May 2008 Articles & Features

Advance Auto


adjust text size: Brain Transplants By the Numbers

By Sam Bell | May 2008

Due to the expense involved, the decision to replace a vehicleâ€™s brain should not be taken lightly. Following the necessary pre-op investigative procedures will verify the necessity, then assure the success of the operation.

Long a staple of cheesy sci-fi movies and horror films, the very notion of brain transplants conjures up images of badly unbalanced â€œscientistsâ€ obsessively working late into the night in their secret underground laboratories. After all the necessary connections have been made, a switch is thrown. While lightning crackles outside, voltages surge and the â€œpatientâ€ is revived! That's almost how it looked at my shop one recent evening.

A little background is in order. The 1967 Volkswagen Type 3 was the first mass-market car in the U.S. to utilize an electronic control unit (ECU) to regulate fuel delivery. The last vehicles with nonelectronically controlled carburetors were produced in the mid-1980s, and by 1990, all new vehicles sold in this country featured electronically controlled fuel and ignition systems. The so-called brain box was here to stay. Despite some early problems, the vast majority of automotive ECUs in use in the 21st century are quite robust and reliable.

This article deals with fairly complicated diagnostic issues. It is not, in fact, for the faint of heart, but for top technicians who don't flinch away from complicated work. Where I refer to the schematic diagrams represented in Figs. 1-5, please take the time to carefully examine the circuits in question. It will help you do automotive brain surgery on vehicles that may roll into your bay next month...or perhaps tomorrow.

Neuroanatomy: Sensors & Motor Nerves

Brains and brain-boxes alike need some way to perceive the world around them. We humans rely on vision, hearing, touch, smell and taste. Computers have to make do with all kinds of sensorsâ€”temperature and pressure sensors, position and speed sensors, specialized oxygen or air/fuel sensors, knock sensors and a host of assorted miscellaneous sensors reflecting simple binary states (think brake on/off or a/c request) or sophisticated multimodal states (think yaw, pitch, roll and acceleration).

Fig. 3 on page 37 shows a portion of the powertrain control module (PCM) wiring schematic for a 1999 Toyota Corolla VE. There are five sections in all. Let's look at a few of the input circuits in detail. At terminal VC/wire 1B (arrow A) we see a 5-volt reference signal sent out to be shared by the MAP, TPS and VPS (tank vapor pressure sensor). Each of these sensors also shares a sensor ground, E2/9B (Fig. 4), via the splice at IB, and each provides its own signal return (at PIM/2B, VTA/11B and PTNK/7B, respectively.) It's clear that none of these signals can ever exceed the reference voltage even if unplugged, and that their minimum possible values must always equal or exceed ground voltage. The total voltage in each circuit (that is, the sum of the voltage between the reference voltage and the signal, added to the voltage between the signal and the ground) is equal to the original reference voltage. Thus these three sensors are examples of voltage-splitting circuits.

Take a look at the ECT and IAT sensor circuits at terminals THW/4B and THA/3B (arrows B and C). Each has its own 5-volt reference feed. This time, each circuit features a negative temperature coefficient (NTC) thermistor and a sensor ground, again at E2, via the IB splice. The thermistors function as voltage-dropping resistors to report operational temperatures needed to adjust the base spark timing and fueling programs. At very cold temperatures, or when the circuits are open, the reference voltage will be dropped only a little or not at all. Very hot conditions or a shorted sensor circuit will result in substantial or total voltage drop. Such circuits are called voltage-dropping circuits.

Arrow D at NE/4A (Fig. 5) points out a third type of circuit used for the crankshaft position sensor (CKP). This input takes the form of a supplied AC voltage signal whose frequency varies with engine speed. A â€œmissing toothâ€ reluctor provides additional information on the basis of which the crankshaft's exact position can be calculated. Toyota labels this sensor NE, which stands for â€œNumber, Engineâ€â€”or, as you and I would think of it, rpm. The G2 or CMP signal is similar, but features only one hump. These are examples of voltage trigger circuits.

Next, look at the circuit indicated by arrow E at STA/11C (Fig. 3). This is an example of a pull-up circuit. An applied voltage detected here indicates that the starter is engaged. Finally, look at OX1, the upstream oxygen sensor terminal, shown here with arrow F at 8B (Fig. 4). This is a pull-up/pull-down circuit. A value above the PCM's internally generated reference voltage (.45 volt for this circuit) is interpreted as a pull-up rich condition, while a voltage lower than .45 is a pull-down indicator showing a lean condition.

Our human brains rely on motor nerves to carry out our intentions as we interact with our surroundings. Similarly, automotive brain boxes need some way to implement the decisions they reach based on their programming and the inputs they have received.

Fig. 4 also shows a few of the actuators under the PCM's control. Arrow 1 at #10, #20, #30 and #40 (12A, 25A, 11A and 24A, respectively) indicates the injectors. A separate driver within the PCM completes each injector's ground circuit, causing the injector to spray a precisely calculated amount of fuel just outside the intake valves. Similar drivers are used to actuate a variety of electromagnetic relays and solenoids.

The twin DIS coil packs are under the control of integrated igniter modules which, in turn, are controlled by their own PCM driver circuits in Fig. 5 at IGT1/20A and IGT2/19A (arrows 2 and 3).

The idle air control valve in Fig. 2 features a rotary vane. The vane position is controlled by a duty-cycled magnetic coil, responding to the commands of a driver at RSO/10A (arrow 4).

The Diagnostic Conundrum

Follow me as I relate the Corolla's story. The car was towed in with the complaint â€œdied while driving.â€ Naturally, the car started and ran as soon as the tow truck dropped it off. There were no codes or pending codes in memory, and the data stream values all seemed normal both KOEO and KOER.

Great, I thought. It's cold and snowing, and I have a car with an intermittent dying problem. Good thing my cell phone's charged up! I drove around for 20 minutes and returned to the shop. I let the car idle in the back bay (with an exhaust hose on it, of course!) for 21â„2 hours, when it thoughtfully stalled ten minutes before closing time.

There were no codes, of course, and the stall was not preceded by any roughness or unusual noises. Rather, it seemed almost as if someone had simply switched the ignition off. This time, though, the Corolla didn't restart. We quickly determined that there was no spark and not even a signal from the PCM to the coil and igniter assemblies. It was a diagnostic conundrum.

Were we missing some critical input, like CKP or CMP? Was it an ignition switch problem? A faulty ground? By now it was past closing, so I decided to pack it in and try to pin it down the next day.

The next day, the car idled flawlessly for over four hours before suddenly dying. Again, there was no spark. As we saw previously in Fig. 5, the PCM controls the coil packs through their respective igniters via two separate wires. The odds of both signals disappearing by coincidence were incredibly small. Scoping the CKP (the waveform in Fig. 6 on page 72) and CMP sensors showed apparently normal waveforms even during cranking with no spark. I hadn't yet accessed the PCM to verify that the signals were making it all the way there, so I couldn't rule out a wiring problem. Or could I? I powered up my scanner again, thinking I could check the rpm PID during cranking, but I was unab