redneckvtek

Hello all.

I have a 96 HX coupe that was in a minor front end accident this year.

I had a s2000 steering wheel when it happened (for 3 weeks ) and the airbags deployed. It was in a snow-storm and I was 10 miles from home, so I removed the airbags from the dash and tried to start the car. No go, since the airbag fuse was blown. I replaced this fuse and drove the car home.

Fast forward 6 months and I have fixed all the body damage and located two new airbags (Stock civic wheel/airbag and a used passenger bag). I knew the SRS light would remain on until I replaced the module, but I did not think the old SRS module would deploy the new airbags. I drove the car for about 2 months with the SRS light on, the old module installed, and the new airbags installed and plugged in.

Reference Attachment 1, FSM table of various SRS modules.

My car, being a early 96 car, has a NEC SRS unit (77960-S04-N81)

The other day, I was able to locate a SRS module from a 1997 car (Siemens brand, 77960-S02-A81-M3, week 40/97 production date) I installed this module according to the FSM instructions. Reference attachment 4, photo of Siemens SRS Module.

Upon attempting to crank the vehicle, the starter would spin the motor but the PGM-FI would not allow the engine to start.

I attributed this to the wrong SRS module, and continued my hunt for the appropriate SRS module and re-installed the factory SRS module. The car immediately fired right up.

Later, I found the appropriate SRS module, a early 97 car with a NEC 77960-S04-N81 module. Reference Attachment 3, Replacement 97 NEC SRS Module.

A comparison of the modules is in Attachment 2.

This module exhibited the same behavior as the first when installed. The car will crank but not start.

I did some further diagnosis, and payed closer attention this time. I was able to read the SRS codes from my original SRS module, and the module did not display any codes (no blinks. Bulb Check, Off, On). Obviously, the SRS light stays illuminated when driving for some reason though.

The new module also did not display any codes when checked, but when the ignition is left in run for a long period of time the bulb remains on even after the bulb check period has expired. A SRS light reset will fix this issue.

The funny thing is that the Check Engine Light will not go off with the new SRS module installed. In addition, the OBDII connector will not connect to a code reader during this time.

Something in the old SRS module is providing information to the PGM-FI allowing the car to start and instructing the PGM-FI to communicate with the OBDII tool. The new SRS module is not allowing this for some reason?

My current thoughts are:

Clockspring is faulty?
I got unlucky and bought 2 faulty SRS modules?
I missed something? If Fuse No.23 was bad, would it cause this symptom? If Fuse No.13 was bad, would the car run?

Former User

Fuse 13 is likely blown.

redneckvtek

Thanks for the response RonJ. I will check that as soon as possible.

Does anyone have experience in the internal workings of the SRS Module?

If Fuse 13 is blown, it would mean that the bad (deployed) SRS module is internally shorting +12V from Fuse No.23 to Fuse No.13 on the SRS side in order to provide power for the PGM-FI. Somehow the activity of deploying the airbag has caused this internal short. I will test this with a DMM to verify and report my findings.

If one or the other is bad and I replace that fuse, am I at risk for deploying the new airbags with the old SRS module installed?
If I find a bad fuse, I will install the replacement (good, undeployed) module before replacing the fuse.

I also plan on checking the resistance across the clockspring to verify that it is still good.

Former User

Fuse 23 has nothing to do with the engine starting/running.

Fuse 13 supplies voltage to the main relay and fuel pump, in addition to the SRS unit. If you unplug the SRS unit, the engine should still start and run. If fuse 13 still blows with the SRS unit unplugged, then there is a short to ground in the fuse 13 wire.

Stay focused on fuse 13 rather than the SRS unit.

redneckvtek

The confusing part to me is that I haven't replaced any fuses since the day of the accident. Without doing any fuse investigation, Old SRS module works, New SRS Module doesn't work.

Car Cranks/Runs/Drives with Old SRS Module
->Disconnect Battery
->Unplug SRS Module
->Connect Battery
Car starts but will not crank
->Disconnect Battery
->Plug SRS Module
->Connect Battery
Car Cranks/Runs/Drives

I appreciate your help and will definitely post back when I figure this out. I will check Fuse 13/23 at lunch, and will start troubleshooting again once I get home.

Former User

What exactly does this^ mean?

Just to be sure, are you saying that if you unplug the old SRS unit, the engine cranks but won't start? But when you replug the old SRS unit, the engine now cranks and starts?

With the old SRS unit unplugged in which case the engine presumably cannot start, does the fuel pump prime for 2 seconds when the key is turned from OFF to ON(II)?

redneckvtek

Old SRS Module = Original in car, caused airbags to deploy during wreck in Feb 2014

New SRS Module = 77960-S04-N81 from 1997 EX bought used
New SRS Module = 77960-S02-N81-M3 from wk 40 of 97, Siemens Brand, Bought used

Checked at lunch and Fuse 13 was in fact blown. When I was repairing things in a blizzard on the side of a road after the wreck I only replaced fuse 23 apparently.

I am assuming (dangerously) at this point that (With the New SRS Module installed) the MIL light staying on after the "Bulb Check" period has elapsed is the PCM's method of indicating a malfunction.

Edit:
If Fuse 13 (7 in the attached pic 3) is not receiving Power, then the PCM will not ever receive power, since fuse 13 is the +12v lead for the trigger side of the relay that provides the PCM power from fuse 44 (PGM FI Main Fuse, No. 6 in attached pic 3)

Looking at the wiring for the MIL Light, the PCM Provides GND to illuminate the light. Most PCM's i have played with default to GND output with no power, which would explain why the MIL remains illuminated with a New SRS module installed.

Thats all I can check at work unfortunately. I will report back once I get home and can experiment more.

See attached files 1/2 for SRS Module wiring references.

See attached file 3 for PGM-FI Main Relay wiring.


What I am anticipating is that Pin 3 and 7 are shorted on the OLD SRS Module. That seems to be the only way that the car can be getting power to the fuel pump currently. The only thing that feeds power to the Fuel Pump is Fuse 13, and the only connection between the downstream side of the fuse and the PGM-FI Main Relay is the branch that leads to the SRS Module


Ron, I cant thank you enough for your assistance so far! I think you called it on your first response, very impressive.

Former User

Reasonable theory.

lostforawhile

someone sticky this, as it's likely happened to more then one person, I'm sure the OP is not the first person to go crazy trying to figure this out.

redneckvtek

I wanted to follow up on this.

Ron was absolutely correct, Fuse 13 was blown.

I removed the faulty SRS Module and ohmed out pins 3 and 7. I was only shown a open circuit. The fault must be triggered by some internal relay or other action. I disassembled the faulty SRS module and took some photos.

Looks like a surface-mount resistor is burnt up, but I am not positive.

I replaced Fuse 13 and the car would then start with no SRS module installed. I then installed the replacement SRS module of the same part number and everything functions as it should.

Thanks!