by steve bodofsky pushing electonsfigure 2: if you used a breakout box to measure the ford epc...

36 GEARS January/February 2007

You can’t compare apples and oranges.”No doubt you first heard that expression from

one of your teachers, the same way I did. As I remember, it had to do with when we began learning the metric system; they’d give you a word problem that involved measurements in both feet and meters. Then, when you screwed up by trying to add the two dissimilar measurements together, the teacher was quick to remind you: “You can’t compare apples and oranges.”

It’s a very real problem, with dramatic consequences. In 1998, NASA made that exact mistake while writing the guidance software for the Mars Climate Orbiter. Nine months later they lost contact with the probe, when it flew too deep into the Martian atmosphere and burned up. All because of mismatched specs: meters and feet… apples and oranges.

I can hear you already: “That’s all really interesting, but what does it have to do with diagnosing electrical sys-

tems?” More than you might think:

The other day I got a call from ATRA Technical Director Lance Wiggins. “We need a sidebar for this issue’s Street Smart article,” he said. “We want everyone to perform the electrical diagnostic test exactly the way Mike Brown explains it. Any other test procedure could give inaccurate readings.”

“I’m not sure I understand,” I replied. “Why can’t techni-cians use whatever diagnostic procedure they’re comfortable with?”

“The problem is, technicians try to use their breakout box to test the signal voltage for the EPC, and end up with entirely different voltage readings.”

PUSHING ELECTRONS

Figure 1: In this issue of Street Smart, Mike Brown wants you to measure the voltage drop across the Ford EPC sole-noid. This shows how much voltage the solenoid uses when

energized.

Figure 2: If you used a breakout box to measure the Ford EPC solenoid, you’d actually be measuring the voltage drop between the control wire and ground; that’d show you how

much voltage was left after the solenoid.

by Steve Bodofsky

Know What You're Measuring Before Comparing the Specs

Pushing Electons Comparing Apples and Oranges:

36pushingEnew.indd 3636pushingEnew.indd 36 12/27/06 1:34:21 PM12/27/06 1:34:21 PM

transtar press plcd.indd 3transtar press plcd.indd 3 9/15/06 4:02:49 PM9/15/06 4:02:49 PM


Pushing Electons

“Well sure: Mike’s asking them to measure the voltage drop across the solenoid windings (figure 1). The breakout box lets them measure the voltage drop between the EPC con-trol signal and ground (figure 2). That would create a different value. Not less accurate; it’s an entirely different mea-surement. Like comparing apples and oranges.”

“Exactly. So we’d like to tell the readers that they have to use the diag-nostic procedure exactly as written, to avoid any confusion.”

Okay, I can see his point. That’s a fair evaluation… for some technicians.

But many GEARS readers are more than capable of understanding the different methods of checking an electrical cir-cuit, and how to determine acceptable ranges based on the published specs. I have to believe that: Otherwise there’s no point to continue Pushing Electrons. Then I’d have to go out and get a real job.

So, if you’re uncomfortable with analyzing basic electrical circuits and dealing with the math involved, alwaysfollow the exact diagnostic procedure as written, such as the one in this issue’s Street Smart… and skip the rest of this article.

But for the rest of you, let’s look at the EPC circuit on the ’92 Ford E4OD:

According to Mike’s article, the normal voltage drop across the solenoid in Park at idle should be right around 4 volts — his specs were more precise, but I’m going to round them off a bit, to make the arithmetic easier to follow.

To use any other means of measur-ing this circuit, we have to know two other conditions:1. The solenoid resistance, and…2. The applied system voltage during

the test.Knowing that — and having a

basic understanding of circuits — is enough to calculate pretty much any other spec you should require.

For this example, we’ll assume the electrical system maintains a fairly consistent 15 volts with the engine run-ning. The EPC solenoid resistance is supposed to be between 3 and 5 ohms; we’ll use 4 ohms, again for simplicity’s sake.

Voltage MeasurementsThe EPC circuit is basically a sim-

ple circuit, with a little twist. Back in the September 2005 issue of GEARSwe covered simple circuits. The first principle of simple circuits was that the circuit will use all of the voltage available to it. Probably a better way of saying that would be that the circuit’s resistance will use all of the voltage available to it.

But when Mike measured the volt-age drop across the solenoid it was only about 4 volts. Why isn’t the solenoid using all of the voltage applied?

That’s the twist: The principles of a simple circuit only apply to a completecircuit. The EPC circuit has a pulsed

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GEARS January/February 2007 39

ground; the circuit is only complete while the ground is applied. During the off time, the circuit is open, so full system voltage passes through the solenoid.

How much voltage should be left on the control wire if the solenoid is only using 4 volts? With 15 volts applied, that leaves 11 volts. So, if you were measuring the voltage drop between the control wire and ground, you should see about 11 volts; not 4.

Take a look at the EPC voltage table in Mike’s article. At stall, EPC voltage drops to zero. How would that affect the voltage drop between the control circuit and ground? It would rise to system voltage, because when the EPC solenoid isn’t using any of the voltage applied, it’s because there’s no ground applied: All of the voltage is passing through the solenoid. If you were using a breakout box to check the control circuit voltage, you should see 15 volts at stall.

What this means is, if you wanted to measure the EPC voltage using a breakout box, you’d simply have to subtract the published spec from the system voltage to get the comparison specification. Not all that tough: most of us learned how to subtract in first grade!

Duty CycleOkay, so far we’ve just been testing

voltage signals. But the EPC control circuit uses a duty-cycled signal to con-trol that EPC voltage. How can we use a duty cycle measurement to monitor the EPC signal?

For duty cycle we have to move up to higher math: division. Remember, duty cycle is the percentage of time that the circuit energizes the solenoid. So, to calculate the duty cycle, we must divide the published voltage spec by the applied voltage. In this case, we’ll divide 4 volts by 15 volts; the result is 0.266666… or 27% for round num-bers.

To check your results, work them backward: what’s 27% of 15 volts? That’s 15 x 0.27, which equals 4.05 volts; close enough, when you consider we’ve been rounding our numbers.

At stall, the published voltage drop is zero: zero divided by 15 is zero… exactly the duty cycle we should have

That’s the twist: The principles of a simple circuit only apply to

a complete circuit. The EPC circuit has a pulsed

ground; the circuit is only complete while

the ground is applied. During the off time, the

circuit is open, so full system voltage passes through the solenoid.

if the control circuit is turned off, allow-ing pressures to rise to maximum.

To measure the duty cycle (figure 3):• Connect your meter’s positive lead

to the solenoid control wire.• Connect the negative lead to a

good chassis ground.• Set your meter to read duty cycle.

Your meter should display the duty cycle of the signal controlling the sole-noid. Compare that to the measurement you calculated, based on the published specs.

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Pushing Electons

FrequencyEvery now and then someone feels

the need to discuss signal frequency when explaining an EPC circuit.

Let’s be very clear: Signal fre-quency has absolutely no effect on EPC control circuit testing or measurement. In virtually every case, the frequency of the signal will be constant. And even if it varied, it wouldn’t affect current flow, voltage levels, or, most importantly, solenoid operation.

The only reason to consider fre-quency is if you suspect component wear: the slower the frequency, the more likely there is to be wear in the solenoid or valve body components. That’s because a slow signal allows the components to move back and forth during operation; almost like a vibra-tion being induced into the component. That vibration is more likely to cause wear in the solenoid and pressure con-trol valve components. A faster signal will reduce the vibration, so there’ll usually be less wear.

That’s the only reason to consider the EPC signal frequency during diag-nosis.

Current FlowI’ve said it before, but it bears

repeating: A current check is one of the

most valuable tests you can perform on an electrical circuit. It not only shows whether the signal is correct; it also verifies the circuit itself. And, because current flow is constant throughout the circuit, you can measure it anywhere in the circuit. All you have to do is clamp your current clamp around one of the

EPC solenoid wires, wherever it’s con-venient or easy to reach (figure 4).

But once again, to use a current measurement, you have to do a little math:

Remember Ohm’s Law? It proves that you can calculate any circuit char-acteristic as long as you know the other two. We know the solenoid’s resistance; that’s one characteristic. The second characteristic is voltage; for that we can use the published specs.

So, assuming a 4-volt drop across the solenoid, and a resistance of 4 ohms, the current flow should be 1 amp: 4 volts divided by 4 ohms. If the voltage were lower — say 3 volts — our current flow would be ¾ of an amp: 3 volts divided by 4 ohms.

At stall, the voltage drops to zero. Zero volts divided by 4 ohms equals zero amps. Easy, right?

For some technicians, the easiest way to check an EPC circuit is to fol-low the directions precisely and com-pare their measurement to the published specs. But there’s no reason you can’t use another testing method, as long as you know what you’re actually measur-ing… and what you’re comparing that measurement to. Otherwise you may find yourself comparing apples and oranges!

Figure 4: The advantages of a current test are that it indicates the condition of the solenoid and circuit, and you can take the measurement wherever it’s convenient

(shown by the red highlight). But now you have to calculate the amperage spec by using Ohm’s Law.

Figure 3: To use a duty cycle measurement, you’d still connect between the control wire and ground. But now you’d need to calculate the duty cycle

spec based on the published voltage spec.


GEARS January/February 2007 41

1. Following the published test procedure ensures:A. That you’re testing the circuit properly.B. That your test results should match the published specs.C. That you’ll be able to discuss your results easily with the

ATRA HelpLine.D. All of the above.

2. If you measure a circuit differently from the published pro-cedure:A. You may be measuring a completely different characteris-

tic of the circuit.B. Your results may not match the published specs.C. You may create confusion when calling the HelpLine.D. All of the above.

3. The test procedure in this issue’s Street Smart asks you to:A. Measure the voltage drop between the power feed and the

EPC solenoid.B. Measure the voltage drop across the EPC solenoid.C. Measure the voltage drop between the EPC solenoid and

ground.D. All of the above.

4. Tech A says you can check EPC voltage using a breakout box on the vehicle discussed in Street Smart.Tech B says, if you use a breakout box, the voltage won’t match the specs in the article.Who’s right?A. A onlyB. B onlyC. Both A and BD. Neither A nor B

5. To convert the published voltage specs to duty cycle, you must:A. Subtract the spec from the system voltage.B. Divide the spec by the system voltage.C. Multiply the spec by the system voltage.D. Add the spec to the system voltage.

6. The benefit of measuring amperage is:A. You can measure anywhere in the circuit.B. It tells you more about the circuit’s condition than a volt-

age or duty cycle test.C. Both A and B.D. Neither A nor B.

7. To use an amperage measurement, you have to know how to apply:A. Ohm’s Law.B. Kirchoff’s LawC. Newton’s First Law of MotionD. The Law of Gravity

8. To calculate the amperage spec, you need to know:A. Applied voltage and solenoid voltage drop.B. Applied voltage and solenoid frequency.C. Applied voltage and circuit resistance.D. The length of the wires being tested.

9. Tech A says you can calculate signal frequency if you know the voltage drop across the solenoid.Tech B says signal frequency has no effect on measuring the computer command to control the solenoid.Who’s right?A. A onlyB. B onlyC. Both A and BD. Neither A nor B

10. Which of these statements is true?A. If you add apples and oranges, you’ll get fruit cocktail.B. If you subtract apples from oranges, you’ll get marma-

lade.C. If you multiply apples by oranges, you’ll get grape jelly.D. If you divide apples by oranges, you’ll get Ohm’s Law.

TEST for Pushing Electrons; Apples and Oranges

Answers: 1. D; 2. D; 3. B; 4. C; 5. B; 6. C; 7. A; 8. C; 9. B; 10. A

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by steve bodofsky pushing electonsfigure 2: if you used a breakout box to measure the ford epc...

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