My friend recently purchased a 1991 Mazda Miata from its original owner, in immaculate condition and with incredibly low miles. Except for one issue: the Air Bag light on the instrument cluster was indicating a trouble code. The number of blinks of the light indicates the fault and the light was flashing 10 times, meaning the System Down Fuse had opened. This fault keeps the air bag system from working resulting in reduced safety in the event of a crash, so it’s important to take care of. Used modules can run around $100, with new computer modules starting over $200.
As these cars are getting to be over 20 years old, these module issues have been known for a while and the cause identified: faulty electrolytic capacitors cause the thermal fuse to blow, disabling the system. Capacitors are at the root of pretty much every electrical problem, it seems. We decided to try repairing the module after some research that showed it’s a common problem with a fairly straightforward fix.
This rest of this article demonstrates modifications to your car’s occupant restraint system that are not approved by the manufacturer and if executed improperly could very likely result in your serious injury or death from the air bag failing to deploy, or deploying unexpectedly.
The repair involves sensitive components which can be damaged by even slightly improper handling, furthering the risk of an unexpected failure.
This information is provided only for experienced automotive and electronics technicians as an academic exercise, and KF7LZE is not liable for any consequences arising from following or failing to follow these instruction.
The module is a little blue package that lives up near the steering wheel on the 1990-1993 Miatas. This part wasn’t used in the entire range of the first generation’s production due to revisions that happened along the way. It was also used in similar years of the Ford Taurus, the Mazda RX-7, and there may also be other Ford and Mazda cars using the same air bag module which have similar faults.
We unmounted the board from its housing, then got down to business by removing the bad capacitors:
If one capacitor of a set is bad, it’s very likely the rest of them will be soon. This board uses 5 x 100uF 35V electrolytic capacitors and 3 x 10uF 35V capacitors, all rated at 105°C, along with an assortment of other components that aren’t subject to failure the same way. The System Down Fuse is the long, red-tipped object parallel with the connector on the right side of the photo. A quick continuity check revealed yes, it was in fact open.
From the black spots around the bottom of C7 (center-left in the photo, lower right of R42) you can clearly see the electrolytic fluid had leaked from the bottom of the cans and etched the board a little, but the damage wasn’t that bad. Rubbing alcohol took off some of the residue, but it’s more cosmetic than operational damage and since this board lives inside of a plastic housing itself located inside the steering column nobody is ever going to see it. Replacing the capacitors was very straightforward: de-solder pads, pull old caps, insert leads, and re-solder pads. My Hakko de-soldering tool makes this job very easy, but with any tool it’s important not to overheat the joint or the traces could de-laminate from the board and that usually means the part is destroyed.
Here’s the thermal fuse on its housing. The fuse is the center component, and wrapped around it is a flexible trace completing a circuit between the two center pins. I’m not sure what purpose it serves, but it makes a complete circuit so it’s important to save it. It might be a current sensing winding around the fuse to send a signal when the inflators are triggered, but that’s just speculation based on its placement. Be very careful – on this one, the foil contact pads on the flexible trace split from the mounting points and it was very frustrating trying to get them back together during reassembly.
As you can see, the foil came off the terminal when it was removed. Not good, but not the end of the world either – it’s fixable.
Here’s another view of the board, showing the replaced capacitors mounted up.
Repairing the foil trace was a delicate process. The original connection was a very small contact area, and when it snapped off it removed a bit of foil. There was a thin layer of resin-like insulation over the remaining portions of the foil wrap that needed to be scraped off to expose the bare metal underneath. We first tinned the metal contacts on the fuse mount body, then heated the foil contact pad from the back while applying pressure to force the foil pad into the metal terminal. Once the solder starts to flow, remove the heat but continue to apply pressure – rolling the iron back so it wasn’t applying heat, but could apply pressure as the joint cooled. This took quite a few tries – looking at it wrong the first few times caused it to break off, taking a little more foil off each time. It finally made a good, solid connection and we wrapped it back around the new thermal fuse.
There was a lot of controversy about the thermal fuse replacement on various Miata forums while we did pre-op research. Most commenters who have attempted this repair in the past have been held up on a lack of information about the part number for the thermal component. I can only assume the flexible trace wrapped around it completely destroyed the part numbers on the fuse for most other modules – this particular car has been garaged its entire life, so maybe it was luck. For a safety-critical part such as this one, it’s important it have the right ratings or it could fail to allow the air bag to ignite in a crash or cause an electrical fire after a crash. Several people suggest to replace it with a 1/4W 10 Ohm fusable resistor, a standard metal-film resistor, or a standard fuse instead. I don’t recommend this shortcut.
…replacing it with a standard fuse, metal film resistor, or whatever would circumvent its primary function. Thus, the bottom line is that replacing it with anything but an identical item would risk air bag deployment at the improper time…
After unrolling the film from the old thermal fuse, there were still some very faint, but readable, part numbers listed – very surprising. It’s a Motorola part, which goes well with the Motorola controller chip onboard. Part number 4283A.
The bag the old fuse is resting on gives away the next step a bit. When looked up in the NTE Cross Reference Search, 4283A brings up a modern part number replacement that happened to be stocked at my favorite local electronics shop, Vetco. It’s an NTE8139: 141°C, 15A thermal cut-off. That’s a definite part number! And it’s available in modern production. No need to worry about replacing it with a different part and changing the operation of a safety-critical circuit when an identical component will do!
This bears repeating: the air bag system down fuse in a first-generation Mazda Miata is an NTE8139.
The new fuse gets installed and soldered into place, wrapped again in the original flexible trace that covered it before. It’s very important to remember this is a heat-sensitive device, and you’re soldering to it. We only applied heat for about 10 seconds max at a time, and it was just barely enough to get it to take the solder. The first 10 seconds were a cold joint with the solder holding it in place, the second 10 seconds reflowed the cold joint to be a proper joint to ensure the fuse wouldn’t open up while it was being installed. If you overheat the thermal fuse, you’ll destroy it.
Finally, clip the extra-long leads down to size, reinstall the board in the housing, the housing in the car, and fire it up: The light came on briefly at the start like it should, then blinked off. NO CODES! The air bag system passed all self-diagnostics. We think it’s ready to protect him in the unlikely event of a collision, but there’s of course no way of testing that short of crashing the car. The controller thinks the system is fine, and the parts were replaced with identical new replacements, but full system functionality testing is impossible.
The total cost of the project was about $8 worth of parts, and a couple hours standing over a soldering iron.
This is a fairly straightforward rework job, but you must take special care to not overheat the thermal fuse when installing the replacement or it will fail. I’d recommend using an a solder clip or other heat-sink between the terminal and the fuse body, and it really helped to have two pairs of hands working on this to hold the fuse in place while the other person soldered. If you don’t have a friend who can hold it in place, definitely use a soldering assist device.
And remember, use this information at your own risk. You should not attempt this repair yourself, and KF7LZE is not responsible for the consequences of failing to follow these warnings.
My apartment has what I like to affectionately call the “free pile”, where neighbors leave things they were throwing out but might have too much value to throw in the compactor outright. I’ve left a few things there myself, and have made a few decent finds like a new microwave or a Samsung 225BW LCD with bad capacitors in the power supply. This time it was a Dell Inspiron 1545 laptop computer with a nice carrying case and power adapter just left sitting. Naturally I brought it home with me, and it didn’t take long to figure out why it had been abandoned:
That is what a broken LCD looks like. You can tell from the pattern on the screen where the break happened – I think this laptop was stepped on, rather than dropped. The cracks originate from the center; if it had been dropped you’d see a crack originating at a corner or side most likely. I hooked the computer’s VGA output up to my television to double-check the rest of it worked – and everything else seemed fine – so it was off to eBay to find a replacement part. I managed to come up with a pulled replacement screen (with a webcam! from one trim level higher) which set me back about $80 shipped; it arrived this afternoon and I set to work.
It’s not difficult or scary to replace an LCD (or most any other part in a well-designed laptop), you just need to be good with a screwdriver and go slowly so you don’t rip any thin wires out of their sockets pulling on something too hard. This entire process – from deciding to begin through powering on successfully – took only about 30 minutes, and that’s because I was stopping as I went along to take photos.
The first step is to remove the trim panel from the top of the keyboard which hides some of the connectors, and lets you access more screws. On this model there is a notch on the right side for you to insert a screw driver to gently pry it away from the case and up. It takes a little force, and there are a few snaps as the plastic clips come unhooked leaving a plastic trim piece free.
Along the top of the keyboard there are 3 screws which had to be removed to allow the keyboard to fold up and away. The ribbon connector underneath has a black retaining clip holding the ribbon cable in; to free this cable, pivot the black piece of the connector vertically towards the front of the laptop and it will snap the ribbon out.
With the keyboard out of the way, we can turn our attention to the hinges and start working to remove the monitor. Before you can take the hinges off, you need to very carefully disconnect the cabling. The hinges are in the top left and right of the computer and are held in by two large top-side screws, and one screw on each side from the bottom.
In case you’ve ever wondered why laptop speakers sound so bad, here’s a visual. I included a quarter for size reference.
The next step happens on the bottom of the computer. Open the access panel with 4 screws, and carefully disconnect the black and white wires from the wireless card. I used needle nose pliers to get a better grip. You could also put more RAM into the computer if you wanted while you’re down here, this model has two slots but only one is populated from the factory (2GB).
On the bottom, also remove the two screws from the top left and right marked “D” near either side of the battery.
Flip the laptop back over and route the black and white wifi cables up through the hole in the chassis and away.
The LCD connector is a flat-panel connector. To remove this one, pull straight up on the tab and it will lift right off.
Now all the cables are free – unscrew the 4 remaining large screen mounting screws and set it aside.
Hooking it all up is the exact opposite of taking it apart. Mount the screen and screw in the 4 top-side mounting screws and reconnect the LCD1 cable – and CAMERA1 cable if your monitor has that. Then route the wireless antenna cables down the way you took them out to the back of the computer, and reconnect the MMCX connectors. Then close the bottom access panel.
Finally, put the keyboard back together, snap the black edge connector into place, three screws, and snap the plastic trim piece back into place.
And with the laptop all closed up, it’s time to fire up!
Apparently this laptop belonged to a neighbor named Victoria, and since I don’t have her password and there isn’t an admin account on the computer, I’ll be wiping the operating system and installing my own copy of Windows on it from scratch. Now the dilemma is, whether I want to keep this and use it for myself, or if I want to sell it! That’s not a bad problem to have.
Feel free to leave a comment or question!
A recent random assortment of ideas:
It’s easy to get media content from a computer to a television, but it’s difficult to get any kind of external media back into a computer. I have a fairly involved computer setup at home. My workstation is connected to two 20″ monitors and a 46″ LCD. A friend was in town for the weekend, and we were looking for an easy way to make a YouTube video play from his laptop onto my monitor, without actually taking away my control of it. I have two sets of input devices, one at my desk and one near the couch, to control the computer when it’s being used for media. We ended up just switching off who had control of the single mouse cursor between the two input devices. The ideal solution? Either multiple mouser pointers, assigned to the unique input combinations – or a VGA capture interface to make another computer’s input appear in a window. VGA capture devices exist but are absurdly expensive, and there’s no multi-pointer support in the operating system. Most laptops these days have dropped an analog TV output in favor of HDMI or DisplayPort outputs meaning they can’t be connected to an inexpensive TV-capture card.
At about the six month mark, as expected, my phone has started acting up. This has been the case with most every smartphone I’ve owned – a Nokia running S60, an HTC 8525 running Windows Mobile 6.5, a G1, and now my G2 running Android. Reliably at about the six month mark they’ve all started acting up in ways that make me suspect the hardware is failing: lockups and reboots, screen glitches, data disappearing off the memory card randomly and visible dust between the LCD and the glass so deep in the phone the only way I can see it getting in is via osmosis. Hard reset doesn’t fix the problem. Have even high-end smartphones become disposable commodities?
Hallicrafters 8R40 Upgrade: On the back of my bench I have a Hallicrafters 8R40 radio receiver from 1953. It will be a good performing radio when I’m through fixing it up, and I’ll be using it to try and pick up long distance contacts. One thing about far away faint signals is that they can be tough to hear even with the audio turned all the way up, the 8R40 only has a single-ended 6V6 output that maxes out around 4.5W of audio power into a not-that-efficient speaker. From Parts-Express, I bought a few Dayton Audio DTA-2 Class T amplifier modules, based on a Tripath TA-2024 chip. They’ll pump out about 20W of power. I’m planning to use one of these modules inside the back of the Hallicrafters to allow it to drive an external speaker at 20W in a reversible modification. I expect I’ll split the detector’s output and use the module amplifier for the external speaker only; leaving the build-in speaker powered by the tube.