Do It Yourself Truck Repair, Case Study No. 2 – The Author’s Nightmare

I am researching this case study on a 1994 Ford F-150 that showed up having some serious engine performance issues, when it actually turned out to be a simple problem that most technicians rarely consider, much less test. I had maintained and repaired this particular vehicle for over 2 years, including extensive work on the front end, suspension, fuel delivery system, and even an engine replacement. All recommended services were addressed and completed immediately. My relationship with this client was very good and he explicitly trusted me for the repairs of all his vehicles.

The truck arrived at my shop with the engine running very bad only when it got hot. It had been adjusted less than 6 months before. I noticed that a new alternator had been installed and not me. I remembered that the client had told me that he was on vacation a month ago and had installed it because it failed while traveling. I have inspected the primary and secondary ignition system for signs of failure. These systems worked well. I hooked up my scan tool to retrieve any service code, where none. I ran a “key on engine off” and “key on engine running” self test with my scanner. No codes were retrieved. Then I went into data stream mode and looked at the critical inputs and outputs of the motor control. All readings appeared normal except for the upstream O2 sensors; both indicate “rich fixed”.

I immediately attached my lab oscilloscope and checked the condition of the upstream O2s. They were both at .75 – .9 volts and not “changing”. From the sound of the engine and the condition of the upstream O2 sensors, I was inclined to believe that the vehicle possibly had an ignition timing problem. Then I plugged in my sync light, disconnected the “sync jumper”, and checked the base sync. Was dead. I reconnected the sync jumper and checked the PCM sync control. It was all over the map! I couldn’t get a stable reading, even at rest. This led me to do tests on the ignition module and pickup coil assemblies. These components also tested well.

At this point, I had about an hour and 15 minutes to diagnose this truck. My practice has been to stop after an hour, reevaluate and brainstorm or research after this point. My shop had online access to a repair database, plus we had hard copies of the repair tracking information (I recommend both for any shop!) And I spent some time researching the possible causes of this symptom. One possible cause showing the exact symptoms was that the distributor shaft had become “magnetized” and was interfering with the performance of the ignition modules to control engine timing. A simple test was to remove the distributor cap and rotor and, with a non-magnetized piece of steel, check if the distributor shaft is magnetized. It was! Problem solved.

I ordered a remanufactured Distributor from my local parts supplier, called the customer with my diagnosis, and obtained authorization to do the repairs. I installed the distributor, reset the ignition timing, checked the “closed loop fuel control” with my scanner, tested the vehicle and invoiced the repair order. The customer paid his bill, picked up his truck, and continued on his way.

The next day, the customer called me and told me that his truck was doing the exact same thing, also sometimes it was working poorly and at other times it was working well. And it “seemed” to work best when it was cold. I took some notes and told him to bring the truck. When he arrived, the truck was working fine, but the customer decided to leave it with me because he wasn’t sure of its reliability and didn’t want to be stranded somewhere if it broke down. He also inquired about past repairs and the bill. It has been my practice to compensate any of my clients if I misdiagnosed a vehicle and explain that their previous invoice would apply to this repair (a practice I wish more shops would do!) If I had in fact misdiagnosed the problem.

Over the course of the next 3 days, every spare moment I had was spent diagnosing this truck. Sometimes it would show the problem and other times it would work fine. It is estimated that I spent over 8 additional hours trying to solve this problem. Countless hours in my spare time researching. He had even dusted off an old Ford OEM Repair Manual looking for clues! The customer had called multiple times to get updates, hoping it would be fixed. My customer service skills were at stake, not to mention my reputation and let’s not forget my professional pride!

My frustration level by then was over 10.5. I finally swallowed my pride, picked up the phone, and called one of my “mentors”, one of the many people who had trained me on the proper procedures for dealing with these kinds of issues. After describing the problem and the steps he had taken to correct it, the first words out of his mouth were: “Did you check the alternator, with your lab oscilloscope, for excessive AC voltage output?” As soon as he spoke those words, this light went out in my head! I said to myself, well, I won’t say what I said to myself because it wasn’t pretty.

Note: Most vehicles run on 12 volts DC. The battery is 12 volts DC. The alternator produces AC current and internal electronic components convert AC voltage to DC voltage and regulate the amount of voltage that goes to the battery to keep it charged (usually 14.5 volts DC).

I set up my lab oscilloscope and tested the alternator. AC voltage output exceeded 1 volt! Simply put, AC voltage has no place in a DC voltage system! A “rule of thumb” is (and this may vary depending on who you ask) no more than .3 VAC. A quality new or remanufactured alternator will have an AC voltage output in the millivolt range (less than one-tenth of a VAC). An inferior quality remanufactured alternator had been installed. The alternator was producing too much AC voltage and causing the problem with this truck. I replaced the alternator with a quality remanufactured one, tested its performance and performance, and sent the customer on his way.

Note: When testing AC output, always test on the alternator, never on the battery. There are also quality battery / charging system testers available, I prefer the lab oscilloscope.

Needless to say, this test was added to my “Standard Test Procedures” regimen. It’s fast and takes less than 5 minutes! I highly recommend that all shop owners and technicians run a set of written tests on any engine performance issues. It doesn’t have to be anything fancy, 2 or 3 pages of standard tests to rule out (or discard!) Certain components. Also, some of the major parts suppliers offer free or low-cost testing procedures, use them as a guide to tailor them to your own needs. I also recommend that all store owners, who do not have one, make the investment in their business and acquire some kind of repair database. Either online access, CD or DVD. With the time and money you save, it will pay off in the long run!

Proper testing and knowing how to test is paramount. I urge all shop owners and technicians to get properly trained and certified. We are professionals! Every year new vehicles become more technologically advanced and we have to keep up with this technology. We have no choice!

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