Author Topic: Faulty fuse??  (Read 613 times)

Offline fiddler

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Faulty fuse??
« on: June 21, 2024, 02:05:42 PM »
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. 
Ricky

Online bd

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Re: Faulty fuse??
« Reply #1 on: June 21, 2024, 02:36:39 PM »
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.
« Last Edit: June 21, 2024, 03:05:21 PM by bd »
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline VileZambonie

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Re: Faulty fuse??
« Reply #2 on: June 21, 2024, 06:27:57 PM »
Put a penny in it  8)

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Offline fiddler

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Re: Faulty fuse??
« Reply #3 on: June 25, 2024, 06:07:03 PM »
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!
Ricky

Offline Mike81K10

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Re: Faulty fuse??
« Reply #4 on: June 25, 2024, 06:28:07 PM »
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.
« Last Edit: June 25, 2024, 09:06:16 PM by Mike81K10 »
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Offline fiddler

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Re: Faulty fuse??
« Reply #5 on: June 27, 2024, 04:53:56 PM »
So just to let you know what happened..  It turns out the rfi filter (Not shown in any schematic) shorts out after awhile running.  My guess is temperature sensitive.  I was never able to catch it shorted so I had to reconfigure wiring a piece at a time to eliminate suspects.  After bypassing it and wiring the motor direct it ran the air for a full trip to the dump and back, so I am pretty sure it is fixed.  I bypassed the rfi filter and hooked everything else back up.  I will run it for a few days before ordering a new rfi filter.  (I usually use my ipod and bicycle speaker in the truck so the rfi filter is pretty much not needed anyway, but in the interest of keeping EVERYTHING working....   Thanks for all of your inputs.  Anybody got a part number for that rfi filter??
Ricky

Online Shifty

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Re: Faulty fuse??
« Reply #6 on: June 27, 2024, 06:24:03 PM »
Is it HEI?
87 V20 Standard Cab Longbed (current)

87 R30 3+3 Longbed (days of yore)

98 C2500 ext cab longbed

Offline fiddler

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Re: Faulty fuse??
« Reply #7 on: June 28, 2024, 12:17:46 AM »
It has been converted to hei.  Thanks.
Ricky

Online Shifty

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Re: Faulty fuse??
« Reply #8 on: June 28, 2024, 11:01:37 AM »
RC4 in Standard Ignition, RR231 in Echlin (NAPA), or if you know someone with olde GM stock, then 1892260. 
87 V20 Standard Cab Longbed (current)

87 R30 3+3 Longbed (days of yore)

98 C2500 ext cab longbed

Online bd

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Re: Faulty fuse??
« Reply #9 on: June 28, 2024, 12:40:49 PM »
Shifty, those part numbers are for the terminal block in an HEI distributor.  I believe Fiddler is referring to the noise filter that connects to the blower motor.  It is discontinued GM P/N 477371 (attached image), which is just a 330 µF, 50 VDC electrolytic capacitor conveniently packaged in a durable steel can that connects between the blower motor positive terminal and ground.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Online Shifty

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Re: Faulty fuse??
« Reply #10 on: June 28, 2024, 01:48:02 PM »
Ah, I misunderstood.
87 V20 Standard Cab Longbed (current)

87 R30 3+3 Longbed (days of yore)

98 C2500 ext cab longbed

Offline fiddler

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Re: Faulty fuse??
« Reply #11 on: June 29, 2024, 01:37:42 PM »
Yup..the cap on the blower motor.  Just wish they had showed it on the schematic/wiring diagrams.  Probably the first place to go for those symptoms.  I will see if I can find a replacement.  If not it obviously runs better without it!  Thanks everyone!
Ricky

Offline fiddler

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Re: Faulty fuse??
« Reply #12 on: June 29, 2024, 11:46:34 PM »
Just one last note.  I took the capacitor and rigged it on my bench using a resistor as a load to duplicate the blower motor.  I used a 12 volt lab supply and a resistor decade box.  As the setup "ran" I saw no increase in current draw, so I went out to the yard to do some yard work.  After about an hour I came back in to find the power supply had tripped.  The cap had shorted and tripped the over current on the supply, and shut everything down.  The cap is still not showing a short at ambient temperature.  Since new parts ore so expensive now that they are made of unobtainium I am going to open the steel case and put a new cap in the case.  These things go for as much as 200 bucks now, and I can get a new cap for about 3 bucks shipped to the house.  They used to be less than a buck when I retired!!  Again, thanks for all your help.   
Ricky

Offline VileZambonie

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Re: Faulty fuse??
« Reply #13 on: June 30, 2024, 07:10:57 AM »
Here's one for $45 bucks new https://www.ebay.com/itm/155208991763
,                           ___ 
                         /  _ _ _\_
              ⌠ŻŻŻŻŻ'   [☼===☼]
              `()_);-;()_)--o--)_)

74 GMC, 75 K5, 84 GMC, 85 K20, 86 k20, 79 K10