Example field problem illustrating the diagnostic process

Awhile back I outlined a process I call VEHICLe , a general procedure for diagnosis and repair. Now, I would like to present an illustration of the process in action. What follows is documentation of an actual road call and the method used to diagnose and repair the problem. It is fairly representative of the type of problems encountered in the field, although this one turned out to be much simpler than many.

To review, VEHICLe stands for :

V:  a VISUAL check is the first step.

E:  check the EASY things next.

H:  HOOK up a scan tool for problems on a system monitored by the vehicle computer.

I:  ISOLATE the problem to a smaller area within the system.

C:  CHECK each item only once.

L:  the LAST action is to always verify the repair.

One Saturday morning a mechanic was dispatched on a road call for a tractor that wouldn’t run. The information given was that it would start, but would immediately blow a fuse and quit running. On arrival the mechanic found a fairly late model Western Star hooked to a trailer sitting on the exit to a manufacturing parking area. The driver confirmed the information already provided, and had removed the fuse panel cover and indicated that a 30amp fuse marked “eng ign” was blown. The fuse panel consisted of two parts, a main panel with most of the fuses and breakers, and another smaller one just to its left, but under the same cover. The blown fuse was one of those in the smaller panel.

After confirming with the driver that no recent work had been done on the tractor, the mechanic did a visual check under the hood, at the firewall, and under the cab/engine. There were no signs of harness chafing, damage, broken wires, or unlatched connectors. The blown fuse was temporarily replaced with a 30amp circuit breaker, to see if the unit would run until the breaker heated up. The tractor immediately started and stayed running. After a few seconds of running, the mechanic felt the breaker and found it cold to the touch. At this time he noticed another breaker in the same panel that was tripped. This was marked “tlr”, and after it was reset, it tripped again after about 3 seconds. Reset was tried again with the same result. At this time the driver volunteered that he had needed to reset breakers in the other panel marked for the trailer tail and clearance lights.

Since a problem in the trailer was indicated, the cord was unplugged from the trailer and breakers reset, after which they again tripped. When the cord was unplugged from the tractor receptacle, all breakers stayed set. The breaker in the ignition circuit was replaced with a fuse, which did not blow. The problem was obviously in the trailer cord.

The cord was a heavy duty unit with plastic end plugs, but the trailer end plug had been replaced with a metal one. When attempting to take the plug apart for inspection, the strain relief was found to be in pieces and the pieces fused to the ends if the pigtail wires. Obviously when the plug was put on, the installer cut the cord sheath too short and instead of the strain relief trapping the sheath, it clamped on the wires and over time and with movement they shorted out. The ignition circuit obviously powered the trailer hot lead (the blue wire), and that shorted, too.

The plug was cut off and replaced, the lights checked for function, and the truck run for a few minutes to confirm the repair. Total time elapsed was about a half hour, not including travel.

This story illustrates several important concepts to keep in mind during any diagnostic challenge. First, start off with a visual check to eliminate anything really easy. In this case nothing was seen, but it is remarkable how many times merely looking will solve a problem. Second, when you look at something, keep your vision wide and look around the area you are looking at. If the mechanic in this case had not noticed the tripped breaker near the fuse that was already blown, it probably would have taken longer to see the trailer circuits as contributing to the problem, and the driver may not have thought to mention the other problems he had. Third, isolating the problem to either tractor or trailer was easy here just by pulling the cord, but notice that the trailer was eliminated first, then the cord. This is a small point here, but it would have prevented backtracking if the problem had turned out to be in the trailer. Last but far from least, everything was checked one last time to be absolutely sure the problem is fixed. Nothing is worse than having to travel back to a job to fix what you failed to get the first time around, just because you didn’t check.

Hopefully this case was interesting to beginning mechanics or anyone else who may wonder what we do. It kind of illustrates the diagnostic mindset a person has to develop to be successful. Unfortunately, there is no book or chart to tell you how to do this, the only way to really learn by experience, and everybody develops the skill to a different degree.