Question: 96 Coupe AirBag Light / DERM Issue Help Needed

[blainelocklair]

Hello everyone,

I just bought a 96 C4 last week, 53k on the clock. Nice overall. White with tan interior.

It has the airbag light on in the DIC. I ran the diagnostics, and it shows "ERR" in the A section of the diagnostics (see attached). I tried resetting from A7, but no joy.

The car was in a front-end accident back in 2005 and was repaired. The light may or may not be related, hard to say after seven years.

I believe that the DERM is bad. My questions are:

* Do you guys agree with my diagnosis?
* Can I buy any DERM that is make/model year compatible?
* Do they have to be programmed by a dealership, or are they plug-n-play ready as-is?

Thanks for all of your help. This is my 2nd Vette, I haven't owned one in two years. I've surely missed being a part of the Vette community!

Blaine

 

[Hib Halverson]

I'm hesitant to agree with your diagnosis. Do you have the '96 Factory Service manual? If so you need to run the diagnostic check in Section 9 which covers the SIR.

The fact that when you use CCM diagnostics to look at SIR faults and all you see is "ERR" and not fault codes has me thinking you may have a problem with the CCM instead of (or perhaps as well as) the SIR. Typically, when "ERR" is displayed, that means the CCM cannot communicate with other modules.

It could be that the lack of communication is a CCM problem and that SIR light is one because the DERM cannot "talk" to the CCM.

I would either get a TECH2 which reads SIR faults directly then follow the diagnostic instructions in the FSM or take the car to a service facility which has a scan tester which supports SIR on a 96.

Good luck

 

[blainelocklair]

Follow up:

I checked the fuse block in the dashboard. There is no 5a fuse in position nine labeled "crank air bag". There is also no 15a fuse in position 34 labeled "air bag". Obviously, I'm on to something here.

My question is: should both places have a fuse in them? Sometimes cars have places for fuses but they don't actually have a fuse in them.

Thanks!

 

[Hib Halverson]

Ah...try putting fuses in those two positions and see what happens.

The car did not come with those spots empty. Someone removed those fuses for a reason, perhaps because the SIR system has a problem. That your seeing ERR on the IP display may be because, with the fuses out, the DERM cannot communicate with the CCM. If you reinstall the fuses and the SIR light is still on, at least with the DERM having power, you have a better chance of diagnosing it.

Do you have the Factory Service Manual?

Also, the SIR is an important safety system. If you bought this car from a dealer, the first thing I'd do is return it and ask that the SIR system be repaired and any other problems with the car's electronics be fixed.

If you bought the car from a private party, some states require owners to disclose problems and you could go back to the seller and ask them why they did not tell you the SIR was disabled. If the repairs are expensive, which they could be, you might put some pressure on the previous owner to rebate some money to you.

 

[blainelocklair]

Progress!

I installed the two fuses, and here's what I found.

When I turned the ignition on, and when I started the car, the airbag light flashed six times each. It would then pause for about three seconds, and then turn back on and stay on. IIRC, it's supposed to flash seven times each time and then turn off.

I didn't get the time to check the diagnostics, so I'll do that this evening when I get home. Pesky work thing gets in the way of having fun, doesn't it?

Hib, thanks for all of your help up to this point. Looks like I'm getting close to finding the culprit. I'll be sure to buy a FSM soon, I'm sure it will be a valuable asset.

Thanks,
Blaine

 

[gmjunkie]

Progress!

I installed the two fuses, and here's what I found.

When I turned the ignition on, and when I started the car, the airbag light flashed six times each. It would then pause for about three seconds, and then turn back on and stay on. IIRC, it's supposed to flash seven times each time and then turn off.

I didn't get the time to check the diagnostics, so I'll do that this evening when I get home. Pesky work thing gets in the way of having fun, doesn't it?

I'll be rolling close to Summerville SC Later today heading to Fla on my annual "Fix my Buddy's Cars and Bud Shootout" trip!~!
:L

If you like I could maybe swing a little off course and see if snappy can pick it's brain,I'll have all of my Diagnostic tools with me in my service vehicle!

 

[blainelocklair]

I'll be rolling close to Summerville SC Later today heading to Fla on my annual "Fix my Buddy's Cars and Bud Shootout" trip!~!
:L

If you like I could maybe swing a little off course and see if snappy can pick it's brain,I'll have all of my Diagnostic tools with me in my service vehicle!

That would be great! Thanks for your offer of help. I don't have the diagnostic resources to solve this issue unless the on-board stuff can solve it. Having the real-deal diagnostics would definitely guide me in the right direction and save money.

I'm about a mile or so off of I-26 at exit 199B. If that jives with you, I can send you my number and addy by PM or Email.

Thanks!

 

[Hib Halverson]

Ok...six flashes then it stays on. Clearly, the previous owner, confronted with the need to properly diagnose the SIR, just pulled the freakin' fuses.

What the SIR light should do is one of two sequences...

If you just key-up, but don't start the engine, it should flash seven times then go off. It it stays on there's a problem. It may also flash four times and if it does, there's a problem.

If you key-up then start, it should stay on during cranking but, once the engine is running, it should flash six times then go off. If it stays on there's a problem.

To properly diagnose that system you need the FSM and a scan tester which supports SIR. There are (sigh) six pages of diagnostic charts in addition to the charts for specific SIR DTCs.

I'd start by taking the "gmjunkie" up on his offer.

I'd be interested to read what the junkster finds in your DERM memory.

 

[blainelocklair]

I'd start by taking the "gmjunkie" up on his offer.

I'd be interested to read what the junkster finds in your DERM memory.

Thanks for the updates. I'll definitely take him up on his offer if he can make it to me.

 

[gmjunkie]

I'm about a mile or so off of I-26 at exit 199B. If that jives with you, I can send you my number and addy by PM or Email.

Thanks!

I'll be leveing here about 3:00,I take NC/SC 9 to I-26 and then east to I-95 south,That takes me about 4 hours!

PM me your # and I'll PM mine!

 

[Hib Halverson]

CAC sponsor, Zip Products, has FSMs. You need their PN B-541. It's a little pricey at 169 bucks, but if you're going to seriously get into DIY service of that car, you need it.

Aftermarket service manuals sold to consumers (Haynes, Chilton, etc) probably won't cover SIR and if they do, the information will not be complete and may contain inaccuracies.

 

[gmjunkie]

It's going to be closer to 4:30 before I get out'ta here!~!!

 

[blainelocklair]

It's going to be closer to 4:30 before I get out'ta here!~!!

No problem, I'll be at the house. Thanks again for your help!

Blaine

 

[blainelocklair]

No problem, I'll be at the house. Thanks again for your help!

Blaine

Progress!

First, a big "THANK YOU!" to gmjunkie for stopping by. He's a good guy, and I am very grateful for his time and assistance. Now I can say I've met someone on CAC in person!

Here's what we found:

The error code from the SIR is 42 - Energy Reserve Voltage Too Low. It is a historical and current code, and reappeared immediately when cleared.

His recommendations to clear the code were to first remove the negative battery cable before doing any work, and then:

* check the yellow plug for the airbag under the steering column, and to unplug and replug it several times to improve the connection.
* remove, clean, and reinstall the frame connecting points and airbag sensors on the front frame under the hood. One on each side, sand them down a bit then coat with dielectric silicon grease and reinstall.
* clean and treat all ground connecting points. I have the list of all 16 grounds for the 96 C4 so that I can eventually get them all.

He said that in his experience, more often than not faulty connectors and grounds are the causes of problems instead of circuit board failure. It sure is a helluva lot cheaper to clean connectors than to replace boards, so I'm happy about that for sure.

The good news was that there were no misfires and no historical or current codes on the CCM. And I have to say, that Snap On diagnostic scanner is really nice! Looks like it would make a fine eBay purchase addition to the home mechanic tool kit for any modern-era car (with the correct carts and adaptors).

So, looks like I've got some cleanin' to do this weekend. Assuming all goes to plan, I'll be able to get at least some of this done, and I'll keep this thread up-to-date as I make progress.

Thanks again!

Blaine

 

[Hib Halverson]

Code 42 sets when the DERM determines there is a problem, either low voltage or unacceptably long charge time, with the 36v, reserve power supplies which "fire" the bags during a crash, if power from the ignition feed to the arming sensor is lost.

My guess is cleaning grounds and sensor mounts is not going to solve that code, but it might.

If you clean the grounds and sensor mounts, clear codes and 42 resets, and you decide to continue to work with the problem yourself, you MUST have the FSM, a DVM and either a TECH1 or TECH2 scan tester. Also you MUST follow the service manual instructions for disabling and servicing the SIR system. Incorrect service procedures or improper diagnostic equipment can cause unintended air bag deployment and that may injure or kill you.

Note that the end results from the various diagnostic info in the FSM relating to DTC42 are either repair of shorts in various SIR circuits, replacement of the arming sensor or replacement of the DERM. I say this only to make the point that while code 42 is possibly set by a grounding issue, it's not probable.

Lastly, rather than disconnecting the battery, why not use the service manual procedure for "safeing" the system which is, on a '96, to simply turn the wheels straight ahead, remove the ign key, then remove the air bag fuse and disconnect the two yellow SIR connectors which are inside the trim panel beneath the IP. You're going to clean those two, anyway, but LEAVE them disconnected while you clean other SIR grounds and sensor mounts. Once you've done that, reconnect the two yellow connectors, replace the trim, replace the air bag fuse then key-up and see if the SIR light stays on, or not.

Good luck.

 

[blainelocklair]

Ok, after battling pneumonia for two weeks, I'm finally tacking this issue again.

I pulled the driver's side airbag sensor, and it looked like this:



Here's the frame where it was bolted down:



Cleaned up:



The cleaned frame, along with the two grounds removed and cleaned:



Here's the passenger side sensor. Much cleaner than the driver's side.



The frame (could only get one hole photographed):



Cleaned sensor:



Cleaned frame:



I coated all ground and frame areas with GM dielectric silicon grease prior to reinstallation. Powered everything back up, and...

... the light is still on.

The more I've read into the FSM, and the more I learn about the DERM, I'm inclined again to suspect the DERM here. It seems that if there's an energy reserve voltage low issue, and the DERM stores and manages this energy, the DERM is likely at fault. I'm guessing the DERM keeps the reserve current in capacitors, and if they're on the DERM, that's gotta be it. There is the possibility that the dual pole arming sensor is bad as well, but I"m leaning toward the DERM.

Any input is always appreciated. I can get both parts for around $50 each used, so it's not a bank breaker. I just hate throwing parts at something without a plan to get it working first. Wastes time and money.

By the way, that passenger side sensor was a bitch to get out. I managed to do it without taking out the heat/AC blower, but not by much.

Thanks,
Blaine

 

[toptechx6]

No personal experience but have been told this company repairs DERM's, just another option in case you do not want to buy used, good luck.

Airbag Computer

 

[blainelocklair]

No personal experience but have been told this company repairs DERM's, just another option in case you do not want to buy used, good luck.

Airbag Computer

Thanks man. I've got no issue with buying used, as long as it works. Hell, the whole car is used and 16 years old. Guess it can't be too bad to put in something used.

Before throwing money at the problem, I'll be sure to check the DERM to make sure the dual pole arming sensor and all required connections are in place.

All advice is welcome!

Thanks,
Blaine

 

[blainelocklair]

* check the yellow plug for the airbag under the steering column, and to unplug and replug it several times to improve the connection.

Tried this today with both plugs, light is still on. I still think it's the DERM or the dual pole arming sensor.

All advice welcomed!

 

[blainelocklair]

I ordered a used dual pole arming sensor a couple of days ago from Corvette Recycling. I should have it in the middle of this upcoming week. I'll get it swapped out and will report back. They had a good price and shipped same day, by the way. Seem like good folks.