A/C Fuse Melted in the Fuse Box

[dieselkid84]

I'M Back, Well here goes a question for a way to fix the melting fuse box, this happen tens years ago and now again. Ten years ago I replaced the fuse box and wires to the a/c fuse from a wreaked truck. Has some one had this problem and what fix did you do.

Oh, this is on a 1986 F-250 idi

 

[Clb]

Bad ground's

 

[icanfixall]

If this happened often my suggestion is cut loose the wires behind the fuse panel. Then install an inline fuse large enough to cover the amp draw. But best idea is to find out why the large amp draw is happening.

 

[hce]

I had something similar happen on a old combine with glass tube fusses. Fuse would not blow instantly, instead get hot and slowly melt the fuse link. The contact for the fuse on the fuse box were dirty, causing low amps across the fuse but resistance and heat at the point of contact on the fuse. Cleaned fuse box and problem went away.

 

[Clb]

To Bandaid this you could install a slow blow fuse,or circuit breaker.
Find the issue.
As above the bad contact on fuse is causing high current flow to fill the need, same as bad grounding.

 

[hce]

To Bandaid this you could install a slow blow fuse,or circuit breaker.

You must be registered for see images attach

 

[chris142]

The blower may be pulling too many amps.

 

[Clb]

The blower may be pulling too many amps.

But why????
Again...

 

[ShadetreeV]

I'll vote for bad grounds .

 

[DrCharles]

Let's get a grip, everyone. "bad contact on fuse is causing high current flow to fill the need, same as bad grounding" makes no sense from an electrical standpoint.

Ohm's Law: Voltage = Current * Resistance. And Power = Voltage * Current.

A bad contact at the fuse will cause heat at that contact due to the resistance there with current flowing through it = power dissipated in the contact.

A bad ground will have voltage drop between it and the actual ground (chassis, usually). So there will be less voltage available for the connected device. With any load (resistive or DC brush motor) that device will draw LESS current, not more. It will cause heating at the bad ground terminal due to the drop there. It will NOT cause a fuse in the battery feed to heat up!

Chris142 is quite right. Brush-type motors with very worn bearings, brushes or commutators tend to draw more current then designed. Here is an example: the BMW E34 (5-series from '89-'95) is notorious for this. The stock blower fuse is 30 amps, and the HVAC blower normally pulls about 10 amps on high speed (with a somewhat higher starting surge, 17 amps IIRC). As the blower wears it starts to pull more current, but usually not enough to blow the fuse. Unfortunately someone did not design heavy enough wires, and the wiring harness sometimes catches fire under the dash and burns the car to the ground!

The very first thing anyone should do when acquiring an E34 is to put a 20 amp fuse in that slot. And replace the blower if the fuse ever blows.

Now I haven't checked the wiring diagrams to see if our trucks feed the HVAC blower throught the fuse in question, but if so it's a legit explanation.

 

[Thewespaul]

This^ you are getting a parasitic load. Aka something is drawing power from a circuit when it shouldn’t be. Blower motor is protected by the thermal fuse in the resistor pack.

 

[genscripter]

+2 on the parasitic load diagnosis.

A few years back, I had a bad ground on my taillight wiring for my 98 Jetta TDI. When the taillight ground corroded in the trunk, the dash wiring would "close" the circuit and all the indicator lights would glow. Odd really.

So when searching for the grounds causing a wiring issue, CHECK ALL GROUNDS in the vehicle everywhere. It might be from a really odd circuit.

 

[DrCharles]

In my opinion that's overthinking the problem. There's an old saying in medicine, "When you hear hoofbeats, think of horses first, not zebras". Works for EE's too.

Most likely causes are dirt/corrosion at the fuse contacts causing heating, or the load itself drawing more than design current but not enough to blow the fuse (as in the example I gave).

Yes, there could be some bizarre parasitic circuit from a bad ground - but again, how could that cause a higher current draw through the fuse in question?

Time for some measurements!

 

[jim x 3]

Dielselkid,

To fix this you need some diagnostic equipment so you can measure, a wiring diagram for your ’86 F250, and a good description of the symptoms.

You mentioned that this problem occurred 10 years ago, what was the cause and the fix at that time?

Part of your previous fix was to replace the fuse box and wiring, did you do that properly? E.g. does the factory wiring diagram still match the wiring in the truck? Are the fuses the right size? Are the wires the right size? Were your splices to existing wiring well done with good electrical continuity and good insulation with no possibility for shorts?

What was the rest of the fix 10 years ago?

Is the AC fuse heating up and is that the sole cause of the recent melting? What size is that fuse – is it the right size? Is the fuse clean and making good electrical contact in the fuse box? What are the major loads on that circuit? Is the wiring from the fuse box to the loads intact – e.g. no damage to insulation at turns and pinch points. Are the major loads drawing the proper amount of power?

In case the heater blower is on the problem circuit, note that the previous info (above) that the heater blower is protected by thermal fuse in the resistor pack is erroneous, at least for my 1988. The resistor pack (and thermal fuse) are bypassed when blower is switched on HIGH. Your wiring diagram will show if this is true for your 1986.

You should find and fix the underlying problem rather than adding a bandaid or blindly changing suspected items. If you have these resources and can answer these questions you can determine the cause of the problem and you will know how to fix it.

Good hunting.

Regards,
Jim Jim Jim

 

[baja-Dean]

What I did for my vintage vehicle that we know all have ghost problems is wired in a fused relay to all high amp devices like AC compressor clutch, and blower for the high side, ya I do not have the option of blower at low speed but not important to me. I did this under the hood at the device with good wire. Thus relieving the old wiring of its strain of high amp requirements.