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RonDwyer82-931
Joined: 10 Sep 2007
Posts: 35
Location: San Leandro, Ca.
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 Posted: Mon Nov 12, 2007 5:25 am Post subject: Electrical Test, or how to start a fire in the weirdest way |
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Ok, so I spent the last few weeks acquiring parts to replace my intake elbows, WUR and cycle valve. Got it all done last night and go to start it. new battery, only a few months old and it won't crank. Pump, lights everything is fine. Must be a weak battery so I charge it overnight. This morning it barely cranks and I smell smoke. Can't find it, then I look to the the R. rear and see wisps of smoke coming up from what i though is probably near the fuel pump. I look underneath expecting to see a smoldering pump and OMG glowing, and genuine flames!
I ran for the hose, doused it and then crawled under to see what burned. Expecting to see a fuel pump, wiring harness or some other 2-week to repair component wasted I get off easy with a burned stainless steel brake line. Flammable brake lines?????!!!!
No electrical devices anywhere near the brake line. Anyone figure this one out yet? I was stumped. There is not an electrical line within feet of this, yet the source of the fire was indeed the brake line! I figured that cranking had something to do with a massive current flow. The brake line was touching the rear crossmember. Why would the brake line be conducting electricity? Was this somehow part of the starting problem?
Thinking a faulty ground path from the starter, I took another look at the battery, the ground to chassis was connected. So I look at the engine to chassis cable at the firewall by the distributor. The cable was not re-attached to the coil bracket when I re-installed the coil. It was overlooked in the darkened cave of my garage amongst the rest of the inhabitants of that area. Never in my wildest dream would I imagine the current path in this case to choose the braking system, but there it is.
The good thing is that seeing the fire probably saved me from having an accident had I somehow been able to discover the ground problem and start the car. The teflon hose inside was likely compromised last night and would probably have failed under use. The engine ground cable is now connected, and the car once again runs fine, although a trip to the coast is out of the question for this afternoon.
Have to take the Saab today. Might as well pull the gearbox and do the input shaft seal while I have it up in the air to do the rear brake lines. Bad weekend for car projects, or things with engines. Bulldozer steering clutches froze up from a year of sitting so I didn't sell it yesterday like I planned.
Anyone ever seen anything like that before?
As a follow up: The source was the point where the line is swaged into the fitting. There must have been a weak connection. I'll post a picture when I pull it out. |
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Tigger937
Joined: 11 Apr 2004
Posts: 919
Location: PCA Milwaukee Region
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| Posted: Mon Nov 12, 2007 8:06 am Post subject: Re: Electrical Test, or how to start a fire in the weirdest |
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| RonDwyer82-931 wrote: | The brake line was touching the rear crossmember. Why would the brake line be conducting electricity?
Thinking a faulty ground path from the starter, I took another look at the battery, the ground to chassis was connected. So I look at the engine to chassis cable at the firewall by the distributor. The cable was not re-attached to the coil bracket when I re-installed the coil.
As a follow up: The source was the point where the line is swaged into the fitting. There must have been a weak connection. I'll post a picture when I pull it out. |
Current always forms a loop. The source is the battery and the return point is the battery. No surprises there. Without the engine to chassis cable, the return current is forced to find another path back to the battery. Since the brake line was touching the rear crossmember, it acted as the return path for the starter current back to the battery. HIGH AMPEREAGE! The point where the brake line was touching the rear crossmember was probably corroded/painted/dirty, i.e., much resistance. P = I^2 x R, Power = Current squared times resistance. To illustrate, assume I = 50 amps and R = 1 ohms (conservative numbers). That's 2500 Watts or twenty-five 100 Watt light bulbs! Your toaster probably consumes 1000 Watts. Massive current combined with massive resistance = massive power = massive heat. I suspect you have a pretty good burn mark where the brake line was touching the rear crossmember. You may want to consider changing your brake fluid as I'm sure it was heated beyond it's temperature breakdown limit.
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1981 931 (Concours)
1982 931 (Daily Driver)
"Think outside the box" |
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Ozzie
Joined: 12 Mar 2005
Posts: 4448
Location: Townsville, Qld. Australia
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| Posted: Mon Nov 12, 2007 9:01 am Post subject: |
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I= Volts/Resistance.
Power = Current (I) x Volts (V)
If you increase the resistance the current will be LESS given a constant voltage.
As V is constant (battery) a high resistance will decrease the current (as in bad earths and stuff not working) where are low resistance will increase the current (melted wires, blown fuses)
As water will find the path of least resistance so will current.
EG, Short out the terminals of the battery with a wire (and reducing the resistance) and try to hang onto it or a knife as my kids done once and got a swift boot for it.
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance |
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RonDwyer82-931
Joined: 10 Sep 2007
Posts: 35
Location: San Leandro, Ca.
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| Posted: Mon Nov 12, 2007 3:32 pm Post subject: |
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| Actually the weak link was the swedged connection and that heated up and caught fire. As for the equations, glad we have people on staff to figure that stuff out and keep us out of trouble! |
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!tom
Joined: 28 Aug 2006
Posts: 1941
Location: Victoria, BC Canada
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| Posted: Mon Nov 12, 2007 3:40 pm Post subject: Re: Electrical Test, or how to start a fire in the weirdest |
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| Tigger937 wrote: | | P = I^2 x R, Power = Current squared times resistance. To illustrate, assume I = 50 amps and R = 1 ohms (conservative numbers). That's 2500 Watts or twenty-five 100 Watt light bulbs! Your toaster probably consumes 1000 Watts. |
Interesting. Since V=I x R, you have a non-standard battery. Everyone else uses 12 volt batteries -- you've got a 50 volt battery in your car if your made-up numbers mean anything.
If you're going to assume some conservative numbers, then maybe choose some realistic ones too.
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78 924 NA
5-lug |
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Ozzie
Joined: 12 Mar 2005
Posts: 4448
Location: Townsville, Qld. Australia
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| Posted: Mon Nov 12, 2007 4:36 pm Post subject: |
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The equations are easy enough
eg 100w bulb /12v = 8.333 amps.
transpose V= I x R to R= V/I = 12v / 8.333 = 1.44 ohms.
to get 50 amps the resistance is real low.
12v/50a= .24 ohms
_________________
Porsche 924 1984 (UK import) NA
Its AUTO and its BLACK
Montego Black on black/red
Engineer of Electro/Mechanical Systems Maintenance |
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Tigger937
Joined: 11 Apr 2004
Posts: 919
Location: PCA Milwaukee Region
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| Posted: Tue Nov 13, 2007 6:59 am Post subject: Re: Electrical Test, or how to start a fire in the weirdest |
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| !tom wrote: | | Tigger937 wrote: | | P = I^2 x R, Power = Current squared times resistance. To illustrate, assume I = 50 amps and R = 1 ohms (conservative numbers). That's 2500 Watts or twenty-five 100 Watt light bulbs! Your toaster probably consumes 1000 Watts. |
Interesting. Since V=I x R, you have a non-standard battery. Everyone else uses 12 volt batteries -- you've got a 50 volt battery in your car if your made-up numbers mean anything.
If you're going to assume some conservative numbers, then maybe choose some realistic ones too. |
Oops, right you are! My apologies. Try I = 50 amps, R = .1 ohms for 250 Watts or 2.5 100 Watt bulbs. Thanks for pointing that out.
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1981 931 (Concours)
1982 931 (Daily Driver)
"Think outside the box" |
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