay box might look like for your year though sorry.

Ok, so replacement of the OBC module has brought some sanity. No more phantom beeps or killing the engine; however the 12 button OBC inside still doesn't work. I tried another one that was known good and it didn't work either. Since there doesn't seem to have been an OBC relay in 1983, I don't know where to go with this? I have been throught the schematic numerous times. What am I missing?