Do It Yourself Truck Repair, Case Study #2 - Author's Nightmare

I'm dredging up this case study about a 1994 Ford F-150 that appeared to have some serious engine performance issues, when in fact it turned out to have a simple problem most technicians rarely consider, let alone test for. I had maintained and repaired this particular vehicle for over 2 years, including doing extensive work to the front end, suspension, fuel delivery system and even an engine replacement. All recommended services were promptly addressed and completed. My rapport with this customer was very good and he trusted me explicitly for repairs to all his vehicles.

The truck came to my shop with the engine running very rough only when warmed up. It had been tuned up less than 6 months prior. I did notice a new alternator had been installed and not by me. I remembered the customer had told me he was on vacation a month ago and had it installed because it failed while traveling. I inspected both the primary and secondary ignition system for signs of failure.

These systems checked out fine. I hooked up my scan tool to retrieve any service codes, there where none. I ran both a "key on engine off" and "key on engine running" self tests with my scanner. No codes where retrieved. I then went to data stream mode and observed the critical engine control inputs and outputs. All readings appeared normal except for the upstream O2 sensors; both indicating "fixed rich".

I immediately attached my lab scope and verified the condition of the upstream O2's. Both where at .75 - .9 volts and not "shifting". By 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 issue. I then attached my timing light, unplugged the "timing jumper" and checked base timing. I was dead on. I reconnected the timing jumper and checked the PCM's timing control. It was all over the map! I could not get a stable reading, even at idle. This lead me to conduct testing on the Ignition Module and Pick Up Coil Assemblies. These components also tested fine.

At this point I was roughly an hour and 15 minutes into the diagnosis of this truck. It's been my practice to stop after an hour, re-evaluate and brainstorm or research after this point. My shop had on line access to a repair data base and additionally, we had hard copies of repair tracking information (I highly recommend both for any shop!) and I spent some time researching possible causes for this symptom. One possible cause that displayed the exact symptoms was that the distributor shaft had become "magnetized" and was interfering with the Ignition Modules performance in controlling engine timing. A simple test was to remove the Distributor Cap and Rotor and with an unmagnetized piece of steel, check if the Distributor Shaft is magnetized. It was! Problem solved.

I ordered a re-manufactured Distributor from my local parts supplier, called the customer with my diagnosis and got authorization to do the repairs. I installed the Distributor, reset the ignition timing, verified "closed loop fuel control" with my scanner, test drove the vehicle and billed out the Repair Order. The customer paid his bill, picked up his truck and went on his way.

The very next day the customer called me and said that his truck was doing the same exact thing, additionally it would run rough sometimes and at other times ran fine. And it "seemed" to run better when it was cold. I made some notes and told him to bring the truck in. When he arrived the truck was running fine, but the customer decided to leave it with me as he was unsure of its reliability and did not want to be left stranded somewhere if it broke down. He also inquired about the previous repairs AND bill. Its been my practice to compensate any of my customers if I misdiagnose a vehicle and explained to him that his previous bill would be applied to this repair, (a practice I wish more shops did!) if in fact I had misdiagnosed the problem.

Over the course of next 3 days, every spare moment I had was spent trying to diagnose this truck. Some times it would exhibit the problem and other times it would run fine. I had an estimated 8+ additional hours spent trying to get this problem solved. Countless hours in my off time researching. I had even dusted off an old OEM Ford Repair Manual looking for clues! The customer had called numerous times for updates, hoping it was repaired. My customer service skills were on the line, not to mention my reputation and lets not forget my professional pride!

My frustration level by then was topping out at around a 10.5. I finally swallowed my pride, picked up the phone and called one of my "mentors", one of the many persons who had trained me in the proper procedures of approaching these types of problems. After describing the problem and the steps I had taken to correct it, the very first words out of his mouth were, "Did you check the alternator, with your lab scope, for excessive AC voltage output?" As soon as he uttered those words, this light went off 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 are "powered" by 12 volt DC. The battery is 12 volt DC. The Alternator produces AC current and internal electronic components converts this to AC voltage to DC voltage and regulates the amount of voltage going to the battery to keep it charged (usually 14.5 volts DC).

I set up my lab scope and tested the alternator. The AC voltage output was in excess of 1 volt! Simply put, AC voltage has no place in a DC voltage system! A general "rule of thumb" is (and this may vary depending upon who you ask) no more than .3 VAC. A new or quality re-manufactured alternator will have an AC voltage output in the millivolt range (less than a tenth of a VAC). A sub-standard re-manufactured alternator had been installed. The alternator was putting out too much AC voltage and causing the the problem with this truck. I replaced the alternator with a quality re-manufactured one, tested its output and performance and sent the customer on his way.

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

Needless to say, this test was added to my regimen of "Standard Testing Procedures". Its quick and takes less than 5 minutes! I strongly recommend to all shop owners and technicians to have a set of written tests to conduct on any engine performance issues. It does not have to be anything extravagant, 2 or 3 pages of standard tests to rule out (or rule in!) certain components. Additionally some of the major parts suppliers offer testing procedures for free or little cost, use them as a guide to tailor it to your own needs. I also recommend to all shop owners, who do not have it, to make the investment in their business and purchase some sort of repair database. Whether online access, CD or DVD. With the time and money you will save, in the long run, it will more than pay for itself!

Proper testing and knowing how to test is paramount. I urge all shop owners and technicians alike to get properly trained and get certified. We are professionals! Each and every year new vehicles become more technologically advanced and we have to stay current with this technology. We don't have a choice!

Chris is a retired Navy Veteran, who owns Advanced Diagnostic Services. Specializing in mobile automotive diagnostics and technician training. He currently writes on a wide variety of subjects and for 2 of his local news publications.

Advanced Diagnostic Services, Mobile Automotive Diagnostics and Technician Training

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