Audi 80 B4: Repair Mini-Warning System/Alarm (Solution)

Hi there,
Does anyone know the impedance of the miniature speaker in the mini-check system (Audi 80 B4)?
I'm measuring approximately 18 ohms on the remaining portion of the original coil (which might be distorted by contact resistances, especially with the fine wires). This would suggest a value of 16 ohms.
Currently, I've installed a replacement part with 8 ohms because it's difficult to find one with 16 ohms.
hg
Herbert.

Edit 14.9.2013:
a) Title changed, see the following post.
b) Based on the length of the speaker coil wire and the wire gauge (0.05 mm), the impedance is 16 ohms.

While the following report concerns a rather rare model, it might still be helpful to some readers precisely because replacement parts may no longer be available.

Problem description: The warning buzzer for "forgotten" appliances either doesn't activate at all, or only activates with a delay and initially at a low volume.

The intended function of the warning siren:
- three warning tones if the oil pressure switch (0.9 bar), which should close above approximately 2000 rpm, does not close (dynamic oil pressure control).
- Continuous tone when a device ("forgotten" consumer device) is left on (e.g., radio, lights), the ignition is off, the key is removed from the ignition, and the driver's door is open.

I assume that the expansion of AI and the dismantling of the mini-check system are already understood.

The circuit board (see image in the attachment) showed no abnormalities when viewed from the component side. The buzzer itself is a miniature speaker with a diameter of 29mm. There was no signal at the speaker connector (measured using a DMM with a test lead temporarily soldered in parallel to the speaker). When removing the circuit board from the enclosure, the speaker separated from the housing.
On the soldering side, the solder joints of the electrolytic capacitor C43, 10µF (in the image at the top left), appeared black and were breaking away from the circuit board.
I installed a new electrolytic capacitor with wire jumpers connected in parallel to resistor R44. Additionally, I restored the connections to the traces leading to resistor R45 (top right) and integrated circuit IC73, contact 52, using solder.
After the repair, with the device operating normally, there were 4 volts AC present at the open speaker terminals, indicating that the circuit was functioning again.
A suitable speaker with an impedance of 16 ohms was not readily available, so a type Ekulit LSF-23 M/N/T with 8 ohms was used instead (pictured on the right). Although the diameter is 23 mm instead of the original 29 mm, an adapter ring could easily be carved from the housing of the old part. The originally used spring clip that secured it no longer fit due to the smaller diameter of the speaker. Now, a suitable rubber disc is placed on top, and the speaker is fixed in place by the upper part of the mini-check system's housing.
Since an 8-ohm load would likely put too much strain on the amplifier circuit, a 5.6-ohm resistor was added in series with the speaker to get as close as possible to the original 16 ohms (visible to the left of the speaker).
Result: The speaker is working again, and the volume is good. There is 1V AC at the speaker terminals on the circuit board while it's operating, which means the speaker, with its maximum power rating of 200mW, is not being overloaded.
After complete reassembly, a full functional test of the mini-check system was performed for verification purposes.
- Continuous tone with a forgotten load.
- The oil, brake fluid, and coolant warning lights illuminate when the ignition is turned on and then turn off when the engine starts.
- When the 0.9 bar oil pressure switch is opened (with the connector disconnected), 3 beeps and a warning light will activate.
- Warning light illuminated for low brake fluid, low coolant (connector disconnected and bridged).
- Check light illuminated indicating engine overheating (connector disconnected, corresponding contacts bridged).
hg
Herbert.

Great report!