But like father time, it’s time to look into the future and make a move toward real world diagnosis. The kinds of diagnosis where your primary tools have wires, and don’t just get thrown in your toolbox. The kinds of tools that cost $2500 to buy and $500 to update. So what is it about diagnosing that gives you the biggest problems? The time it takes, the cost of your new tools, or is it your lack of confidence? Maybe it’s all of the above or maybe it’s none at all. Either way you say it, it’s time to move on.

In this issue we’ll cover techniques used to diagnose some of those problem vehicles quickly, using your scan tool and your ability to choose. There are a lot of common calls we get at the ATRA Technical HelpLine and each one of them takes the technician through a different diagnostic routine.

Unfortunately, you can’t use your scan tool on some vehicles. In that case, you’ll have to diagnose them the manual way, one sensor at a time. Here’s an example.

Consider the Toyota that comes to the shop with a Vehicle Speed Sensor (VSS) code. You replace the sensor, but the code reappears, causing your frustration level to peak. Here is a technique you’ll want to remember: Toyotas typically use two sensors for their VSS system. Consider it a backup system, Toyota refers to them as Vehicle Speed Sensor #1 the back up sensor and Vehicle Speed Sensor #2 the primary sensor. VSS #1 is cable-driven and located behind the dashboard in the speedometer head. VSS #2 is located on the transmission (figure 1).

Figure 1

Both sensors are used and compared by the Transmission Control Module (TCM) or Engine Control Module/Unit (ECM/ECU). If the ECU reads an incorrect signal from one of the two sensors it’ll trip a code. Most VSS #1’s are driven by a cable, so don’t overlook the cable as a possible problem. If it’s frayed or dry it can cause a number of different problems, such as a bouncing needle, a noisy speedometer head or no movement of the speedometer head.

To skip the headaches, disconnect the transmission speed sensor. If the code goes away and the vehicle drives correctly, repair the wiring to the TCM, ECM or ECU, or replace the #2 sensor at the transmission. If you’ve already replaced the VSS sensor, more than likely the problem is in the wiring.

If replacing the sensor didn’t change the vehicle’s operation and the code is still present, reconnect the transmission VSS sensor and remove the cable at the transmission. If the vehicle shifts now, more than likely the speedometer head is faulty. Reconnect the cable, and check the signal going to the ECU from sensor #1 (figure 2) to be sure.

Figure 2

Obviously the easiest test is to disconnect the sensor. Within ten minutes you should have a pretty good idea of where to look for the problem.

These examples are helpful hints for diagnosis, and should be much easier with scan tool capabilities. Let’s see how to use a scan tool for diagnosing a 1996 Ford, with an E4OD transmission. It has harsh shifts and code P1728 in memory.

P1728 is a component slip code. It’s usually caused by a slipping torque converter clutch, but not always. Before you pull the unit and replace the converter — and have the customer come back in the next few days with a code P1728 again — let’s look at the easy way to diagnose this problem.

After clearing the code (figure 3), connect your scan tool to the vehicle, locate the live data screen (figures 4a, 4b). The PIDs (Parameter Identifications) you’re interested in are TCC% and TCC-MACT (RPM). In some cases your scanner will not have TCCMACT shown on the scanner and it may be necessary to watch the RPMs very carefully. TCC% is the level of amperage or the duty cycle the PCM (Powertrain Control Module) is using to control the torque converter clutch. TCCMACT is Torque Converter Clutch Application Modulated Actual (RPM); whew, that’s a mouthful, but a need-to-know PID. At full lockup, the TCC% should be between 90% and 100%. At the same time, the TCCMACT (RPM) should drop from about 350 RPM to 10–0 RPM at full lockup.

Figure 3	Figure 4a	Figure4b

Will this tell you whether the Toque Converter is slipping or if the code is being caused by an internal component? The TCCMACT will tell you whether the torque converter is slipping, but you’ll have trouble telling whether an internal component is slipping. If the TCCMACT (RPM) drops to at least 10 RPM when the TCC% is near 100%, the torque converter can’t be slipping, but if its higher it doesn’t mean it’s the Converter clutch it may be an internal component. The next check is to look for a slip from the internal components.

Clear the code, and drive the truck on the rack for 10 to 15 minutes. If the code resets you can bet you have an internal problem. The reason for the rack drive is so the torque converter can be applied with no load on it. Now the computer won’t see a slip unless there’s an obvious leak in a clutch.

If the code only resets on the road, make certain you take your scan tool with you and get a movie or freeze frame when the code sets. Document the toque converter percentage (TCC %) and TCCMACT. It’s possible for the converter to fail only when hot, after a long drive. Valve wear, clutch wear, fluid breakdown, and overloading can all cause converter clutch problems. Again, within a few short minutes, we have a direction to look.

More often than not, we tend to look a little too deeply and miss the big picture. How many times have you pulled your hair out on a 4L60E truck with a 2-4 burnt band? You’ve over-hauled it once or twice for the same problem and the customer claims, "It’s been doing this thing for a while, but I just don’t have the time to drop it off." What’s "this thing"? "This thing" is second gear starts. In other words no power to the solenoids because the ignition switch is bad. Yeah, you know that problem now, but did you know it before you fell into its trap?