I have an 87 Mustang,4cyl. 5 spd. The electric cooling fan is wired to run all the time when the key is turned on. I’ve been trying to get it back to normal and have gotten as far as the Haynes manual’s test of the instrument gauge and resistor. The book says to set an ohm meter to 10 ohm and plug one probe into the coolant temp. sensor wire, ground the other probe to the engine and turn the key on. The gauge should read in the hot range. Then dial it to 73 ohm and repeat the test and the gauge should read in the cold range. Since my gauge doesn’t move one bit, the book says it’s either the instrument resistor or the gauge itself.
My question is: Where did those insane Ford engineers put that resistor?
The cooling fan circuit in your vehicle works like this.
When the ignition switch is in the run position it supplies voltage to the cooling fan relay at the primary side. The coolant temperature sensor on the right side of the engine closes at 220 degrees to ground the primary side of the relay. This then causes the contacts in the relay to close which supplies voltage from a fusible link located at the starter motor to the cooling fan motor.
Why you’re messing around with gauges and resistors is beyond me. But then, you’re following a Haynes manual.
Tester
My question is: Where did those insane Ford engineers put that resistor?
Most likely on the backside of the gauge cluster. You’ll need to pull the gauge cluster and repeat the test on the gauge itself.
As for the cooling fan, you’ll need to diagnose that circuit separately as described by Tester. There is one CTS for the gauge and another for the cooling fans. The CTS for the gauge is a variable resistor type that makes the gauge function through a temp range. The other for the cooling fan is a simple on/off switch that triggers at a preset temp.