Vehicle Diagnostic and Troubleshooting Process Revisited

        Hello. I have thought for some time that someone should provide a general and universal vehicle diagnostic process guide. After writing most of this post, I searched the internet for such a guide. I felt sure someone had written a general diagnostic procedure and I not only found one, I found one containing exactly the information I wished to present and written better than anything I could have produced. It is written by Steve Litt and is called “Twenty Eight Tales of Troubleshooting”. Below is the link to the site where the e-book can be purchased. The site outlines the process, but I highly recommend buying the book. It is not a technical manual, but is written more like a collection of short stories. It is an entertaining, easy read, but contains the philosophy of the general troubleshooting process. Anyone who is serious about becoming a better mechanic or is just beginning a career will find it well worth the few dollars it costs. I also believe this information should be part of the curriculum in any repair education program.

                                                           troubleshooters.com

Rather than reiterate the information presented by Litt, this post will add my views on some aspects of the vehicle repair industry that inhibit efficient, timely, and cost effective  repair.
I think the most interesting aspect of the vehicle repair industry is diagnostics. Many laypeople believe that diagnostics is merely reading codes off a computer and changing a part. Some see mechanics as “grease monkeys” or glorified parts changers. We in the industry know otherwise, but realizing the full importance of good diagnostic procedures doesn't develop until we have been around awhile and have seen mistakes made and money and time wasted. I am particularly interested in how decisions are made during the process of vehicle repair. It seems as if good diagnosticians are born with or develop the ability to a high degree, while others just don’t seem to “get it”. I am not talking here about the nuts and bolts of the process, not of how to check circuits or to find a failed component in any specific case, but of how to make sound troubleshooting decisions.

        In the Nanoscience Technology program from which I have my degree, we had a class in Design of Experiments, or D.O.E. This focused on optimizing, analyzing, and repairing systems of deposition or removal of materials onto silicon wafers. The process chamber pressure, gas blend, temperature, and time are systematically varied and the result of each recipe is measured. In this way an optimal process is arrived at or a problem identified. This type of analysis doesn’t really apply to the vehicle repair situation, but what does apply is that diagnostics can be taught and that a procedure and careful documentation should be part of the diagnostic effort. 

Several challenges exist which make vehicle diagnosis a unique situation. Communication problems often exist between customer and service writer, between service writer and mechanic, and when another tech takes over the job or if the vehicle comes back again. Lack of good troubleshooting information, and gaps or incompleteness in that information also contribute to difficulty in making an expedient repair. Inadequate or obsolete equipment or instruments often exist in even well-equipped shops. Lastly, technician inexperience, lack of training, or outright laziness have always been problems.

One contributor to the communication problem is the lack of a common language between the customer and shop personnel. Technicians tend to be very specific with the words used to describe an unusual noise, for instance. A squeak is different from a squeal or a whistle, and a tapping noise is different than a knock or a rattle. Customers however, sometimes use some of these terms interchangeably or with different meanings.

        Another aspect of communication with the customer arises when a vehicle is brought in with more than one failed component or multiple conditions. It is not uncommon for a customer to continue driving a vehicle that has a driveability or starting problem, and this can cause a problem for the mechanic in diagnosing the problem or in determining the problem’s root cause. An example could be a truck with a long crank-to-start when cold. As long as he can get the truck running, the owner may continue driving it until one day it won’t crank. A mechanic diagnoses and replaces the failed starter, but if he doesn't know about the hard cold start, he won’t address the root cause of the failure and the vehicle will be back again. This specific problem as well as similar ones is a common cause of customer dissatisfaction as well as unnecessary warranty work. Multiple failures of components across time can sometimes lead to baffling troubleshooting sessions and difficulty getting to the root of a problem. The only way to address these problems is getting complete information from the customer at first contact. 

        Mechanics or service writers need to be sure that they know exactly what the customer is experiencing and that the symptom can be reproduced. The customer may need to be prompted by specific questions about what happened and when. The mechanic must be given enough information about the problem and when that problem happens that he can reproduce it and find a solution. This sometimes means that the contact person must ride with the customer to experience firsthand what is going on. The knowledge about the problem that the service writer experienced needs to be put on the work order. If another tech takes the over to complete the job, the first mechanic must provide complete information as to what was done so that no time is wasted by duplication or missing information. Complete communication, preferably in written form, must be provided by all parties involved. If I print a diagnostic procedure from an information source, I like to check off each step and write down any meter readings or measurements I get from doing that step. That way I keep track of where I am if I take a break and if someone else takes over, he knows what has been done.

        Lack of good or complete diagnostic information is another issue, especially for tough problems. Probably the only time you will ever get anything like complete information is at a dealer working on that dealership’s brand. Of course, much OEM information is available to independent shops, but the cost tends to be high for independent shops so many of us don’t have access to it. Non-OEM sources, in my experience, can contain spotty, incomplete, or even false information. Of course anything is better than nothing, but no matter what the source, you must be able to think for yourself and watch for something that doesn’t make sense. You also must know enough about the system you are working on to be able to fill the information gaps a source might have and in some cases to solve a diagnostic problem without any formal information. 

In addition to the problems described above, a vehicle engine is made up of several subsystems designed to work together. The mechanical, electrical, electronic, high and low pressure hydraulic, cooling, intake air, and exhaust each play a role in keeping a modern engine running as it should. In contrast, a computer system outside of a network has only two systems, hardware and software. Granted, a software program can be thousands upon thousands of lines of code, but it is still one system. The interconnection between different systems in a vehicle means that a fault in one can cause symptoms in another, often masking the root cause.

        Nearly every diagnostic process will require some kind of equipment or instrument, and you will need to provide at least some of it yourself. I would say every mechanic needs, at minimum: a good digital multimeter, a set of backprobe pins or leads, a dial indicator, a caliper or micrometer, and some kind of gauge for air and fluid pressure checks. These are pieces of equipment that a shop may not provide due to the difficulty of keeping them in good working order when they are used by everybody. The cost of all these items really isn’t that great, especially when compared to the many sockets, wrenches, and other hand tools most mechanics own. If you spent just a few hundred dollars on this group of tools, you would be getting quality products that would last for many years, if not the rest of your career. Of course, the cost of diagnostic laptops or scanners is beyond the reach of most mechanics, and any repair shop that wants to repair vehicles will have one. A scanner to read codes and display a generic datastream would be the bare minimum. OEM diagnostic programs are expensive and need periodic updating for late model vehicles, but have more capabilities than a simple scanner. Most shops should have both, as a scantool is good for quick checks of a system or for use on the road, whereas an OEM program will allow the user to do things like actuate relays, check sensor function, run injector disable tests, and reprogram vehicle modules. Wireless internet access is also very beneficial, especially with OEM laptop software. For these reasons, a shop that owns just a generic scantool limits itself to the most basic diagnostic jobs. 

        Technician knowledge, experience, initiative, and persistence are the most important factors allowing efficient and profitable diagnostic work. Without them, the best equipment and information is all but wasted. Trouble is, many mechanics don’t seem to take the initiative to keep themselves up to date and learn new skills. Worse yet, some of us don’t have the basic knowledge needed for effective diagnostic work. I am always shocked when a seasoned mechanic doesn't know how electrical relays work, or how variable sensors provide information to a control module. In addition to being compulsory, most dealer training is thorough and in-depth, but most independent shop mechanics need to upgrade their skills themselves. In this internet age it is easier than ever to find needed information, and there should be little excuse for lack of knowledge.  If you work at an independent shop, you have a few years before starting to see new technology show up on the vehicles you service, and tips and other information will become available from aftermarket sources and online. 

        The diagnostic process itself must follow a systematic and logical path in order to arrive at a solution. By systematic I mean that the process has sequential steps in proceeding from a general observation of a problem to increasingly specific tests that narrow the search until the answer is found. Logical means looking only at components having to do with the system in question, and then allowing each step in the process to guide subsequent efforts. In an earlier post I outlined something I call the VEHICLE diagnostic process. It can be used as a very general jumping off point for further refinement. It goes as follows:

The VEHICLE diagnostic process

First - law #1:  Make sure the simple, cheap, easy things are right before moving on!

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 ECM.

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

C: CHECK each item only once. (unless you changed something)

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

E: EVERY problem has a simple solution.*

*simple doesn’t necessarily mean easy to find. And rarely a problem may have multiple causes. But most difficult problems are simple ones hiding behind layers of distraction

Please check the original post if you want elaboration on each of the steps. Note that this process is systematic and logical, and it is also very similar to Litt’s 10 step process. Incidentally, this post except for this first and this last paragraph were written before I ever came across “Twenty Eight Tales…” Every successful vehicle diagnostician I have ever worked with follows the same basic process in working a problem, so it appears that whether through trial and error or by being taught, we all find pretty much the same set of tools.  

Thanks for reading today, and good luck in your efforts.

forum posts

Hello all. To everyone who has posted to the forum page on The Toolbox, I apologize for not replying. I don't know why I haven't been getting notifications of new posts, but until I figure it out I will monitor the forum frequently. Also feel free to join and reply to posts if you have the answer.

Thanks for visiting, and please keep posting.

Bruce Christopherson 

Caterpillar C7 inframe rebuild

       Hello everyone, and welcome to the latest from The Toolbox. This post is on a Caterpillar C7 rebuild. They are known for problems with broken compression rings, resulting in massive blow by and oil slopping from the breather tube and even out the oil fill at high rpm. The C7 Acert is the latest version of the 3126, the HEUI injected motor found in many medium duty brands. This one was in a 2005 GM C6500 which sold at auction because of the cost of repair. The new owner bought it for $5000 before he asked or found out about potential problems. “Let the buyer beware” was never more meaningfully applied to a situation. He wasn't so happy with the sale when he found out it would cost another $5000 to repair the motor. Still, 10 grand for the truck was not a terrible deal, as it was equipped with a liftgate and was cosmetically and mechanically sound but for the engine.
This is the second time I have rebuilt one of these in situ, with the engine in place. The problem is that the cylinders are cast in place and are part of the block, instead of being replaceable as in the International DT 466. This means much greater cost because the block needs to be bored to accept a new liner, then the liner is bored to size and honed. I have seen other forums suggest that this process will reduce displacement, but that is the result of not understanding the process. The engine block has enough material to allow a liner of sufficient size to bore to the original size, so stock pistons are used for replacements. The real problem is that a service with a portable boring bar needs to be called in for machine work, and the machinist needs about 3 feet of headspace above the block for the boring tool. It a C6500, there isn’t enough space to get both hands on the engine, let alone get the head off or work on the deck. This meant the cab had to come off. Other repair alternatives are replacement with a reman engine or block or with a salvage engine. One could also remove the engine, strip it, and take it in for the machine work, but that would really add to labor cost, not to mention the extra time to outsource that work. The problem with those routes are that to pull the engine you would need to remove the hood and radiator assembly and still need to at least lift the cab off the frame and then change over a bunch of parts, so I don’t think you would save anything in labor. Besides, I am not impressed with the workmanship of any rebuild I have seen, and a salvage motor could have the same problem at any time.
Removing the cab may sound daunting, but it is done all the time at salvage yards and as this was a hydraulic brake truck, there were no airlines to deal with. I had the cab off and dropped onto a stack of pallets in 4 1/2 hours. A forklift and a 3 inch ratchet strap through both doors and another to the firewall support did the trick. The other 3126 I did was in an older Ford Louisville, where cylinders 2, 3, and 4 needed to be resleeved. By removing the front engine crossmember and taking the rear trans mount loose, I was able to tilt the engine forward enough to clear the firewall with the boring equipment.
Once I tore down the engine I found the top rings in cylinder 3 and 4 were broken and those cylinders badly scored. The others had absolutely no wear ridge at the top and no scoring, so just cleaning them up with a hone would save the customer considerable money. Note that on this engine the piston oilers need to be removed for any work to be done on the block. The oiler nozzles stick up into the bottom of the cylinder and they will be damaged by the bore head or the hone if they are left in place. If there is no ridge at the top and no scoring you can feel with a fingernail, you can just break the glaze with a hone and reassemble with a new piston and rings. The machinist came and relinered the two cylinders, then bored the new liners to size with a deck plate bolted to the block. Installation of the head must be simulated for the final bore sizing, or the pull of the head bolts will pull the cylinders out of round when the head is installed.
Before honing, I pushed a new top ring 1/3 of the way down each of the cylinders with a piston to check end gap, and all were within spec. Using a Flex-hone, (or berry bush) hone and a liberal amount of 10W-30, I honed each cylinder for 20 seconds at about 60 up and down cycles/minute. A finish hone for about 10 seconds at120 cycles/minute gave me a good 45 degree finish hone at the speed my air drill ran at. It is important not to overdo the honing, as all that is necessary is to have a good pattern for the rings to seat to. I was also using a brand new hone that cut pretty aggressively.
After all the machining swarf was cleaned off the inside of the engine and the crank it was reassembled as usual, keeping in mind that there are a couple of fussy things about assembling this motor. The piston oilers can be put in after the pistons and rods are in, but there is more room to work if it is done before the pistons are in place. You need to hold the oiler up tight to the block when you put the bolt in, or the end will bind in the block oil passage and it will bend, then the oil will leak out around the oiler instead of going through the nozzle. That will certainly cause the pistons to overheat or the wrist pins to seize. If the nozzles are in place when the pistons are installed, care must be used so the rods don’t hit them on installation.
It is worthwhile to spend a little extra time making sure everything is hooked up correctly and well assembled before the cab is set back down. There is very little room in this engine compartment to work on anything once the cab is back on. Luckily this engine ran and survived a 10 mile test drive with no problems, so I didn't need to go back and tweak anything.