I drained my battery down to pretty much zero volts, and had to replace it (long story short). After I put the new battery in, the airbag light was on. Is that normal? I took it in to my local Jaguar dealer, who told me that they didn’t have the diagnostic tools to investigate it.

 

Brendan made a excellent work up on the airbag system and there might be some information there

see below file

 

I finally got around to doing those tests, and found an issue, but don’t know how to resolve it. If anyone can point me in the right direction, that would be great!

After removing the airbags and testing the control module, I found the following anomalies:

Section 8 step 2&3 both read 0 volts. I think that means the Safing Sensor feed is not working.

(Incidentally, Section 8 step 3 passed, and returned bales as expected)

Section 6&7 (crash sensors) steps 1&2 both passed, although from memory, the measured voltage in all instances was about 1V below the battery voltage. If this is an issue, I could recheck it.

Assuming the issue is with the safing sensor feed, what would my next step be?

Thanks!

 

Dig into the safing sensor. You can check continuity of the wires and probably test the function of the switch itself.

Keep in mind that a number of different faults can cause the thermal fuse inside the control box to blow. If that happens, a new control box is needed, unless a technician can replace the thermal fuse for you. I followed this excellent guide all the way through but in the end I found the fuse blown.

 

How do you check that? Take it out and open it up?

 

You might have put a charger on the cars battery backwards or installed the cars battery backwards and blown the internal fuse

I personally have not opened up the airbag control unit, but I did install my battery in backwards once

I have yet to connect my replacement airbags as an operating system as they were blown by the previous owner in a crash , so they worked as designed

Now I have a white tape on the battery with an arrow pointing fwd to catch myself

By installing the battery in backwards and hooking it up you will blow certain regular fuse box fuses like the stereo and instrument cluster

With a charger ( and a smaller reversed voltage difference ) you may not see these fuse block fuses blow but possibly the one of interest in the airbag control unit

The charger will run at a voltage higher than a fully charged battery of 12.75 volts

 

Yes. Below is a photo of the thermal fuse coupled to a 15 ohm resistor (F1). Too much heat through the resistor and the fuse blows (open circuit). Its a safety feature to prevent a fire after a crash and deployment.

You should still check out the safing sensor, because an actual fault that blows the thermal fuse, if not corrected, will blow the new unit's thermal fuse!

I meant to say that I didn't find any faults with my airbag system, but that fuse was still blown. Could be age related or as Parker says, an electrical glitch of some kind. In your case the dying battery. I put in a new Ford Taurus control box from the mid-90s, same unit, no faults since.


Airbag thermal fuse alongside 15 ohm resistor. Fuse often fails

 

<QUOTE> I put in a new Ford Taurus control box from the mid-90s, same unit, no faults since.


Airbag thermal fuse alongside 15 ohm resistor. Fuse often fails[/QUOTE]

Thanks! Added to wrecking yard scavenging list...

 

" You should still check out the safing sensor, because an actual fault that blows the thermal fuse, if not corrected, will blow the new unit's thermal fuse! "

Thanks

 

So there was no other sign that the thermal fuse was broken when you ran those tests in the “airbag for dummies” document? How did you know it was the thermal fuse?

 

I’m going to have a good look at the safing sensor next time I get a chance, is it accessed from inside the vehicle, behind the trim?

 

There is 1 safing sensor by the right front door hinge at the end of a yellow conduit , look under the the stteering column and up along the right-side wall for your right-hand steering model

There are 2 impact sensors by the front grill and again yellow conduit

All 3 are drawn the same way on paper in the wiring guide so you probably should check all 3

 

I had a good look, so many wires…… The yellow conduit looked like it went beneath an ECU or some kind of control module in one direction, and disappeared into darkness in the other direction…. I’ll regroup and give it another go. Any more tips very welcome.

 

The Safer sensor is above the ECU tucked away high up along the right wall as it gets hidden by the dash components

The 2 front crash sensors If I recall are black square boxes tucked in close to the centerline of the car , you get to then from the top

I think there was headlight current sensors in the area to confuse things

you might be able to check them from the control module connector but then that is buried also

The airbag control module is labeled as SRS ( Supplemental Restraint System ) on page 24 of the 96 wiring guide

Editing

What you don't want to see is .................................................. ...........

24-3.pdf (denso-ten.com)

 

The Safing Sensor is actually directly behind the Engine ECU which must be removed to access it. This picture is RHD car.

 

Yes correct. Pin 15 is the feed to the Safing Sensor, so if there is no voltage there then that will cause a problem.

The feed at Pin 15 originates at Fuse 17 in RH Heelboard.

Testing of the Power and Ground at the Air Bag control unit is covered in Section 4 of the Dummies guide. So assuming that you have carried out those tests and you have B+ volts at Pin 13 at the module, then it’s a fair conclusion that the control module is faulty.

 

Today is the day for me to play with mine , but it is in a already fired bags state with new / old replacement airbags not connected but installed

So my observations would not possibly be a ready to fire again as a zero state ready to go system

But I will be checking the internal resistor and have a in town big selection parts store source to replace the resistor coming Monday , and possibly triggering the system ( bang ) to watch it fire off with a needle meter on a bag connector and again a internal resistor check

 

Someone made a YouTube video on the removal for a RHD no glove compartment X300

In the end looks like 2 / 10 mm bolts under the blue air bag monitor module under the frame bracket angle, now where did I put my 10 mm socket

I did find the safing sensor behind the engine ECU

 

Confirmed: Airbag module thermal fuse blown.

Removed and opened up Airbag Diagnostic Module.
Tested the fuse (F1) for continuity, blown.

Interesting that it was the safing sensor test that showed up the blown fuse.

Thanks for all the help on this one. I'm not sure why it happened when the battery drained (coincidental?), or perhaps something happened when I gave it a jump. In any case, I can’t find any other faults.

 

OK, got everything back up and running again, albeit with the airbag simulators, took it for a drive, light stayed off. Reinstalled the airbags, the light’s now back on, and the fuse in the module had blown. Oh well.