Mechanics

AIRBAG MODULE CHANGE

2001 Jeep Cherokee

Electrical problem
2001 Jeep Cherokee 6 cyl Four Wheel Drive Automatic

Hi there,

I have Jeep Cherokee 4.0l 2001 60th aniv. Model. My airbag light came on out of blue few weeks ago. I had a full service at Chrysler in the meantime and it was acknowledged that my airbag module has gone bust (needless to say, they asked  £670 for new one to install).

So, I got second hand one (working tested), same part number etc, and I am just about to call my local mechanic to do the job for me.
Question: If airbag old module has been changed to working one, will airbag light still stay on? Does main ECU need to be re-setted again after job is done?
Would it help if battery is disconected first (I am sure my local guys know that anyway, but I thought I'll ask)?
I am just concerned a little because my local mechanic does not have proper Jeep diagnostic tools.

Your advice and help is very much appreciated

Cheers
Alex
Avatar
AlexCH
March 3, 2010.



By " tested", I hope you mean it was working when removed from the donor vehicle. There is no way to test a module except by the rebuilders with special equipment.

I won't argue your mechanic's findings, but these modules have a REAL low failure rate. It would be nice to know what led them to that conclusion. When it detected a problem, it turned on the red warning light and shut the system down. The bag(s) will not pop in a crash. Once the fault is gone, the light will still turn on during the seven second self-test procedure each time the ignition switch is turned on, then go off. This has nothing to do with the engine computer, if that's what you mean by " main ecu".

How did your mechanic come to the conclusion of a module if he doesn't have the equipment? This sounds fishy. A MUCH more common problem is a broken clock spring under the steering wheel. It's a wound-up ribbon cable. As it continues to break, eventually the horn and cruise control will also not work. I'd suspect that first, but I would really want to read the diagnostic fault code first. If I'm right, the code will be " open detected in squib", or " open initiator". That's the firing circuit in the steering column, including the clock spring.

I've never run into a defective air bag module that I can recall. The only other reason to replace one is for liability reasons after air bag deployment. The " Safing" sensor could develop a tiny arced spot on the internal mechanical sensor, and Chrysler, (or any other manufacturer), worries that in the event of another crash, the sensor might make contact in that exact same spot and not trip. Some vehicles now use that internal sensor as their only sensor, but older vehicles had two additional impact sensors by the headlights. To pop the bag, either one of the front sensors must trip at the exact same time as the safing sensor.

Caradiodoc

Caradiodoc
Mar 3, 2010.
To add to my story, if you want to replace the module yourself, which will prove if either I or your mechanic is correct, you can do this yourself. You do not need to disconnect the battery, although, for liability reasons, it is specified in the service manual to do so. My coworkers an I have replaced hundreds of clock springs and never had a problem. Every electrical connector in the squib circuit is bright yellow. They will have more interlocking tabs than normal, their pins are gold-plated, and most importantly, a shorting bar touches the two terminals together anytime a connector is pulled apart.

A nine volt transistor battery is more than sufficient to pop an air bag for demonstration purposes. When sliding your butt across the seat, if you develop and feel a static electric shock, that is at least 3,000 volts. THAT'S what we worry about accidentally popping an air bag if we aren't careful. As long as the shorting bars are not tampered with, you can toss the bags around like a football and they won't pop.

The same shorting bars exist in the air bag connector, the squib connector at the base of the steering column or under the clock spring, and the connector at the module. All those safety devices are useless when the clock spring wires break. That leaves an open circuit that is subject to static electricity. The air bag assembly will become perfectly safe to handle when its connector is unplugged. That activates its shorting bar in the connector.

Basically, turn the ignition switch off, wait two minutes for the storage capacitors to discharge, (just a little extra precaution), unplug all the connectors, plug the new module in, and be sure to bolt it into the same bracket in the same orientation. Angle is critical for it to only pop above the design speed. You don't want it to pop to protect you from bumping into a shopping cart in the parking lot at 5 mph! : )

With the new (or old) module, watch the red warning light very closely when you turn on the ignition switch. It will light for the seven second self-test, then, if it continues to stay on steady, the self-test never completed. That is indeed a module problem. If the light blinks off for a fraction of a second, then comes right back on, the self-test completed and passed, then a problem was detected in the circuitry other than the module.

Also, be aware there are two different fused power circuits for the module. That's required so if the fuse blows for the module, there is a second backup power source to turn on the warning light. Many drivers are not in tune enough with the normal operation of their vehicles to watch for or interpret the light during the seven second self-test, and they wouldn't notice the light never turning on.

On minivans, one of those two fused circuits is for the rear wiper. The purpose for doing that is in case the rear wiper motor shorts rendering that safety system inoperable, you might never notice until you need it. The blown fuse will be evident by the air bag light telling the customer to get the vehicle serviced. Don't overlook a blown fuse.

Caradiodoc

Caradiodoc
Mar 3, 2010.
Hi caradiodoc, thanks for your thorough reply and advice.

It wasn't my mechanic who point it out that module is dead, it was Jeep people (from Chrysler) when I had my Jeep serviced last week.

I just want to take my car now to my local garage (not Jeep specialized) to replace airbag module with another one I bought (second hand).

I will keep you posted

Cheers
Alex

Tiny
AlexCH
Mar 3, 2010.
Hi, Thank you for using 2CarPros. Com. We appreciate your donation and look forward to helping you in the future.

Tiny
2CarPros Linsey
Mar 5, 2010.