Trans pressure, fuel pumps, HMMWV upgrades, jacks and more in this military vehicle ‘Tech Tips’ installment

A: It seems as if you have a dedicated and historically valuable project going on with your HMV collection. There isn’t much one can predict about an automatic transmission that is over 70 years old. If the oil or ATF looks clean and there are no metal particles in it, and if the unit seems to shift properly out on the road, and if there is no slop or hesitation from a standing start or when shifting from forward to low-range reverse, about the only other test I can think of would be a pressure test. I performed such a test on one of my M211s (the 10-wheeled version of the M135) by jacking all the vehicle’s wheels off the ground and supporting the axles on heavy wooden blocks so I could “drive” the truck while stationary. Get some kind of mechanical pressure gauge capable of reading at least 250 PSI. Connect the gauge to the main test port on the transmission. Army Technical Manual TM 9-8025 -2 (transmission manual) may be helpful. With the engine and transmission warmed up, the transmission oil pressure in neutral at engine idle should be between 90 and 110 PSI. In reverse, the pressure should be between 180 and 220 PSI. Out on the highway with the engine in the mid to high RPM range and the transmission in fourth gear, transmission oil pressure should be between 95 to 110 PSI. Other than extensively road-testing the truck, and making sure it doesn’t drag its front wheels when backing up, this is about the only other test I know of to get some idea of the transmission’s condition, Of course, as with any older vehicle, and especially if it has been sitting a long time, various problems such as leaking seals are likely to occur if one starts driving a lot.

A: In most cases, yes, though some civilian fuel pumps were not double-acting. This which means they didn’t have a vacuum pump feature for the windshield wipers.

The U.S. Army is responding to soldiers’ requests for new equipment to enhance combat operations and increase safety by installing five new upgrades to HMMWVs at forward repair sites. These include a fire suppression system, improved seat restraints, an intercom system, a gunner’s restraint, and single movement door locks. While these upgrades will initially be installed in HMMWVs, the Army is adapting some of them to other medium and heavy tactical vehicles. Eventually, the entire tactical fleet will receive the fire suppression system. New gunner restraints and the intercom system will be installed on all vehicles with gun-mounted turrets, and most tactical vehicles will receive the new seat restraints.

Using your vehicle, HMV or otherwise, to jump-start another vehicle can be very hard on your generator or alternator unless it’s done right. An alternator or generator was not designed to charge a vehicle’s battery while the starter was cranking and drawing a lot of power. If the other vehicle’s battery is dead, hook up your jumper cables — taking all the usual precautions — then run your engine at moderate RPM for at least five minutes to charge the other vehicle’s battery. Then shut your engine off while the other driver cranks his. Don’t let yourself be talked into running your engine while the other guy is cranking because it “saves time.” It might save him time, but that won’t help you if your generator or alternator gets fried!

A: No. An M38 dipstick is about two inches shorter than an M38A1’s, so it won’t be useful in determining the proper oil level in your ‘A1, though you could weld or braze on additional length. The correct length of M38A1 dipstick is 27&19/32 inches, measured from the lip of the cap to the end of the rod. The military encountered this problem when the M38 and M38A1 were in service together. While the longer M38A1 stick didn’t fit in the M38 engine, the M38’s shorter stick did fit in the ‘A1 — as you found — resulting in some massive oil overfills. While we’re on the subject, and for FYI. some M37, M42, M43 and M201 trucks came from the factory with oil filler pipes that were too short. The correct length for these is 14&1/8 inches.

The correct jacks for many large U.S. military vehicles are usually just off-the-shelf hydraulic jacks. For example, the correct jack for a CCKW or DUKW is a 3-ton bottle-jack, while the White halftrack uses a 5-ton model. 7- and 8-ton bottle jacks were specified for the White Scout Car, Dodge WC63, the Corbitt 6-ton and the Mack NO, to name just a few. When choosing a jack, remember that for a cargo vehicle a jack must be strong enough to raise it when fully loaded. Fortunately for HMV collectors, the appearance of bottle jacks hasn’t changed much over the years, and a new jack can be painted OD or flat black for added authenticity.

A: My suggestion is, it’s probably better not to, or you may be wasting paint, as well as having to paint your truck all over again. While your grandfather is correct that OD paint was sometimes made in the field this way, such homemade OD is often unstable, and you may find your G-506 turning strange shades of green within a few months.

A : Your G-506 is indeed probably an early production model, which used the civilian style instrument cluster — too bad about the missing data plate. It used to be standard practice in many states to civilian-register military vehicles by the year they were released from the government, rather than by the actual date of manufacture. This was especially common after WWII, and is probably why your truck is registered as a 1946. If you’re good at wiring, you can replace the civilian style instrument cluster with one from either a later model G-506 or from a closed-cab GMC CCKW. However, your truck would be more historically valuable if you kept it original. You might consider doing research to find an accurate date of manufacture. The data plates were usually mounted on the glove box doors of 1940-41 trucks, then above the windshield on later models. You should be able to obtain reproduction data plates from some of our advertisers, as well as manuals.

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

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