Military Vehicles Magazine Tech Tips: knocks, tachs, enhanced HMMWVs and more

Q: I wear out the pages reading your Tech Tips. I’m a small-time Jeeper since the 1950s, mostly CJ-2s, M38s and M38Als. Question about my M38: when I back up, the brakes lock. Pull forward and it’s OK. I put brakes on it three or four years ago. Before I pull the brake drums I wanted to hear from you. My transmission shifts OK, except sometimes it’s hard to get it from 4-wheel drive back into 2-wheel drive. If I stop and go into reverse with a little clutch movement, it slips into 2-wheel drive and away I go. I broke down in Prescott, Arizona a few years back with my M38 ... naturally at 5:30 p.m. at a stoplight. The jeep had no spark. After a couple of days checking and rechecking and wearing out all the Irish bloopers I knew, a friend of mine suggested it was the capacitor in the distributor: All-knowing me said they never break down. I had to eat those words! Also, my M38 has never had a working instrument cluster.

Keep on writing the good stuff. — R.L. White

A: Thank you for your kind words about Tech Tips. In regard to your question about the brakes locking when you back up your M38, first, are you sure it’s the brakes? There are cases where the main shaft in the transmission becomes loose, either because of badly worn bearings or the nut on the output gear to the transfer case backing off, which may cause a jamming condition in reverse. This has also occurred on other vehicles such as early Nissan Patrols. If you’re sure the problem is in the brakes, it sounds as if something in the brake shoe mechanism is either missing, broken, or incorrectly installed. If you have a manual this should be easy to spot. Also, make sure you have the correct shoes for your jeep. Some drum brake systems have a leading shoe and a trailing shoe, so also make sure these are correct. It’s also possible that the brake shoe return springs have lost their proper tension from being overheated or because of old age.

Most modern drum brakes (“modern” includes your M38) are self-energizing. This means that after one steps on the brake pedal and the shoes make first contact with the rotating drum, they are pulled tighter against the drum by its rotation. So, if something is broken, missing, incorrectly installed — or if the shoe return springs are weak — one or more of the brake shoes may be pulled into tight contact with the drum when you’re backing up, causing them to jam. With this in mind, I would first make sure — with a manual — that everything is installed correctly in the brake shoe mechanisms, and that the shoes are adjusted properly so they don’t drag on the drum.

If all seems OK in this respect, my next move would be to install new shoe return springs and see if this solves the problem. Oil or grease on the brake drums may also cause the shoes to jam, as can dirt or sand in the drums, as well as corroded or sticky wheel cylinders. It’s normal for the drive-train to build up tension in four-wheel drive, which can make it hard to shift out of four-wheel drive. Likewise, it may be difficult to unlock locking hubs. I discovered this on my first jeep, an M38. I also discovered that backing up a little way—a few feet or so—usually released this tension, making it easy to shift out of four-wheel drive as well as unlock the hubs.

In regard to the capacitors or resistors in M-Series waterproof distributors (or any other distributors, for that matter) they all eventually fail so it’s a smart idea to carry spares. Badly burned ignition points are one indication that a capacitor—often called a condenser — has failed or is failing.

As to your M38’s instrument cluster, if none of the gauges work (except, I assume, the speedometer) I would first suspect a bad ground between the instrument panel and the dashboard. If the panel turns out to be properly grounded to the dashboard, I would then check that each gauge is properly grounded to the panel by removing the mounting brackets and cleaning them to be sure they were making good metal-to-metal contact. If that didn’t work, I would then check that all the gauges get power when the ignition switch is turned on. After that, one would have to start testing individual wires to each gauge and to the sending units. And, though it seems unlikely, there is a possibility that all the gauges are bad. Lastly, check the sending units.

“Reliability Enhanced HMMWVs” have been specifically modified for harsh environments and increased payloads. These improvements are specific to the M1151, M1152 and M1165. Reliability Enhanced HMMWV packages include: Geared Fan Drive (angled fan);. New Cooling Stack and Shroud; Rear Differential Cooler; New Power Steering Pump; New Shock Absorbers; New A-arm Bushings; New 3-Piece Frame Rails and Cross-members; Increased Capacity Service Brakes with Quick Pad Removal; Redesigned Reduced Effort Steering Geometry and Linkage; New 24-Bolt Wheels with Increased Load Rating; Reengineered Geared Hub Assemblies; New Parking Brake; Reengineered Airlift Brackets.

Q: I’m the owner of an M43. I need to re-pack the wheel bearings. “TM 9-8030” says to use “Drift 41-D-1535-25 and Handle 41-H-1397” to remove the inner bearing cone and oil seal. Any idea where I could get such a beast? Failing that, can anyone tell me the dimensions so I can have one made up?

I enjoy your magazine. — Gary Rafferty

A: If I understand you correctly, you could always search for the proper tool, but you can use just about any suitable drift punch to drive out the bearing race and oil seal. Lay the wheel down with the rear oil seal underneath and use a suitable drift punch to tap the bearing race downward evenly. This will also push out the oil seal. Tap the race alternately — first on the left then the right (or vice-versa) so it doesn’t cock sideways and jam. Likewise, you can install the race (or a new one) by tapping it back in with a drift punch, being careful not to damage the polished bearing surface. It will be apparent by feel when the race is properly seated. A wooden dowel will work as well as any special tool to install the oil seal.

I’ve never liked “skinny” steering wheels, and at age 16 after driving my first HMV, an M38, about 1,200 miles on a trip to Death Valley, Calif., I liked them even less. Not wanting to sissify my jeep with a one of those “European Style” leather steering wheel covers available in those days, I made a field modification by slitting a length of heater hose so it fit over the steering wheel’s rim, then wrapping it tightly with several layers of black electrical tape. This significantly increased the thickness of the wheel rim and gave a very comfortable grip while looking like something a soldier might have done. I liked this so well that I’ve done it on almost all the vehicles I’ve owned. Try it, you might like it, too.

Whether civilian-owned or in actual military service, many HMMWVs don’t have the right fuel-injection pump installed, and/or the fuel-rate calibration is incorrect. This results in either over-fueling, which wastes fuel, produces black smoke, and is detrimental to proper engine lubrication, or under-fueling, which results in lack of power. Proper fuel delivery, either with the correct pump and/or the right calibration, is determined by the amount of equipment loaded onto the vehicle, and/or the type of variant. Installing the correct pump with the proper calibration is especially important for our hobby’s HMMWV owners who often re-configure a vehicle.

Simply put, if you lighten a HMMWV that has a fuel pump calibrated for heavy weight, you are wasting fuel. Likewise, if you add significant weight to a HMMWV without re-calibrating the pump, the vehicle’s performance may be degraded.

Q: The GMC 302 engine in my 1954 M211 is making a knocking sound. All my mechanically minded friends say it is a rod knock and tell me I have to rebuild the engine or it will blow up. I know my truck has very low miles and is in good condition so worn-out rod bearings don’t seem possible. Is there any way of being sure what is causing this sound? — Hal Mitchner

A: I wish I could give you a quick answer, but accurately diagnosing engine sounds takes many years of experience, and even then it can be difficult. You’re right in saying that many people call any knock or rattle a “rod knock.” A guy once told me that his 1944 Willys MB had a rod knock. In a way it did. However, this particular “rod knock” turned out to be a big bubble on a front tire that was hitting a tie-rod end every time it went around.

Here are a few basic tips and procedures: Invest in a cheap stethoscope at an auto parts store. Or, you could try a piece of hose stuck in your ear, but two ears are usually better than one. Diagnosing engine noises is nothing more than eliminating possibilities. First, eliminate all the sound-making accessories on your truck, such as the generator and air compressor by removing their drive belts one at a time. If the noise goes away, then obviously the cause was one or the other — with a worn air compressor being the most likely. If the noise is still there, you should be able to hear it more clearly by not having the accessories going. This also applies to accessories such as power steering and vacuum pumps, as well as air-conditioner compressors.

Your M211 has an in-tank electric fuel pump (if stock), but for others who own HMVs with mechanical pumps, eliminate that possibility by putting your stethoscope on the pump while the engine is running to see if it’s making the noise. Try to track down the noise with the stethoscope tip (or the end of your low-budget hose) held firmly to various places on the engine. Spend a full 10 minutes putting the hose all over the engine, not just where the sound is loudest. Try to envision the parts moving inside the engine. You want to train your ear, so don’t get in a hurry — except to be sure that the engine doesn’t overheat. A trained ear can tell you which piston is slapping or which rocker arm is clacking.

Rod knocks are usually loudest at rpm over 2,000. Feathering the gas pedal may produce a back-rattle between 2,000 and 3000 rpm. Bad rod knocks may double knock if enough rod bearing material has been worn away to let the piston touch the cylinder head in addition to the connecting rod bearing knocking on the crankshaft. If this is the case, there will be a hard metallic knock from the loose rod bearing with an alternating and more muffled aluminum knock from the piston hitting the cylinder head.

Finding out which cylinder has the noisy parts may be done by removing the spark plug wires at the plug end and grounding them one-by-one. Be sure to ground them or it produces a shock in the ignition coil. Be careful where you ground them or you could produce a shock in yourself. If the noise changes when a plug wire is shorted to ground, it’s usually safe to assume that the problem is in the lower-end parts of your engine: piston, wrist pin, connecting rod, or connecting rod bearing. The reason the sound changes is that when you short the cylinder plug wire, you are cutting out the combustion explosions that are slamming the piston downward and making the loose connecting rod bearing knock against the crankshaft journal. If the sound was a piston slap, no explosion in the cylinder affects how the piston is slammed sideways against the cylinder wall, so the sound will change or go away.

It is common in cases of rod-knock or piston slap that more than one piston pin or rod bearing is loose. If the sound doesn’t change when you ground the spark plug wires, then suspect parts other than lower-end. Valve train noises generally are loudest up to 1,500 rpm. Mechanical lifters are often noisy, while dirt contamination in a sludged-up engine is the usually the cause of hydraulic lifter noise. Low oil pressure also causes hydraulic lifter noise, but your M211 has mechanical lifters.

Q: I have been an off-roader for years with my mostly stock M715 cargo truck. I was out camping recently and saw my first Unimog. The vehicle was climbing a very steep slope. I have never seen any vehicle move that slowly under power! What kind of transfer cases do these vehicles have?

A: A Unimog doesn’t have a transfer case in the conventional sense. The gearing is all built into the transmission. I suspect the `Mog you saw had what is known as agricultural gearing. While capable of the same top speeds as any other `Mog, a unit with this type of gearing can move so slowly you have to watch a while to be sure it’s moving at all.”

Q: I have a 1963 Willys M676 with the Willys flathead six-cylinder engine. Is there any way to hook up a mechanical tachometer?
— Clyde B.

A: Just about anything is possible if one has the skills and determination (and usually money) to do it. Most mechanical tachometers on gasoline engines were driven off the distributor, which means you would have to locate a distributor with a tach-drive that will fit your Willys 226 engine. Mallory ignition used to make aftermarket distributors for many engines, and a tach-drive was often optional, so that’s one possibility. While I doubt that Mallory still makes distributors for your engine, you might find a used one by searching the web.

Those 226 engines were also used in marine and industrial applications. I saw one in a boat that had a tach-drive distributor, so I know they exist. A lot a folks don’t know that the parts of many older distributors will interchange, so if you’re good with tools you might be able to custom-build a tach-drive distributor. 

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