The Transfer Case Vampire

Transfer Case and Trans Oil Change Article

In September of 2001 AM General instituted a Customer Satisfaction Campaign (AMG R0101) having to do with the cooler loop located in the transfer case (tcase). Here's a little background on how the system works. Dexron III automatic transmission fluid from the transmission is pumped (by the transmission) out to an oil cooler located on the radiator stack in front of the truck where it is cooled. It's return trip is routed through the tcase cooler loop and back to the transmission. The fluid in the tcase which is also Dexron III never mixes with the fluid in the transmission. This circulation has the effect of warming up the tcase until it reaches the temperature of the transmission. After a period of time friction will heat up the tcase until it's hotter then the transmission. The cooler transmission fluid flowing through the heat exchanger in the tcase is supposed to cool the tcase down.

This arrangement is designed to rid the transfer case of excess heat. In practice this system doesn't work too well. If you drive your truck over 65 mph in the summer the transfer case temperatures will rise to the point where it will burn the oil (Dexron III) and accelerate the break down of plastic / nylon internal parts. I recomend that you fill your transfer case with synthetic Dexron which has a much higher breakdown temperature.

The problem occurs when the tcase cooling loop springs a leak. The transmission fluid that is circulating under pressure through the loop is forced into the tcase which bleeds the transmission dry which, of course destroys it. That's why it's called the vampire. Just an aside, the term 'vampire' was coined by Scott Weiser back in 1995 when this first happened to him. As fluid continues to leak the tcase will fill up with the Dexron being pumped out of the transmission. At this point the tcase can be damaged (overheating) because it is overfilled. Next, the oil with nowhere to go is forced up the tcase air vent line which terminates in the air cleaner. Note: It can also be forced out of a service plug in the tcase and start leaking from the bottom of the truck. The Dexron then will drain back down the other vent lines contaminating the oil in the geared hubs and differentials. By this time your truck will probably be dead in the water.

The cooling loops were developing stress cracks at the points where they were welded to the inlet and outlet fittings. The cracks are caused by vibration and incorrect installation by New Venture, the company that manufactures the tcase's for AM General. When the outside nuts were tightened on the coolers the rectangular area with the weld wasn't prevented from turning. This caused the loop to 'torque over' putting stress on the weld which vibration eventually cracked. The fix is twofold. First a piece of angle was added to the assembly to keep the loop ends from turning. Second another tab was added along the length of the loop which supports the loop preventing it from flexing.

The new cooling loop pictured on the right has a light blue band around one of the hose connections.  This band is visible from the outside.  If you have the band, you have the new loop. If you don't have the band, you don't know, since the band may have come off during installation.

If your truck is still operational and you are lucky enough to catch the 'Vampire' in the act before any damage you can still drive your truck if you have no choice. Otherwise get it towed to the nearest dealer. The first thing to do is to check the fluid level in the transmission. If you can still measure it you may be in luck. Next open the top fill plug of the transfercase (with a drain pan below) and see if ATF pours out. This is the definitive symptom. Once the oil has drained close up the tcase. Next you must pull the two lines that go into the tcase from the transmission and oil cooler and join them together leak-free any way you can. I was lucky when it happened to me. I discovered a nice red puddle of oil under my truck in the garage. Luckily I was only down 1.5 quarts of ATF. Using hose clamps and misc. fittings I was able to jump the loop out of the circuit. I filled up the transmission, checked for oil in the air cleaner and was able to drive over 100 miles to the dealer to get the loop fixed.