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Airbag (SRS) - Capacitor Replacement

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Introduction
The Supplementary Restrain System (SRS) ECU suffers from ageing capacitors the same way as many other ECUs in the NSX. Two things makes it a special issue: The SRS is a safety device and a permanently lit warning light means failing the regular vehicle checks in most countries.
Especially the first generation ECUs (cars with only a driver's Airbag) tend to fail by shorting an important MOS-FET inside the housing which complicates repairs.

Diagnostic
Due to the limited diagnostics of the SRS ECU (only the warning light indicates an error and no codes are available), a failure is typically observed by the warning light in the cluster not turning off after six seconds. The next step is to check the corresponding fuse. If it's blown and keeps blowing the most common reason is capacitor failure or water damage due to a blocked AC evaporator drain.

Preparation
Removal of the SRS ECU from the vehicle requires special caution. Even though the system should only fire if the sensor in the ECU and one of the crash sensors behind the dashboard experience a crash like acceleration it's better to be safe than sorry. Disconnecting the battery and waiting at least 30 minutes should ensure the backup-power in the ECU is drained and the system can't fire any more.
The ECU is located under the centre console and has to be removed towards the right or left hand side of the vehicle - depending on whether it's an LHD or RHD. In case of RHD, the unit is removed towards to the left.
The mounting screws are held in place with strong thread locking compound. Unscrewing them can strip the nut from the inside of the bracket and cause additional trouble.
To avoid this, a small torch (e.g. from a gas powered soldering iron) can be used to heat up the screw to ~100 C which softens the compound sufficiently and allows the screw to be turned with moderate force. Removal of the yellow connectors is not easy but not an impossible task, either.

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located behind the metal bar
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opposite side view
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successful removal
Please ensure the ECU is handled with care - the sensors inside are mechanical devices that can be permanently damaged if they experience too much force (e.g. due to dropping the ECU on a garage floor).

To open the housing remove the corresponding screws. They are secured with regular strength thread locker and can be opened without special treatment. Early build year ECUs can be identified by their single PCB design and the single large backup capacitor. Later years feature a double PCB design and multiple backup capacitors.

Repair
Replacements of the large orange backup capacitors are very hard to source but no leakage has been reported yet. Capacitor replacement is therefore focussing on the smaller size capacitors. For the inspection and repair of PCBs contaminated with capacitor liquid, please refer to the corresponding section in the Climate Control Unit entry.

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1997 double PCB design
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bottom PCB
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replacements installed
Note that the SRS PCB contains large copper areas and therefore requires a powerful and regulated soldering/desoldering iron for removal and installation of the capacitors - replacement should only be attempted with proper equipment.

Name Replacement Specification - Build Year 1997
C6 EPCOS B41888C7108M000, 35 V, 1000 F, 10000 h @ 105C, ↥ 30 mm, ⌀ 12 mm, RM 5 mm
C9, C4 Nichicon PLX1V101MDL1, 35 V, 100 F, 3000 h @ 125C, ↥ 12 mm, ⌀ 10 mm, RM 5 mm
C12 Nichicon PLX1H220MCL1, 25 V, 22 F, 3000 h @ 125C, ↥ 12 mm, ⌀ 5 mm, RM 5 mm

Dry, Aluminium Polymer Capacitors are suitable for this purpose but hard to source in case of 1000 F. Above selection contains a regular but long living replacement type. It does not feature the third pin used to fasten the capacitor to the PCB and therefore needs to be securely bonded to the PCB, e.g. by means of Epoxy glue.

Note that in case of damage to the MOS-FET on the PCB, a PCB clean-up and repair, followed by replacement of the capacitors is not sufficient. Exact replacements seem to be hard to source but exchange types might be suitable as well as the transistor is probably used for switching purpose only.

Ensure that the battery is reconnected after the ECU is safely mounted in the vehicle to avoid unexpected deployment of airbags/seat belt tensioners and potential personal injury.


Important Notice

The SRS ECU is a safety device. Repair, as well as preventive capacitor replacement, is your own responsibility.

A replacement part should be the preferred way of repair. 1993 and later ECU can be used in cars without passenger airbag or set belt tensioners if the corresponding wires to the non-existing actuators are bridged with resistors - therefore please consider all available options before deciding.

Updated 21-02-2021 at 03:49 PM by Heineken

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