I am trouble shooting a fault in my heated windscreen and wanted to check the windscreen relay worked (R5). It's identical to the ignition relay (R1) so I thought I'd pop the windscreen relay into the ignition relay position to see if the car still started (to confirm the relay worked).

However, I noticed that when I removed the original ignition relay the car would still start (with no relay in place). What is the purpose of the ignition relay? I had expected to be unable to start the car.

I don't have any problems with the ignition - I'm just curious about what ignition relay does!

If you leave that relay out, what doesn't work?

Without wiring diagrams or other knowledge (that may be on here) I can't say?

10 minutes ago, Shearers said:

If you leave that relay out, what doesn't work?

Without wiring diagrams or other knowledge (that may be on here) I can't say?

With the ignition relay out the car still starts and runs as normal. I was expecting the car to be unresponsive to turning key in ignition (no start and no dash lights).
 

When I remove the ignition FUSE the car won’t start and no dash lights come on. I had expected the ignition relay to be essential it that same way. 
 

 

Yes but... if the relay (despite the name and without wiring diagrams) serves any useful function, something on the car won't work, just sayin?

4 hours ago, Nic Williams said:

I am trouble shooting a fault in my heated windscreen

Have you got a digital multimeter and do you have FORScan ?

It should only take a few minutes to find the reason for it not working if you have the right equipment.

I have a multimeter but no FORScan. 

my trouble shooting so far….

Battery voltage is 12.54V (engine off)
60A fuse in engine ok. So are the pair of 30A fuses in the passenger box. 

The windscreen relay appears ok with continuity across pins 1&2 and the switch across 3&5 is open. However I’ve not yet checked the switch closes when 12V applied. 

No volts at either of the 30A fuses when I switch on (but button light works) so I don’t think it’s the windscreen itself that’s the problem. But I’ve not checked connections under the trim yet.

Thinking I’ll try a new relay to rule that out but after that I’m stuck for cheap/easy ideas.

Some say there’s a fail safe to stop heated windscreen working if battery power isn’t high enough. Might have to try new battery if new relay doesn’t solve it. 

3 minutes ago, Nic Williams said:

so I don’t think it’s the windscreen itself that’s the problem.

I have to ask the obvious. Are you 100% sure that it has the heated glass windscreen fitted. I've seen many a car that has had the windscreen replaced with a basic one.

For both half's not to be working it has to be:

1. Wrong screen fitted.
2. Screen connections are not plugged in
3. Control fault with the GEM/BCM (depends of the vehicles age as to what is fitted)
4. Heated screen switch has a fault. Just because it lights up, doesn't mean it works.

I can see the wires embedded in the screen so it’s a heated one. I’ve only just got the car so don’t know if it’s ever worked so I suppose it could have been fitted without connecting. I’ve not checked behind trim on A pillar yet. 
 

what could I check with the forscan? Would that pick up a GEM fault? 

If you use FORScan it would tell you if there are any DTC's for the screen circuit. Also you should be able to set up live monitoring to see if the circuit is being commanded on when the switch is operated.

Common causes of the screen(s) not working is that it's too warm outside, I don't think that will the problem just now 🤣 or that the battery state of charge is too low. On your model I'd expect that as long as the battery was 65% charged or better then that should be OK.

Would a cheap OBD like this pick up the fault? 
 

https://www.ebay.co.uk/itm/256272498789

1 hour ago, Nic Williams said:

Would a cheap OBD like this pick up the fault? 

Short answer would be no. If there are DTC's for the front screen they will be Ford specific and so a generic code reader will not see them.

For £25 you can get a genuine vLinker FS cable to connect a laptop up and then use FORScan (which is free).

How much information does ForScan provide if the Vlinker did pick up a DTC associated with the heated windscreen. Would it be enough for a garage to know what needs doing and provide me with a quote to fix? I need a quote to make a claim on my warranty but I don't want to spend loads on diagnostics in case its not covered.

If I get no DTCs does that rule out a problem with the GEM talking to the heated windscreen? Or could that just mean there's no fault code for this type of thing?

Would you expect most garages to have a ford specific OBD and would they do a quick scan for free so they can get the work if a fault is found? Or do they charge upfront for a scan like this?

I've never explored this before so apologies if some of my questions are a bit basic.

1 hour ago, Nic Williams said:

Would you expect most garages to have a ford specific OBD

Virtually no garages except Ford have OBD readers that will get Ford specific DTC's.

1 hour ago, Nic Williams said:

I need a quote to make a claim on my warranty

Did you buy this car from a garage ?

If you are planning on getting someone to repair the heated screen system then you need an Auto-electrician, which most garages don't have.

I'll see if I can find any wiring diagrams for the operation of the system. At the moment assuming that everything is fully connected up as it should be, then the problem is the heated front screen relay is not being turned on. If it was then the screen would be working.

The reason for the relay not turning on basically comes down to one of three things:

1. A faulty relay. - Replace it with a new one
2. A faulty switch - test the switch contacts to ensure correct operation
3. A fault in the BCM - Should be detected by FORScan when running a module self test.

Are you ready to do some fault finding with your multimeter ?

Let's get started......

Attached is all the information you should need. The aim of the game is to find out why relay R5 is not turning on. When the ignition is turned on, one side of the relay coil is supplied with 12V positive via fuse F1 (7.5A) in the passenger compartment fuse box. The other side of the relay coil is supplied the negative from the Body Control Module (BCM).

So either the relay is not getting the 12V positive supply, or it's not getting the negative from the BCM.

Remove the relay from the fuse box. Turn the ignition on and start the engine. Set your multimeter to Volts(dc) and connect the negative lead to a good solid earth point. Use the positive lead to check pin 1 of the relay base in the fuse box. You should get a reading of 12V positive.

Next, keeping the positive lead of the multimeter on pin 1 of the relay socket, move the negative lead to pin 2 of the relay socket. You should get a reading of 12V between the two pins. -- I expect at this point you will find that you don't get 12V. If this is the case then it is because the BCM has not commanded the relay on by providing the negative needed.

Let me know how you get on with the above testing, and then I'll take you through the next steps.

Thanks unofix. Here's what I discovered

F1 fuse is missing! I've popped in a 7.5A fuse at its now working. Relay clicks when I press the button and windscreen starts to clear.

Without this fuse I was only getting 3V to pin 1. Not enough to switch the relay.

What a relief and a very cheap fix!

I'm kicking myself - I'd wondered if this missing fuse was a problem in a different post. The owner manual calls it "ignition, rain sensor and heated front screen". Since I didn't have rain sensors and my ignition was fine I presumed it was only needed in a different model. Mine is 2010 fiesta. I didn't have the wiring diagrams so could not be sure it was involved.

I wonder why someone removed it in the first place?!

Where did you get the circuit diagram from @unofix? Is there a manual I can download somewhere? 

5 minutes ago, Nic Williams said:

Where did you get the circuit diagram from

I could tell you, but then I'd probably have to kill you 🤣

Oh alright then, here is the link to my personal Google drive with the full Ford schematics for the 2012 Mk7 Fiesta.

Use it wisely young Jedi, and may the heated screen be with you 😀

https://drive.google.com/drive/folders/1LROazf4_k5cbgKvXx44SN7cpvh4fNAk0?usp=sharing

Thanks @unofix  you’ve been fantastic!  Excellent forum. 

12 hours ago, unofix said:

Oh alright then, here is the link to my personal Google drive with the full Ford schematics for the 2012 Mk7 Fiesta.

https://drive.google.com/drive/folders/1LROazf4_k5cbgKvXx44SN7cpvh4fNAk0?usp=sharing

Can I pinch that?

39 minutes ago, mickywrx said:

Can I pinch that?

By all means, you're very welcome 👍

53 minutes ago, unofix said:

By all means, you're very welcome 👍

Thank you, kindly. 😎

Great advice and info from @unofix but in most cases all of the auto electricians I follow strongly recommend load testing of any possible wiring faults using a test lamp as a multimeter can give very misleading readings if there is a high resistance connection/green crusties etc.
This gives a demonstration (if I find the even better one, I'll edit this post):
https://www.youtube.com/watch?v=xfohKS-HFGg&t=11s

Edit, this one is even better!):
https://www.youtube.com/watch?v=A9yErVxZBQ0&t=811s

I'm seriously thinking of getting a similar test lamp...