Tech Tips – December 2016

by Steve Turchet

Click, Clunk, CRUNCH

I have a 1951 Willys M38. It is my second car, and I drive it to work five days a week – about a 20 mile round trip.

A few weeks ago, I started hearing a faint clicking sound in the transfer case. It was slow at slow speeds and got faster when I drove faster. It didn’t sound very serious, so I didn’t do anything.

Then a week ago, the transfer case started making a loud, clunking sound. It was strange, because I know the gears turn fast. Thissound was slow and regular. It did speed up when I drove faster.

This does sound serious. Do you know what it is? Did I screw up by not checking out the clicking sound when it started? – H. Morris

I suspect that the clicking sound you first heard was a chipped tooth on one of the three gears that transmit power through the transfer case: The main shaft gear, the intermediate gear, and the output shaft gear. I’m sorry to have to say this, but you should have stopped driving your Jeep, pulled the transfer case covers, and looked at the gears right away.

What probably happened is that the chipped tooth on one gear fractured again, and the broken piece broke a tooth on another gear, which in turn broke a tooth on the third gear. This will only get worse if you keep driving!

The gears will continue to break each other’s teeth until, with a CRUNCH, they will finally strip to the point that your Jeep won’t move.

The reason the clunking sound was slower than what you would imagine for spinning gears is because the same gear teeth don’t mesh with every revolution. The clunk only occurred when the broken teeth came together.

The damage is already done, and the only fix is to replace all three gears. You might as well install a new intermediate shaft and bearings while you’re at it.

Little Sparky

I have a 1963 M37. It is a show truck and I don’t drive it a lot and I keep the batteries disconnected and on a trickle charger. I’m curious to know why there is a little spark whenever I connect up the batteries again even though all the switches are off? – Dale

On many HMVs, such as the M37, M38 and M38A1 with Autolite voltage regulators, a very small amount of power is drawn from the batteries at all times to keep the regulator’s filter condenser charged. For a vehicle that’s driven regularly this isn’t a problem, but you’re doing the right thing by disconnecting your batteries when your truck sits for long periods, because even this tiny current drain would eventually run them down. The tiny spark is caused by the regulator’s condenser charging up again when you connect the batteries.

Baffling Batteries

I have an M715. I replaced the driver’s side vacuum windshield wiper with a 12 volt electric unit that I run from a wire off one battery. This might be a dumb question but I can’t understand why you can get 12 volts from one battery when two are connected together for 24 volts? – S. Martin

As my dad used to say, the only dumb questions are the ones that aren’t asked. Each battery cell produces two volts, and DC (Direct Current) only flows in one direction. This concept is less confusing if you think of a 12-volt battery as being six 2-volt batteries connected in series. On old-school rubber-cased batteries, the cell connectors were out on top in plain view. Since DC current only flows in one direction, if you were to put a voltmeter on each cell connector, you would find two volts at the first cell, four volts at the second, six volts at the third, eight at the fourth, ten at the fifth, and finally twelve volts at the last. This, by the way, was a method of checking old style batteries to see if a cell had gone bad.

Likewise, if you connect two 12 -volt batteries in series, this gives you 24 volts at the “second” battery. And since DC current doesn’t flow backwards, you can get 12 volts from the “first” battery.

Sometimes Cool Is Not

Owners of vehicles with Multifuel engines, such as the M35A1, should be aware of engine temperature at all times. Running your engine at low RPM or driving off before the engine is fully warmed up means that all the fuel isn’t burned in the cylinders, leaving carbon deposits on the valve stems, which may cause burned valves. Unburned fuel also leaks past the piston rings and into the crankcase, diluting the oil.

A cold running engine also causes condensation to form in the crankcase, which produces metal-eating acid as well as sludge in the oil galleries and valve cover. Unburned fuel also “slobbers” out the exhaust pipe and blows onto your truck’s canvas.

Always make sure your Multifuel engine is fully warmed up before driving off, and if it still seems to run cold, check the thermostat. For extreme cold weather operation, a radiator grille cover may be required.

Hot HMMWV No-No

I live in New Mexico and have a HMMWV. It is often hard to start when the engine is hot. I have heard that our troops have the same problem and pour water on the fuel pump to cool it. Do you think it would help for me? – Brian

It might help your engine start... but it could also kill your fuel pump. And, unlike our troops, you probably don’t have a motor pool to replace it.

If you pour water on the pump to cool it, the outside of the unit contracts but its delicate innards do not. Don’t do this. It isn’t the same concept as pouring water on the fuel pump of an M715 when it vapor locks.

Send your favorite Tech Tip or question to Steve Turchet at military@aimmedia.com