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Discussion Starter · #1 ·
The wiring, relay and switch were all present, so I installed new fogs today [Bosch Pilot 150] on my Canadian-spec '89 9kT. I'd checked the wiring before and everything appeared OK - the relay closes and the switch goes bright when pressed. With the fogs installed, though, the 15A fuse blows. I tried just one light and it works OK. With two connected, the fuse blows. I've checked the lamps for shorts. Can anyone suggest what the problem is?
Thanks,
Rod.
 

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What's the wattage of the lamps? Assuming 100W lamps your approximate current draw will be just over 8A per lamp. Therefore 2 lamps running off the same 15A fuse will pop it! If this is the case, either run each lamp from separate fused feeds or upsize the fuse (but only if the cables are sized sufficiently to do this)

HTH

Julian
 

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If the fuse pops instantly you must have a short (or a huge current flowing). The rated fuses will run well over their capacity without blowing or even getting to warm.

I've run fuses 50% over rating without problems.

Once pulled over 170A off a 100A rated 3 phase fuse box for 5 days without a twitch. Those fuses are 1/4 inch aluminium bars

Andrew
 

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Discussion Starter · #4 ·
Thanks guys. The lamps are standard H3s -- 55W each. If I understand correctly, that's a total 9-10A draw, so the 15A fuse should be OK. Guess I'll keep looking for a short.
Cheers,
Rod.
 

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Discussion Starter · #5 ·
Thanks guys. The lamps are standard H3s -- 55W each. If I understand correctly, that's a total 9-10A draw, so the 15A fuse should be OK. Guess I'll keep looking for a short.
Cheers,
Rod.
 

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Discussion Starter · #9 ·
Nothing other than the espresso maker....
Here's an explanation I received on the TSN BB:
"A 15 amp fuse is really to small even though the math says it should work. A fuse 1.5 times the load rating is reasonable depending on the type load. The fuse is to protect the circuit from smoking in case of a short. Therefore the wire size, relay capacity, etc must be be capable of handling the fusing current of the fuse. Lamps have a high inrush current when turned on due to the low resistance of the filiment when cold. Measure the resistance of your lamps and then do the math to determine max inrush current. You will be surprised. So it's a race between when the fuse blows and the lamp lights up and reaches it's rated wattage."
http://www.saabnet.com/tsn/bb/9000/index.html?bID=147349
I guess it makes sense...
Rod.
 

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If the 15A fuse blew instantly you're probably drawing over 23A current, so the 20A fuse may blow fairly soon.

If nothing else is run off the fuse, the only thing I can think of is that at least one bulb is out of spec.

If there were no foglamps on the car before and the fuse is only supposed to supply the fog lamps, I suppose a previous owner could have used the supply for something else.

Another thought that will need an answer from the experts. Would Saab have put a fuse in the box if it wasn't needed? They don't usually fit fuses to redundant circuits these days, but were they doing it (or not doing it) in 1989?
 

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I didn once many moons ago have one lamp that would regularly blow fuses.

I eventually found that the +12v rubber/plastic insulation was cracked just where it entered the base of the bulb, sometimes shorting.

Andrew
 

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Discussion Starter · #12 ·
The car did originally have fog lights because everything was still there -- except the lights. I simply plugged in the new ones and replaced the fuse. I also changed the relay but the old one is still good, too. Why the 15A fuse is too small is strange, but why is beyond me.
Rod.
 
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