Airbag ECU - Capacitor Replacement

Same Topic, next ECU please: This time it's the Airbag Control Unit. There are several threads and reports related to the SRS fuse being blown even though the wiring is OK. In most cases this seems to be caused by leaking capacitors that potentially take out a transistor and a voltage controller. A similar issue can be observed when the AC evaporator drain is blocked and/or disconnected and water leaks into the airbag ECU housing.
All those reports are from early NSX build years. Over time seat belt tensioners and then a passenger airbag was added, the inside of the airbag ECU changed each time. This write-up is from a 1997 JDM NSX.

Before working on any parts of the Airbag system, disconnect the battery and wait for at least 30 minutes. This ensures the ECU's emergency backup capacitors are fully discharged. As the system is relatively simple it could be sufficient to give it enough of an acceleration (shock, rumble, blow, etc.) and the SRS components would be activated - not a pleasant thought.

Notice: Working on the safety related systems of the NSX it's on your own risk - just to be sure having mentioned that :)

To remove the ECU, first roll back the driver's carpet. To do so the foot rest (if equipped) needs to be removed. The bolts might be tight but their base is strong so there seems to be no risk in stripping anything.
Be careful with the heater duct, it can potentially break, especially the pins holding it to the carpet.

Attachment 13890

The first two of the four screws holding the ECU in place are visible now. As Kaz and others reported, the screws are secured with very strong thread locking compound.
Simply using a lot of force is going to strip the nut on the inside of the bracket and then things get really annoying.
The best approach seems to be heat. After using a small gas torch (e.g. from gas soldering iron) to heat up the head of the screw to around 100 °C it can be turned with a moderate effort. If they should become stuck again - just re-heat.

Next stop is the passenger side. Two things should be noted: The heater duct is secured with a screw (the driver side was not) and there is no crash pipe coming down so the ECU can be removed from this side only.
It's still a fiddly task as three connectors have to be detached and somehow moved out of the way.

Attachment 13891

After all that's done the ECU is finally removed from the vehicle.
Please handle it with care as the sensor inside seems to be of a mechanical type and could be damaged if it experiences too much force (e.g. when dropping the ECU on a concrete floor).

Attachment 13892

All screws in the housing are secured with "normal strength" thread locker and can be opened without special treatment. It's advisable to loosen the ones holding down the PCB in a round-robin fashion as the conformal coating may prevent the washer from separating and thereby lifting the PCB. If such a screw is fully unfastened while the others are still tied down it can cause excessive stress to the PCB.

Attachment 13893

The top PCB is of no interest as the large emergency backup capacitors are usually trouble free and an exact replacement is quite difficult to source.
When opening the ECU it's easy to see that it's high quality work by NEC. The conformal coating is shiny as a mirror and all components are mounted beautifully and are well secured.

Attachment 13894

The following capacitors are going to be replaced:

• C6: 35 V, 1000 µF, axial, 30 mm height, 12 mm diameter
• C9, C4: 35 V, 100 µF, axial, 105 °C 12 mm height, 10 mm diameter
• C12: 25 V, 22 µF, axial, 12 mm height, 5 mm diameter

Taking care of the available space, opting for dry Aluminium Polymer capacitors where possible and ensuring a temperature rating of at least 105 °C this digikey shopping list was created.
Note that the PCBs contain large amounts of copper, a strong soldering and de-soldering iron is required to remove the old and install the new capacitors.

C6 is originally equipped with a third pin to secure it to the PCB (there is no electrically connection). Such types are very hard to source, the replacement type is therefore secured to the PCB by using Epoxy glue.
After completing the work the conformal coating is repaired by means of a small brush. All screws are secured with fresh thread locking compound and the unit is ready to be installed in the car.

Attachment 13895

Please not that the battery may only be connected after the housing is safely secured to the vehicle frame.

Installation is even fiddlier than removal. I made good experience by leaving all connectors detached, slide the housing into position from the passenger side. When moving it forward as far possible there is enough room to attach all connectors working from the driver's side and to stow the two small connectors to the holder on top of the ECU.

New thread locking compound (normal or extra strong) is used to secure the mounting screws.
After re-attaching the battery and switching the ignition on, the SRS warning light should go off after roughly six seconds - which it did in my case.