A breakout box (BOB) simplifies test equipment hookups and can make (or break) an OBD II diagnosis. Here’s how, when and where to deploy this handy piece of kit.

Since OBD II compliance became mandatory in 1996, technicians have benefitted from the uniformity of the data link connector (DLC) and have become accustomed to codebased diagnostics covering an increasing number of systems over the years. OBD II has evolved as new and more efficient communications protocols have been added. The systems have become increasingly robust.

But what do you do when your scanner won’t communicate with a vehicle? The vast majority of communications issues are caused by a “simple” blown fuse. Many older scanners were powered directly from the DLC, so if they didn’t wake up when plugged in, the easiest check was to plug them into another vehicle. If they powered up then, there must have been something wrong with the first vehicle, right?

The photo on page 26 shows the pin layout of a standard DLC as viewed from its open end. Note that it may be mounted in any orientation, so pay attention to its trapezoidal shape and the position of the connector latch. Pin 16 should always have battery power, while pin 4 should be connected to chassis ground and pin 5 to sensor ground. Testing should be done using an appropriately sized connector pin (Metripack male terminal 150 series, GM Part No. 12047581, for example) or by backprobing from the other side. Using a conventional electrical probe on the front side may spread the connector’s terminal, causing problems both now and later. A better approach, and one much easier on your neck, is to use a breakout box (BOB) like the ones shown here and on page 28.

So you plug in your DLC BOB and there’s no power on pin 16. I leave it to you to locate the correct fuse and replace it. If it blows again, be sure to look at the wiring diagram; that fuse may also protect other components, often including the power-point with the shorted-out phone charging adapter in it, or the lighter socket with a penny at the bottom.

Once you’re sure you have both power and ground, but still have no communication with the PCM, try contacting other modules. Many scan tools offer menu items such as system scan, system selection, module polling, health check or the like. If any module responds, check it for DTCs and particularly for U-codes—communication fault codes. They’re always manufacturer-specific, so a U-1000 code may mean different things in different years, makes and models. Make a note not only of any codes found but also—and equally importantly—of which module responded with that code. You may use this information later to help pinpoint the problem area (see sidebar “Hey ‘U-All’” below). If any freeze frame or failure record data are available, capture and record these as well.

If you’re still unable to establish communications with any module, recheck using a generic interface. This is often available under a separate main menu entry in your scanner, often labeled OBD or EOBD. The generic interface may use a different set of pins (and therefore different wires) to communicate with the PCM. You may now be able to access basic generic data or retrieve relevant DTCs, possibly including U-codes, which were previously unavailable under a manufacturer-specific interface. (Another bonus of datastream under a generic interface is that no substituted values are allowed. This can provide important clues if, for example, a Vref circuit is shorted to ground.) Still no love? Try another scanner if you have access to one. Once in a while you’ll run into a vehicle that just doesn’t like something about your scanner but behaves normally with another one.

Break It Out

If there’s still no communication, it’s time for a “triple breakout.” First, break out your DMM and run a couple of quick voltage drop tests. Start by turning the ignition off, then unhooking your scan tool and hooking up a memory-keeper cord with an LED. (The memory keeper is a cord designed to run between the DLC and the power-point “lighter-style” socket on your jump box. In this case, you’re not connecting the power point end, but are using the LED as a dummy mini load on the circuit. It’s not much of a load, so if you get even slightly weird results, step up the load with a small wattage bulb wired between pins 4 and 16 of your breakout box. You may omit using the cord if your BOB is equipped with its own LEDs.)

With the positive lead of your DMM on the battery positive under the hood and the negative lead on terminal 16 of the BOB, set your meter for Min/Max and crank the engine. Then move your negative lead to the battery ground terminal and the positive lead to DLC pin 4 on the BOB. Select Min/Max and crank the engine again. In each case, the maximum voltage drop should be well under .5V. If it’s not, clean or repair connections as needed and repeat. Most known-good values are less than .1V.

Second, you need to break out the books, consulting your repair information system about both supported generic protocols and manufacturer-specific communications. On late-model cars, these will sometimes involve more than one set of pins on the DLC. Make a note of the pins involved in each type of communication. Reconnect your scan tool, paying careful attention to the ground indicator on pin 5 if your BOB is equipped with LEDs. If the light changed, read the sidebar “Grounded for Life,” on page 28.

Third, break out your scope. Depending on the communication protocols employed, you’ll need to hook up to the appropriate pins on the BOB. Voltages may be as low as 1.5V, as high as system voltage or somewhere in between. Some protocols are in the slow-as-molasses kB/sec range, while modern CAN systems may be much faster. Good practice is to set up your scope to capture anything below 20V and simply look to see what you get. You can home in on the signal by changing voltage and time bases.

What are you looking for? I’ve included a couple of scope captures (Figs. 1 and 2) above, but this is yet another instance of the need for known-good reference data. After all, if you’ve never looked at known good data from known-good vehicles, how will you differentiate between the good and the bad?

Communication Faults

These known-good scope captures are from two very different systems—ISO 9141 and CAN, respectively. The scope capture in Fig. 3 on page 31 is just plain ugly, but it serves to illustrate an important point. A number of students have asked me over the years why they couldn’t just use their DMMs, and it’s a valid question. While a DMM may be fast enough to capture true Min/Max values on a high-speed CAN network, it can’t find the little glitches that are sometimes the only clue to the underlying problem, and it’s very easy to become confused. The average voltage in this illustration is about 3.85V, a value not out of the ordinary for a working 5V communications toggle. But, as the waveform shows, something is drastically wrong.

I recently fixed a late-model VW where a faulty SRS module showed a believable average communications line voltage of about 9.2V. Had I relied only on my DMM, I might well have taken that to mean a square wave with an average duty cycle of about 25% and moved on. Looking with a scope, however, I discovered that the module was faulty; the 9.2V was a flatline response with no modulation whatsoever. (The local dealership had been unable to fix the car after three attempts and over $1200 of ineffective “repairs.”)

Fig. 4 on page 31 shows a normal charging system waveform you might have captured between DLC pins 16 (B+) and 4 (chassis ground). A weak battery coupled with a worn alternator may sometimes produce a waveform with sufficient AC ripple to drown out legitimate communication.

For some purposes, such as battery replacement and parasitic draw testing, the BOB shown in the left photo on page 28 is my go-to BOB. Its built-in digital voltmeter displays battery voltage, confirming that vehicle power is reaching the DLC. I test my memory lead’s internal fuse by plugging it into the BOB before I plug it into my jump box. And since I normally store the memory lead plugged into my jump box, I always know from the lead’s LED that the internal fuse is good before I put it into service. Using the BOB also allows a more easily accessible disconnect if I need to make a direct ammeter connection at the battery to measure a suspected draw.

However, it’s important to know that the BOB itself may be a bit of a power hog. The one I just mentioned draws about 35mA. For most vehicles, 50mA is about the upper limit for resting draw, so if the vehicle you’re testing has an actual draw of 25mA or higher, you might think you had a problem if you measured 60mA at the battery. Do yourself a favor: Measure the BOB’s current draw, then record it on the BOB’s case for future reference.

Here’s the easy way to measure a BOB’s current draw:

•Make sure the battery you’ll be using is fully charged.

•Plug the BOB’s two leads into one another. •Make sure your DMM is in the OFF position.

•Unplug the red lead from the DMM and move it to the AMPS jack.

•Connect the red lead to the battery positive terminal.

•Connect the black (COMM) lead to terminal 16 of the BOB.

•Run a jumper from the battery negative lead to BOB terminal 4.

•Run an additional jumper from the battery negative to terminal 5.

•Set the DMM to the AMPS scale.

•Read and record the current draw as shown on the DMM.

•If your BOB has multiple modes, switch to each one in turn and record the value.

Scan tool communication failures are often frustrating, but most can be traced to simple causes. When U-codes or parasitic draw issues raise their ugly heads, your breakout box should become your go-to tool platform. As with most tools, a bit of practice will enable you to get the most out of your breakout box. I’ll remind you once again of my standing advice: At least once a week, take 10 minutes to scope a known-good signal on a known-good vehicle. Don’t wait until you have a problem in front of you.