Grinding gears, polishing compound, ignition coil resistance and more in this installment of Military Vehicle’s ‘Tech Tips’

A: It might, and you should check for proper clutch pedal free play. However, most M38A1s had Warner T90 transmissions with synchromesh second and third gears but no synchromesh in first. It can be hard to shift into first after using reverse, when at a complete stop, or moving faster than a walk, unless you learn to double-clutch.

A: Thank you for your service. Check the player’s capstan. You will probably find several layers of the “eaten” tape wrapped around it. This will speed up the playing. By the way, it’s seldom the fault of the player if tapes get eaten. The tape itself, if not subjected to temperature extremes or otherwise damaged, will last indefinitely. What usually happens is the tape breaks where it’s joined by the magnetic sensor strip (what triggers the player to change tracks) because the strip has deteriorated from age. This material is available from online sources. You should never play any vintage 8 track tape — even if it’s still sealed in its original wrapper — without first replacing the sensor strip. Likewise, you should check the condition of the foam pads and the rubber tape pinch roller. Replace them if necessary… and it is usually necessary.

Automobile polishing compound will quickly and thoroughly clean your vehicle’s windshield and window glass of old crusted stuff, bug splats, and paint overspray. It also works great on mirrors, reflectors, and head and tail light lenses. Car wax will brighten and preserve plastic lenses.

Some M151s exhibit a mysterious little problem when idling: Sometimes the engine simply shuts off as if someone had flipped a switch, but it can usually be started again instantly. If this has been happening to your MUTT, check the fuel safety shut-off switch. It’s set to kill the electric fuel pump if the engine oil pressure falls below 10 psi. Of course, check your engine oil pressure too, and make sure it isn’t actually dropping below 10 psi at idle, especially when it’s hot.

A: There are two common types of 12-volt ignition coils used on most vintage U.S. vehicles. One type has a 3-ohm internal resistor and is used without an external ballast resistor. The second type has a 1.5-ohm internal resistor and is generally used with an external ballast resistor. If so, it should be wired with an extra wire on the “+” terminal that will feed it a full 12 volts when the starter is cranking — usually from the starter solenoid. If not wired this may, you may have problems getting the engine started. Some people have used 1.5-ohm coils without an external resistor, but the coil may become extremely hot in operation and will probably fail prematurely. While it is normal for coils to heat up, they should never become too hot to touch. Using a 1.5 ohm coil without an external resistor may also cause burned distributor points. In either case, buy a good quality coil. There are a lot of cheapies these days which may only last a few months (if that) as well as causing other problems such as hard starting and /or engine misfiring. On the other hand, good quality coils often outlast the life of a vehicle.

A: I believe that the six-bolt Dodge and Unimog wheels will, indeed, interchange. The pattern should be the same — six on 7-1/4” center — but do some checking. Or, if possible, try to actually switch a wheel before making any purchases.

A: Such groves almost always face down so that oil drains back into transfer case or transmission instead of collecting around the seal and leaking out.

A: A more accurate term would be “water resistant vehicles,” like watches being advertised as “water resistant” rather than “waterproof.” The same would apply to all M-series vehicles, not just HMMWVs. Even if fitted with a fording kit, driving a vehicle under deep water is considered a last-resort event. It’s a very stressful ordeal for the vehicle and meant to be over with as soon as possible. Despite sealed dashboard instruments, lights and switches, as well as special gaskets and 0-rings in air-cleaners, carburetors, starters and generators/alternators, the majority of seals and gaskets on M-series vehicles’ engines, transmissions, transfer cases and axles are of the conventional type. They are designed to keep oil and other fluids from leaking out , not to prevent water from leaking in. In other words, these gaskets are meant to withstand pressure from within, not from without. Also, the special seals and gaskets on M-series vehicles, including the battery caps, are not designed to withstand submersion for long periods of time. In addition, most M-series vehicles have a fording valve that uses engine crankcase pressure to pressurize their transmissions, transfer cases and axle differentials when fording, so their engines must be running to provide this pressure. Simply put, there are just too many places for a vehicle to leak. If an M-series vehicle — including a HMMWV — was parked under water for any length of time, you would likely find its engine, transmission, transfer case, and axles, as well as its batteries and possibly the fuel tank, full of water when you attempted to start it.

A: The best precaution would be not to do it. Most smart people who drive off-road or out in the bush and who don’t have breakdowns or get stuck, use the philosophy of, “do I really need to go there?” before attempting to climb a mountain or cross a river. Aside from asking yourself that question, you should get a manual that covers the procedures for deep-water fording your jeep. Driving any vehicle under deep water is not a casual event; it requires proper preparation as well as service after the fact... such as checking the engine, transmission, transfer case, axles, batteries, fuel tank, and brake master cylinder for water when you’re back on dry land. Also keep in mind, that just because you recently installed a fording kit, that, no guarantee that the seals, gaskets and 0-rings on your M38A1 are in good enough shape to withstand deep water pressure. If, in spite of this advice, you want to go ahead with this adventure, you should have another vehicle standing by that is capable of rescuing your jeep. Also, be sure to remove the fan belt, because the fan blades may be bent forward and cut into the radiator from trying to pull water instead of air.

If the fuel solenoid diode (NSN 5961,1-180-5634) burns out in your HMMWV, TACOM LCMC recommends replacing it with a NSN 5961-01-593-3791 diode. This provides better electrical protection. You can tell the two diodes apart by the protective shrink sleeve on each. The new diode has a blue sleeve. The old diode, sleeve is black. The old diode is still being used, but it should only be applied to the horn circuitry for vehicles with serial number 255269 and above that have the Smart Start System. So both diodes will be added to the HMMWV parts TMs in a future update. The main reason for the original diode, failure is that vehicles are being improperly slave started. That causes a transient voltage spike that can damage other electrical components as well as the diode. 

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