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I don't believe it ever actually locks up. Get it hot and running at idle then cut it off the fan should stop with in a few seconds or a few rounds. With the engine off but hot, spin the fan be hand, it should have a reasonably amount of resistance and it should only spin about 1-1.5 rounds when you spin it with some force. With the car off grab the fan and check the bearing for slack. If it fails any of these I would replace it.





A 1970 would likley have a "thermal" fan clutch. It has a little coil in front that senses temperature and tends to engage or "lock up" meaning it slips less when hot. If it fails the tests above, replacement is needed. My one experience with parts store clutches on a DD - torqflow brand - was not good and I went with OE. The MP re-pop setup from Mancini works well on my 1970E. The kit I used comes with a 5 blade "low drag" fan which worked fine in my application, however, with cooling problems I would go with the OE fan which is usually bigger/more aggressive or an even bigger fan like the 7 blade used on mid/late 70's BB Chrysler's (JY) or the 6 blade fan from Bouchillon.