Motor Age Garage: Boggy Territory

Jan. 1, 2020
Aging vehicles with quirky electronics are the spice of a diagnostician's life.

Right after computers first began to appear on vehicles, my parts supplier gave me the opportunity to attend a Computer Command Control class in Houston at the General Motors (GM) training center. It was 1981, and the questions my classmates asked then-GM instructor Ellen Smith went something like this:

"What's going to happen when one of these cars gets 100,000 miles on it? Won't it be a disaster trying to keep all this computer stuff straightened out?"

Ellen replied (quite truthfully) that a properly maintained computer-assisted engine would run as well at 100,000 miles as it did when it was new. She cited several specific examples of cars she knew about that proved her point, and we've all seen thousands of them since.

The questions the mechanics fired at Ellen continued: "Will it start if the computer craps out?"

Ellen disconnected the computer on the Buick trainer vehicle and turned the key. The car started and ran, albeit with the mixture control metering rod in the full rich position and without proper ignition timing control, but the car could be driven that way. Granted, today's cars can't.

But Ellen believed in computer-controlled fuel and ignition, even when the independent shop techs like me weren't so sure it was a good idea to let the computer handle that stuff.

In the here and now, we know that today's fuel economy and emissions standards would be virtually impossible to meet without sophisticated electronics calling the shots, and they've spread their tentacles into virtually every part of the vehicle. Some new Hyundai models even have electronically controlled engine mounts.

But what about those high-mileage cars, such as the Lexus SC300 I wrote about recently that came in blowing gasoline steam out the tailpipe because of a computer problem nobody wanted to tackle? We managed to fix the fuel problem with a remanufactured powertrain control module (PCM), but that same tired old Lexus SC300 came in a couple months later needing a $450 power steering hose.

And what about the thousands of unbilled labor hours veteran techs like me have put into diagnosing older electronically controlled fuel systems on cars that weren't worth the time and didn't deserve the effort because the customer wasn't willing to pay anyway? Where should the line be drawn? How much money should a willing customer be expected to spend on a car that has drifted past the point of obsolescence?


This 1995 Talon wasn't quite a junker, even though it was 12 years old, but it was dead as a doornail. The owner called me when we were swamped with work and holding final exams. Her Talon had died and had been sitting for a while, and I told her it would have to sit awhile longer before we could have a look at it. She didn't seem to be in too much of a hurry for the car. As far as I was able to tell when she had it towed to us, nobody had touched it.

This vehicle was one of those peculiar transitory Mitsubishi on-board diagnostic II (OBD II) compliant units that needs a special adapter cable to talk to the enhanced room in the PCM. Our scan tool has a special "Y" cable that connects to the OBD II data link connector (DLC), as well as the Mitsubishi data link; it is mounted about 6 inches from the OBD II connector. The OBD II generic room – which sometimes has codes that don't show up in the OBD II Enhanced room – can be accessed via the universal 16-pin hookup. However, the Talon wouldn't talk to us either way. To a tech like me that likes to grab data quickly and spring into action, this is annoying beyond measure.

In a no-communication situation like this, it's imperative to determine if the PCM has power and ground at the appropriate pins. This PCM had those feeds, but there was no ignition and no fuel pulse forthcoming. Could it be a bad crank position sensor? Maybe, but that alone wasn't likely to cause a no-communication problem.

This was a set of circumstances where a scan tool isn't much help, but it was obvious that the PCM had died. There was no reference voltage anywhere either, and with the idea that a shorted three-wire or Hall-effect sensor might be neutralizing the reference voltage along with everything else the PCM was trying to do, I found that disconnecting the pertinent sensors did no good.

Generally, my experience has been that the whisper of current provided by reference voltage circuits does no permanent harm to the PCM; it simply causes a no-start. For example, it isn't all that unusual to have the Differential Pressure Feedback EGR (DPFE) sensor short the 5-volt reference line to ground internally on a Ford. It kills the car along with scan tool communication, but doesn't hurt the PCM. I worked on a police Crown Victoria that had the reference voltage wire, which fed the fuel tank pressure sensor, scratching on a bracket near the fuel tank. It would stall during right turns.

Well, even with the suspect sensors disconnected (crank and cam were still in the loop), we still had no spark and no fuel pulse. Removing the PCM from the Talon, I plugged it into a 1995 Dodge Neon I use as a trainer car because the pinout was almost exactly the same. The only difference was that the Talon has a separate computer for the transmission and the Neon PCM controls the torque converter.

With the Talon PCM installed (I didn't try to start the Neon), the scan tool wouldn't talk to the Neon either. That was another indicator that the Talon PCM was cooked. I had a bad feeling about trying my Neon PCM on the dead car. A replacement PCM was only $200 from the dealer, but it would be on backorder for about three weeks before I could get one.


Installing the replacement PCM, my students managed to start the Talon, but before I could get to the car, it had died again. In a perfect world, we would have connected the scan tool as soon as the PCM was replaced, but a couple of my guys jumped the gun and allowed this angry little Eagle to eat the new PCM for lunch. This was turning nasty.

Have you ever been at this fork in the road? What now, especially since no salvage yard in our part of the state had an engine controller for a 1995 Talon? Would an exhaustive check of every pin for power, ground and/or shorts to other wires be a viable test? Maybe, but what if the problem was intermittent? After all, the car did start and run for a few minutes before it cooked the replacement PCM.

Well, it just so happens that I have a very expensive piece of equipment that was designed to find problems like this. I put it to work.

First, I called the dealer and reordered PCM number two.

Second, I rented the necessary cable to connect my outdated Snap-On/Sun Dynamic Data Collector (DDC, originally produced by Edge Diagnostics) to the Talon.

This pricey little box, called a "pod," cost my department many thousands of dollars and is the size of a small suitcase. It came with an interface card that had to be installed in a Windows PC with an Industry Standard Architecture (ISA) slot.

No modern PC comes with an ISA slot. That's how old this DDC unit is.

The DDC pod doesn't pull diagnostic trouble codes (DTCs) or talk to the serial link at all; it simply connects between the vehicle's PCM and harness just like a breakout box. That's why I had to rent the cable for two days. The cables are vehicle-specific, and because they haven't done any updates on this equipment, you can't get one for just any and every vehicle. The DDC checks each individual pin and sends the resulting information via a special cord to the PC-mounted interface card, which operates through the PC's motherboard and video card to display the data on the monitor in a colorful fashion using special software.

With the Talon selected, the software designers had been diligent enough to give the program full knowledge of what each and every pin on the 1995 Talon PCM should read in ohms and volts. With that data at its disposal, the DDC performed a "Sweep Test" on all the circuits connected to the PCM while I stood back.

When it was done checking resistances, the DDC told me to turn the key on and upon checking the volts at each PCM pin, the DDC software flagged all the reference voltage lines in red; in effect, there was no reference voltage that amounted to anything. Pin 44 feeds 9 volts out to the injectors, and that pin was dead as well.

This piece of information was obviously an effect rather than a cause. The source of this disastrous failure remained obscure, masked by the lack of voltage at all the sensors.

I took a chance and plugged my 1995 Dodge Neon controller to hopefully regain reference voltage, then I retriggered the sweep test. The results looked a lot different this time around. I had reference voltage, but the throttle position (TP) sensor input line was dead.

Disconnecting the TP sensor and doing another test, I found that the TP sensor line read more than 4 volts with the sensor disconnected. Had the voltage remained low, I would have figured it was a wiring problem. As it was, the TP was the only faulty input. Why it might destroy the whole circuit within the PCM remains to be figured out, but there it was.


The TP sensor was $85, and with the new PCM installed, everything was fine. We drove the car around quite a bit to make sure it was OK. The owner paid for one PCM (DaimlerChrysler graciously warranted the first one) and a TP sensor. The encounter was a victory, but finding the problem wouldn't have been so tidy without the DDC. As it was, I spent $100 of somebody else's money just to rent the cable.

I hadn't previously seen a TP sensor fry a PCM that way, but this time around, that appears to have been the case.

We managed to get this sporty little Talon going for less than three hundred dollars – the territory was as boggy as any I'd seen, but we managed to pull this one out.

Richard McCuistian is an ASE-certified Master Auto Technician and was a professional mechanic for more than 25 years. Richard is now an auto mechanics instructor at LBW Community College/MacArthur Campus in Opp, AL. E-mail Richard at [email protected].

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