30 GEARS January/February 2007

ElectroMagnetic Interference (EMI) is one of the mysteries of automotive troubleshooting.

Unless you know what EMI is and con-sider what it can do to sensitive elec-trical signals, it’ll be difficult — and sometimes impossible — to fix some problems on computerized vehicles. And keep in mind that some EMI-relat-ed problems take time to solve, so don’t volunteer to fix some weird problem in an hour.

EMI is defined as an electrical signal that interferes with another sig-nal. Typical EMI signals can come from external sources such as bad grounds, bad alternators, high voltage spikes from ignition systems, various unshielded signal generators (MAS or O2 sensors), high powered transmitters (aftermarket amplifiers), or faulty spike protection diodes.

EMI can also be generated within a module and affect circuits directly. And one faulty module can also affect other modules indirectly.

Under the right conditions, EMI can come from high powered trans-mitters such as those found near air-ports. Nicola Tesla proved this almost 100 years ago. He showed that with

enough power and the right frequency, you could transmit signals and voltage across long distances through the air. This principle is what makes TV, radio and microwave transmissions possible. So what’s to stop airborne EMI signals from interfering with cars and trucks? Something to think about the next time you cruise by Area 51.

To start with, let’s cover all the sim-ple ways to eliminate or verify whether EMI is present. Then we’ll look at a very interesting EMI problem that concerned Tony, an ATRA Member in Colorado. Let’s get started:

Most driveability problems turn out to be a simple sensor or wiring problem. Obviously, there shouldn’t be any codes present in any module. But if your diagnosis doesn’t reveal the reason for a customer’s problem, con-sider EMI. At this point you may have observed an erratic or strange signal with your scan tool, or while monitor-ing a circuit. Even if you’re not sure you have an EMI problem, you can still try to eliminate or reduce EMI.

Eliminate Obvious Sources

Check all grounds, including the

battery cables (figure 1).To verify a ground circuit:

• Connect your meter’s negative lead to the negative battery post.

• Backprobe the suspect ground wire with your positive test lead, as close as you can get to the device it’s grounding.

• Key on, engine off.• Switch on the high beams, and as

many accessories as you can. This will load the grounds with current, and make a weak ground more likely to show up.A good ground should have a volt-

age drop of less than 0.10 volts. Disconnect any aftermarket ampli-

fiers, radios, battery isolators, or con-verter devices. Look for any aftermar-ket chips attached to the PCM or related wiring. Look for any engine harness or computer-related wiring or connectors that may have been crushed in an acci-dent, damaged while being backprobed, cut, or misrouted too close to the alter-nator or ignition wiring.

Shut off as many devices or sys-tems as possible. These include the alternator (see figure 2), cruise control, ABS brake module, A/C system, and suspension. Try to disable as many sys-tems as you can, and still be able to run the engine; if necessary, drive the car. As you disable each system or device, see if the problem is still present.

If you want to test for EMI as you disable systems or devices, connect a high quality lab scope or graphing meter to the VSS or TPS input signal. To pick up EMI, the lab scope should have a minimum sample rate of 10 MHz and at least 1 MΩ impedance. Most automotive lab scopes, such as the Modis or Vetronix MTS 5100 include 2-4 channels, so you can monitor sev-eral inputs at the same time.

DRIVEABILITY

Tracking Down EMIby Dave Skora

Figure 1: Check battery ground cable

30Dave.indd 3030Dave.indd 30 12/27/06 11:58:55 AM12/27/06 11:58:55 AM

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32 GEARS January/February 2007

NOTE: If the problem only occurs under certain conditions, such as when driving near an airport, you’ll need to drive the vehicle to the possible source of EMI and test for it.

In the case of the module itself

causing EMI, you’ll have to get creative. Depending on the signal being affected, you could try signal substitution. For example, if the suspect signal is a tur-bine speed sensor (TSS), try switching the VSS and TSS wiring. Now see if

the VSS signal has EMI and the TSS has a good signal (of course it will read higher than a VSS). Another method would be to jumper the input signal from a sensor on another vehicle.

HINT: Most sensors (TPS, ECT, MLPS, VSS, O2, etc.) share a common sensor ground. Always check for excess voltage on the sensor ground circuit. Frequently you can locate the source of the short to voltage by disconnecting each sensor one at a time. When the sensor ground circuit drops to 0.10 volts or less, you found the faulty sensor.

If the circuit or display on your scan tool still has EMI, it’s likely the computer is at fault. But before chang-ing the computer, double check all grounds. Make sure all possible mod-ules are disconnected. For stubborn EMI, try connecting a known good sen-sor right to the module and monitor the circuit with a lab scope. If you detect a faulty signal, it almost has to be coming from the module itself.

Before we condemn the module, let’s look at a real world EMI problem we alluded to earlier, from Tony in Colorado. He had a 2001 Ford truck with a 5.4L V8 engine. The truck had erratic shifts and set DTC P0503 (VSS intermittent). Tony did all the right things: He confirmed the complaint. He

Tracking Down EMI

Figure 2: Unplug alternator to eliminate stray A/C voltage.

Figure 3: Normal plug gap ignition wave pattern

Initial Plug Firing

Plug Firing Across Gap

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GEARS January/February 2007 33

cleared the code to see whether it came back. He changed the VSS, expecting this to fix it.

It seemed to work for a while, but the code and shifting problems returned. Tony connected a lab scope directly to the VSS wiring at the PCM. He observed stray signals that came and went, even with the truck standing still.

Finally he called the ATRA HelpLine. We suggested he check the grounds, disconnect any aftermarket devices, and disable the alternator. Nothing helped.

Just by chance, another late model Ford truck came in with the same type transmission. He couldn’t swap the PCM because it had a different engine. So Tony ran two wires from its VSS over to the first truck’s PCM. The lab scope still displayed the erratic signals. This proved the problem was related to the vehicle or PCM; not the VSS circuit. I asked Tony if the truck had any engine problems or codes. He said no, and was going to check the truck further.

Tony called the next day with news of the fix. He called a friend at a Ford dealer who mentioned some problems with the ignition system. Ford calls it COP, for Coil Over Plug. Tony learned the truck had the original plugs, which were worn out. After installing the new set of plugs, the VSS signal was rock solid.

Later I found Ford’s TSB 05-22-8. It turns out that the COP coils are controlled directly by the PCM. If there’s excess inductive voltage from the spark plug firing, it could generate EMI. Once inside the PCM, it ends up interfering with other signals, including Tony’s VSS.

To illustrate this, I captured a pri-mary ignition wave pattern from a COP on a 2002 Explorer with a 4.6L V8 engine. I explored the (See figures 3 & 4) idea of checking the first image with a normal spark plug gap of 0.052". The second is with the plug gap opened to 0.110".

Notice the initial firing voltage for the primary circuit is about 300 volts on both waveforms. But look at how short the firing time is for the plugs with the large gap, and the coil oscillations are reduced. Last but not least, notice the erratic voltage spikes (noise) occurring during the coil off-time. Just image

how much noise (EMI) this would cause with 8 worn out spark plugs.

W h i l e it does take s p e c i a l i z e d equipment and knowledge to verify EMI, it’s important to know that it does exist. Since most vehicles rec-ommend spark plug changes at 100,000 miles, a lot of people forget about them alto-gether. It’s not unusual to see vehicles with 150,000+ miles and still have their original set of plugs. The simple way to eliminate EMI from COP ignition systems is often simply to — drum roll please!

— change the plugs. Thanks Tony for following

through.

Figure 4: Excess plug gap ignition wave pattern

Initial Firing

Plug Firing Across Gap

Erratic Spikes

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