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Mr. Crossfire
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16,162 Posts
Discussion Starter · #1 ·
I need a 12v source from the transmission harness to trigger a relay to my trans cooler. Anyone know which sensor or wire I could use, I know its a long shot. FYI I have a TH400 so harness isn't being used folks.
 

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Worlds Fastest Street HD Truck
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14,677 Posts
Why do you need it to be from the trans harness?

One of these would do exactly what ya need lol
 

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Mr. Crossfire
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16,162 Posts
Discussion Starter · #5 ·
My trans cooler is mounted to the x-brace in the back of the truck. Need a 12v source to trigger the relay so when I start truck fan automatically come on. My trans harness is near my front bracket to my long bars.
 

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Pretty simple: carefully tap your fuel pump positive wire. If it's going to a relay you will be fine. Be sure to use a good ground. Even when the pumps are not operating at full voltage you will have 12v at your relay with a chassis ground.

Enjoy!
 

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Mr. Crossfire
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16,162 Posts
Discussion Starter · #7 ·
Pretty simple: carefully tap your fuel pump positive wire. If it's going to a relay you will be fine. Be sure to use a good ground. Even when the pumps are not operating at full voltage you will have 12v at your relay with a chassis ground.

Enjoy!
Not being smart but is it smart to touch a line that powers the fuel pump? Wont that take away voltage from my pump (I have single pump).
 

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If you are not using a factory pump setup, I would shy away from this... Unless you know what your current draw is and know that it's sized correctly for the wire.

Let me pull a diagram and see what I can come up with.
 

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Tap the violet/white wire going to the trans. It's power for the 4R100 solenoids.

Another GREAT option (already relayed and set up for 30A current) is to tap the red wire going to the trailer pigtail.
 
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