Gunfreak25
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This is often an overlooked part of repowering these old GMC's. But it is vital to the life of your transfer case. The GMC tcase engages the front axle automatically, when the rear tires begin slipping (around 6% faster than the front). When traction is regained, the front axle is automatically disengaged. The unit that does this is called a "sprag". The front sprag for the front axle must be told which way to drive. This is what the little shaft on the front of the Tcase is for. This is called the "reverse shifter shaft". It is spring loaded, and when the linkage is removed it automatically pulls the shaft inwards. Inwards means the Tcase is in forward or "drive" mode.
How it worked, was the original Hydramatic shift linkage when shifted into reverse, would PULL on this little shaft which told the sprag to drive in reverse. This also gave you front axle power in reverse, but once returned to the "drive" position it returned to the normal operation (rear axle power only).
The biggest issue when repowering a GMC is the fact that you are loosing the original Hydramatic shift linkage. This is the linkage that also controls the shifter shaft on the Tcase.
I spoke with 2 men at Memphis Equipment today, who have a lot of experience in these GMC transfer cases. I was strongly advised to NEVER drive the vehicle in reverse without shifting the Tcase into reverse. While it may drive okay for a few feet, it will eventually cause things to bind. I was told the GMC sprag unit is different from the REO sprag unit, the GMC's does not have teeth. But rather a dog style overrunning clutch which is much more reliable and rarely gives issues.
Basically, if you do not need front wheel capabilities you can just remove the front driveshaft and forget about the need for shifting the front sprag on the Tcase. Otherwise, just rig up a cable style linkage that will pull the shaft out when you want to drive in reverse. If your really good at fabricating, you can probably even use most of the original linkage coming off the Hydramatic shift tower. You'll have to play with how far you must pull the shaft out. It's a little sensitive.
To answer a couple common questions. It is perfectly normal for the front ouput yoke on the Tcase to "click" when rotated "forward" by hand. I noticed this when I had my front driveshaft removed, but was told it's completely normal.
Lastly, there's a rumor floating around that you can convert the Tcase to a 2 speed unit. I was told this is very involved, it requires removing a center bearing and installing a new gear inside. Most fire departments when repowering GMC's would use an auxiliary Tcase behind a transmission to drive the stock GMC tcase, this would give you low range again.
The GMC tcase is extremely strong, I have never heard of one breaking. 9 times out of 10 any Tcase issues are solved by adjusting linkages. It's a good design, but can be a little tricky to understand at first. The TM explains it to a T.
How it worked, was the original Hydramatic shift linkage when shifted into reverse, would PULL on this little shaft which told the sprag to drive in reverse. This also gave you front axle power in reverse, but once returned to the "drive" position it returned to the normal operation (rear axle power only).
The biggest issue when repowering a GMC is the fact that you are loosing the original Hydramatic shift linkage. This is the linkage that also controls the shifter shaft on the Tcase.
I spoke with 2 men at Memphis Equipment today, who have a lot of experience in these GMC transfer cases. I was strongly advised to NEVER drive the vehicle in reverse without shifting the Tcase into reverse. While it may drive okay for a few feet, it will eventually cause things to bind. I was told the GMC sprag unit is different from the REO sprag unit, the GMC's does not have teeth. But rather a dog style overrunning clutch which is much more reliable and rarely gives issues.
Basically, if you do not need front wheel capabilities you can just remove the front driveshaft and forget about the need for shifting the front sprag on the Tcase. Otherwise, just rig up a cable style linkage that will pull the shaft out when you want to drive in reverse. If your really good at fabricating, you can probably even use most of the original linkage coming off the Hydramatic shift tower. You'll have to play with how far you must pull the shaft out. It's a little sensitive.
To answer a couple common questions. It is perfectly normal for the front ouput yoke on the Tcase to "click" when rotated "forward" by hand. I noticed this when I had my front driveshaft removed, but was told it's completely normal.
Lastly, there's a rumor floating around that you can convert the Tcase to a 2 speed unit. I was told this is very involved, it requires removing a center bearing and installing a new gear inside. Most fire departments when repowering GMC's would use an auxiliary Tcase behind a transmission to drive the stock GMC tcase, this would give you low range again.
The GMC tcase is extremely strong, I have never heard of one breaking. 9 times out of 10 any Tcase issues are solved by adjusting linkages. It's a good design, but can be a little tricky to understand at first. The TM explains it to a T.
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