Fiesta heater diagnosis help please
Discussion
Hi
The wife's V reg Fiesta has developed an annoying fault - the heater is stuck in the hot position. i.e, you get hot air regardless of the dial. And it's not very nice in this weather!
I know the heater control valve on the bulkhead is prone to failure on these but that's been replaced. So I presume it's the controller circuit board that's failed.
Google seems to be giving conflicting info on voltages but I can see it's supposed to be a pulse modulated signal to the valve, and itseems that the default postion of the valve is HOT so pulling the connector makes no difference. (i.e, 0V = hot, pulsed in between & whatever the max steady voltage is = cold)
Ideally, before forking out, I'd like to test that the valve is in fact ok & it must be the controller feed but I'm wary of slapping 12V on it as I don't know the maximum that the controller circuit normally delivers as the steady state COLD signal - anyone know? Some Googling suggests 12V, some 7V and all sorts else!
Or anyone got a cunning bodge to persuade the valve into the Cold position for the time being? Or has any definitive answers on the electrics feeding the Fiesta heater valve?
Cheers.
The wife's V reg Fiesta has developed an annoying fault - the heater is stuck in the hot position. i.e, you get hot air regardless of the dial. And it's not very nice in this weather!
I know the heater control valve on the bulkhead is prone to failure on these but that's been replaced. So I presume it's the controller circuit board that's failed.
Google seems to be giving conflicting info on voltages but I can see it's supposed to be a pulse modulated signal to the valve, and itseems that the default postion of the valve is HOT so pulling the connector makes no difference. (i.e, 0V = hot, pulsed in between & whatever the max steady voltage is = cold)
Ideally, before forking out, I'd like to test that the valve is in fact ok & it must be the controller feed but I'm wary of slapping 12V on it as I don't know the maximum that the controller circuit normally delivers as the steady state COLD signal - anyone know? Some Googling suggests 12V, some 7V and all sorts else!
Or anyone got a cunning bodge to persuade the valve into the Cold position for the time being? Or has any definitive answers on the electrics feeding the Fiesta heater valve?
Cheers.
Eggman said:
Worst way, you could just bypass the heater matrix - undo the hoses from the bulkhead and join them together with a couple of Jubilee clips and a bit of metal tubing.
I'm liking the bodge factor here 
Well Mrs PBF wouldn't even know how to open the bonnet so she'd never know - and then in winter just connect it up again!
No idea if it is the same but I had a 97 Mondeo and the rotary dial is basically connected to a gear that pushes and pulls a cable running to a lever on the side of the air box in the driver footwell. The cable kinks and ceases to work causing the arm to lodge either hot or cold. I tied the arm in the well in the right position and later took the dash apart and simply straightened and oiled the cable. It was fine as long as you did it slowly.
If it's electronic then no idea.
If it's electronic then no idea.
Pot Bellied Fool said:
Hi
The wife's V reg Fiesta has developed an annoying fault - the heater is stuck in the hot position. i.e, you get hot air regardless of the dial. And it's not very nice in this weather!
I know the heater control valve on the bulkhead is prone to failure on these but that's been replaced. So I presume it's the controller circuit board that's failed.
Google seems to be giving conflicting info on voltages but I can see it's supposed to be a pulse modulated signal to the valve, and itseems that the default postion of the valve is HOT so pulling the connector makes no difference. (i.e, 0V = hot, pulsed in between & whatever the max steady voltage is = cold)
Ideally, before forking out, I'd like to test that the valve is in fact ok & it must be the controller feed but I'm wary of slapping 12V on it as I don't know the maximum that the controller circuit normally delivers as the steady state COLD signal - anyone know? Some Googling suggests 12V, some 7V and all sorts else!
Or anyone got a cunning bodge to persuade the valve into the Cold position for the time being? Or has any definitive answers on the electrics feeding the Fiesta heater valve?
Cheers.
Are there three wires going into the actuator or two? Don't know about the fiesta, but a lot of NA GM vehicles use a two-wire pulse duration modulation system that recalibrates upon ign - runs to each end, looks for the increase of current at the end stops, then remembers how long a pulse is required to take the flap from open to closed. If this is the case you simply give it 12v until it opens, then leave it disconnected. Do you have the old valve to rip apart? I doubt there's any fancy electronics in there - I'd hazard a guess it's simply a nasty cheap motor and all the driving is done at the ECU. The wife's V reg Fiesta has developed an annoying fault - the heater is stuck in the hot position. i.e, you get hot air regardless of the dial. And it's not very nice in this weather!
I know the heater control valve on the bulkhead is prone to failure on these but that's been replaced. So I presume it's the controller circuit board that's failed.
Google seems to be giving conflicting info on voltages but I can see it's supposed to be a pulse modulated signal to the valve, and itseems that the default postion of the valve is HOT so pulling the connector makes no difference. (i.e, 0V = hot, pulsed in between & whatever the max steady voltage is = cold)
Ideally, before forking out, I'd like to test that the valve is in fact ok & it must be the controller feed but I'm wary of slapping 12V on it as I don't know the maximum that the controller circuit normally delivers as the steady state COLD signal - anyone know? Some Googling suggests 12V, some 7V and all sorts else!
Or anyone got a cunning bodge to persuade the valve into the Cold position for the time being? Or has any definitive answers on the electrics feeding the Fiesta heater valve?
Cheers.
Fiesta mk V - I have the same problem of constant hot air. I disconnected the feed to the valve and connected a capless bulb and started the engine. The bulb stayed on steady (bright) for all selections (cold to hot - hot to cold)of the rotary temp selector so I'm wondering if the temp selector is at fault as the valve was changed last year. Anyone got any ideas please? Many thanks
Same with the Mrs KA, they just all go this way after a while.
A coolant change will help the next one work for a bit longer, but it's just a matter of time. We're on our 3rd one now. It's a simple enough job, tbh for the hassle of checking the voltage etc I'd just stick a new one one now and be done with it
If you wanted to bypass it there's 4 pipes that need linking up, bit of metal pipe and jubilee clips would do it, you'll see where the join is when you look at the valves. The plastic push fit onto the pipe gets brittle and they'll probably crack away from the heater valve, so a pair of long nose pliers might be needed to pull the bits out of the rubber pipe
A coolant change will help the next one work for a bit longer, but it's just a matter of time. We're on our 3rd one now. It's a simple enough job, tbh for the hassle of checking the voltage etc I'd just stick a new one one now and be done with it
If you wanted to bypass it there's 4 pipes that need linking up, bit of metal pipe and jubilee clips would do it, you'll see where the join is when you look at the valves. The plastic push fit onto the pipe gets brittle and they'll probably crack away from the heater valve, so a pair of long nose pliers might be needed to pull the bits out of the rubber pipe
Edited by andy-xr on Monday 9th March 14:18
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