42 GEARSMay/June2007

Since the last edition of Hands on Diagnosis, I’m happy to report that I’ve regained some

of my speed in the rebuilding aspect of my job. As far as the diagnostic part, I’ve had quite a few interesting prob-lems to work on.

In this issue, we’ll cover three vehicles with bad computers. I’ll explain the different diagnostic steps and why I followed them. This will allow you to use these tests on vehicles with different problems.

When diagnosing a computer problem, poor grounds and improper supply voltage to the computer can cause many symptoms. These symp-toms can mislead you into believing the computer is bad when it isn’t. Always test voltage and grounds before condemning the computer. The voltage and grounds were fine in all three of these examples.

Right or wrong, diagnosis in the real world is a balancing act between using proper testing to pinpoint the problem, and throwing parts at it while crossing your fingers. It’s all about fix-ing the vehicle as quickly and inexpen-sively as possible.

There are many factors to con-sider when choosing between an edu-cated guess and a diagnostic process. But when diagnosing a bad computer, always use some type of diagnosis to verify the problem: Some of those computers cost over $1000!

1994 Chevrolet C1500 with a 4L60E

This vehicle originally came in with a broken drive shell. We rebuilt the transmission and installed all new

HANDS ON DIAGNOSIS

by Larry Frash

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44 GEARSMay/June2007

Hands on Diagnosis

solenoids and an internal wiring har-ness. During the road test the transmis-sion had very harsh engagements and upshifts.

There were no codes, and accord-ing to the scan data, Force Motor cur-rent was 1.13 amps on the desired and actual PIDs at idle. This indicated the computer was commanding the correct signal for normal line pressure at idle.

Usually the next test would be to connect a pressure gauge to the trans-mission to verify line pressure, but I was pretty confident it was high. At this point there were two likely possibilities: Either the new Force Motor was defec-tive or the computer was bad and lying to the scan tool.

It could have gone either way, but it was quicker to check Force Motor current than to pull the pan and install another new Force Motor.

I looked at the wiring diagram and found the computer pin numbers for the Force Motor High and Low circuit. I connected a current clamp around one of them (figure 1).

A current clamp is a valuable tool because it’s a non-invasive way to check current flow. If you don’t own a current clamp you can still check current with your ammeter, but you’ll need to cut one of the wires and connect them through your ammeter. Obviously the current clamp is a much easier way to check current; it saved me about 20-30 minutes on this job alone.

According to the current test, the actual current was 0.15 amps while the scan tool was reading 1.13 amps. But what could cause the current to be so different from the scan data? Low voltage or high resistance in the circuit can cause low current, but the computer should have recognized either of these conditions, and displayed low current in the scan data.

My conclusion was, since the scan data was different from the actual computer command, the computer was most likely the cause. Unfortunately I couldn’t think of another test to verify this, but installing a rebuilt computer solved the problem.

This is a good example of why you should never trust scan data 100%. If the computer is bad, it can easily send false information to the scan tool. By comparing scan data to voltage and cur-

rent tests, you can often isolate the root cause of the problem.

1994 Saturn with a TAAT Transaxle

This vehicle came in with a code for the Line Actuator electrical circuit. I cleared the code, but it came back intermittently when I turned the key on. This type of electrical circuit problem is either the computer itself, or the actua-tor circuit, which includes wiring, con-nections, ignition voltage, actuator, and the actuator circuit board.

In this vehicle, all actuators have a resistance specification of 4.5 – 5.5 ohms. The wiring diagram reveals each actuator receives key on voltage through its own fuse (figure 2).

Since the only trouble code being

set was for the Line Actuator, I focused on that circuit. With the key off and the computer disconnected, I measured resistance from pin F1 of the computer connector to the Line Actuator fuse. The resistance was 5 ohms, so the cir-cuit was within specs.

Next, I checked voltage at pin F1 with the key on. There were 12.6 volts, proving the fuse was good and the cir-cuit complete.

Because many electrical problems only happen when the circuit is under load, I performed a current draw test. Keep in mind these actuators are pulse width modulated; the computer never completely grounds them during nor-mal operation. But for a quick test you can ground the actuator, as long it’s only for a few seconds.

Figure 2

GEARS May/June 2007 45

Battery voltage was 12.6 volts. Ohm’s Law (voltage ÷ resistance = amperage) says each actuator circuit should use about 2.5 amps when fully grounded. To do this I supplied ground through an ammeter to pin F1 with the key on. The amperage was correct, which proved that at the time I tested the circuit, it was good. If the circuit was good, the computer must be bad.

But remember, although this code set often, it was still intermittent. The circuit could still be at fault. I wanted to verify the problem further.

Since the only trouble code was for the Line Actuator circuit, the computer saw the other actuator circuits as good. Because all actuator circuits are identi-cal, I switched the 2nd Actuator (pin F2) with the Line Actuator (pin F1) at the computer connector (figure 3a, 3b). To be on the safe side, I ran the resistance and amperage tests on the 2nd Actuator circuit before I switched them.

The logic of this test was simple: If a code for the 2nd Actuator set, it would mean the Line Actuator circuit was bad because that circuit was now switched to the 2nd Actuator pin of the computer.

But as soon as I turned the key on, the original Line Actuator code set. This proved the computer was at fault; the code never reset after we replaced it.

This may seem like a lot of work, but all this testing took just over an hour and the tests were important. Although the current draw test is much more accurate, it’s important to check the resistance in the circuit first, because a shorted circuit can damage your meter.

Switching circuits at a computer can be a useful test for other types of problems and vehicles. The important thing is, if you switch circuits at the computer, the two circuits must be the same type and resistance. For example, you could switch shift solenoid circuits on a 4L60E, but you should never switch the TCC Solenoid Circuit with the 3-2 Control Solenoid.

1998 Chevrolet S-10 Pickup with a 4L60E

This vehicle came in setting a code for a 3-2 Control Solenoid circuit error. I also noticed the engine was running very poorly. This was a new job and the customer also wanted a service, so

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46 GEARSMay/June2007

to save time I replaced the 3-2 Control Solenoid and the internal wiring har-ness while the pan was off. At the same time I verified the ignition switch was supplying B+ to all solenoids.

After the work, the code still set as soon as I started the engine. With proper voltage to the solenoid, a new harness and new 3-2 Control Solenoid, the only two things left in the circuit were the computer and the wire that connects the computer to the 3-2 Control Solenoid.

If this were an intermittent problem — one that occurred once or twice a day — diagnosis could have taken hours. In that case, I probably would have replaced the 3-2 Control Solenoid wire between the computer and the

transmission connector and gave it a shot. But this code seemed to set every time I started the engine.

I disconnected the computer and turned the key on. Using a wiring diagram, I located the computer pin that controls the ground side of the 3-2 Control Solenoid. There was system voltage to this pin with the key on, indicating the circuit was complete. In other words, system current was traveling from the ignition switch through the 3-2 Control Solenoid and to the computer seeking ground.

To test the circuit further, I grounded this pin through an ammeter. With a solenoid resistance of 22 ohms and 12.7

volts, the current in this circuit should be around 0.58 amps. It measured 0.56 amps.

The circuit tested good, so once again I was looking at a possible bad computer. But this computer was fairly expensive so I chose an additional test to verify the diagnosis.

Since there isn’t an identical circuit to the 3-2 Control Solenoid, I did the same thing a different way: I cut the 3-2 Control Solenoid wire about an inch from the computer connector and connected the end from the computer directly to a new 3-2 Control Solenoid. I connected the other wire of the new 3-2 Control Solenoid through a 10-amp fuse directly to the positive battery post. This duplicated the entire 3-2 Control Solenoid circuit, while eliminating the original circuit, except the computer.

The instant I started the engine, the code came back. Because I knew the cir-cuit I created was good, the computer had to be the problem. We installed a rebuilt computer and not only was the code gone, the engine ran like a champ. Needless to say, the customer was very happy.

As you can see, in each of these vehicles, I used different styles of logic and testing to check and verify bad computers. You can use these types of tests for many different electrical diag-noses. The key is knowing the circuit you’re testing and test you’re selecting are valid for that particular problem.

Well that’s all for this edition of Hands-On Diagnosis. Diagnostics can be a very frustrating and time-con-suming task, but keep in mind that with each vehicle you diagnose comes understanding that will help you diag-nose the next one.

Hands on Diagnosis

Figure 3B

Figure 3A

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