Tech Tips: Voltage, Cables, Batteries, Knocks and More

A: I have to respectfully disagree… not in theory, but from practical experience. In the late 1970s I had an M37 with its stock 24 volt electrical system using two 12 volt batteries in series. I put a cab-over camper on the truck, and the camper’s interior lighting as well the its exterior clearance lamps were 12 volt. I also installed a CB radio, AM/FM radio, and 8 track cartridge player in the truck’s cab, all of which were 12 volt. I connected the positive input of all the 12 volt lighting and accessories to the positive terminal of the “first” of the truck’s two batteries. Ground was common with the truck’s 24 volt system. I drove that truck for about three years on a daily basis, including a trip from Tucson, Arizona to Seward, Alaska and back, lived in the camper for about a year in the Arizona desert, used the 12 volt lights and accessories every day. and never had any problems with the batteries, which were charged by the truck’s stock 24 volt generator. Battery charge as indicated by the truck’s voltmeter was always 22-24 volts engine off, and 26-28 volts engine running. I bought both batteries new (Sears Die-hards) before setting out for Alaska. and they were still strong when I finally sold the truck. On the other hand, this might be a case where results vary depending upon how such a system is used; and I did occasionally switch the batteries so that both were alternately used as the 12 volt source… which simply seemed like a good idea.

A: Yes, the convex sides of the shoes face the engine’s flywheel. If think about it, this makes sense because when the starter is engaged the yoke and shoes lever the starter drive toward the flywheel so the convex curve makes the action smooth. Many of these types of starters will still work if the yoke is installed backwards, but that wasn’t the way they were designed.

A: Thank you for the useful tip, Clell!

Batteries are often some of the most misunderstood and neglected components of collector or hobbyist military vehicles. Batteries are often blamed for every vehicular electrical problem from slow engine starting to headlights that only burn as bright as birthday candles. However, these problems are more often caused by dirty, loose or corroded connections and wiring. Just as often, while a battery may indeed be low – meaning not fully charged – it’s not because there is anything wrong with the battery… except that its owner didn’t properly service and maintain it. Thousands of perfectly good batteries are replaced every year because they’re thought to be the cause of hard-starting or electrical problems when the actual cause is something as simple as loose or dirty terminal connections. Look at your battery cable clamps for gray fuzz or corrosion every time you open the vehicle’s hood, and make sure the clamps are tight by trying to move them by hand. If your vehicle’s battery or batteries are located somewhere other than the engine bay, check the battery cable clamps for gray fuzz or corrosion at least once a month. Corrosion can also form out of sight between the battery post and the cable clamp. This is usually a kind of hard glaze that has to be scraped or wire-brushed away, so remove the clamps occasionally to check for this. While there are various products to spray on or apply to battery clamps to prevent corrosion, wheel bearing grease seems to work just as well.

A: I wish I could give you a specific answer, but accurately diagnosing engine sounds takes years of experience, and even then it’s difficult… especially not being there to hear it. Low mileage is no guarantee that an engine doesn’t have problems, but you’re right in saying that many people call any knock a rod knock. Here are a few tips and procedures. Invest about $20.00 in a 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 begins with eliminating possibilities. First, eliminate the accessory sounds, 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. Try to track down the noise with the stethoscope tip (or the end of your hose) held firmly to various places on the engine. Spend a good amount of time putting the hose or ‘scope all over the engine, not just where the sound seems loudest. Try to envision the parts moving inside the engine. You want to train your ear so don’t be in a hurry, but be sure the engine doesn’t overheat with the belts off. Rod knocks are usually loudest at RPMs over 2000. Feathering the gas pedal may produce a back-rattle between 2000 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 often 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. 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 the 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’re 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’s 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 are generally loudest up to 1500 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.

A: While I wasn’t in WWII - due to the fact that I wasn’t born yet - and so never got a chance to drive a brand new GPW or MB, it’s been my experience that these types of windshields usually leak to some degree, especially at higher speeds and during heavy rain. My solution has usually been duct tape, which is easily removed when no longer needed, and is also available in dark green and even OD. When I drove my M37 to Alaska, I used black silicone to stop the windshield from leaking during the rainy summer months.

A: If your M38A1 has all of its fording system components - as could be confirmed with a manual - then installing a new fuel pump is simply a matter of reconnecting all the lines and fittings that came off the old pump. I looked though my manual, but you didn’t say if the mystery port is on the fuel pump section (bottom) or on the vacuum section (top) of the pump. Both sections have “in” and “out” ports. If the port you describe is on the mid-section it may be for a vent line. Also, keep in mind that many rebuilt or reproduction components, such as fuel and water pumps, may have extra ports or fittings for different applications, though the component itself may work fine without some of these connected to anything. Work the pump’s lever a few times while holding a fingertip over the mystery port. Does it “suck” or “blow” when the pump is activated? If it does one or the other it would be a good idea to consult a manual before plugging it. If it does nothing it may be for a vent... but again, it would be best to consult a manual before plugging or ignoring it. That’s about all I can tell you with the data I have.

A: Thank your for the KISS. With a little ingenuity one can make an attachment for a grease gun to get up in there and lube the clutch fork socket. Just be careful not to get grease on the clutch.

A: You may be on the right track about the brake shoes heating up and dragging. I’ve seen this happen on many vehicles in which new brake shoes were installed or old ones adjusted, and were adjusted too tightly when cold. Most things expand when they get hot, and this includes brake shoes. If they were adjusted too tightly when cold, they will drag against the drum and heat up when the vehicle is driven. And the more they heat up the more they drag. Adjust the shoes cold so you can just hear them rubbing lightly against the drum when you rotate a wheel by hand. My method is to first adjust the shoes out firmly against the drum until I can’t turn the wheel by hand. This seats the shoes and takes up any slack in the components. Then I back them off until they just barely rub. By the way, just because a master cylinder is new doesn’t mean it can’t be defective. I’ve found a lot of defective new, rebuilt, and reproduction components right out of their boxes. If you’re sure that shoe adjustment isn’t the problem, the next thing I would suspect would be the master cylinder. A good rule to follow if one has problems after replacing any parts anywhere on a vehicle is to first suspect the new stuff and/or anything that was changed, altered, replaced or adjusted. I’ve also found that replacing one part of an assembly can throw the other old parts out of wack.

A: Thank you for your service. In most cases the adjusters can be installed either way, though the correct position, as your manual shows, is with the stars facing the front of the vehicle. This allows easier access on some MUTTS, and the stars will turn in the correct direction when making brake shoe adjustments.

A: More likely than not, yes, and if so you should replace it as soon as possible because it will only get worse until the bearing finally seizes up. Since pulling the transmission and bell-housing is not a minor job on your M38, you should probably consider replacing the whole clutch assembly while you have it apart, and also possibly have the flywheel resurfaced. Be sure to install a new pilot bushing. You may also want to install a new front oil seal on the transmission while you have it out.

A: That gasket was only there to enhance your M38’s deep water fording capabilities by keeping water out of the clutch housing. If you don’t have a fording kit and don’t plan to ford your jeep, it’s not essential. However, if you want your M38 to be correct, you can make a suitable gasket from thin gasket paper. Silicone will work for waterproofing, but of course they didn’t have it in 1952.