I just purchased my 1999 Ford Ranger XLT 4.0L V6 about two months ago, and have done several repairs on it since then. None of them have been super major repairs. Anywho. But, the cold and hot gauge on the dash does not work. It's not worked since I bought it. I just assumed it was needing a sensor since it was so old of a truck. I just had the coolant temperature sensor replaced as well as the thermostat in the truck. All the wiring and sensors look good. I even thought maybe it was the gauge cluster on the dash. So I put another now it from another ranger. Still does not work. Instead of going up the gauge goes down. Way past the cold line. Any advice as to what this could be? I'm stumped on it.

 

There are two temp units on a fuel injected engine
ECT sensor, 2 wires, 5 volts only used by the computer

Coolant temp SENDER, can be 1 or 2 wires, 12 volt and only used by dash board gauge

The Sender will have a red/white wire(and maybe a black/white wire as well in 1999)
Disconnect the connector and use a jumper wire to Ground the red/white wire to battery negative or any GOOD ground on the engine
Get in the cab and turn the key on
Watch the temp gauge, it should go to HOT, if so then red/white wire and gauge are OK
If not then red/white wire or gauge is disconnected/bad

If Gauge did go to HOT then you may have the wrong sender, or if its 2 wire the black/white wire is not a good ground
To test black/white wire do the same test as before but just jump the red/white wire to the black/white wire, gauge should go to HOT if black/white wire is a good ground