A n electrical circuit is usually identified by the presence of a wire that is terminated at both ends with a connector of some kind. A portion of the vehicle, be it the fender, frame or dash, is not usually seen as an electrical conductor, but does serve that exact purpose on our vintage military vehicles. The metal parts all have to be connected electrically to provide for a reliable operation of the lights, motors, relays, solenoids, instruments, radios and other systems that make up the complete vehicle.
Beginning at the negative battery terminal, each metal part is bonded to the next with special “ground straps” or nut and bolt assemblies with paint penetrating washers to complete this part of the electrical circuit. From the standpoint of radio shielding and reduction of electrical noise, it’s vital that all the parts of the vehicle are electrically tied together.
GROUNDING AND GAS-TIGHT CONNECTIONS
The first time the author was introduced to the term “gas-tight connection,” was in the early 1970s when the so- called wire wrapping method began to be an accepted way to make connections in electronic assemblies. The electronic components were mounted in sockets that had a square gold-plated pin for each terminal. A wire wrapping tool was used to firmly wrap the fine insulated wires around these sharp edged terminals. As the wire was wrapped, the insulation was pierced and a metal-to-metal contact resulted. The tension of the wrapped wire made it a permanent assembly. The metal-to-metal contacts were referred to as “gas-tight connections” since no oxygen could enter the joint and oxidize the metal. Another example of a gas-tight connection is the crimp-type connection used in connecting wires and terminals.
Two clean copper wires simply twisted together will conduct electricity from one wire to the other. However, exposure to air, moisture, etc. will quickly corrode the copper surfaces and make the joint something less than optimum. Wire terminals and conductors are plated with tin, silver or gold for corrosion protection. Wire terminals, such as the ring, fork, quick disconnect etc, can withstand high continuous currents if you assure gas-tight connections and the size of the terminal can carry the same current that the wire is rated for when it is matched with the wire size.
The external/internal tooth washer or “star” washer is the washer of choice when grounding metal assemblies. The washer/nut/bolt on the left came from a 1953 M37 and the washer must be replaced when the parts are put back together for proper paint penetration. The washer on the right is a typical hardened cadmium plated star washer.
The trick to securing a long lasting joint is to keep the adverse effects of the environment away from the area where the two conductors are joined. This can be done several ways, but he most popular, by far, are crimping or soldering. Either technique properly done will do the job. It is not necessary to do both.
PAINT PIERCING WASHERS
In a nut and bolt assembly, the torquing of the nut applies a tension to the bolt (cap screw), which holds the assembly together and the use of a lock washer may seem redundant. However, when the bolt serves as an electrical return path for a light or lights, then a toothed washer must be used to penetrate the paint for a metal-to-metal contact. The washer of choice is the external/internal tooth washer, which must be hardened and plated, usually with cadmium or zinc.
Figure 1 shows the washer placed under the head of the bolt. The wiping action of the nut makes the contact on the other side. For the most reliable connection, use two washers, one on each side of the assembly.
Take a close look at your typical 1950-1960s military vehicle, and you will see how these washers are used to make a solid electrically continuous path from bumper to bumper. Vehicles from that time period seem to show fewer compromises with the basics.
This illustrates the importance of the star washer. It must be used to penetrate the paint for a good electrical connection.
When you disassemble these vehicles for painting or parts replacement, replace the special washers with new ones. The original washers may no longer serve their purpose in making the electrical connections. The sharp edges of the washer must dig into the metal for “gas-tight” connections. This is particularly important when the pieces are primed and painted before assembly. Think of it as replacing old frayed wiring on the vehicle.
When a ring terminal on a grounding wire is part of the assembly, place the toothed washer against the painted surface, then the ring terminal under the head of the bolt. Turn the nut to tighten. If the head of the bolt must be turned to tighten the assembly, place a flat washer between the ring terminal and the head of the bolt to keep the relatively soft ring terminal from getting deformed. An example of the importance of this washer is demonstrated by Figure 2, where the incorrect placement of the washer results in a “non-functioning” wrecker crane.
When several ground wires are connected to the same bolt/fastener and that bolt/fastener comes loose for some reason, the electrical ground path may or may not go directly to the negative battery terminal. It may return via other lights or devices, thus creating problems that are confusing and difficult to troubleshoot.
This figure from TM-8000 shows the different ways bonding can be made to work. Think in terms of providing a low resistance path from one part to another when using the external/internal tooth washer.
It is better to use a “bus strip” to terminate multiple ground wires since each return is handled by a separate screw. Look at the bus bar(s) inside the electrical distribution/breaker box in the basement of your house for an example of this. These bus bars are also used in the shop van variants of the deuce-and-a-half.
Multiple grounds terminated at the same point, may result in odd circuit behavior if loose. One lamp may be grounded via another lamp or device.
Typical symptoms of this problem include lamps that are turned on with a lower intensity and also come on when not supposed to. These electrical part-to-part connections are vital for good radio operation. Quoting TM 9-8000 Principles of Automotive Vehicles (page 18-3), “Bonding is the term applied to the method of electrically connecting individual metal sections to each other and to the frame or hull of the vehicle. Such bonding is necessary to provide an easy path for grounding static charges. Bonding is accomplished by internal-external tooth lock washers and by bond straps. The better the connection between metal parts, the greater the effect in preventing interfering waves from being thrown off to affect radio reception.”
Figure 3 shows a number of ways bonding can be accomplished. Note how the tooth type washer is the actual electrical path from one piece of metal to another. This may explain why you will find grounding (bonding) straps in curious places, like from the frame to the radiator, hood to the firewall and from eng ine valve covers to the firewall on your vehicle.
The grounding path for most electrical/electronic vehicular devices is via the chassis parts. Specific examples follow.
TURN SIGNAL CIRCUITRY
The turn signal flasher unit comes in a number of different enclosures, some are metal and some are plastic. The internal circuitry is not grounded to the housing, but a separate wire must be connected to solid ground, usually via one of the mounting screws for the unit (Figure 4).
The turn signal flasher unit is not grounded via its case and must have a good external ground connection provided by the wire marked “C.” A typical mounting is shown here with the “C” wire grounded by one of the mounting screws. Note that the manuals rarely suggest that a good ground connection must be made. As you can see, the description simply calls for one screw and two washers. It’s up to you to make sure a good ground exists.
The turn signal switch, which is mounted on the steering wheel column, is grounded via the hose clamp. This ground path is only for the indicator light and does not affect the flasher unit operation.
THE FUEL TANK
The primed and painted fuel tank on the M35 deuce-and-a-half truck is supported by two brackets and held in place by lined metal straps. There is no metal-to-metal contact here and the ground is provided by one wire (#79) from a connection on the frame, which is plugged into the fuel pump connector. The connector housing is the ground. The housing is in turn electrically tied to the fuel pump mounting plate via three studs.
The fasteners used to mount the housing must include external/internal (star) washers to properly ground it. The fuel pump mounting plate is in turn grounding the fuel tank itself through its ten mounting screws. The screws used here may be of the “binding head” design, which works well in conducting electricity when the surface is not painted.
Since the fuel tank is grounded via these screws, it may be advisable to add star washers to the assembly if there is any measurable resistance between the pump mounting plate and the tank itself. The ground path for the fuel level sending unit is through its mounting screws to the tank, to the fuel pump mounting plate screws to the connector housing and finally via wire #79 to the frame. A non-functional fuel level sending unit may simply be due to a poor ground connection. Also, as you can see, if the fuel pump isn’t running it may not help to just check the fuse. The ground path to the frame must be intact.
THE FRONT COMPOSITE LIGHTS
The fender mounted front composite light assemblies on the M35 series trucks are grounded through the housing to the guard and from the guard to the fender and from the fender to the frame via mounting screws. Looking at the individual parts, you will find that they are primed and painted before assembly. Notice that the same screws are used to hold the separate parts in place.
Here is another example of the manual only showing the hardware used in an assembly. The importance of putting it all together so that a good solid ground results from the lamp housing to the fender is not explained. These parts are usually primed and painted before assembly, so you must make sure a ground path is provided when you work on these. Note: Plastic housings have separate ground connections and both mounting screws must be grounded.
The parts list does not show any paint piercing washers for mounting the guard to the fender. The mounting screws serve double duty in that they also hold the wire protector below the fender in place. This offers no reliable electrical path to the fender.
See Figure 5 (from TM 9-2320-211-20P Parts Manual). Though the fender itself is not shown, the ground path must connect to it. If you are having problems with the grounding of the composite lights, include internal/external lockwashers to complete the path from light guard to the fender via at least one screw. You can also prepare a separate ground wire, properly connected to the light housing and to the fender.
The M756A2 pipeline truck bed has 15 marker/clearance lights separate from the host vehicle. The light housings are mounted on brackets extending from the bed frame. Each marker light base is installed using two star washers, one under the head of the screw and one under the nut. This provides one or more ground paths to the mounting bracket. The mounting bracket must be mounted similarly with an internal/external tooth washer under the head of the screw and one under the nut to penetrate the layers of paint. Again, the parts lists do not pay any particular attention to the grounding of the bed to the main frame of the vehicle.
Each instrument on your dash is grounded by its mounting bracket, which has “prongs” to help punch through the paint. The dash panel itself has no separate ground wire and is grounded by the wiping action of the four 1/4-turn fasteners.
The bed mounting bolts must provide the return path for the lights. This is not a common variant of the deuce-and-a-half, but if you happen to own one, you will want to add a ground strap with proper washers for a good ground between the bed and the frame of the truck for reliable operation of the lights.
GAUGES AND INDICATOR LIGHTS
The mounting bracket on the back of each instrument is its ground connection as shown in Figure 6. The lights on the dash are grounded via the mounting screws which must have the external/internal lock washers for a solid ground. There is no separate ground wire for the dash “sub” panel, it is grounded via the four 1/4-turn fasteners.
By the wiping action of these fasteners a ground is provided, however after long periods in storage, this ground path may be compromised. A separate ground wire can be added from a screw on the sub panel to the main dash.
ANTENNA MOUNT GROUNDING
If the antenna is ever used, not just installed for show, it must be grounded properly. A ground strap is installed from the antenna base unit to the “sugar scoop” mount as shown in Figure 7. The sugar scoop is bolted to a spacer, which in turn is bolted to the vehicle body. All mounting screws are installed with paint piercing washers.
Note that no less than three washers are used on the ground strap end attached to the mounting bracket, emphasizing the importance of this ground path. An improperly grounded antenna could result in injury if contact is made to the mount when transmitting.
THE GROUND STRAP
The braided tin plated ground strap is composed of perhaps a hundred individual strands of wire. The reason for using this kind of wire is to offer the least possible resistance to movement. A connection between the engine and the frame must withstand constant movement or vibration.
A connection between two non-moving parts can be made with a less flexible cable, like the battery-to-frame connection. The size of the cable is important. The ground strap from the engine to the frame must be able to carry the full current from the starter motor and the terminals are dimensioned accordingly.
This picture shows how to properly install the antenna “sugar scoop” support. The use of the “star” washer is shown here, but is usually not shown as explicitly in most other exploded views in technical manuals. Study the drawing and think in terms of making a good electrical connection from one metal part to another.
The engine-to-frame ground strap must have the largest possible contact area and the terminations require special preparations. The surface area that makes contact with the ground strap must be clear of paint and grease. A flat washer is placed under the head of the bolt and the bolt is torqued to spec determined by the size of the bolt and its thread (coarse or fine).
The proper order of assembly for the heavy cable or ground strap from the negative battery terminal to the frame is as follows: terminal against the clean frame with a toothed washer between the head of the bolt and the terminal, and toothed washer between the frame and the nut. A toothed washer should not be placed between the frame and the terminal. All non-conductive materials must be removed from the contact area of the frame before assembly.
ACCEPTABLE VOLTAGE DROP
The most effective connection will offer the least possible resistance to electrical current. A quality junction should have a resistance not exceeding 0.001 ohms (one milliohm). This can be achieved when the surfaces are clean and adequate pressure is applied. The pressure is in the order of 1,200-1,500 psi.
The best method to measure the resistance of a ground strap and its connections is simply to measure the voltage drop across it when full-load current is flowing through it. Place one probe of your digital voltmeter/multimeter (DVM) on the negative post (not the clamp) of the battery and the other at the place of interest: engine, frame, fender, etc. The voltage reading on the DVM is the direct result (voltage = current x resistance) of the resistance in the return path. This voltage must not be more than a small fraction of one volt. Even when checking the starter motor ground connection, the reading on the DVM should not exceed a few tenths of a volt when cranking. The measurements must be made when full load current is supplied, so two people are needed–one to run the start motor and one to take the reading on the DVM.
Some voltmeters have an extra feature, that of storing the maximum reading and may be useful if you work alone. I used my Fluke model 16 for this measurement. If the resistance is high, then heat will develop in the junction, which will speed the process of corrosion and increase the resistance futher. After a period of time, the joint fails electrically even if it’s secure mechanically.
There are special low resistance measuring instruments, called “meggers,” for making accurate measurements of resistances in the milliohm range. They are expensive and cumbersome, however, and not readily available to the hobbyist.
THE POSITIVE VOLTAGE
This article focuses on the return path from an electrical light or device, but good connections are, of course, also required on the supply side. The path from the positive battery terminal to the device should offer the least possible voltage loss/drop as well. You may measure the supply voltage at any point of interest, open the cover of a light housing and note the readings when the light is turned on. When the power is supplied via a mechanical switch, the voltage should be at or very near the full battery voltage.
The turn signal flasher is a solid state device and there is a voltage drop across the transistor within the unit so you will not have full battery voltage on the turn signal light bulbs. To bring full battery voltage to the turn signal lights, add a relay: then, this becomes the only load seen by the flasher unit. During turns, the relay operates the lights. This relay also provides an overload protection for the flasher unit.
The MX-7777 electrical transient suppressor is designed to protect the vehicular radio and intercom systems from damaging voltage spikes and reverse polarity connections at the battery. The detail view shows the grounding strap, which is needed to bypass the rubber shock mount. It is similar to the method used to ground the 25-amp voltage regulator mentioned in the article.
You can also reduce some of the load on the internal 20-amp circuit breaker of the 3-lever light switch by wiring the supply to the lights via a separate circuit breaker. A mil-spec 25 amp relay can handle currents of up to about 75 amps (equivalent to a lamp load of 10 ampere). A relay may also be used in the headlight circuit, which will reduce the current load on the circuit breaker inside the 3-lever light switch (for more on this, see “Highway Deuce” in MVM no. 112, December 2005).
GROUNDING SENDING UNITS
Oil pressure and coolant temperature sending units rely on a good electrical connection via the threads of the unit itself. When you install these, use only a single wrap of teflon tape on the pipe threads. Too much tape may cause a break in the electrical circuit. A similar connection exists on the flame heater spark plug, but no sealing tape is used.
The battery connections need special attention. Suffice it to say, the terminals must be clean and the cable clamps torqued properly. A poor connection will keep the batteries from being charged, which in turn means that the generator/alternator must work at its maximum output during the moment when an intermittent connection is made. When that connection is interrupted (by a bad connection), then the full load is “dumped” into the electrical system of the vehicle. A “load dump” will most likely destroy the regulator and possibly other electronic gear that is turned on at the same time. A load dump voltage spike can easily exceed 100 volts. Chemicals called antioxidants are used to keep the terminals free of corrosion and special pre-soaked felt washers are available to reduce the effects of corrosion on the battery terminals.
If you ever wondered why you should remove the negative connection first and install it last when working on the batteries, the reason is simply to keep you from accidentally causing an electrical short with the wrench to chassis when loosening/tightening the positive terminal. With the ground disconnected, you can short the positive terminal to the chassis all day long and not melt the tool or burn yourself. Touching the ground terminal to the chassis doesn’t cause any harm whether it’s connected or not.
Don’t forget the short jumper connecting the two batteries in series. Obviously, those connections must be good too. The battery terminals must be clean before the clamps are attached. Use a special battery terminal cleaning tool to maintain these connections.
DEVICES WITH SEPARATE GROUND WIRES
To protect expensive radio equipment, a suppressor unit should be installed (Figure 8). The MX-7777/GRC is designed to absorb/limit a “load dump” caused by poor battery connections as explained above and also to protect gear from reverse polarity connections and trip a built-in circuit breaker if a short circuit exists.
This unit comes with a ground strap already attached to it, since the rubber shock mounts do not provide a reliable ground path. Just complete the circuit by connecting it solidly to a ground using a nut/bolt/star washer assembly.
The vehicular intercom system (VIC-1) is not reverse polarity protected and it’s recommended that the MX-7777 is used in conjuction with it. The VIC-1 boxes are not grounded through the mounting bolts, but through the wiring within the connecting cables.
If the MT-1029 radio mount is used, make sure the ground strap provided is terminated correctly. The star washer may not work as a “paint penetrating” device here since it’s only a #8 size screw, the paint is simply too thick and must be scraped off before assembly.
The 25-amp voltage regulator has short ground straps for bypassing the rubber shock mounts. The vehicle trailer connector has a separate ground connection, which relies on a ring terminal-to-frame connection for its electrical return path.
The importance of the grounding connections can not be overemphasized. The nut and bolt assemblies have special “paint piercing” washers to provide a low resistance path for the return currents. When the old assemblies are removed the washers must be replaced with new ones to continue to do their job.
Metal light enclosures are usually grounded internally to simplify installations. Plastic and glass (sealed beam head lights), each have a separate ground wire, which must be connected to a solid ground. This “solid ground” is the vehicle body.
Pay attention to the placement of the toothed star washer, so that it can do its job of completing the electrical return path. The order of assembly is important with the washer piercing the paint. Don’t take these connections for granted. Extensive use of fiberglass in a vehicle body/fenders will require special attention to grounding.
Manuals rarely mention a ground connection and when they do, it is only in terms of assembling items with the right hardware to (hopefully) get the connections right. Think in terms of the electrical return path when assembling parts of your vintage military vehicle and it will be properly grounded.