A little look into the Plenum chamber / ECU's

[IT]

Many of us know whats where under the bonnet, but everyone has to start somewhere. I wanted a look at my ABS / ESP ECU today, so thought I'd document the journey.

Heres a typical D2 engine bay - This is an S8 - and the plenum chamber for right hand drive cars is always on the passenger side at the back.




4 fairly obvious crosshead screws (if they're all still there!) will allow you to remove the trim just to the front of the plenum chamber and get a better view. Then it looks like this. Then remove all the rest of the screws circled in red.



The small device clinging on arrowed in green is the 'air quality sensor'. It doesnt normally live here. It should be where the blue arrow is, but the LPG conversion brought wires through so they decided to move it. The air quality sensor sniffs out ecessive particles in the incoming fresh air, and switches to recirculated air (if set to do so) automatically. Auto recirc ia a button on the Climate module with an A with an arrow round it, and was standard from 1997 on all 8's.

Below, the fresh air intake is arrowed in red - This is where air goes into the cabin. it is sucked through the bonnet, and ultimately through the pollen filters (arrowed orange) and past the far left and right sides of the engine bay behind the second bulkhead.



So, once the screws are out - The top lifts up at the top, and slides out forwards. Note this is designed as a watertight container. Water will run from the windscreen over this box. It has rubber seals and must never be left uncovered or poorly sealed. Some of the most important electronics for the car live in there..... You dont want moisture.

You can easily see the Engine ECU on the right, and the Gearbox TCU on the left. Both easily accessible.




There are 4 x 10mm bolts holding the gearbox TCU in. The ABS/ESP (combined) ECU lives underneath this, so I need to remove it to get it.

Remove the 4 bolts, and the Gearbox TCU lifts out, complete with wiring still attached:

Next, we remove the 2 crosshead screws arrowed in orange.

Then, the TCU bracket lifts out, complete with a bit more wiring, and sits on the right.




Finally, we see the bottom of the ABS/ESP ECU. Now, remove the 4 crosshead screws that are clearly visible and it lifts out.....



The plenum chamber is double entry - It has a large square opening in the cabin too, secured by 4 x 10mm screws, which is easy enough to access if you remove the glovebox. This is the best route for running any cabling into the engine bay in my opinion....