Transmission Cooling Lines

It seems that most people fail to understand the difficulty of repairing or the time and expense to repair damaged transmission cooling lines. Both the lines in our M1038 and from our new powerplant were damaged, and unusable.

We could have just substituted hose for the lines. However, tubing is far sturdier, and rated for higher pressures. Consequently, tubing is less likely to fail than a long length of hose. Arguably, there is some cooling effect to be accomplished just from the +/- 20 feet of line.

We determined that it would be virtually impossible to install factory cooling lines without removing the cooling stack and possibly even the powertrain. As a result, we used 3/8″ brake tubing and made our own.

Custom fabricated transmission cooling lines to cooler (new cushion clamps are on order)

We fabricated our cooling lines in two pieces (each line) that joins at the frame near the idler arm. This allowed us to have the complex bends on the front section, and allows for it to be somewhat easily installed. We do not have a 3/8″ beading tool — If we did, we would simply have beaded the ends. However, using a Ridgid 41162 377 flaring tool (which provides the proper 37 degree angle for AN fittings) and used Russell -6 AN tube sleeves and nuts to connect an AN to 3/8″ hose barb. This ensures a leak-free connection.

Although the transmission pressure through the cooler only reaches a maximum of 30 p.s.i., we were still concerned about preventing leaks between the hose and the tubing without having a bead.

Although the AN connectors are not cheap, we already owned the Ridgid flaring tool, and the cost of the fittings was considerably less than a beading tool.

Heater Control Valve

The Heater Control Valve 4820-01-189-2107 [4820011892107] (AMG 12339966) was missing from our vehicle. We researched the different aftermarket valves available and were able to locate a replacement: Four Seasons P/N 74828.

This appears dimensionally identical to one in an M998:

Installation of Cooling Stack

After the fasteners finally came in, we were able to assemble the cooling stack. Although we had attached the shroud to the radiator last week, we were finally able to fasten the oil/transmission cooler to the radiator, and install the power steering/hydraulic cooler over that.

As we have the later version radiator side mounts, we were able to precisely align the shroud to provide for optimum cooling. We were able to connect one of the two oil cooler lines, but will have to loosen the cushion clamps and re-orient the hose so we can connect to the fittings.

We will connect the power steering cooler after all lines have been connected from the pump to the brake unit, winch, and fan clutch. Similarly, as the transmission cooling lines were not serviceable, we will be making replacement lines out of 3/8″ brake tubing with AN adapters to 3/8″ hose. We will post on this once it gets underway.

Wiring: Dash and Underdash

As this HMMWV did not have a dash or gauges installed, we had to identify each wire by number from the wiring diagram. We put tape on each wire and marked where it belonged. We found it simplest to tape the wiring diagram to the windshield to assist in wire location identification.

Through this process, we located where a 3 wire bundle had been cut off. Through the process of elimination, it appears that these wires are for the headlight dimmer switch. We will have to fabricate a new section which will involve soldering and splicing to the existing harness using waterproof marine heatshrink tube..

Leakdown Testing the Deep Water Fording System

Once the Deep Water Fording (DWF) system has been installed, it is important to test the system for leaks.

Instructions for testing may be found in the “Instructions for Installation of Deep Water Fording Kit” at Section 3-14. The testing consists essentially of:

  • Block off engine air intake and exhaust, set selector valve to “vent.” (Note, we are unsure as to why blocking off engine air intake and exhaust is necessary — the only connection the DWF vent system has connected to the engine during this test is the vent at the fuel pump). It is imperative, however, to set the selector valve to “vent.”
  • Disconnect the “42 inch long” hose (CDR valve to selector valve) at the CDR valve.
  • Apply 2.5 to 3.5 p.s.i. into the hose removed from the CDR valve. (Note: there are notes to these instructions found on the internet that have redacted “to selector valve hose” so that it reads apply pressure to the CDR valve. This is incorrect, and doing so would be an attempt to pressurize only the crankcase. Air needs to be applied into the hose leading to the selector valve.)
  • The system must have at least 1 p.s.i. in the system after 1 minute.
  • This will check the entire DWF system.

Notes on this test: 1) If there is only a slight leak, testing at 3.5 p.s.i. might pass the 1 minute test, whereas testing at 2.5 p.s.i. would fail the test. 2) It is best to conduct the test in a quiet area so that any leaks can be heard. 3) We recommend use of hand pump only, not a compressor or regulated air source. 4) We suggest making a low pressure leakdown tester as described here. 5) Application of excessive air pressure will likely “blow out,” dislodge or otherwise damage seals in the transmission, transfer case, differentials and gear hubs. Use of other than a hand pump could potentially cause damage in the thousands of dollars to repair.

Our initial testing went well. The tube to the power steering pump had to be removed and blocked because the cap did not seal well enough. After identifying this as a leak point, air was re-applied and was heard leaking into the engine.

As the only point for air to enter the engine would be the vent on the fuel pump, we suspect the fuel pump has a leak into the crankcase. We will first remove the vent line from the fuel pump and test again. If we are able to maintain pressure with that line plugged, we will have to replace the pump.

Our HMMWV failed the 1 minute test, and allows us to diagnose leak points, and potentially catch a fuel into crankcase or fuel into air filter situation.

We will update in a new post as to our further testing. We also want to further examine the venting circuit to ensure that the selector valve should be set to “vent” when testing at the CDR valve.

Special Tools – Fording System Leakdown Tester

In order to confirm that all Deep Water Fording (DWF) vent lines, transmission seals, differential seals, etc are sealed, a leak down test is required.

Pictured above is a leakdown tester we assembled. We used a 1/4″ NPT brass tee, a 1/4″ NPT x 1/4″ hose barb, a 1/4″ NPT x Schrader valve, and a low pressure gauge (0-15 p.s.i.).

You may note that we used a glycerin-filled gauge. A glycerin-filled gauge adds a level of dampening in an application where shock-loading frequently occurs. It is unnecessary for this application, however the cost was only slightly higher than a non-filled gauge. Of note, filled gauges often have a small brass plug mounted in rubber to allow you to zero the gauge out if atmospheric pressure has raised it from zero.

WARNING AND CAUTION: We highly recommend using a hand pump, such as a bicycle tire pump. Should you put excessive pressure into the system while testing, it is highly likely that you will “blow out” seals, and potentially damage seals in the engine, transmission, gear hubs, and differentials. It takes a very little amount of air to reach 2 to 3 p.s.i., and even use of regulated air may provide a “burp” of air at a pressure sufficient to dislodge or destroy seals.

Wiring: Restoring/Preserving Cannon (Amphenol) Plugs

Cannon plug missing anodizing and with corrosion

Most of the cannon plugs (also referred to as Amphenol plugs) had most of their protective anodizing missing. The internal contacts were good and clean. Rather than spend an incredible amount of time and expense of moving the connectors into new fittings, we opted to preserve the exterior of the plug. We use a two-step process of etching primer followed by a lustreless olive drab marketed by Rapco Parts.


The first step is to use a small stainless steel to remove all corrosion from the outside of the plug. The second step is to apply the etching primer and let it fully dry.

Although the etching primer produces a color that is acceptably close to the original anodizing, primer does not make a good final coat.

The final coat of lusterless olive drab (FS 34086) is very close to the original color of the plug, and having a paint coating on the aluminum housing will extend the service life of the connector and assist in preserving the integrity of the electrical connections.

As a note, we applied the silicone grease to the threads and connections to prevent corrosion of the threads and to provide a slight amount of additional waterproofing to the contacts in the event the cannon plug seal fails.

Wiring: Dash Ground

Use of carbon grease on dash ground

The HMMWV has a reputation for grounding issues. Poor grounding can cause faulty or unreliable gauge readings, and can also cause failure of components including the control box.

We removed and cleaned all connections, and liberally applied carbon grease to all contacts to ensure the best possible connection. After this photo was taken, excess carbon grease was wiped off. This compound is highly conductive, and it is unwise to not clean up after a sloppy application.

MG Chemicals’ Premium Carbon Conductive Grease

Wiring: Silicone Lubricant / Waterproofing

Military Surplus Silicone Waterproofing

Anytime wiring with rubber boots are involved, it is always best to use a silicone lubricant. This assists in assembling the components, protects the rubber, and provides a further degree of waterproofing.

We have almost two cases of military surplus J941-C-5000 (also known as NRL S-75-G), which we acquired in the 1980’s. Since then, we have liberally used the silicone, and despite its age (Manufactured in 1954), it continues to be usable. Not bad for a 65 year old product!

We were unable to find further information of any kind on this product, however we have 30+ years using this material and have had superb results.

UPDATE: NRL S-75-G Interim was developed in World War II as a waterproofing lubricant to prevent machine gun failure caused by moisture freezing in solenoids at high altitudes (-20 F temperatures). (these failures are indicated as affecting the Cal .50 M3 and the 20 mm M24A1 Gun).

“The material is an open chain methyl silicone having a viscosity of 20 cSt at 77°F, and 300 cSt at -65°F, and a pour point of -75°F. Dodecane phosphoric acid (0.1 percent by weight) was added for lubrication. This material was labeled NRL S-75-G Interim.” Per: AMCP 706-26 at page 8-2 (Engineering Design Handbook, Guns Series, Automatic Weapons, Headquarters, U.S. Army Materiel Command, February 1970 [Document unclassified, but not marked as publicly released]).

Wiring: Circuit Breakers

As the breakers currently mounted in the HMMWV are around 30 years old (and have likely gone through countless fordings), we opted to replace them.

Newly installed 15 amp Klixon circuit breakers

These circuit breakers are “Klixon” style 15 amp breakers. The callout in the Parts Manual for these units are 5925-01-430-2318 [5925014302318]. However, this number seems to be obsolete. We were able to locate identical specification breakers under 5935-00-026-4767 [5935000264767]

These breakers have the same external dimensions and the same amp rating as the replaced breakers. The only difficult issue we had is that two of the connections would not release, and had to be replaced.