In the first part of this series ondiagnosing ECUs, we went oversome basic visual diagnosis ofcommon failures. In this, the secondpart of the series, we’re going to goover some basic tests you can performwith a digital multimeter (DMM) thathas a diode test function, and somemore advanced testing using a labscope. We’re going to focus mainly ontesting for some of the common com-ponent failures on ECUs that controlautomatic transmissions.

CAUTION: If you don’t feel com-fortable with testing or repairing anECU, or aren’t confident using a digitalmultimeter or lab scope, consider send-ing the ECU to a company that special-izes in testing and repairing them.Many can be found by doing a websearch on “ECM rebuilding,” or “ECMrepair.”

Before You Begin…Remember to check the basics

first. In the first part of this series wewent over some visual checks; look forthese failures before you start testing. Acracked solder joint can baffle you withan intermittent problem, and the crackmay be so small you can barely see it. Ifyou’re spending time checking forshorted transistors in the ECU, and theboss walks under the car and finds aharness that detours into the bell hous-ing or is wrapped around the exhaustpipe, he won’t be very happy. The pointis, make sure you’ve checked the cir-cuits on the vehicle thoroughly beforeyou go sniffing around inside the ECU.

When looking for a specific prob-

lem with an ECU, you need to have asmuch information as possible. Thismeans looking at a vehicle wiring dia-gram or pin chart to determine whichterminal on the ECU connects to thecircuit in question:

• If it’s a solenoid circuit, what’sthe problem? Is it energized allthe time? Does it energize atall?

• If it’s a sensor circuit, do youhave reference voltage to thesensor? Is the voltage too high?

And don’t forget to check all thepower (battery +) and ground (battery–) pins, as well as ignition terminalsthat are only hot with the key on.

Testing Solenoid Driver CircuitsYou may have heard the term driv-

er mentioned in discussions about auto-motive electronic systems diagnosis. Adriver is simply a power transistorinside the ECU that switches the poweron and off to control a relay or solenoid.Driver transistors are like little solid-state relays: They use a small current tocontrol a large current. The computercircuit inside the ECU can’t handle thehigher current required to control thesolenoid or relay directly. Instead, thecomputer circuit controls the drivertransistor, and the transistor does theheavy work.

The driver transistor is part of thedriver circuit. A driver circuit may alsoinclude a clamping diode or a current-

by Mike Van Dyke

Figure 1

4 GEARS October 2003

ECU-2.qxd 9/12/03 1:21 PM Page 4

borgWhyvo-full.qxd 9/15/03 3:04 PM Page 5

sensing resistor. These components aresusceptible to damage or failurebecause they carry most of the current,and can be overloaded when a solenoidcircuit’s resistance is too low or short-ed. These components can fail andcause the ECU to energize the solenoidall the time, or keep it from energizingthe solenoid at all. It’s more commonfor a transistor or diode to fail shortedthan to fail open, so let’s look at theprocedure to find shorted components.

Figures 1 and 2 show two simpli-fied, generic solenoid driver circuits:one for a ground-controlled solenoid,and the other for a feed-controlled sole-noid. An ECU may have either a clamp-ing diode or a clamping zener diode, orit may have no internal diodes. Lookingat these schematics, you can see that ifa clamping diode or driver transistorshorted, the solenoid circuit wouldshort to power or ground, depending onthe circuit design and which type ofclamping diode it has.

You can perform a quick test of asolenoid driver circuits at the ECU’sconnector without opening the ECU:

• Get a wiring diagram or pinchart for the ECU so you canidentify the pins.

• Unplug the ECU from the vehi-cle.

• Set your DMM to measureresistance, at the 1 megohmrange or higher.

• Connect one lead to the suspectsolenoid control pin.

• Connect the other lead to eachof the battery + pins, one at atime.

• Measure the resistance for eachpin.

• Repeat the test, with your probestill on the solenoid control pin,but now probe each ground(battery –) pin, and measure theresistance for each pin.

A normal reading will usually be4000 ohms (4k ohms) or higher. Themeter polarity — which terminal youprobe with the positive or negative lead— doesn’t matter for this test. Whatyou’re looking for is a low resistancereading; in many cases it can be lessthan 40 ohms. Shift solenoid driver cir-

cuits inside an ECU are usually identi-cal, so test readings from one shift sole-noid pin should be nearly identical tothe others.

If you discover a low resistancereading, you can open the ECU to tracedown the bad component. Follow thetrace on the circuit board back fromwhere it’s soldered to the pin until youcome to the clamping diode or drivertransistor. This is much like following ahydraulic passage through the case ands e p a r a t o rplate in atransmission.You can con-tinue to tracethe circuitthrough theboard andcontinue onthe otherside, testingyour concen-t r a t i o n .Figure 3shows aneasy trace tofollow: Itstays on oneside of theboard andleads to thedriver tran-

sistor and clamping zener diode. The transistor has three terminals.

When you find it, check the resistancebetween all three terminals. If only twoterminals read shorted, the clampingdiode is probably shorted. If all threeterminals on the transistor read shorted,the transistor is probably shorted.

Next, carefully de-solder andremove the suspect component, to testit “out of circuit,” to verify that it’sfailed.

6 GEARS October 2003

Figure 2

Figure 3

Diagnosing ECUs, Part II

ECU-2.qxd 9/12/03 1:21 PM Page 6

1-800-827-7455www.jasperengines.com

P.O. Box 650 • Jasper, IN 47547-0650fax 812-634-1820

36 Months/75,000 MilesNationwide Warranty

Parts and Labor!

Let us...• Help with high liability jobs

• Ease backlog work situations

• Provide the availability you need in time-crunch situations

69-Jasper Ad Aug-03.qxd 7/10/03 2:12 PM Page 69

IMPORTANT: Before you removeany component from the board, alwaysmark it or draw a diagram of it, so youcan make sure you reinstall it facing thecorrect direction.

Testing a Diode or Transistor Out ofCircuit

To test a diode:• Set your DMM to the “Diode

Test” function.• Connect each meter test probe

to a diode terminal and checkthe meter reading.

• Reverse the probes, and checkthe reading again.

A good diode will read about 0.5 to0.8 volts (depending on the meter) withthe negative probe on the “banded,” orcathode terminal (the one closest to thestripe on the diode body), and “OL” oropen circuit with the positive probe onthe cathode terminal. A shorted diodewill read near zero volts both ways.

From a testing standpoint, a typi-cal, bipolar transistor will look like twodiodes connected together, as in figure4. The base of a transistor would be theterminal that connects between the“diodes,” it isn’t always the center ter-minal on the transistor.

Set your DMM to the “DiodeCheck” function and measure betweenall three terminals of the transistor,reversing the test probes with eachmeasurement. Any measurementshould read either about 0.5 to 0.8volts, or “OL” (out of limits). The mainthing you are looking for is any readingthat is zero, or near zero volts, indicat-ing a shorted transistor.

It’s Alive!In this last

test, using a lab scope, we’ll check for a5-volt supply and activity around theMCU (Microcontroller Unit). TheMCU integrated circuit is a little com-puter itself; if it isn’t running, the ECUusually won’t function at all.

You’ll need a 20 MHz-or-higherlab scope and a 40 MHz-or-higher 10xprobe. If your probe is selectable, makesure to set it to the 10x position; thiswill minimize the loading effect on thecircuit. Next, you’ll need to power upthe ECU. You can do this either byplugging it into the vehicle harness andturning the key on, or by connecting a12-volt bench power supply to theECU. If you use a power supply, makesure you connect to all of the batterypositive pins, including those that arehot with the key on, and all of thepower ground pins (battery –). Consulta wiring diagram or pin chart to be sure.

Next, identify the MCU on the cir-cuit board. This is usually the largest orsecond largest integrated circuit on theboard, and it has an oscillator crystalmounted close to it. The crystal willlook like a little metal can, or a dippedcomponent with two or three terminals.Figure 5 shows examples of both typesof crystal. The crystal will usually haveits frequency printed on it; most arebetween 1 and 20 megahertz.

Set your scope to 0.5 volts/divi-sion, DC. Set the time base to 0.5microseconds/division. With a 10xprobe, this will give you 5 volts per ver-tical division and 1 microsecond(1/1,000,000 second) per 2 horizontal

divisions.Connect the ground lead of your

probe to a ground circuit on the board,or the power supply negative. Power upthe ECU, and probe one of the pins onthe MCU oscillator (figure 6). If theMCU oscillator is running, you shouldsee a sinusoidal waveform about 4 to 5volts high (figure 7). This is a good signof life. This particular oscillator is run-ning at 4 MHz, as there are 4 cycles ofthe waveform in 2 horizontal divisions.Probing random pins on the MCU, youshould also find steady 5 volts or 5-voltlogic on some pins. 5-volt logic willlook like a scrambled, messy squarewave on most oscilloscopes (figure 8).

If you don’t have access to a labscope, a logic probe is a cheap alterna-tive for looking for any activity on theMCU oscillator or pins. Radio ShackP/N 22-0303 will work just fine. Alogic probe is also handy to check fordata activity at the diagnostic connectorand on the data busses.

If you don’t have 5 volts on any ofthe MCU pins, the 5-volt regulator cir-cuit may be at fault. Some ECUs have asingle 5-volt regulator that looks justlike a transistor. It may have a “7805”or “78L05” part number printed on it.The regulator should have battery + (12volts) on one terminal, 5 volts on anoth-er terminal, and 0 volts (ground) on thelast. A bad 5-volt regulator or bad inter-nal ECU ground can cause the voltageon 5-volt supply to be too high. Again,look carefully for cracked solder jointsor burned traces on the circuit board.

8 GEARS October 2003

Figure 4

Figure 5

Diagnosing ECUs, Part II

ECU-2.qxd 9/12/03 1:21 PM Page 8

superior1plcd9.qxd 9/16/03 3:19 PM Page 9

Where’s the 5-Volt Reference?In some cases, the ECU will use

the 5-volt reference from another mod-ule. This is common on Nissan vehi-cles. The transmission control moduleon a Nissan very often receives its 5-volt reference signal from the enginecontrol module. This means that if theECM is dead, or you power up a Nissantransmission controller by itself, itwon’t have a 5-volt reference for theTPS and transmission temperature sen-sor circuits. The controller does have aseparate 5-volt supply for its MCU and

internal computer system.As I mentioned in part one,

remember to perform all ECU tests on astatic dissipating service mat whilewearing the attached wrist strap on yourworking hand. If you’re testing with aDMM, set it on the static mat, too.

That pretty much covers some ofthe basic testing you can do on an ECU.In the next issue of GEARS, we’ll lookat how to diagnose and repair of a fewcommon problems on a couple of trans-mission control modules, as well aswhat parts you need and where to get

them. And after we’ve repaired onetransmission controller in particular, itwill be BETTER than new!

10 GEARS October 2003

Figure 6

Figure 8Figure 7

The transmis-sion controlmodule on aNissan very

often receivesits 5-volt refer-

ence signalfrom the enginecontrol module.

Diagnosing ECUs, Part II

ECU-2.qxd 9/12/03 1:21 PM Page 10