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Matt, Mechanical Engineer
Category: Ford
Satisfied Customers: 22908
Experience:  BEng hons Mech engineering, in auto industry 22 years
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1997 Mk 4 Fiesta 1.25 16v. Heater Blowing hot air. I have

Customer Question

1997 Mk 4 Fiesta 1.25 16v.
Heater Blowing hot air.
I have replaced the heater control valve on the bulk head.
Still the same.
I have Checked the voltage to the valve.
0 volts on both cables when ignition off.
2 Cables are attached to the valve
Cable 1 - Purple/Black 12v when at pre-ignition The wires also bells out to the earth.
Cable 2 - Green/Black 0v when at pre-ignition.
Cable 1 & 2 values remain the same even when the Heater control panel is at cold, mid, or Hot.
I replaced the Heater control panel with a second hand one; and carried out the same checks.
Cable 1 - Purple/Black 12v when at pre-ignition. The wire also bells out to the earth.
Cable 2 - Green/Black now 4.56v even when the heater control panel is at Hot , Mid, or Cold.
I am not sure whether cable 1 has shorted out.
Submitted: 3 years ago.
Category: Ford
Expert:  Matt replied 3 years ago.


I would disconnect the electrical connection to the heater control valve ( as having the valve connected will screw with the readings) and put a voltmeter onto it. Turn the ignition on and turn the heater knob to hot. The voltage should be zero or at least very low . Turn the control to cold and the voltage should go up to 12v.

It could be the other way round, but moving the control knob from hot to cold should change the voltage.

If this works, it must be a faulty control valve. New ones are known to fail. If there is no change in voltage, then it must be either the control panel behing the control knob, or the power feeding to that.