Christian P. Koop
For the benefit of those who are not familiar with or have not read the first two parts of this series, I will explain the main reason behind these articles.
History has a way of repeating itself, even when it comes to emergency response vehicle (ERV) maintenance and repair. I have seen many similar issues occur time and time again, and I feel sharing some of the problems I have encountered over the years may help some readers find solutions to issues they may currently or in the future encounter with their fleets.
Some of these fleet defects may have been created because of poor specifications that did not take into account real-world drive cycles, terrain, climate, vehicle weight, or a host of other factors that can affect ERV drivability, durability, and reliability-which in our business can mean the difference between life and death. Some of these issues are easily, or luckily as the case may be, discovered during the acceptance phase for a new fleet. Yet others are from manufacturing or component defects that appear after the units are placed in service. Some may not show up until the units have been in use for considerable time and may take many thousands of miles, hundreds of hours, and many months before they appear. Unfortunately, some of these issues can be very tough to deal with, and finding solutions for them can become paramount for all those involved in the process.
This example shows burned insulation from the harness of a transmission
One area I feel is a major source of ERV downtime and problems that can be very time consuming for technicians to pinpoint is the low-voltage electrical system. Although there have been many improvements in technology over the years with the use of electronics to control engines, transmissions, multiplex systems, electronic pump governors, wireless system components, and a host of others, an incorrectly designed or built low-voltage wiring system can lead to problems in these modern and sophisticated systems that can be chronic and difficult to find and correct-even for experienced technicians.
Over the years, I have seen problems that include incorrectly sized wiring that could not handle a load, poorly crimped connectors that increased resistance and created voltage drops, failure to have a drip loop or a service loop, incorrectly designed or mounted components that would fill with water and fail, and wiring that did not conform to specifications. I will provide some background and examples of the issues caused by these shortcomings and failure to follow important build requirements and procedures in the low-voltage electrical system.
For a number of years, both National Fire Protection Association (NFPA) 1901, Standard for Automotive Fire Apparatus, and the old ambulance standard, KKK-1846-G, which is being replaced by NFPA 1917, Standard for Automotive Ambulances, have required low-voltage wiring that is resistant to heat, abrasion, and chemicals. Basically what this means is that the material used to insulate the copper wiring or conductors must withstand a lot more heat and physical abrasion than the more common polyvinyl chloride (PVC) insulated wire you may find at your local auto parts or hardware store. The wiring must withstand the high under-hood temperatures found in modern automobiles and trucks and is also resistant to chemical degradation from gasoline, diesel, lubricants, coolants, and other fluids that would ordinarily damage lesser materials used for wire insulation.
This wire type is also commonly referred to as cross-linked wire and meets the Society of Automotive Engineers (SAE) J-II28 standard for SXL, GXL, and TXL low-voltage primary wiring and SAE J1127, which is basically the same but for low-voltage battery cable. The maximum end of the temperature range for this type of insulation is 125°C (257°F) compared with PVC, which, depending on the type, may only have a maximum of 80°C (176°F) to 90°C (194°F). In my book, that is a substantial difference. And in my experience, it can mean the difference between having an electrical failure and having a costly electrical fire. This does not mean you won’t have an electrical fire when using cross-linked wire. It just provides more protection.
The wires inside this protective loom burned because they were routed too
There are other wire insulation types on the market that far exceed the capabilities of cross-linked wire. However, for the most part, cross-linked wire is all that is required in most modern automobile and truck circuitry. I say most because the wire used for oxygen sensors and diesel particulate emission sensors has to withstand much higher temperatures than what cross-linked primary wire is designed for. GXL is designed with thin-walled insulation to fit in areas that are tight; SXL has a standard thickness; and TXL-rated wire has a very thin wall and is very light for special applications.
Many years ago, a coworker and I were curious about the advertised properties of this high-temperature rated wire insulation. We decided to test the wire just to see if it would live up to the specifications and compare it to common PVC wire. We took a two-foot length of 16-gauge PVC and SXL wire and short circuited them across the positive and negative terminals of one of the shops’ 8D jumper battery carts. We were both amazed and surprised by the results. The PVC wire insulation immediately melted and started burning. However, the SXL wire did not, and the copper conductor actually burned in two inside the insulation. The SXL insulation got very hot during the test but never melted. This made both of us believers in the significance of the heat-insulating properties of this superior type of insulation. This is extremely important because this extra level of protection can prevent electrical fires. An electrical fire that gets out of control can lead to a complete loss of the ERV.
Even with the standards in place, I have seen manufacturers use the wrong type of wiring to build trucks, and this can lead to unnecessary problems, which, in a worst-case scenario, can end up as a vehicle fire. For those who are directly involved in writing specifications for your department or company, I recommend mentioning the SAE standards for wiring and harness construction in your specifications. Or, you can cite NFPA 1901 paragraph 126.96.36.199, which refers back to these two SAE standards. For those who are involved in the acceptance inspection process, consider taking a hard look at how the harnesses are routed during the build process to ensure proper chafe protection as part of the inspection. Having a “drip loop” for electrical components mounted on the body, chassis, or engine compartment that are exposed to the elements is also very important. Basically this means wiring must be routed to electrical components in such a way that water or moisture will not drain along the wire and drip or wick into an electrical component such as a relay or module.
Many electrical components and their connector sockets are not waterproof, and if the components and their wiring are not properly mounted or routed, water will inevitably find its way inside and ruin the components and cause unnecessary ERV downtime, expense, and headaches for the shops. Even if the water or moisture does not immediately take out the device, corrosion can set in and will eventually cause voltage drop problems in the circuit. Having an appropriate length in the service loop ensures there is a sufficient length of wire to allow cutting the wire or harness to install a new electrical component such as a relay or module without having to splice in an additional section of wire when replacing these types of components. Another extremely important item to remember is to specify as-built wiring schematics. This will be a huge aid to technicians when they are troubleshooting difficult electrical problems. This is an area that can lead to a lot of lost time and frustration for technicians when they are tracing a wiring problem on a schematic that does not accurately reflect the ERV they are working on.
Shown here is an example of physical abrasion damage because of a
I remember visiting an ambulance manufacturer many years ago that was building a large order of rescue trucks for our department. We met with the president of the company and he explained that they were using a new electrical terminal crimping machine to build the harnesses. Several units were in the middle of the electrical harness installation process, and they were easy to inspect because most harnesses were accessible and just hanging from the open ceilings in the rescue modules. I decided to perform a simple mechanical pull test on the terminals, and many easily came apart.
After I reported this problem, the manufacturer discovered that the new crimping machine had not been adjusted correctly. Luckily, the problem was discovered early enough in the build process and was corrected before the units were completed. If all the trucks had been built with the defective electrical crimps-causing insufficient mechanical clamping pressure-we could have had been plagued with many hard-to-find intermittent gremlin-type electrical issues with these units and unnecessary ERV downtime.
Poorly crimped terminals can cause intermittent problems or excessive voltage drops. Voltage drops across terminals can sometimes create enough heat to melt electrical connection sockets or plugs, which can lead to an electrical fire in a continuous-duty circuit. Examples include circuits that remain on the majority of the time the ERV is in service, such as air-conditioning, or a rescue or ambulance that has a refrigerator for drug storage. These types of circuits will develop heat at electrical connections that are loose or corroded or have mechanically damaged conductors but are still connected and not shorting to ground.
Fuses or circuit breakers will not protect the circuit from these types of electrical problems. The fuse or circuit breaker will not blow or trip because there isn’t a short-circuit condition. The exception to this is if the poor connection causing excessive voltage drop is getting hot and is very close to the circuit breaker or actually at the breaker. Generally, the heat generated, if it gets hot enough, will conduct into the breaker, heat the bimetal strip in the breaker, and open the circuit. If the circuit breaker is an automatic-reset type, the circuit may cycle on and off. The complaint from the driver or operator about the problem will generally reflect this condition. Over the years, I have seen hundreds of cases like these, and it takes well-trained, experienced technicians to find and correct these types of issues quickly.
Mechanical Pull Test
Most technicians who have received proper electrical crimping tool training to repair electrical terminations and wire splicing should be very familiar with the mechanical pull test. If a technician is repairing electrical terminals or splicing wires, the first test he should do after crimping a wire termination is try to separate the terminal from the wire by attempting to pull it apart by hand. If it separates, it was not crimped properly, so it was not a good mechanical connection, which generally renders it a poor electrical connection. The problem may not be apparent right after the repair is completed and may not actually appear until much later. If it passes the mechanical pull test, the next best way for a technician to test for proper electrical conductivity is to test for voltage drop. However, remember the circuit must be under load to get accurate test meter readings.
Remember that Ohms law-E (volts) = I (amps) x R (ohms or resistance)-explains what happens in a circuit. As current flows through a resistance or a load, a voltage drop is produced, and that is the amount of voltage consumed by that resistance or load as the current passes through it. Technicians should not forget to use Ohms law as a diagnostic aid when dealing with difficult voltage-drop issues. Another point to remember is that NFPA 1901 paragraph 13.2.1 states that voltage drop in all wiring from the battery or power source to the device or electrical component shall not exceed ten percent.
Keep in mind that high-current circuits, such as the cranking or starting circuit, are allowed to drop up to 0.2 of a volt (rule of thumb) for each side of the circuit, positive and negative. It is also best when repairing or splicing into circuits to use heat-shrink type terminals, particularly if the circuit is exposed to the weather. Heat-shrink type terminals and butt connectors not only seal out moisture and can stop corrosion from forming but also give the copper conductor a better degree of vibration protection as compared to the more common PVC type.
The low-voltage electrical system, if not designed or built correctly at the onset, can be a source of chronic and difficult issues that can lead to unnecessary and costly ERV downtime. These issues are frustrating not only for the users and operators but also for the shops and the technicians. Electrical problems, in general, will always occur. In my experience, this is inevitable. However if the vehicle is specified, designed, and built correctly using good quality materials as required by the minimum standards, ERVs’ in-service time will be optimal.
CHRISTIAN P. KOOP is the fleet manager for the Miami-Dade (FL) Fire Department. He has been involved in the repair and maintenance of autos, heavy equipment, and emergency response vehicles for the past 35 years. He has an associate degree from Central Texas College and a bachelor’s degree in public administration from Barry University and has taken course work in basic and digital electronics. He is an ASE-certified master auto/heavy truck technician and master EVT apparatus and ambulance technician. He is a member of the board of directors of EVTCC and FAEVT and a technical committee member for NFPA 1071, Standard for Emergency Vehicle Technician Professional Qualifications.