Fiesta MK4 Heater Problem, newbie please be gentle!

[spannerman68]

Mornin all,
Just want to start by saying what a wicked site. 27 years in the motor trade and I've just found it, SLAP MY WRIST! lol

Now to the problem.
My better halfs Fiesta mk4. The heater was stuck on hot a few months ago, so I did the usual and changed the control valve on the bulkhead, and all was ok. However, when I removed the old original part it crumbled in to tiny parts and some fell back into the heater pipes. I flushed the system before I fitted the new one and it's been fine. About a month ago the problem came back, I took the valve off and there were bits of the old valve jammed into the opening keeping the valve open, hence a constanly hot heater. I found the solenoid had burt out so I fitted another new valve and flushed the system again. All ok.
NOW! And this is the wierd bit. 3 days ago it started playing up again. Took the valve off, all ok. Nothing stuck inside, arm is moving freely back and forth. So I refitted it, bled it out, all ok. But, from cold all is working fine, heater gets hot, turn the control and it goes cold, set it in middle it gets warm, turn it up and it gets hot, all good. Stop the engine for a second and start it up again and the heater blows hot constantly, until you switch off the engine and let it cool right down, start it up and the heater is fine. Drive it round all day and it fine, normal running temp, turn it down and its cold, up and it hot, no problem. Switch the engine off for a sec and its bleedin hot again, all day. I dont get it!! Ive checked wiring, changed the control panel, even the reastat at the fan. Bled it out jacked up on the o/s where the tank is. I've done everthing I can think of. OR HAVE I? Is there someone out there who knows different, pleeease.
Please bare in mind I'm a senior Master Tech at one of the biggest main dealers (not ford) in London and I do this sort of thing day in and day out. The lion did go from strength to strength but now it has a thorn in it's side lol. The oldies will get that lol.
Any help would be very much appreciated. Or this little car could be the end of me lol.
All the best Ian

[ch427]

only thing i can suggest is that an electrically operated diverter flap is going to the hot position every time it looses power.
im not that familiar with the fiesta set up but how do you control from hot to cold, is it the older type rotary control?

[spannerman68]

The hot is controlled by a soleniod valve under the bonnet that stops the flow of hot water through the matrix. The only flap is to change direction of air flow ie to screen, feet and center cabin. The heat generated by the hot water through the matrix should disperse after a few secs and cool the matrix to allow cold air through, but for some reason the valve is staying open, but only once the ignition is switched off and on again. I tell ya, this is really doing my nut in lol.
I never thought I'd ever hear myself saying, give me a digital ECU controlled climate control anyday. It's a fiesta for crying out loud, hard wirered, open and closed, on and off, old school fiesta, and I can't fix it.

The boys at work are having a great time watching me with this one lol.

Thanks for the reply.

[ch427]

i know those valves are unreliable, particularly if they are a pattern part.
only thing i can suggest is to disconnect it and put a meter across the terminals and monitor the switching voltage when operating the controls and switching off the engine.

[RJP001]

With no power the valve's default position is open. When a constant voltage is received it closes the valve. Between the hot and cold setting on the dash, the electronics behind the centre switch alters the frequency of a pulse sent to the valve, varying the position of the valve with this process.

Mine just gives out a constant voltage when connected regardless of position because the electronics are bust, so I'll be changing the panel on mine in due course.

The neat thing about this to know is how the previous Fiesta had a simple cable linkage that worked fine. All Kas and Focus mk1s are said to suffer from the same problem too as they all share the same electronic board apparently.

[Love my fiesta]

Hi Guys, I have a problem with my 1998 Ford Fiesta's heater.
It's the same as the SPANNERMAN68 except mine is cold when I start but when I turn it up I can't turn it back down,
it constantly pumps out boiling hot air. If we turn control down and the car off and on again it will go back to being cold.
We have changed the HCV twice and the circut board and have just checked the voltage at the heater control panel. Everything is fine but we still have the problem. Any ideas please.I'm new to this site so please be nice.