The Diagnostician's Mission

Have you ever sat and wondered "how is it so hard to find what's wrong with my vehicle!" "Why is the diagnostic so expensive?" "Does it really take 5 hours to find a bad wire?" "Doesn't that fancy scanner tell you what's wrong?"

So what is the Diagnosticians Mission? you might think its all smoke and mirrors but there is a lot behind what makes your vehicle run the way it does. For example I once had a 2008 Ford F-450 that was towed in, customer stated that he was driving down the road as per usual and then suddenly the whole truck shut off, nothing on the dash, no noises. It was like someone removed both batteries and left it for dead.

Where to start? I grabbed my trusty scanner hoping I'd get some idea of the location to which the issue was related. With good batteries and a charger on the truck I had no power and no communication from the CAN High or CAN Low (Controller Area Network (Those are the 2 wires that send and receive all information to every module on the CAN system)) So with no diagnostic trouble codes (DTC's) I have to go in blind. First step is to determine why there is no communication, blown fuse? bad module? terminating resistor? or is it a bad relay, wire or capacitor?

This is where the job can now get a little tricky, you have to know how to safely test every component without further causing unnecessary shorts or damage along the way. For this you need a High impedance meter. This will help minimize and potential damage when in the right hands. Some good brands are Fluke, Snap-on, MAC Tools, MATCO and so many more. DON'T go and grab a $15 multimeter from Princess auto and think its just as good, you could put yourself in a position where one wrong move could short a module that is critical for vehicle operation, plus no one wants to spend a few thousand to fix what they caused.

In my case the vehicle in question had completely burnt the CAN High wire from the the TCM (Transmission Control Module) to the BCM (Body Control Module). That happened due to a short inside the BCM on the terminating resistor causing 12V and a lot of amperage to go through those little wires and fired them instantly. Thankfully it stopped there and didn't continue through every module causing thousands of dollars in damage not to mention potential loss of the vehicle from setting on fire.

With the replacement harness and a new BCM installed and programmed I was finally able to get in with my scanner only to find all codes where related to the BCM failure but without changing the harness and the module I never would have been able to access them.

The mission is always to find the problem as quick as possible and keep the repair costs to a minimum, however in some cases like this one it took nearly 8 hours of checking harnesses; cutting them open, doing pin to pin resistance tests and reading a lot of schematics. The repair only took an hour with programming but the 8 hours to find it is what makes the bill so expensive. Some problems are easier to fix then others but in the wrong hands at the wrong shop you could be paying for a guess, and I can imagine you don't want your mechanic guessing on your dime. Not to mention the potential loss of revenue from not having your truck on the road.

A few diagnostic tips I can give is:

-Always make it a last resort to replace any module, Cummins once did a study on all their ECM core returns and over 70% of them where still in good working order, just had a bad case ground. I can imagine the shops that caused that 70% defiantly told their customers it was a bad ground and just ate the cost on a potentially $500-$10,000 ECM... NOT! likely the customer was treated like a mushroom (kept in the dark and fed S***) and had to swallow huge bills for nothing but a small simple ground... so always double check all your work.

-Try and get schematics, If you know how to read them it'll make the job that much quicker. If you have troubles reading a schematic there are courses you can take online to help and if you have access to interactive schematics then that's even better.

-Beg, borrow or buy a good high quality multimeter for the testing application, I personally have several for many applications, some are for inducing voltages, some are for high amperage, some are for doing pulse width modulation and some are just for voltage testing. So make sure you have the right tool for the job.

-Don't always take a connector view or schematic as gospel. For example; On a Ram 2500 and a Ram 5500 they will give you the same connector view for an ECM plug but where the pins say they should be might not always be true because it is a different application. This is where making sure you know what your doing can make a big difference.

-Wire colors are always handy but when it passes through the firewall, another connector or module the color could completely change. Best way is to test one wire at a time and take your time if your the DIY kinda guy. Some heavy truck brands use all white wires with numbers instead, and some give you nothing but pin numbers on the connectors.

-Understand your codes, Credit to Samsara for making this beautiful break down

The first character is always a letter. It indicates which control system has an issue, and has the following possible values and meanings:

• P (powertrain) refers to the engine, transmission, fuel system, and associated accessories.

• C (chassis) refers to mechanical systems generally outside the passenger compartment such as steering, suspension, and braking.

• B (body) refers to parts mainly found in the passenger compartment area.

• U (network) refers to the vehicle’s onboard computers and related systems.

The second character is a digit, typically 0 or 1, and shows whether or not the code is standardized.

• 0 indicates that the code is generic, standardized SAE (Society of Automotive Engineers) code. Generic codes are adopted by all cars that follow the OBD-II standard.

• 1 indicates that the code is vehicle manufacturer-specific. These codes are unique to a specific car make or model and are typically less common.

• 2 or 3 are more rare and their meanings are dependent on the preceding letter of the code. Most of the time, 2 or 3 indicates that a code is manufacturer-specific, with only a few exceptions.

The third character is also a digit, ranging from 1 to 8. This reveals the subsystem at fault.

• 1 refers to the fuel or air metering system

• 2 refers to the fuel or air metering injection system

• 3 refers to the ignition system

• 4 refers to the emissions system

• 5 refers to the vehicle speed controls and idle control system

• 6 refers to the computer output circuit

• 7 and 8 indicate that the issue is transmission-related

The fourth and fifth characters are read together as a two-digit number between 0 and 99 known as the specific fault index. These characters identify the exact issue of the vehicle.

If you are unsure about what your code means, check with your vehicle manufacturer or your vehicle manual for explanations.

How do you interpret a DTC?

Each DTC corresponds to a specific vehicle malfunction, such as a misfire in engine one. The following common DTC, P0128, would be read like this:

• The first letter is P, which means that the issue lies in the powertrain.

• The second character is 0, indicating that the following code is not manufacturer-specific code. It is standardized and can be interpreted according to SAE definitions.

• The third character is 1, which reveals that the issue is specifically within the fuel and air metering subsystem.

• The last two digits make up the specific fault index 28.

Based on this reading, it’s clear that code P0128 refers to an issue of the engine coolant temperature that’s below thermostat regulating temperature.

So with that information at hand you can see how there are so many possibilities for code numbers and locations.

Lets not forget your average modern passenger vehicle has hundreds or wires, some applications could even touch a thousand. If you pulled every wire and lined them up end to end it would be over a mile long for some cars & trucks on the road.

Take your time and if you don't believe this is something you want to tackle please give us a call and we can book you in for a professional diagnostic.

-This is not a guide for how to "Do it yourself"

-Compex Diesel Services LTD. always recommends you bring your vehicle to a professional for appropriate diagnosis and service.

-If you do attempt any DIY please insure you follow all safety measures when dealing with any Diesel Engine and/or lifting heavy objects.

-Compex Diesel Services LTD. does not recommend or endorse any unlicensed technician to perform the work detailed above, this BLOG post is designed to inform customers.