SRS Unit Capacitor Replacement

crxguy52

crxguy52

Contributing Member
Joined
17 November 2007
Messages
125
Location
Raleigh, NC
My SRS unit recently started blowing its fuse as soon as the ignition was turned on, so I did some research and found that either the unit was full of water or (more likely) the capacitors were leaking electrolyte like the rest of the electronics in the car :rolleyes:. [MENTION=25737]Kaz-kzukNA1[/MENTION] has some very useful information here on the capacitor replacement of the SRS unit. I pulled my SRS unit using this very helpful guide, and sure enough the same 2 capacitors were leaking in mine that Kaz found in the one he was repairing. Note that I was able to remove the SRS unit without stripping the torqx screws holding it in by using heat - the screws have red loctite on them, which is deactivated by heat. I held a small butane torch on each screw for about 45 seconds, then was able to remove the screw with minimal headache.

IMG_5507_small.jpg

Fortunately the electrolyte hadn't reached the transistor next to it, and the extent of the damage was limited what whats in the photo. Since Honda no longer sells this unit, I'm planning on replacing all the capacitors save the large orange one - I'd imagine that has some sort of safety rating that I'm not aware of. I created an image (below) with the capacitor values and ratings from my 91 NSX in case anyone else runs into the same issue. I'm providing this info as reference only, since the SRS unit is a safety device and should be replaced entirely if at all possible.

SRS_Caps.jpg

I found the following parts on Digikey that meet the criteria of the original capacitors (value, voltage, and lead spacing). I was able to find all automotive qualified parts.

Cap: C1, C9
Value: 22 µF
Voltage: 25V
Lead Spacing: 5mm
Digikey Part: P15374CT-ND

Cap: C4, C6
Value: 100 µF
Voltage: 35V
Lead Spacing: 5mm
Digikey Part: 565-3450-ND

Cap: C23, C24
Value: 330 µF
Voltage: 35V
Lead Spacing: 5mm
Digikey Part: 565-3451-ND

Cap: C15
Value: 3900 µF
Voltage: 35V
Lead Spacing: 22mm
Digikey Part: 565-2002-ND

Cart with the above parts and quantities: http://www.digikey.com/short/q8t3d1

I added a replacement for the big orange 3900 µF cap based on Old Guy's information. It won't fit the lead spacing so I intend on bending the leads and laying it down sideways, then potting it down with hot glue.

I will update this post with pictures of the replacement process and results.
 
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In the 1991 version of the service manual it states " electrical energy is supplied to the airbag by the battery, or the backup power unit if the battery voltage is too low.". This is prefaced by "the backup power unit ... located in the SRS control unit". I suspect that C15 is the backup "power unit". If it truly is a back up, you have some judgement related flexibility as to whether you need to replace it as long as failure (such as an internal short in the capacitor) does not disable the unit. However, something to consider. If it does develop a leak it is going to leak out a lot of electrolyte relative to the other capacitors on the board.
 
I didn't replace the very big one as I didn't come across a substitute. And yes, it's the power unit in the case of a crash to still supply power to the SRS system if the 12 V system is destroyed in the crash.

SRS repairs are DIY project only. I'd refuse to repair them for someone else, even with a liability weaver.
 
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goldNSX said:
The problem are the special dimensions.
Click to expand...

I managed to find an automotive qualified part. The problem is that all the caps are tall (~40mm) and have narrow lead spacing compared to what is in there now. I plan on bending the leads so they'll fit in the existing lead holes, then potting it down really well with hot glue.

I updated my original post with the part number and a cart for these parts

RYU said:
My SRS light suddenly came on after many years of being bypassed for a Momo steering wheel. I think my yellow ribbon is still good so maybe it's the SRS module itself.
Click to expand...

This is what happened to me, the light just came on one day. I replaced the fuse and it blew immediately.
 
>Mind sharing where the location of the SRS module can be found?

Mounted on the center hump, under the center console, roughly under the climate control. It hides behind the carpet, you will need a torx and maybe a bit of heat to keep from stripping the bolt heads.
 
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guys, this might be a dumb question but since i've been using an aftermarket steering wheel for years i'd like to simply remove the SRS module.

Does anyone know if the SRS light goes off and if the horn and cruise control still work with the SRS module unplugged?
 
If your car has a safety belt tensioner system (93+ in Europe) you can't remove the SRS module without trouble.
 
RYU said:
guys, this might be a dumb question but since i've been using an aftermarket steering wheel for years i'd like to simply remove the SRS module.

Does anyone know if the SRS light goes off and if the horn and cruise control still work with the SRS module unplugged?
Click to expand...

That was covered by Kaz in the link provided by Drew. The short answer on the SRS light is no. The SRS failure detection circuit, in part, resides in the dash cluster circuit. The dash cluster circuit is looking for a specific status condition on the connection to the SRS module and removal of the SRS results in failure to satisfy that condition and illumination of the SRS failure light. You could probably develop a work around if you have access to an SRS control module and are prepared to dissect the operation of the monitoring circuit.

I haven't explicitely checked; but, I do not recall any interconnections between the SRS module and the horn or cruise control and would not expect that removal of the SRS control module would alter their function.
 
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Just a heads up for those of you considering capacitor replacement. Electrolytic capacitors and halogenated compounds are a bad mix leading to failure of the capacitors. If you are using a flux cleaner on the board after removing the capacitors and are repairing the conformal coating after the solder repair work, make sure to get non halogenated products. Halogenated 'stuff' is getting hard to find, so it should not be that difficult. There are a number of water based cleaners available; but, an application guide that I have for Panasonic capacitors indicates that isopropyl alcohol is OK.

The can of conformal coating that I have in the basement is acrylic based; but, uses n Heptane and Acetone in the carriers. The application guide frowns on the use of petroleum based hydrocarbons for cleaning and in the carriers for the conformal coatings because of the potential to damage the rubber seal in the base of the capacitor. Technically, acetone is a ketone so not on the no-fly list; however, n heptane is definitely a hydrocarbon so not necessarily welcome. However, it is far down on the ingredients list so the amount may not be an issue.

Just something to keep in mind when doing the repair.
 
More desoldering!
What is the process to remove this board/device?
will disconnection of the battery be sufficient? Or do I have to jumper the airbag as well?

Mine blew the fuse only once. right after A/C recharge. and the Air control on recirculate for a day or more. I suspect condensation from the A/C above the unit. Never has blown since, but I feel like I need to replace at least the most common failing capacitors.

thanks again.
 
warrenw said:
More desoldering!
What is the process to remove this board/device?
will disconnection of the battery be sufficient? Or do I have to jumper the airbag as well?

Mine blew the fuse only once. right after A/C recharge. and the Air control on recirculate for a day or more. I suspect condensation from the A/C above the unit. Never has blown since, but I feel like I need to replace at least the most common failing capacitors.

thanks again.
Click to expand...

According to the service manual, the air bag is to have its shorting connector installed anytime the SRS is disconnected. It would be prudent to install the shorting connector prior to disconnection of the harness from the SRS control unit. I am guessing when you say jumper you mean the shorting connector that is referred to in the service manual and should be tucked under your steering column cover.
 
Old Guy said:
You could probably develop a work around if you have access to an SRS control module and are prepared to dissect the operation of the monitoring circuit.
Click to expand...

Or you could just pull the bulb out of the cluster :tongue:. The previous owner of my Integra did that with the check engine light -_-

Old Guy said:
Just a heads up for those of you considering capacitor replacement. Electrolytic capacitors and halogenated compounds are a bad mix leading to failure of the capacitors. If you are using a flux cleaner on the board after removing the capacitors and are repairing the conformal coating after the solder repair work, make sure to get non halogenated products. Halogenated 'stuff' is getting hard to find, so it should not be that difficult. There are a number of water based cleaners available; but, an application guide that I have for Panasonic capacitors indicates that isopropyl alcohol is OK.

The can of conformal coating that I have in the basement is acrylic based; but, uses n Heptane and Acetone in the carriers. The application guide frowns on the use of petroleum based hydrocarbons for cleaning and in the carriers for the conformal coatings because of the potential to damage the rubber seal in the base of the capacitor. Technically, acetone is a ketone so not on the no-fly list; however, n heptane is definitely a hydrocarbon so not necessarily welcome. However, it is far down on the ingredients list so the amount may not be an issue.

Just something to keep in mind when doing the repair.
Click to expand...

Thanks for the info! I was planning on using IPA, so it sounds like that should be fine.
 
Worked on the SRS unit for a few minutes today. Both capacitors were leaking pretty badly and the rubber plug on one had a significant bulge. I was able to clean up the electrolyte and fortunately the damage was fairly contained. I plan on removing the 3 resistors and cleaning any remaining electrolyte off before proceeding. Per one of our technician's recommendations, once I replace the other capacitors I'm going to put some insulating shims under the formerly leaky capacitors, solder the caps on, and conformal coat the damaged area.

IMG_5578.jpg

IMG_5577.jpg

IMG_5583.jpg
 
That is suitably ugly looking. As you do the capacitor replacements on the SRS and the Bose amps and head unit, have you by any chance been recording the vendors of the obviously failed capacitors? If so, is it a common vendor or are the failures not linked to a particular supplier?
 
Old Guy said:
That is suitably ugly looking. As you do the capacitor replacements on the SRS and the Bose amps and head unit, have you by any chance been recording the vendors of the obviously failed capacitors? If so, is it a common vendor or are the failures not linked to a particular supplier?
Click to expand...

It seems less correlated to vendor and more related to capacitance. Most of the caps that were leaking (I've found 2 in the SRS and 5 so far in the head unit) were the larger ones. There were a few smaller ones (4.7µF), but the big ones were the ones doing all the damage

I was able to remove all the caps in the unit and repair the board successfully, by reinforcing the areas that were degraded with some solder and applying conformal coating to replace the soldermask. If I end up doing more board repair in the future I'll likely invest in a fiberglass sanding pen to clean things up a little better.

IMG_5627_small.jpg

IMG_5629_small.jpg

The large orange cap was rather difficult to remove and required 3 irons and 2 people to get out. In hindsight it may not have been worth it - it looks like it's been epoxy filled on the bottom, which is the source of all the leaking caps I've seen to date (as opposed to the vents on the top opening up). Just an observation if anyone else is going to attempt this. Also the conformal coating is alcohol soluble and comes off with some elbow grease and a lot of q-tips.

IMG_5617_small.jpg
 

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