73-87 Chevy _ GMC Trucks > Electrical

Faulty fuse??

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fiddler:
I have a 1974 chevy stepside.  C10.  With factory air.  So two days ago I was driving down the freeway on a hot day with the air going and suddenly it quit.  The blower just stopped blowing.  So yesterday I started debugging and could find nothing wrong but a blown fuse.  I replaced the fuse and I can't find any reason for it to be blown.  I guess the next step is to put a dc ammeter on the circuit to see if it is indeed close to the rated 20 amps of the fuse.  Has anybody had a similar experience?  Do automotive fuses just give up with age??   Thanks for your inputs. 

bd:
As long as they are not exposed to weathering, fuses don't wear out*.  If blower voltage was low because of voltage drops across wire connections, switch, ground connection, etc, the motor will draw greater current that might overtax the fuse.  If the fuse doesn't seat snug in it's socket or if the socket is oxidized the degraded connection can generate heat that effectively lowers the current tolerance of the fuse.

Did the blower quit on high speed only?  Check the inline fuse holder at the firewall junction block for melting.  If it quit on all speeds, the compressor clutch also runs off the in-cab blower fuse so you will need to include the clutch in your diagnosis.

Checking the current demand of the motor and A/C clutch, as well as, voltage drops across the wiring are good starting points.


*EDIT - Rethinking this, if a fuse is repeatedly heated to near the melting temperature of the fuse element, it might alter the element structure sufficiently to decrease the fusing current, but only slightly.

VileZambonie:
Put a penny in it  8)

fiddler:
OK so this is quite a puzzle.  The blower now functions for 20 minutes, or 10 minutes, or 30 minutes....you get the picture, before blowing the fuse.  Fun troubleshooting.  I have rerouted and built bypass jumpers to get the resistor network out of the circuit.  I now have the circuit isolated to just the blower and the fuse.  If it still blows it will have to be the fault of the motor, if not it will have to be the fault of the little radio can that is inline with the power wire to the blower.  I will see how it goes tomorrow.  Thanks for the schematics by the way.  I appreciate your response!

Mike81K10:
To me, it sounds more like the blower motor going bad than a fuse. A fuse will blow if there is a short, which should be checked for. Look for bad looking wiring in the system and chafing. Look for bad spices as well. I would think about installing a new blower motor/fan. You can pull the blower motor and look for any obvious problem with it and the hole it goes in. There could be a problem with the resister (but apparently you bypassed it) going bad or an obstruction were the blower motor sits.

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