Thermostatic Fan Clutch -
If you happen to have a copy of Haynes manual 478, this type fan clutch is illustrated in photos 49.12A, 49.12B, 49.13, and Fig. 2.7.
Info: The center bore is 15/16" in diameter and 15/16" deep. The diameter that the fan fits around is 3-3/8", and the fan mounting bolt holes are on a 3-15/16" diameter circle. The OD of the housing is just a hair under 6".
Note that the type fan clutch shown is used on mid-70's XJ-S's and XJ12's. The later XJ-S and XJ12 came with a different type of fan clutch that mounted with four bolts to a flange on the pulley shaft. And reportedly the very early XJ12's, prior to the introduction of the XJ-S, had a different type clutch that mounted with four bolts. See FanClutch.html for pictures of the late-70's-on 4-bolt style fan clutch. The early XJ12 4-bolt fan clutch is illustrated in the Haynes manual in Fig. 2.6, but as yet there are no teardown photos on this site. Note that Haynes describes Fig. 2.6 as applying to the XJ-S, but that appears to be an error; the XJ-S was apparently never fitted with a 4-bolt fan clutch and a metal fan.
Without further ado, on to tearing this unit apart and taking pictures of the innards. There's no point in trying to rebuild it, as replacement is not all that expensive (especially if you choose a non-Jaguar substitute); the only reason to rip it open is to see how it works!
On the front there is a crimped edge in the circle between the fins at the center and the fins around the outer edge. This crimping was cut off and the unit popped right open. Here's the housing:
The red circle is a square cross section O-ring for sealing the cover to the housing. It is very soft rubber, and was in good condition.
Here's the cover:
Here's the rotor that was in the middle:
And I even took a picture of the back side of that rotor:
The only difference between the two sides is that little circular rib around the hub area, but that doesn't appear to be functional; it's just a machining mark.
Compared to the unbelievable complexity of the 4-bolt fan clutch, this thing is laughably simple! No grooves, no vanes, no compartments, no thermostatic bimetal elements, just a disk rotating within the housing. There's not even a bearing! If there is any thermostatic control at all, it's in the characteristics of the viscous fluid itself.
There appeared to be water mixed with the viscous fluid in this fan clutch. That would indicate a seal failure; in fact, the seal on the back side appears to be NFG. Since the fan clutch was probably initially totally full of viscous fluid, it's probable that some leaked out prior to this water leaking in. Whatever, water in the works would explain the problems that Sowell was experiencing. It may be wise to try to keep the outside of this type fan clutch clean, and when dark goo appears on it, replace the fan clutch. Other than the appearance of dark fluid eminating from the seals, it would be difficult to ascertain whether this type fan clutch is in good condition or not.
It may be possible to replace the seals in this unit; both of them press in from the outside. There is a bit of peening around the edges to secure them in place, but not enough to prevent servicing them. It should be possible to pry the old ones out and press in new ones; the only problem would be making up for any lost viscous fluid while they are out.
It would also be possible to rebuild this unit. After cutting the crimped edge off to separate the cover from the housing, there is simply no need to provide any sort of retention to put it back together; the four bolts that hold the fan to the clutch will also hold the cover on the clutch. Again, the big concern would be in trying to find a suitable substance to fill the clutch with when reassembling.
Of course, by the time you have purchased two seals and a tube of goo,
you're probably going to have spent nearly as much money as a new clutch
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