Fuse Heat

[Bitsyncmaster]

We have all wondered what causes our fuse blocks to melt. Mostly the fuel pump fuse and sometimes the headlight fuse are the common melt downs. I have been doing some testing on my Fan Fail unit which has an ATM fuse for each fan. Here are some results.

With the fan fail powering 15 amps to each fan the fan fail unit will dissipate 3.9 watts of heat. A little more than half that heat is caused by the 20 amp ATM fuses. Running at a room temp of 70 deg. F the unit will feel warm So running in our closed relay compartment in the summer it would probably feel hot if you quickly opened the relay box and felt the heat.

With the fan fail powering 10 amps to each fan the fan fail unit will dissipate 1.44 watts of heat. Notice the heat with 33 % less current is about one third the heat. That is because voltage drop is less along with less current (power = voltage X current).

A fuse is just a metal junction that will melt the metal with a specified current flowing through it. So that metal runs very hot and gets worse the closer you run to the rated current. So all that heat gets transferred down the fuse contacts. Then the mating contacts and the wire crimped to it dissipates the heat.

I’m convinced the block melt down is caused by the weak clamping on the OEM mating connector pins. Remember all that fuse heat has to transfer back into the wire harness.

Note:
I have measured one set of OEM fans that only draw a little over 10 amps each. My OEM fans measured more than 15 amps each. Now I wonder if the fan motors were all the same or if age causes those different currents. I also wonder if the fuel pumps have an age problem which causes higher currents than the normal new current of 7 amps.

I still have my removed fan circuit breaker and the original wires still attached to the breaker that I removed because it is redundant with fuses in the fan fail. I tested the voltage drop at 30 amps and got 1.06 volts drop with the circuit breaker dropping 0.13 volts. The OEM connector pins on that breaker got so hot I could not touch them. So I pulled a little on those wires (pulled the pin) and the voltage drop went down to 0.82 volts. This just proves that weak contact pressure on the connector pins is a major problem. By the way, that 1.06 volts drop is making 32 watts of heat.

[David T]

We have all wondered what causes our fuse blocks to melt. Mostly the fuel pump fuse and sometimes the headlight fuse are the common melt downs. I have been doing some testing on my Fan Fail unit which has an ATM fuse for each fan. Here are some results.

With the fan fail powering 15 amps to each fan the fan fail unit will dissipate 3.9 watts of heat. A little more than half that heat is caused by the 20 amp ATM fuses. Running at a room temp of 70 deg. F the unit will feel warm So running in our closed relay compartment in the summer it would probably feel hot if you quickly opened the relay box and felt the heat.

With the fan fail powering 10 amps to each fan the fan fail unit will dissipate 1.44 watts of heat. Notice the heat with 33 % less current is about one third the heat. That is because voltage drop is less along with less current (power = voltage X current).

A fuse is just a metal junction that will melt the metal with a specified current flowing through it. So that metal runs very hot and gets worse the closer you run to the rated current. So all that heat gets transferred down the fuse contacts. Then the mating contacts and the wire crimped to it dissipates the heat.

I’m convinced the block melt down is caused by the weak clamping on the OEM mating connector pins. Remember all that fuse heat has to transfer back into the wire harness.

Note:
I have measured one set of OEM fans that only draw a little over 10 amps each. My OEM fans measured more than 15 amps each. Now I wonder if the fan motors were all the same or if age causes those different currents. I also wonder if the fuel pumps have an age problem which causes higher currents than the normal new current of 7 amps.

I still have my removed fan circuit breaker and the original wires still attached to the breaker that I removed because it is redundant with fuses in the fan fail. I tested the voltage drop at 30 amps and got 1.06 volts drop with the circuit breaker dropping 0.13 volts. The OEM connector pins on that breaker got so hot I could not touch them. So I pulled a little on those wires (pulled the pin) and the voltage drop went down to 0.82 volts. This just proves that weak contact pressure on the connector pins is a major problem. By the way, that 1.06 volts drop is making 32 watts of heat.

The heat is also caused by the current flowing through the wires. The wire is sized so that it is large enough (enough copper) to handle the current and not get overheated. That doesn't mean they won't get warm. Put many together and it all adds up. Especially the ones that have continuous loads on them and even more the ones that get overloaded because of old, defective loads like a bad fuel pump. Add in crimps on connectors that lose their tension so they don't grab the end of the wire as tightly as they should. Now with all that heat the spring connectors soften and lose their grip on the fuses which further adds to the heat. Now consider that the fusebox has a cover and is in an area where it gets no ventilation. It all adds up to a problem area that few owners ever bother to look at until there is a melt-down. BTW many fuseboxes that I have seen have incorrect fuses in at least some spots. Overfusing is a surefire way to have one! Every new owner should make sure each fuse is the correct size for it's position and that noting is plugged into the hot start relay plug. Smelling something burning while driving on the highway gets your attention quickly! I personally have seen fan fail jumpers melt and it is impressive!

[SS Spoiler]

Anybody ever put a fan to cool the fuse area?

[Bitsyncmaster]

Anybody ever put a fan to cool the fuse area?

That was what a few owners did about 6 years ago (this has always been a problem). Trouble is you need to vent an input air (cabin temp) and vent the output air (hot). Not easy to do with the carpet covering the cover.

[JohnZ]

Anybody ever put a fan to cool the fuse area?

I'm fabricating a new relay compartment access cover that is similar to the old one, but I put a steel grille to make them breathe a bit more. The fact that the carpet will be put over the grille may make you think that it's an useless improvement, but I think that having some holes to dissipate heat is still a good thing.

Moreover I can just lift the carpet if I want the heat to come out, surely with the grille rather than solid wood this process is favored.

:)

I'm still into doing it practically, but I'll post some pics as soon as I will put it into the parcel shelf area!

[David T]

I'm fabricating a new relay compartment access cover that is similar to the old one, but I put a steel grille to make them breathe a bit more. The fact that the carpet will be put over the grille may make you think that it's an useless improvement, but I think that having some holes to dissipate heat is still a good thing.

Moreover I can just lift the carpet if I want the heat to come out, surely with the grille rather than solid wood this process is favored.

:)

I'm still into doing it practically, but I'll post some pics as soon as I will put it into the parcel shelf area!

Many owners use that area (parcel shelf) to put things. It would block any airflow so you have to consider that. Another problem is fumes from the battery. If the alternator voltage regulator goes bad and makes the alternator overcharge the battery you can get a LOT of fumes.

[JohnZ]

Many owners use that area (parcel shelf) to put things. It would block any airflow so you have to consider that. Another problem is fumes from the battery. If the alternator voltage regulator goes bad and makes the alternator overcharge the battery you can get a LOT of fumes.

I know that some owners travel with out the access cover at all, but I don't like this solution since I don't want to put any luggage over the fuses with out any protection but the carpet! Hence the idea of putting a grille.

Besides, lately I've been checking my relay area constantly after the trips and I'm happy with that because it is just warm :) :)

[Bitsyncmaster]

Maybe a vent on the back board and an inlet vent above the battery cover would be best for air (or fan) design.

One thing every one should do is replace the push on terminals on the circuit breakers with ring terminals. I'm surprised my bad terminals on the fan circuit breaker did not cause a melt down there. The 5 foot of 12 AWG wire on my fan circuit breaker test should have only dropped 0.24 volts. So the bad connections dropped about 0.7 volts.

[dn010]

I have 3 or 4 fans in my relay compartment in different locations - I can't remember how many exactly since I haven't been in there for a very long time with the frame swap, engine build, etc.

I did this 10-12 years ago and, believe it or not; even without a vent the airflow has seemed to end my headlight fuse meltdown problem... I have one fan blowing air on the fuse block while others are circulating the air around the compartment. I am using small old ass 12vdc computer fans that don't even compare to moving as much air as "new" computer fans do these days.

[JohnZ]

I have a question: if the headlights are not in use (AKA turned off :sissy: ) is the fuse stressed with current flow?

dumb question yet useful :biggrin:

[Bitsyncmaster]

I have a question: if the headlights are not in use (AKA turned off :sissy: ) is the fuse stressed with current flow?

dumb question yet useful :biggrin:

There is no current flowing through the fuse with the headlight off.

[JohnZ]

There is no current flowing through the fuse with the headlight off.

Ok thanks! But is it the low or high beams fuse getting particularly hot?? Which one?

[Bitsyncmaster]

Ok thanks! But is it the low or high beams fuse getting particularly hot?? Which one?

I think others have had the high beam fuse melt. The high beams are a little more current but I don't know why that 20 amp fuse would get that hot. It may just be a problem with those that have installed higher power headlight. Elvis had done testing and suggested using a 25 amp fuse from what I remember.

[Delorean3610]

Has anyone tried a heatsink system? Or perhaps having an inlet hose for a fan routed along the base of the rear window, with an exhaust hose going out under the bodyshell.

[Farrar]

You already have a small vent behind the battery. Perhaps hot air could be routed out that way with a small duct. Just a thought.