Diagnostic Dilemmas: When Over-Voltages Electrocute PCMs

By Gary Goms. During the past nine months, I’ve had three instances of powertrain control module (PCM) failures on Jeep Wrangler vehicles that were evidently caused by an over-voltage condition.

The first case involved an owner who requested that a local shop replace the fuel pump because his ’98 Wrangler was slow-starting in the morning. The shop found the pump pressure was marginal and replaced the pump. Because the Jeep still had the problem, the owner complained about the fuel pump, first to the shop and then to the local jobber supplying the pump.

When the pump suffered a “complete failure,” the jobber recommended that the Jeep owner contact me. It didn’t take but a few minutes to confirm that the PCM wasn’t communicating with the scan tool.

The second Jeep failure was a ’92 Wrangler suffering from a no-cranking condition in which the owner had tried recharging the battery and jump-starting it, all to no avail. The owner had the Jeep towed to a local shop after he replaced the battery and discovered the Jeep still wouldn’t start.

The shop found a number of burned fuses and couldn’t get the PCM to communicate with the shop’s scan tool. After receiving a call from the shop, I confirmed the no-communication problem and performed the necessary testing to confirm the PCM was at fault. Since cores are in short supply, the PCM was sent to a rebuilder for repairs. After the reman PCM was installed, the Jeep still wouldn’t start.

My third case was another ’98 Wrangler, fully customized for off-roading. Because the owner had spent a great deal of money on the paint and mechanical accoutrements, money was no object in getting the vehicle repaired. Here again, I discovered that I had no communication with the PCM.

Three PCM Failures

The problem shared among these three Jeeps was a failure of the PCM to communicate with the scanner. In the first case, the owner knew the battery was failing, but neglected to replace it until his wife complained that the engine wouldn’t start when she tried to drive it to work.

He placed the battery on a trickle charge and returned at lunchtime to see if the Jeep would crank. It wouldn’t crank, and in frustration, turned his heavy-duty diesel battery charger to full boost. Then he noticed that the fuel pump wasn’t activating after he turned the key on.

When I tested it, the Jeep wouldn’t communicate with my scan tool. After the PCM was replaced, the fuel pump produced the correct fuel pressure.

Although the failure narrative on the ’92 Wrangler was less clear because I wasn’t able to directly interview the owner, the Jeep’s electrical system had obviously been exposed to very high amperages and possibly to reversed terminal polarity.

When the Jeep wouldn’t start after installing the remanufactured PCM, the client shop called in somewhat of a panic because the repair bills were mounting on the vehicle. During the second diagnosis, we noticed that the ignition produced an erratic spark.

The scan tool’s bi-directional controls proved that the coil and PCM were capable of producing full spark. Scope-testing the crankshaft position (CKP) sensor indicated that the CKP signal was erratic. The CKP sensor had either become defective before the battery failed or it was ruined by excess reference voltage when the PCM failed. Replacing the CKP sensor solved the problem.

In the last example, the ’98 Wrangler, was stored for the winter in a garage at a high mountain cabin. The following spring, the owner attempted to recharge the battery with a small trickle charger. Failing that, he called my client shop to diagnose the no-cranking problem.

A severely bulged battery case indicated that the battery had frozen during the winter. After replacing the battery, the Jeep exhibited a cranking, no-start problem and was transported to the client shop. After being called on the case, I found that my scan tool wouldn’t communicate with the Jeep’s PCM. Testing power and grounds indicated a failed PCM. Replacing the PCM remedied the cranking, no-start complaint.

Battery Failure Issues

The common thread in all three case studies is that each PCM failure was preceded by the owner ­attempting to recharge a bad ­battery. Because many recreational Jeeps are stored during the winter in mountain residences, it’s ­understandable why the batteries run down and eventually fail.

­Although the general rule of thumb for parasitic draw is about 50 milliamperes (ma), it’s also a rule of thumb that modern original equipment-specification batteries will last only from three to six weeks in storage before normal parasitic draw will drain them to the extent that they will no longer crank the engine. In fact, many owner’s manuals specify exactly how long a vehicle might be parked before it develops a low-battery ­problem.

In my first case study, I’ve found that a battery with a bad cell will cause cold-startup performance complaints on many Chrysler-built vehicles. I can only speculate, but I think that their adaptive memories are erased when battery voltages drop below the 9-volt threshold during cranking. After start-up, the PCM must then re-learn all of its adaptive fuel values, which might have led the owner to believe that his Jeep had a bad fuel pump.

In the second and third case studies, the vehicles wouldn’t start because the batteries had completely failed. But, how could a failed battery cause the PCM to fail?

Modern Battery Charging

Going back to the first case study, the owner admitted to attempting to start his Jeep by turning his heavy-duty diesel battery charger to full boost. In the second case study, the vehicle electronics had obviously been exposed to high amperages and possible reversal of polarity.

In the third case study, the owner implausibly destroyed his Jeep’s PCM by attempting to charge its battery with a small trickle charger.

All of these PCM failures were likely caused by exposure to high voltage in the electrical system. Keep in mind that a battery acts as a capacitor that absorbs amperage and buffers voltage spikes. Without a battery in circuit, the vehicle’s electrical system is completely exposed to voltage spikes and high charging voltage. While most modern PCMs are protected from high-voltage spikes, none can withstand prolonged exposure to high charging voltages.

To protect modern electronics, modern battery chargers are limited to about 16 open-circuit volts. Because older battery chargers often exceed 20 open-circuit volts, it’s easy to see why even a small trickle charger can ruin a PCM when a battery is taken out of circuit by a corroded terminal or open-circuit cell. In this situation, the PCM can be exposed to very high charging voltages the second the ignition key is turned on. So my hypothesis is that all three of these vehicles were suffering from an open-circuit at the battery even as their respective owners were attempting to charge the battery with an obsolete or defective battery charger.

A Word About Diagnosing Failed PCMs

When diagnosing any PCM for a non-communication issue, it’s important to make sure that the VIN data is correctly entered and the diagnostic connector is powered and grounded correctly. With OBD II vehicles, I’ll either select “generic” or “global” on the scan tool just to make sure I don’t have a data entry problem. In other cases, I connect a professional “generic” scan tool to double-check communications issues. When diagnosing OBD I vehicles, I’ll try to manually retrieve flash codes before condemning the PCM or ECM.

It’s always important to consult a wiring schematic and make sure that all power and ground connections are clean and functioning correctly. I often use a small, homemade bulb tester to load both power and ground connections with about one-half ampere of current flow.

In some cases, I brush a very small amount of electrical conductivity enhancer like Stabilant 22 on the PCM pins just to make sure I have good pin contact. In other cases, it’s advisable to check the tension fit on individual pins, either with a special tool, spare mating pin or drill size.

Last, be aware that a shorted sensor can interfere with PCM communications by pulling down the PCM’s reference voltage. There are anecdotal cases in which a shorted VSS sensor had defeated scanner communication on some Jeep vehicles. In this case, it might be helpful to consult applicable technical service bulletins to help locate the suspect sensor or, as a last resort, start disconnecting the various sensors.

Whenever diagnosing any potential no-communication problem on any OBD II PCM, remember that the root cause of any PCM failure can be an over-voltage condition in the vehicle’s electrical system. In all cases, I recommend bench-testing the alternator for voltage output and AC voltage ripple before starting the vehicle just in case the initial PCM failure was caused by a defective voltage regulator or diode.

Before releasing the vehicle to the customer, check all lighting systems, accessories, communications equipment, and other solid-state equipment for functionality. Taking these extra steps will help prevent return failures and future customer complaints.