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After building an engine that was ready to make big power, and ready to tow bigger loads, cooling concerns are always in the mind of anybody towing with an indirectly injected (IDI) diesel. Unlike direct injection (DI) diesels, the combustion actually takes place inside the cylinder head of an IDI diesel, so there is a tremendous amount of heat transfer that takes place. This places a larger strain on the cooling system. When pulling a decent load up a 6% grade, the stock 87 gpm pump and single thermostat simply doesn't provide enough flow to keep the engine cool; that's where the fan clutch comes in. The bi-metallic coil reaches a temperature that causes the clutch to engage and turns the fan at a much higher rate, thus drawing a greater amount of air over the radiator. GM had their clutches built by Borg-Warner, and the calibration was set for somewhere around 210 degrees, but since it takes longer for the space between the radiator and bi-metallic coil to heat up to that 210 degrees, the clutch usually wouldn't engage until the gauge reads around 220 or 230. Then, once the fan clutch had turned on and the roar could be heard from under the hood (and it is quite a roar!), the temperatures would plummet back toward their standard range. On a regular pull, one could experience highs near 230, and then would quickly drop to 190 once the fan clutch had engaged. This kind of sudden and wide variance leads to increased stress on the head gaskets, and eventually a possible compression leak. GM recognized the shortcomings of the stock cooling system and gave the 1997 model year 6.5TDs a 130 gpm water pump and dual thermostats. This kept the heating and cooling during towing within a more stable range, but with the stock fan clutch retained, the variance was still too great. We had a 110 gpm pump for our v-belt 6.2L, a dual thermostat housing with 180 degree thermostats; now it was for a lower temperature engagement fan clutch. Enter Bill Heath, owner and operator of Heath Diesel. It's not a problem finding a lower temperature engagement fan clutch for a 6.5TD, but those wouldn't work since they're a counter clockwise rotation clutch. After talking with Bill, it was suggested that we try the Hayden severe-duty fan clutch that was designed for clockwise operation.
The new clutch bolts onto a standard GM four bolt pattern, and installs in minutes. Simply unbolt your old fan a clutch as an assembly, seperate the clutch from the fan, and reassemble with the new part.
After testing, we noticed that this new clutch did engage at just over 200 degrees, and bring the water temperature back down to around 190. It was noted that the clutch would not engage in 4th gear with the torque converter locked up. Only with the transmission in 3rd (direct) gear and the rpms at about 2700 would the fan clutch engage.
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