Servicing fiber optic systems

Have you ever watched an Internet video on a slow-speed connection? It’s really frustrating! There’s massive amounts of data that must be sent and received and it just doesn’t work properly on systems that can’t handle the load.

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It’s the same in automotive applications – if there are massive amounts of data involved, without the right set-up the system just won’t work properly. That’s why fiber optic systems were first used on vehicles: to send and receive data in media and body systems. Fiber optic systems could handle the massive amounts of data these systems needed and transferred it quickly and at the speeds required for proper operation – after all, it’s tough to enjoy a media system provided through a low-speed, poor quality connection.

Not only are optical systems used in media systems, some European models have also used fiber optic systems to transmit data from one module to another for their passive safety systems too. Even though the optical bus system isn’t being used in extremely high volumes it’s still worth understanding how the system works, what goes wrong, and how to fix it properly because the system is definitely out there on the road and it might roll into a service bay near you.

Why it’s used
There are some good reasons why the system is used, besides its capacity to handle large amounts of data and transfer massages quickly. For example, fiber optic systems are lighter than traditional wiring, which is important to manufacturers trying to reduce vehicle weight and improve fuel economy to meet tough CAFE standards.

And the signals involved won’t degrade or vary in the circuit.
But there are disadvantages to the system. One of the biggest drawbacks is the fragile nature of the cables involved. Fiber optic cables aren’t as resistant to rough handling as traditional wiring and they need special care and handling to prevent damage during vehicle service and repairs – and it doesn’t take much damage to cause faults and malfunctions.

There’s also been concern about cables aging poorly, something that doesn’t happen with traditional circuits. And then there’s the cost of the system – it’s more expensive than traditional systems even though the cost of the components is decreasing.

But despite the disadvantages optical buses aren’t uncommon and if you haven’t worked on one yet you might soon. Here are some tips to make the process go smoothly.

How to Recognize It
Since fiber optic systems must be handled with care, it’s important to be able to recognize the system when you encounter it so you can service it properly and avoid damaging the components. 

It’s easier to identify the system by looking for stiff, rigid casing than trying to identify an optical bus by its color alone because depending on the manufacturer the outer casing can be black, green, yellow, orange or orange with heavy white plastic coating and can look similar to other vehicle systems.

Another way to identify the system is by the looking at the connectors used – the connectors in a fiber optic system look very similar to traditional connectors but the contents are different: the connectors contain ferrules instead of terminals. (“Ferrules” are the end attached to the cable.) They’re small and round and could at first glance be mistaken for regular terminals. They must be handled with care to avoid breaking the ends.

Of course the best way to know if a vehicle uses an optical bus is by checking the service information system to find out for sure.

How it works The biggest difference between fiber optic systems and traditional circuits is the way data is sent and received: fiber optic systems use pulses of light and thin plastic fibers rather than electricity and traditional wiring to transmit data between modules and sensors. The cables are coated with thin cladding that reflects light along the thin fiber core and allows the signal to be transmitted. The cable’s outer layer consists of insulation to protect the inner core.

In the fiber optic system, electrical signals are converted into pulses of light that are sent and received along a data bus, and received and converted back into electrical signals by the receiver at the other end.

This is similar to the CAN bus systems that have become common on newer vehicles. Another similarity to traditional CAN data bus systems is the system set-up: the optical bus can be set up in either a ring configuration (typically for media systems), or in a star-shaped configuration (typically for safety systems) with bi-directional signal capability (therefore using a single fiber to transmit messages). Messages travel along the bus from module to module as with CAN, but at a much faster speed than the traditional system.

One diagnostic tip: if you suspect that one of the modules on the bus is staying awake or otherwise malfunctioning and causing a battery drain, don’t disconnect the battery before you start your diagnosis. Doing so may cause the faulty module to reset and function normally, hiding the problem and possibly driving you nuts.

How to Inspect It
Just as with any other system, begin the diagnostic process with a good preliminary inspection, which includes checking service bulletins, fuses, power and grounds (a fiber optic system will still have an electrical component that must be checked for proper operation).

On fiber optic systems, it’s also wise to look for signs of recent repairs to the vehicle (especially recent body repairs), and checking out what else doesn’t work on the vehicle since this can provide valuable clues to speed up diagnosis (look up what, if anything, all the problems have in common such as a common ground or a common pass-through point in the body).

And, of course, inspect the cables, connections and components themselves.

When inspecting the cables, look for any sharp bends that would cause the delicate fibers inside to break or even just tight bends that would distort or degrade the signal across the sharp angle – light can’t travel around corners so the signal won’t transmit properly through sharp bends. The system is very sensitive to misrouted cables. If a harness has been moved or relocated it might cause problems in the system. Also look for heat damage, signs the cable was or is compressed or stretched, punctures, cuts, looseness or anything else obvious. Check the insulation carefully. The signals sent along fiber optic cables can be lost or degrade if the cable’s insulation is damaged, much more so than with traditional wiring harnesses.

The best way to test a fiber optic cable is by using the special tool designed for that purpose.

While in theory you should be able to check the integrity of a fiber optic cable by shining a flashlight down one end and checking that the light exits at the other, in reality there are a few problems with this diagnostic method.

The first problem is that it can be extremely difficult to maneuver a flashlight to shine down the cable and then check for light at the other end. The cables are delicate and the last thing you want to do is cause further problems in the system. And if you did manage to do this you would have a hard time gauging the intensity of the light shining out the other end – and intensity is as important as presence.

Think about fuel pressure and volume – even if there’s enough fuel pressure present, the engine won’t run if the volume of fuel is too low. In a fiber optic system the light may well be there, but as with the case of the fuel pressure it may not be enough for the system to operate correctly. For these reasons, the best way to be sure that the cable is OK is by using the correct tool for the job.

For the connectors’ inspection, check that the connector ends are securely locked in place, not just seated in position. Just like with traditional wiring systems, connectors tend to be the weak links in circuits, especially if they’ve been disconnected and reconnected a few times. If they’re not plugged in properly the weak contact may not be enough for the system to operate correctly.

Here’s another diagnostic tip: if you’re searching for the cause of an intermittent or elusive problem and you suspect the connector, try not to wiggle or disturb the circuit too much at this early diagnostic stage.  You might inadvertently repair the problem by wiggling something and have no idea what fixed the problem, and that means you can’t be sure it won’t come back! Note your suspicions and recheck the connector when you can monitor the signal or operation properly.

When checking the components themselves it’s important to do some research on the system you’re servicing before you start unplugging anything because on some models the components on the bus can only be disconnected and reconnected with the battery disconnected – look in the service information for this warning before you begin to prevent problems later on.

Repairing Fiber Optic Cables
As with any wiring harness repair on a vehicle, check service information to see if it’s OK to repair a damaged wiring harness before you start. Certain harnesses can’t be repaired; they can only be replaced. It’s also interesting that some manufacturers will permit only a single splice (connection) in a fiber optic cable to maintain the integrity of the signal, so in other words if there’s already a splice or the cable would need two splices for the repair, the cable must be replaced rather than be repaired.

If the manufacturer doesn’t support the idea of repairing the system, don’t take chances, just replace the cable to maintain the integrity of the system and the safety of the vehicle and to keep your professional reputation intact.

Some cables, however, can indeed be repaired and there are special kits available to do just that. Only use the correct tools – never twist the fibers together or worse (shudder!) use a butt connector. Using side cutters and tape to fix the cable won’t work either, and the resulting damage may not be possible to repair. A wire cutter will actually squeeze and distort the cable and therefore distort the light signals, which might affect circuit operation. Use the proper tool that won’t damage the end.

If the fiber can be repaired there are a few important things to keep in mind to ensure that repair is successful. For example, it’s important to maintain the cable’s original length. If the cable is too long or too short it can affect signal transmission and the system won’t work properly after the repair. If a section is damaged, a new section of similar length must be installed. Even minor variations in length are enough to cause problems so keep the length unchanged to prevent headaches later on.

It’s also important to achieve a perfect fit between the ends of the cable. Any distortion in the cable or mating surface will mean distortion in the signal and problems in the system after the repair. It’s important to keep the repair surgically clean for the same reason – grease can distort the signal too. With this in mind, be sure to cut both ends of the cable cleanly and prepare the ends using the supplies in the repair kit. Then crimp the connector in place using the tool provided to avoid damaging the fiber.

Also, during these repairs it might help to think of fiber optic cables as little tubes that carry light pulses, rather than as high-tech wires. Treat the cables as you’d treat delicate tubes – be extremely careful not to bend, kink, pinch, scrape or stretch the harness during installation because just as with tubing, the damage that results may not be repairable. Any “stressing” of the cable can distort the signal and negatively affect system operation.

Likening the cables to tubes is also something to keep in mind when you’re routing the cables into place. It’s important to keep the harness as straight as possible and follow the manufacturer’s original routing. The minimum radius of a bend should be at least an inch (some manufacturers have a minimum radius of two inches, about the same as a pop can) to prevent damaging the cable or blocking a portion of the signal – and the damaged cable may not be repairable.

Remember to be careful of the insulation too – damaging the insulation can affect the signals sent thought the fiber itself.
And of course, don’t break the ends or lenses (if applicable) off of the cable. The ferrule should be handled carefully to prevent distortion or damage when connecting or reconnecting components.

Conclusion
Even though fiber optic systems aren’t used in high volumes, they are used on European vehicles to send and receive data so it’s worth knowing how the system works, how to work safely around it, and how to repair it when something goes wrong. It’s also important to know a few diagnostic tricks and tips to make the job go smoothly.

The systems may seem advanced and high tech, but they’re actually not that complicated and there’s money to be made diagnosing and repairing the systems. Understand the system, look things up instead of guessing or assuming, and use the proper tools and you’ll be well on the way to successfully working on these systems.

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