MV Starter Circuits

The importance of the starter system in our vehicles is obvious, yet we tend to take it for granted and only bother checking it when it doesn’t perform. In part it may be because of a lack of knowledge of the operation of the system, so perhaps after reading this article you will have more appreciation for it.

This discussion is common to most vehicles, but with some emphasis on the multifuel engine starter system.With the exception of the battery/generator gauge, there are no monitoring devices in the starter circuit.

We do rely on our hearing intensively during the engine start operation to listen for confirming sounds; after all, that’s how we know when to release the start button—we hear the engine run. The problem is that other sounds, the dreaded “click” or “chatter”, the whining sound or the sound from a laboring starter motor, a sudden crackling sound perhaps, or the absence of any sound may leave us perplexed; what does it all mean?

This article will explore some of the details of this important system and hopefully provide a guide in understanding and troubleshooting it; troubleshooting in the sense that you will understand the way the parts work together but not as a step-by-step procedure for finding a particular problem.

Perhaps the most important idea is that the operator can cause damage to the system more often than not because all the components are hidden from view and controlled by a simple, normally open, pushbutton switch. There are no knobs to tweak, no adjustments to make, except for turning the key or pushing that button and hoping the engine will start.

We can, however, improve the odds for success by being aware of the factors that help start the compression ignition engine.

THE STARTER SYSTEM CIRCUIT

In early systems, easy-to-understand, electric starting systems consisted of a foot operated lever, which engaged the starter and then connected it to the battery in one single move. Not much can go wrong here; no relay malfunction to be concerned with. It was the simplest electric circuit possible, consisting of one heavy duty switch.

When the starter motor couldn’t be mounted within reach of the operator, a remote system had to be devised, including an electrical solenoid and relay to take the place of the driver’s foot, plus some additional wiring and a relay for control.

Now, we have lost the original “sense” of what is going on. We rely on the solenoid to engage the flywheel and the relays to route power to the starter motor in the correct order. We listen for the engine to crank and start. Nobody wants to hear a “click” or a “chatter” from the starter.

These sounds are actually warnings that something isn’t right, not unlike “dummy” lights for low oil pressure and so on: sounds, unfamiliar to the driver, who used his foot to operate the starter. Even when the batteries runs low, the start motor at least tries to turn the engine over.

With the “remote” system, there are several devices added to replace the foot operated starter. A dash mounted switch energizes a relay, which in turn energizes the solenoid mounted on the starter motor. If the batteries are poorly charged or poorly connected, we may not even hear the starter motor run. The solenoid requires a fair amount of power and may reduce the battery voltage level all by itself.

THE SOLENOID RELAY

The solenoid relay, as it is referred to in the technical manuals, has two jobs to perform. First, it moves a small gear (pinion gear) to a new position, so that it engages the much larger gear on the engine flywheel. Following that, it closes a contact, so that the full battery power is applied to the motor. Most wiring diagrams don’t show it, but there are two windings: one coil-grounded via the motor windings (pull-in coil) and one grounded directly (hold-in coil).

So, there are two separate windings making up the solenoid: one heavy gauge winding to pull the pinion gear forward via a lever, and one lighter gauge winding to hold the contacts close to operate the starter motor. Note how the pull-in winding is grounded via the motor and causes the motor to rotate at a slow speed to aid in engaging the flywheel ring gear. It is also effectively shorted out when the solenoid relay contact closes moments later, while the holding coil stays on until the operator releases the start button. This is good to understand, because it can explain the “chatter” fault.

If the holding coil is not able to do its job, we basically have a “bell” circuit, where a contact is repeatedly opened and closed. The holding coil ground wire is often an external wire and can be checked for continuity. The two windings furthermore “buck” each other when the start button is released and no significant inductive kick is produced. That could harm the contacts of the starter relay, which energizes the solenoid relay.

PROTECTIVE CONTROL BOX

Many 5-ton trucks feature an additional relay, which senses a reverse battery polarity condition to protect the charging system on those vehicles. It’s housed in the, so called ‘protective control box’, along with the start relay and some control circuitry to inhibit the start operation if the engine is already running.

The relays must be able to handle the 35 amp inductive load of the starter solenoid. The starter is operated like any other starter, with the voltage from the dash switch applied to the solenoid coils (wire #74), except for the conditions imposed by the circuitry while the full battery power is applied to the solenoid contact terminal (wire #6) directly, as usual from the batteries.

Having heard about problems with these boxes, the author cut one open to investigate. The box is a welded steel enclosure with a large gasketed MS type connector at one end. This unit had much rust and corrosion on the inside.

When electronic circuitry is packaged in this manner, and getting warmed up and cooled down, only a hermetic seal will keep moisture from getting “pumped” in. A rubber gasket will not do the job.

Also in the unit examined, a 1/8 NPT port in the housing was plugged and painted over. Perhaps that simple vent could have prevented the accumulation of moisture.

Power relays, particularly relays that are not hermetically sealed with an inert gas, have minimum current switching ratings, that is, they should not be energized without a load. The current with the inevitable arcing help burn off oxides from the contact surfaces to make a good low resistance connection. The reverse polarity relay in this control box is an unsealed “high power” relay, which is energized without a load connected, so there may be a potential problem with this relay making a connection also.

Another variation to the start circuit is where a neutral safety switch may interrupt the flow of electricity if the transmission is not placed in neutral. This circuit also shows a direct connection of the ground wire from the battery to the starter motor (wire #7), thus eliminating two connection points when grounded via the frame of the vehicle.

COMPONENT CURRENT RATINGS

The parts of the starting system are, of course, already selected and installed from factory, but there may come the time when we have to make substitutions and to that extent, it is necessary to be aware of the current ratings of the individual components.

The 24-vdc starter motor requires anywhere from 200 to 400 amps depending on condition of the engine crankcase oil and ambient temperature. This means that the cables from the batteries to the starter should be at least “0” gauge in size (military part number M13486/1-14).

The current draw of the solenoid pull-in and holding coils is about 35 amps and since it’s an inductive load, a relay rated at about 50 amps is suitable for this application. The only reason for using this relay is to reduce the relatively heavy wiring otherwise needed to run them to the dash and the start switch would have to be rated accordingly.

If you would like to make your own measurements and determine what the actual currents are in your particular starting system, you will need a special device known as a “shunt”, which is wired into the high current circuit. It is nothing more than a resistor with a very low resistance, 1/10th of a milliohm in this case. The voltage drop across it is measured with a digital voltmeter and converted to amps. The shunt shown in the figure produces a voltage of 50mV for a current of 500 amps and is suitable for our application (military part number MS91587-7). It is installed permanently on some newer vehicles and used with automatic diagnostic/troubleshooting equipment (STE/ICE).

The start switch on the dash “sees” less than one amp, so there are plenty of choices here. To extend the life expectancy of the start switch, a diode should be installed across the coil terminals of the start relay as shown in figure 8. This is common industry practice to kill the inductive spike that is generated in a relay coil when the switch is released. The voltage spike will otherwise cause damaging arcing across the switch contacts. Slave cable connectors are typically rated at 250 amps.

CABLES AND CLAMPS

The instructions in the Technical Manuals are always based on the assumption that mil spec components are used throughout, so when you are troubleshooting any part of the starting system, look first at anything that is not mil spec. A substitute part is often a compromise.

So, when the TM suggests “clean connections”, that implies the correct battery, battery post, battery terminal adapter, correct nut and bolt (lead or zinc chromate plated), cable lug (tin plated and crimped to spec), battery to starter cables (“0” gauge with tin plated strands) and the correct torque applied to the bolt and nut assembly. Correct torque in this case is really something you have to get a feel for; tighten until snug only.

When all connections are done correctly, there should be no more than 0.001 ohms of resistance between the starter and battery. The same standards apply to both positive and negative connections. That translates to a voltage drop of only a few tenths of a volt during cranking. Figure 9 shows the specifications for the battery terminal adapters (clamps). The plating and materials that are specified will minimize corrosive action.

There is a difference between the clamps used for the positive and negative battery posts. They are marked “+” and “-” respectively. There is a difference in size to reduce chance of installing the cables with reverse polarity. New clamps will fit snuggly to the clean battery posts. Install the lugs under the head of the bolt to eliminate any distortion when tightened.

The “0” gage cable (M13486/1-14) is rated at 245 amps and is stranded for maximum flexibility and current carrying capacity. 1,045 individual tin plated strands of 30 gauge wire make up the cable. Tin plated lugs at the cable ends are crimped.

A poor connection has a high resistance, but not high enough to supply power to lesser loads like dash lights, gauges or even the “pull-in” coil. When the contact to the motor is made, the voltage drop across a poor connection will be enough to keep the motor from turning.

Since the starter motor is the biggest user by far of battery power, the cables, both positive and negative, should ideally go directly to it from the battery and continue from there. The wires from the solenoid terminals to the start relay should be 10 gauge or better to handle the current requirements.

EFFECTS OF A LOW BATTERY

A poorly charged battery will result in poor starting performance. The cranking rpm may not be enough to start the engine. Under the best of circumstances, the rpm should be 200-300.

If the batteries are not up to par, especially in cold weather, the rpm may only be one-half of that. A high cranking rpm is important for the multifuel engine, which does not rely on glow plugs to help warm the combustion chamber.

A fully charged 6TN/6TL battery has a 120 amp-hr capacity (20 hours at a 6 amp rate). It has a discharge rate of 600 amps for 30 seconds at 0°F, before voltage drops to 7.2 and the reserve capacity is 200 minutes, which is the time it can supply 25 amps before the voltage drops to 10.5 at 80°F.

When the starter is first engaged, the required torque is at a maximum and the current draw may be 300 amps or so. Once it starts spinning, the current draw is reduced, so it’s important that the batteries can maintain a minimum voltage at the starter terminals of about 22 volts to prevent overheating of the motor. The motor uses less current when spinning and the rpm is dramatically reduced for voltages below 22 while the current draw remains high.

Observe the Battery/Generator gauge while cranking the engine. It is wired directly to the battery and will give you an accurate voltage measure as long as all connections are in good condition.

If the voltage drops below 22 into the red area, release the start button and tend to the batteries. If there is any abnormal (=high) resistance in the cable connections or solenoid contacts, they will also overheat and possibly self destruct. If the solenoid relay contacts overheat, they may even weld together and remain closed after the start button is released, complicating the situation.

The allowed time to operate the starter and the cool-off period varies from TM to TM. This time may be as short as 10 seconds or as long as 60 seconds and the cool-off period may vary from 10 seconds to two minutes.

This duty cycle does depend on how quickly the heat builds up in the starter. For slow, high torque cranking in cold weather, the heat builds up faster and shorter crank times must be observed.

STARTING SYSTEM WARNINGS

The operator’s manual for different vehicles state clear warnings about overextending the cranking time. There is no real agreement as far as how long to run the starter or how long the cool-down period should be, but here are some examples:

“Do not operate the starter continuously for more than 1 minute. If the vehicle does not start immediately, allow for a 2-minute cool-off period.” (TM 9-2320-209-10)

“Do not operate the starter longer than 10 seconds at a time. If engine fails to start immediately, allow a 2-minute cool-off period before operating the starter again.” (TM 9-2320-211-10)

“If engine does not start in first 30 seconds of cranking, release starter pushbutton and wait 30 seconds before attempting to start engine again.” (TM 9-2320-392-10-1)

“Do not crank engine more than 30 seconds continuously. If engine fails to start, place master switch in “OFF” position and allow for a 1-minute cool-off period.” (TM 9-2320-230-10)

When starting your personal car, the radio and other electronic devices are automatically shut off. Likewise, in our vintage vehicles any radio or intercom system must be off during the starting operation to prevent damage from severe electrical transients that may occur.

“Never start your vehicle while the radio or radios are ON.” (TM 11-5820-401-10-1).

On engine start, there are often voltage transients produced, which can cause interference with or damage to electronic devices, in particular, if there are poor connections in the system. There is one other electronic device currently available, the push-button light switch, which is replacing the 3-lever switch in newer vehicles. When it is substituted for the mechanical switch, it is in fact always “ON”. Some of the electronics is always exposed to battery power. This may be a problem when the starting system is not in perfect order and to adhere to the instructions above, in the author’s opinion, there should be a switch installed, so that the feed to the electronic light switch can manually be turned off.

CRANKING RPM

The starter motor pinion gear to flywheel gear ration is about 10:1, so for a needed starting rpm, the starter must run within the range of 2,000 to 3,000 rpm. The diesel, multifuel or compression ignition engine relies on the heat from the compressed air to ignite the fuel and a minimum of about 200 rpm is needed to get it started.

The starter motor running without a load will reach 5,000 rpm while using only about 50 amps to do so. When more torque is required, the rpm drops and the current draw increases. Any resistance in the circuit, including battery internal resistance will reduce the voltage and in turn increase the current draw.

When conditions are ideal, the engine is cranked at about 250 rpm and the current draw may be 200 to 300 amps. In cold weather, conditions are at their worst, battery capacity is reduced, crankcase oil viscosity is high, fuel is cold and as a result the engine is more difficult to start while the starting system has less to work with. Current draw may reach 400 amps and the cranking speed may still be near the minimum required.

The multifuel engine falls in the category of “Cold Start” diesels and the very high 22:1 compression ratio makes it possible to start it without any aids down to about 10°F, but the starting system must be able to do its job.

COLD WEATHER STARTING AIDS

Often, by looking at means and devices used to enhance the operation of a piece of equipment at environmental extremes, the weaknesses of the system are exposed and the starting system is no different. The multifuel engine falls into the category of cold start engines, needing no glow plugs to start, but only a very high compression ratio. The 22:1 compression ratio of the multifuel engines compares favorably to the famous Lister engines, but the engine must turn over fast enough to heat the air or it won’t start.

To ease the stress and strain on the starting system in cold weather, various preparations can be done, such as heating the coolant and/or the crankcase oil pan. The multifuel engines are fitted with oil filter canisters that drain back into the oil pan for heating. A part of the “Arctic Kit”, the fuel burning coolant heater is designed for this task. It will heat the coolant while the exhaust from the heater is routed to a shroud surrounding the oil pan.

Heating the oil pan indirectly helps the starting process by reducing the resistance to cranking and also improves the time to oil pressure. The warm oil improves engine lubrication at startup, which will reduce engine wear. Heating the coolant effects the upper portion of the engine, where it improves chances for ignition. It also heats the intake manifold during this warm-up process.

For less extreme weather conditions, several, perhaps less desirable methods can be applied to help start the engine, the intake manifold heater, the ether start kit or simply, additional batteries: “less desirable” because it’s more of a brute force approach for the starter motor with high viscosity oil and a cold engine.

The fuel burning intake manifold heater, “flame heater”, uses air in the process and should be used sparingly while cranking and at low rpm only. The ether starting aid adds a precicely metered amount of starting fluid to the intake air to induce ignition. Some fuel filters are equipped with a heating element to improve chances of ignition.

HAPPY BATTERIES

Automotive lead acid batteries don’t like hot weather. They perform best at around 80°F and the life expectancy is drastically reduced at elevated temps. They age faster in use or simply at rest. At cold temps they will live longer, but can’t perform very well. So, for storage, keep them cool and fully charged. When ready to use, heat them for best performance.

Always keep your batteries fully charged, which will prevent sulfation at any temperature and also keep them from freezing at low temps. Your vehicle charging system works on the batteries in a series configurations, which is not ideal unless both batteries are identical in all aspects.

A good maintenance charger monitors and restores the charge to the batteries individually. These are powered by solar or house current. Devices used for desulfation are usually not maintaining a charge and are also not needed if the batteries are kept fully charged, since sulfation will not take place under those conditions.

The batteries should also be heated before attempting to start the engine to maximize their capacity. If they have been sitting in cold weather for a period of time, the original capacity is restored by heating. The self discharge rate of the battery is actually reduced by cold soaking. When using an electric heating pad, it is only necessary to warm the batteries right before starting. Keeping the batteries hot continuously will only increase the self discharge rate.

When heating the engine oilpan, it’s also best to heat only for a short period of time before actually starting the engine. Keeping the oil heated continuously may cause evaporation of water, which will condense on cooler parts within the engine and possibly create a corrosion problem.

As a guideline, every 15°F rise in temperature will cut the battery life in half. A lead acid battery, which would last for six years at 80°F, will only be good for three years if operated at 95°F. Theoretically, the same battery would last no more than one year and a half at a temperature of 110°F.

SUMMARY

The starting system is completely at the mercy of the operator and there are no fuses or overload protection here and really only one single gauge to monitor its operation. So, there is no surprise that he or she can easily destroy several of its parts by a sort of “roadrage” attitude when there is a problem.

A cold soaked engine creates an extra load on the starter while the batteries are at their worst, performance wise.Implement as many of the cold weather starting aids as possible to maximize the life of the starting system, and “check batteries daily”.