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Heated rear screen! Aaaarrgghhh!!!!

This is a real puzzler!!!! 1994 Ford Granada hatchback. Rear heated screen not working. Switch on dash working fine. Element in screen undamaged and is making a circuit. There is a small box of tricks in these Fords in the tailgate. It acts as a demister unit and radio aerial amplifier. (These cars use the screen element as an aerial). When you flick the switch there is power getting to the box of tricks. I disconnected the two cables that come from the screen element into the box of tricks. I tried the two connections on the box that these cables plug into and power is present there when the dash switch is on!!!!! But as soon as you attach the cables to these connectors the power disappears! Why? I even tried replacing the box of tricks for another one but that didn't behave any differently. I cannot understand this at all. Any ideas?

Jamie

Reply to
<the
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I remember something on my old Capri like that (but it was a fair bit older than your Granada) - there were two or three metal contacts set into the bodywork where the tailgate shut, and a corresponding number of sprung switches on the tailgate itself. They used to get grubby and not work properly so needed a good cleaning from time to time. Perhaps yours has them and one of them is the earth for the demister?

Si

Reply to
Mungo "two sheds" Toadfoot

Is it like the Escort and Fiesta where the power goes through some brass plates on the botom of the tailgate, and some corresponding sprung pins on the boot lid? The springs can get jammed in the "up" position and hence fail to make a circuit.

Cheers,

Robert

Reply to
Robert R News

same thing on my dads escort,(1998) one pin refuses to come out, but still works, for now!

Reply to
Tom Burton

connections

If you are checking voltage at the screen termination points and they read ~12v when disconnected and the ~0v when connected, I would say there is a connection problems somewhere..

If you r getting 12v with no load (heater disconnected), but nothing when loaded (heater connnected), take a look at the switch or relay controling it.

Although it might be switching of with no load. it may well be failing under load....

A fault with the demister would show 12v when under load and 12v when not loaded, or the fuse would blow.

sQuick..

Reply to
sQuick

sQuick....

Thanks for the info but I'm a little lost. ~12v never appears at the screen termination points at any time whether cables are connected or not. But the heated element is OK as it's making a circuit. I am getting ~12v output from each of the terminals on the box of tricks but that disappears and goes to ~0v as soon as you connect the cables from the heated element.....and as soon as you disconnect the element cables the multimeter shows ~12v again. Any ideas? By the way...last year the heated screen failed for about 3 months then miraculously fixed itself...this would suggest a loose connection.

Jamie

Reply to
<the

Test the output from the box with a bulb not a meter, if the voltage drops to zero then you've got a high resistance connection in or before the box.

Reply to
Duncan Wood

If this box is the same as the one I had in the back of my old escort It could be suffering from dry joints inside.

IIRC the rivets on the box have to be drilled out to open it up then its just a matter of resoldering the dodgy joints.

I would say this would be worth a look as the problem you are describing would indicate a high resistance joint somewhere.

sQuick..

Reply to
sQuick

Had this problem with an old Chevette..

HRW worked OK on it's own but with the lights on - no way!

After a bit of head-scratching traced to a slack fanbelt - a quick tweak to get the right tension, all OK!

connections

Reply to
R. Murphy

sQuick

I have tried replacing the entire unit with another one and it makes no difference. I have also resoldered the two main joints inside the box that are connected to the two cables that go to the screen element.....no better. This is a real puzzler. Any other ideas?

Jamie

Reply to
<the

Do what Duncan suggested with a bulb. If the bulb fails to light properly, try it from the 12 volts to earth. If it still fails to light, it's most likely a faulty switch, although it looks OK with very little current being drawn through it ! You could confirm that by pulling the switch out, and shorting the appropriate contacts on the back. HTH

Andy Pandy To e-mail, address hopefully, self-explanatory !

Reply to
Andy Pandy

Andy

Pardon me...but I am a little slow when it comes to these sort of things. What type of bulb? Any car bulb such as a sidelight bulb? And where do I connect it to try it? Do I connect the two bulb contacts to the two contacts on the box of tricks? If it lights up but only very dim then do I try connecting one of the bulb contacts to earth? Sorry about this but I really am an amateur. Regarding the switch, the light on the switch lights up OK when switched on but can I assume that does not necessarily mean the switch is working correctly?

Regards

Jamie

Reply to
<the

Preferably one with a similar wattage to the rear screen heater, for most cars a headlamp bulbs appropriate

Well it depends on the box, one of the contacts should already be ~ earth, the other 12V

ALas no, that'll still come on even if the switch is a bit dicky. However, it's more likely to be the earth connection between the screen/box & the car. Assuming the spring contacts are working as per previous posts

Reply to
Duncan Wood

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