4.0 coolant temp wires

[Hitchner2009]

Anybody know what the wire colors are for the coolant temperature on the 4.0 swap from a 2.9? I think my schematic says there a gray/black wire on the 4.0 harness and a black/white on the 2.9. Just wanted to make sure I'm reading this right.

 

[RonD]

There are two temp units on all fuel injected engines

The dash board temp gauge uses a 1-wire temp Sender
The computer uses a 2-wire temp Sensor.

2-wire on 4.0l looks to be Green/red and Grey/red

1-wire can be tested by grounding it, Temp Gauge needle should go all the way to hot with that wire grounded.
If oil pressure gauge goes up then it is your oil pressure wire.

I think the temp sender and the oil pressure switch are the only 1-wire senders

 

[adsm08]

And the wires for the gauge are red/white on both engines.

 

[Hitchner2009]

sweet deal, thanks guys. i will get it done probably tomorrow. have an odd feeling the gauge might be bad because i swear i hooked up red/white on the truck and it didnt work. the gauge didnt work with the 2.9 unless it idled for a long time then it read a temp

 

[Hitchner2009]

Actually the wire turned out to be Gry/Red on the 4.0 Harness lol

 

[Hitchner2009]

i lied, im stuck on this. i spliced both red/white wires and nothing happened. i know the sensor is good. The schematic on the 4.0 says lt grn/red, thats the same wire for the alternator......really confused on this. i wired in the red/white wire to the 4.0 lt grn/red wire and the temp gauge stays consistent with engine temp. but my alternator light stays on in the dash. the truck still charges, but i think the light is being illuminated because of the power from the dash gauge going into the harness.

 

[Hitchner2009]

Black/Lt Blu seems to have been the wire. I grounded it at the connector and the needle pegged, and it stays where it should when running the truck. Any input though would be great in case i messed this up

 

[RonD]

The Battery Light works by current flow.
Any light bulb works this way, if I hook up 12v to one wire on a light and 0volts(ground) to the other wire on the light, the light will come on because current is flowing through the filament.
If I hook up 12volts to each wire on the light it will not light up, because there is no current flowing through, both wires have equal voltage.

When you turn on the key, 12volts should flow from ignition switch to/thru the Battery Light and then to the alternator which is 0 volts since it is not turning and producing voltage, so battery light is on.
When you start the engine a working alternator will produce voltage so Battery Light goes off, equal voltage on both wires.
With alternator running system voltage is now 13.6volts(approx.), so both wires on the Battery Light are 13.6v so no current flow and light stays off.

If alternator starts to fail and it's voltage output drops below 12.5volts(battery voltage) then Battery Light comes on because current is flowing thru the bulb from Batteries 12.5v to alternators less than 12volts.

This Battery Light wire is also the ON/OFF switch for alternator.
An alternator is built much like an electric motor, so it will draw power in some cases if left connected.
So the 12volt/key on turns on the alternator, with key off alternators rotor is disconnected so can't act as an electric motor.