My '99 XK8, which uses the older IDS diagnostic system reports a problem with the RCM (restraint control module). There is no communication but also NO CODE or flashing lights of any kind.

The passenger seat controls do not work at all so I've operated under the assumption that that is the heart of the problem. Obviously it would be good to know if there's a real problem with the airbag system BEFORE an accident but since I don't drive that car all that much and even less so with a passenger I have not taken the time to dig deeper.

None of the passenger seat switches works so aside from the mechanical seat back release the seat doesn't move at all. Incidentally that makes removing the seat extra difficult because I can't raise it enough to easily get to the hold down bolts.

If I'm right, and the problem does lie with one of the modules under the seat what's the most likely problem I should be looking for...a broken solder joint, burned circuit, etc. ??? Since I'm getting no codes or lights all I have to go on is the failure to communicate with the RCM

 

The standard approach is to swap the passenger side module with the driver side module and see if the problem moves with the module. These modules are notoriously prone to failure. These modules are attached to the bottom/front of the seat, and are subject to some flexing with the weight of the occupant. As the circuit board is resting bare on the hard plastic case, the electric tracks on the outer edge wear out and fail. The fix involves tracing these tracks and replacing them with a wire to/from each of the end points. This involves fine but low-tech soldering. There are forum members that fix these for a fee as well.

If nothing is working, check the fuses, obviously. The seat heaters are also controlled by the same module, so you can test it out for other possible clues.

The other option is to try and locate the RCM and check the fuse and wiring to it. I assume IDS is trying to "talk" to this module by sending a data packet, but is not getting a response in the time allotted, likely indicating a failed or powered down module. Could also possibly be that IDS assumes you should have this module, but in reality your car does not have it. Is this the first time this error comes up?

Best of luck, keep us posted.

 

When you mean with the RCM the Airbag module, then the reason for not being able to communicate could be in the interface you use. As far as I know only the original VCM is able to connect, not the mongoose versions, at that is what I experience om my car.

 

That may be what I'm experiencing as I'm using SDD 130 through a Mongoose cable and while it has the IDS program as well if you're correct about the VCM being needed then I'm never going to know if there's a problem with the air bag system until it doesn't work.

I would think given the importance of the airbag systems capabilities that it's codes would be available to a simple OBDII reader.

In my case since the passenger seat module is obviously bad swapping it with the driver's seat is an exercise in futility or at the very least just asking to get aggravated. I'll need to pull it and either repair or replace it to A) get that seat to function and B) to see if it what's causing the RCM failure.

 

If you pull up the electrical diagram for the '99, Fig 19.1 has all of the detail. Assuming this RCM is in fact the airbag module, it looks like it is wired separately from the rest of the modules. So, this airbag module is not on CAN, SCP or ISO. It is on it's own on pin 3. My guess is that this module is using the slower SAE J1850 VPW typically used by GM vehicles which works on just one pin. It should normally go on pin 2, but as the car is also using the faster SAE J1850 PWM (Ford SCP), pin 2 is already taken.

In other words, it looks like the '99 is using a airbag control module that "speaks" GM and needs to be wired separately from the modules that "speak" Ford. So unless the external cable to the laptop knows this, no diagnostics is directly possible.

So, the original VCM is just a cable, an alternative to the Mongoose cable? Does the VCM work with IDS/SDD running on a laptop, too? If so, it means IDS already knows how to diagnose the airbag (can speak GM already), so a wiring adapter might be possible...

 

The original VCM is not a cable alone, it is bigger and has more electronics than the Mongoose version. The VCM works with all older and newer versions of IDS at least until 131 as that is the one I use.

The airbag module still provides info to the IC, in the form of blinking lights, archaic but at least you do get some info.

 

I understand the use of lights blinking in a coded fashion as Chrysler (and others) were using such communications at least by the early to mid 80's. What I don't know (or have been too lazy to look-up) is which 'light' does the actual blinking. Since my passenger seat obviously has a bad module I have never seen the light over the glove compartment come on, although I can't say as I have ever made a point of staring at it.

I just assumed that like my wife's 2004 XJ8 that light only comes on to signal that the passenger seat has judged that there's a person sitting in that seat because of their weight.

Would that be correct?

I also assume that if there indeed was a problem with the airbag module that the 'blinking light' would be in the main instrument cluster where the major system lights are that cycle through their ignition sequence when the key is in position 2 (1 being when the key is first inserted).

Would that be correct?

Is it your belief that the airbag module, despite its critical importance, DOES NOT set a code readable by an OBDII scanner?

 

My driver's side seat module was bad, so I managed to get it out without removing the seat. Took it to a local TV repair shop, and paid 20 cash for the solder work it needed. Then I stuffed it back under the seat and it works fine. I do suggest a bead of silicone on the board so it prevents the case rubbing the the electronic lines away again.

 

How'd you remove the module without getting the seat out? Since I can't raise the seat with the motors I can hardly get my meat hooks under it let alone disconnect, unbolt, and remove it.

 

There's two bolts that hold the module to the frame that hangs down under the seat. I cheated and took a photo of the area with my phone first. I'm not flexible enough to turn myself into a pretzel and squeeze in front of the seat. But working blind you can feel the nuts. Worst thing is the bolts are built in to the module, so once the nuts were off I used a large screwdriver to bend that frame bracket so I could get the bolts past it to remove the module. On mine there was enough wire length to the 3 wiring harnesses to be able to pull the module out far enough so I could unhook the harnesses. Taking the module apart you have to be careful, the plastic tabs holding it together like to break. Then you can take a good look at the circuit board and figure out which one(s) are broken. I'm not too good at finessing a.soldering iron, so a cheap trip to a TV repair shop got it fixed by the run wires past the break trick. Then I hooked the thing back up, stuffed it back under the seat. One caveat, many swear words were used too.