Author Topic: Heater fuse keeps popping  (Read 246 times)

Offline tale

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Heater fuse keeps popping
« on: January 08, 2019, 09:06:30 AM »
Hello all!  This is my first post.   :)

I have a '79 C10 long bed w/ 350.  Technically it is now my son's.  My father-in-law bought it new in the fall of '78 and has given it to him for his 16th birthday.  It runs good, but we have a few issues that we need to fix. 

I've noticed that the fuse for the heater has burned through.  It was a 30 amp fuse.  I'm not sure if that is the proper amperage or not, because it melted the plastic around the fuse the last time it burned.  So that is the first question.  What amperage fuse should the heater use?

Secondly, I'm just hoping to get some tips on how to troubleshoot a heater that is burning fuses.  I have a decent electrical background, but I'm not sure what is the best way to proceed.  Can somebody give me some advice of things to check?

Offline bd

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Re: Heater fuse keeps popping
« Reply #1 on: January 08, 2019, 09:46:05 AM »
Welcome to the site.

With or without A/C?  The fuse should be a 20-amp ATC/ATO.  Are you saying the heater fuse location in the factory plastic fuse box under the dash has melted?  Temporarily power the blower motor directly from the battery using a 30-amp fused, 12-gauge jumper and measure the current draw of the motor.  Perform a voltage drop test between the motor frame and the battery negative post.  If the voltage drop across the ground is less than ~0.3 volt and the motor draw is <20-amps then connection problems between the fuse and its fuse box terminals may be causing excessive heat concentration in the fuse and the surrounding fuse box cavity.  Systematic voltage drop tests will isolate where electrical connections are poor, creating excessive circuit resistance and heat buildup.  Then it's a matter of cleaning connections and/or replacing terminals to remove that resistance.
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 Rapid Roy

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Re: Heater fuse keeps popping
« Reply #2 on: January 08, 2019, 11:12:16 AM »
Welcome
1974 Cheyenne 10 LWB 350/TH350/AC/4 BBL Quadrajet

Offline tale

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Re: Heater fuse keeps popping
« Reply #3 on: January 08, 2019, 02:53:49 PM »
Thanks for the quick responses!  It used to have air conditioning, but it isn't hooked up anymore.  My son would love to fix that, but I think it will do him good to live without it for a summer or so.  He still thinks it is a necessity here in North Carolina. :)

The plastic where the fuses are inserted is the part that I'm referring to that melted.  On one side of the fuse is a concave circle where the plastic melted.  The other side is fine.  Is it common for fuse blocks to need replacing after 40 years?  I don't see any corrosion from a quick inspection, but I wasn't able to easily remove the fuse block to look at the back side.

I'll give those checks a shot and report back.

Offline JohnnyPopper

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Re: Heater fuse keeps popping
« Reply #4 on: January 08, 2019, 03:02:18 PM »
Welcome from SoCal!

Not usual for fuse blocks to fail unless someone puts the wrong fuse in.

Sounds like 30a is what melted your block where a 20a should have been. Darn that pesky DC!

Assuming the fuse slot will no longer hold a 20a fuse, you may have to go with an outboard fuse holder for that circuit.

Following BD's plan should get you to the root of what shorted out.


Offline bd

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Re: Heater fuse keeps popping
« Reply #5 on: January 08, 2019, 04:57:47 PM »
Under typical circumstances, plastic fuse boxes don't deteriorate.  Chances are high that a poor connection existed between the original 20-amp fuse and its socket, resulting in excessive localized circuit resistance, voltage consumption and heat production.  The heat deteriorated the connections even further and the "lay solution" was to install a higher ampacity fuse.  For awhile the 30-amp fuses increased the intervals between failures.  Nevertheless, eventually, the heat melted the replacement fuses, each time damaging the fuse box a little bit more.  Of course, this scenario is purely hypothetical and predicated on the idea that the blower motor, as an example, isn't shorted; hence, the various checks and measurements that I originally recommended.  The solution(s) that you implement will depend on what you determine to be the cause(s) from your tests.
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: Heater fuse keeps popping
« Reply #6 on: January 08, 2019, 06:48:28 PM »
Let me ask this, when you put a fuse in, how quickly does it blow? Does it matter what speed you have it set on?
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Offline 75gmck25

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Re: Heater fuse keeps popping
« Reply #7 on: January 10, 2019, 12:31:01 PM »
I know they made some wiring changes in about '78 when power windows and locks became and option, so maybe the wiring for the A/C blower also changed.  However, in my '75 if a truck had A/C, it has only a 20 amp fuse in  the fuse block.  In addition, under the hood there is a 30 amp A/C fuse that is inline from the power junction to the A/C relay. 

On my '75 the A/C fuse is in a wire running right across the top of the firewall from the power junction next to the brake booster, over to the relay in the A/C plenum.  This 30 amp fuse and the A/C relay are what provides the high speed for the blower.

I had problems with the 30 amp inline fuse holder melting, and solved it by replacing it with a new blade fuse holder and wire.  I guess the old fuse holder had too much corrosion and resistance, which let it heat up.

Bruce