The 'E' Type Forum

Investigating why my rear window heater doesn't work the S2 wiring diagram shows the supply to the relay comes from fuse 5 (for the side and tail lamps). But as fuse 5 draws power through the headlamp switch it means the window heater won't work without the headlights on. Rather bizarre!
My thoughts are to route the relay supply from a different fuse, perhaps fuse 3 which only seems to supply the horns.

Any comments? (Except that if I had looked at the wiring diagram before then this anomaly would have become obvious!)

Thanks
Geoff

Yes,very strange isn't it. I imagine it is a mistake in the diagram. I would take the lead to W1 on the relay ( shown as a red wire ) from the downstream i.e. fused side of a fuse fed by white wires which should be ignition switch controlled . The hrw should then always be available, but be incapable of being left on and flattening the battery

Hi Geoff yes it does look strange....looking at the diagram the heating element operates via a connection at C2 through the switch and an inline fuse to the live unfused side of fuse 7... with only the warning light looking like it works through the relay and fuse 5 .I wouldnt move any wireing till you find out how the circiut works.....mine works ok so will have a look tomorrow and re post....unless someone comes along who knows...you could find and check the inline fuse... Steve

Hi Geoff...found this on the forum..shows the wireing slightly different on tne relay to the diagram i have and assume you have...also read the note...there is a dimmer circuit to dim the warning light when you switch on the lights, thats the wire from fuse 5 the the relay.. Steve

Hi Christopher and Steve, wires fully checked and indeed the power to the relay comes via the headlight switch. Switch the lights on and the relay works!
The inline fuse is for the hrw switch and comes from the power supply input (white) to fuse 7.
The book's diagram is wrong showing relay C2 connecting to both the switch and screen, instead of switch and warning light.
Will search for a sensible white ignition switched fuse to go into

Thanks

Geoff missed out the llink on the previous post.. viewtopic.php?t=1507&highlight=heated+rear+windscreen note that the wire from fuse 5 is not the supply it provides a parallel circuit to dim the warning light when the light switch is operated so its not too bright at night.. Steve

Steve, thanks for that link, it's a better diagram than in the manual.
Sorry to disagree with your comment about it dimming the warning light, but the red wire only gets power when the headlights switch is on, certainly on my car, with no power to it from switching on the ignition.
Will do some more investigation to try and get to the bottom of it.

Amended to add: if my understanding of relays is correct, putting power to W1 by turning on the switch causes relay to close connecting C1 to C2. Which to me means that the red to C1 must have power, and I can only see that having power via fuse 5 when the headlights are on. Please correct me if I'm wrong.

Cheers, Geoff

Hi Geoff....if you read on that link the warning light has a dimmer circuit (its in the owners handbook)..it is also noted on the wireing diagram by the relay it says incorperating resistance ...... with the lights off you get a bright heated screen warning light, when you switch the lights on warning lamp is put in parallel with the side lights via the fuse 5 connection this results in less current so a dimmer warning lamp....if you have no other power to the switch check out the inline fuse.......the way the circuit is arranged means that with the light switch on you will have power at the relay (for the dimmer cct) but the main supply is via live side of 7 and the inline fuse....Steve

So relay is merely for dimming the light not switching power to the hrw.
Inline fuse is in good order, so lack of power to the hrw must be some other gremlin. What joy working on old cars is :?

It is. Pure joy in fact, with every prospect of successful work using only basic knowledge and tool. Try even finding, let alone reading/trouble-shooting a wiring diagram for modern cars. They have multiplexed harnesses linking dozens of computer logic modules mostly coded together and completely impenetrable or repairable. Even things like changing headlight bulbs is often a dealer-only job.

I just realised by reading this thread that maybe why I think my HRW does not work is because I have not switched on the lights..!

I did a bit more digging around regarding wiring diagrams.
First looked at my S1 4.2 Coventry Auto Components wiring booklet - it doesn't even have the HRW in it! (I do also know of at least 2 other errors in it).
Then looked at the various versions of the wiring diagrams on the Forum Knowledge Base.
A few of them have the wiring wrong for the HRW.

My take on the whole thing is that the HRW can only be on with the ignition on because the switch and relay are fed via a white wire - white wires generally denote an unfused feed which has been switched via the ignition switch (green wires are fused from the ignition, brown straight from the battery - unswitched, unfused).

The HRW switch feeds W1 terminal on the relay coil and also the warning light. The contacts of the relay are C1 and C2, therefore the main load red wire comes to C1.
Note: the wiring diagram version being referenced is the S1.5, I think, and it is a bit confusing because you can easily think that W1 is joined to C2, which it is not. Other versions of the diagram are drawn better.
However some other versions show the red main feed going to W1 and the switch wire going to the relay contacts - very confusing and wrong.

The really confusing thing seems to be that you can turn on the HRW switch and the warning light comes on, therefore you think the HRW is on. However it isn't on at all until you also switch on the lights.

I've given up trying to understand the warning light dimming because the red feed from F5 has to be battery voltage because it is also feeding the side lights.

Please correct me if I've misunderstood anything...

Hi Steve.....the relay in this circuit does not act as others on the E type, it does not take a small current across the winding to put out a larger current across the contacts......it you look at the diagram it says that the relay is a 6RA incorperating a resistance.......put on the HRW switch and the main live via the white from the live side of fuse 7 goes to the HRW......the warning light operates via an internal connection of the relay contacts...and comes on bright.....for night driving whan you switch on the lights the feed from fuse 5 on the red wire to W1 operates the relay..a resistance is put across the contacts to dim the warning light....the relay is part noC26651.....an internal diagram of it would help if anyone has one....Steve

Hello All, as the originator of this and by default the cause of the associated thread, I thank you all for suggestions, help, advice and all.
I've not been able to get to do further investigations, and it may be that some of what I posted earlier was due to my mis-interpretation of the various wiring diagrams.
I.E. does the relay power the HRW or merely dim the lights?
Next chance to thoroughly check my car is Tuesday, when that is done I'll post results.
In the meantime please accept some of my investigations and postings as possibly being incomplete.

There do seem to be some differing wiring diagrams and I get the gut feeling that some are not necessarily correct for any car and have been based on assumptions.

Again, many thanks

Geoff

Just removed relay from my S2...the winding is 70ohm approx and the contacts are normally closed...when relay is operated the contacts open and there is a resistance of approx 50ohm across them.....there are no connections between the contacts and the winding.......

After further investigation it seems that my HRW relay is merely for dimming the warning light when the headlights are put on, not for powering the HRW itself.
Now I had been assuming the opposite and thought there was a duff relay causing the rear window not to heat up, and hence my postings for help to sort it out.
The 6RA relay is wired as per the manual with red to W1 coming from the headlight switch to power the dimmer circuit. Black (earth) to W2. C2 has the blue/white from the HRW switch and delivers 12 volts when switched on and C3 has the red/brown to the warning light. Switch the headlights on and 12 volts goes to W1, the warning light dims.
What seems to be my problem is an open circuit between the blue/white (or is it white/blue?) at the switch and the rear window itself.

Will investigate that another day.

Thanks to all for their input, especially Steve (mgcjag).

Geoff