Rep Power: 221

2001 LX Civic sedan, auto trans. 102,000 miles.
Hit by a deer Dec. 23 2011 that took out the radiator.
The trans started slipping last week.
I checked the coolant tonight and the color is brown. The color was green.
The trans fluid is where it has been since I replaced the fluid.
The engine oil level has dropped a little. The oilpan gasket leaks.
From what I have read on a google search the consensus is that the mixture is too weak causing corrosion BUT this engine has an aluminum block NOT an iron block. So that point is moot.

Why is my coolant brown???

 
Rep Power: 221

Anyone?
Is this problem so common that I just need to search better?

 
Rep Power: 285

You said you're not losing oil, aside from what would likely be normal as a result of the bad oil pan gasket, but I'll ask this anyway, any brown sludge?

The only possible causes I know for brown coolant are rust (not possible in this case), a blown head gasket, or an improper mix.

 
Rep Power: 812

you either have trans fluid (coming in from the trans cooler inside the radiator) or engine oil in the coolant. it doesnt take much to cause contamination.

 
Rep Power: 518

Or cruddy coolant.
Rust from lack of antifreeze/too much water? That wil turn it brown.