Fuses on relay board, over amped Options

[Tom]

I have followed two AAR shorts the past 12 months that caused extensive damage to the ignition harness in both cases. Just read a thread on a member losing his car to a fire. Don't know if this was AAR related or not.
Some things I have learned.
The first fuse ( rated at 8 amps) is not being used. Goes to pin #9 of the 12 pin connector at the rear of the relay board.
The second fuse is 25 amp, and fuses the heated blower, fuel pump, AAR, and defroster for rear window.
The heater blower draws about 5 amps, haven't measured the fuel pump- probably near the same, AAR draws less than 2 amps, probably the greatest draw would be the defroster.
Most don't have the rear defroster, so a 25 amp fuse isn't necessary if all is working properly. A 16 amp would do the job and probably blow before a fire in the harness if any one of the loads became shorted to ground.
What are your thoughts and knowledge about this?
Since the front fuse isn't being used, I'm thinking of moving the wire for the AAR from pin 12 to pin 9 and using the front fuse for the AAR. If I do this I will mark the fuse as AAR and I will make a correction to add to my car's Haynes book to note the changes. I will use 8 amp fuses for both.
OK, let me know what you think.
Tom

[SirAndy]

let me know what you think.

As far as fuses go, the lowest you can get away with without blowing them under normal operation.

Too many amps in the fuse is like not having a fuse at all. (IMG:style_emoticons/default/blowup.gif)

[Spoke]

16A fuse should work ok but I would carry the 25A with you since it operates the fuel pump.

What do folks think the cause of the AAR fires is?

If you're worried about the AAR shorting and causing a fire because of the 25A fuse, you could always use an inline fuse to the AAR of like 5A or so.

[Tom]

Spoke,
At the bottom of the AAR where the wire goes in it get chafed and when it comes into contact with ground, the wire (18 or 20 ga) just can't handle the current it is fused for. When I had mine off several years ago, I noticed the chafing and heat shrinked it.
I recommended that to one member and he installed an inline fuse and all works well. His had shorted to ground at the AAR case and fried his ignition harness. Just trying to use what is there and the forward fuse doesn't seem to feed anything, so why not use it for the AAR with a smaller fuse. Then an 8 or 16 amp could be used for the rear fuse for the fuel pump and heater blower.
My feeling about the problem is that circuit is fused for 25 and the wires feeding the current to the loads are only rated for 20 amps or less, so the wire becomes the fuse - starting a fire in the wiring harness.
Tom

[Dave_Darling]

Don't forget that the start-up current draw for some things can be larger than the steady-state draw. It is possible that the fuse is sized with that in mind. Make sure to carry spares of all three sizes (8A, 16A, 25A) with you just in case.

Moving the AAR to its own fused circuit sounds like a good idea. But remember that it is supposed to get power only while the fuel pump is running. I'm not sure your re-wire will do that; it may wind up powered the whole time the key is on. That's not a big difference, but it is a difference.

--DD

[Tom]

Dave,
Actually I thought of that and most of the time my key is on, the engine is running. I suppose there are times others leave the key to the on position without the engine running , but I feel that is a bad idea even for normal points ignition, never mine electronic ignitions.
Thanks for the reply,
Tom

[JeffBowlsby]

Tom, Factory wire gages and ampacities from this source:

http://www.expertelec.co.uk/cable/pdf/TRI.pdf

mm^2, ampacity
0.5, 11 amps - AAR
1.0, 17 amps - Fuel pump
1.5, 21 amps - Heater Blower
2.5, 30 amps - Rear window defrost

[Tom]

Jeff,
Thanks, your info is as always valuable. Maybe I'm chasing a dead end, but I think the design is not so good, and may have caused some electrical issues in these cars. Just too many fires that "could" be traced to a root cause of over fusing a circuit.
Thanks,
Tom

[Mike Bellis]

Tom, Factory wire gages and ampacities from this source:

http://www.expertelec.co.uk/cable/pdf/TRI.pdf

mm^2, ampacity
0.5, 11 amps - AAR
1.0, 17 amps - Fuel pump
1.5, 21 amps - Heater Blower
2.5, 30 amps - Rear window defrost

Jeff, with all due respect. I have to disagree with this web site. 53A on a 6mm2 (10awg) wire is extreemly dangerous. That rating is right before the wire melts down. It's also based on AC current. DC current takes a larger wire to be efficient. The NEC (National Electrical Code) states a maximum of 30A for a 10 awg. Additionally you are only allowed to use 85% of the maximum ampaicity. With AC, the current flows on the outer edge of the wire. As frequency goes up the effect is greater. DC current uses the center of the wire to conduct. A 10Awg in a DC application can push and effictive 20-25 amps. Any more current and the wire starts to heat up. This was proven by Edison and Tesla. Edison needed a 3" copper conductor to send DC power 1/4 mile to light up the neighborhood. Tesla (father of AC) used a small copper condutor to send AC power to the next town over. Edison thought Tesla was an idiot and blew him off. Westinghouse saw his genious and the rest is AC history.

Automotive electrical systems have the smallest allowable wire gage that is safe. As the wire ages, resistance grows in the wire from corrosion. Wire has a life span. Our cars are 30+ years old. The wiring has reached its life span. Any current flowing on the system has to work harder to get to the load.

At this point in our car life, they should all be rewired with new copper. Since there is no source for harneses (except Jeff B's FI harness) any wire replaced in the system should be upgraded to a larger size. This will help the electrical system not work as hard. The other concern with our cars is the ground plane. Without going too deep, our cars are now grounded by rusty metal and spot welds. Some simple upgrades include added ground straps from the battery and larger alternator charge wires.

[JeffBowlsby]

Hey Mike...what you say makes a lot of sense, I know you are a *real* electrician, so I believe you....I assumed those values on that website were max ampacities before derating, they looked high to me too, but understood them to maximum ratings.

The mm^2 numbers I posted are the factory wire sizes...I convert them to the next largest AWG wire for my harnesses and use AWG wire, which adds a little margin of safety.

Got any more realistic ampacities for these old wires per the original question?

[Tom]

Well, I decided to just lower my fuses to 8A. Started and ran the car several times with no problems,
On my car I don't have the defrost for the rear window, nor is my heater fan hooked up and the 8A fuse seems to be just fine for the load for the AAR and fuel pump. I will, however carry extra fuses just in case.
Thanks to all who answered.
Tom