rear screen heater problems

Late 1990 200 Tdi 90.
Rear screen heater wont work.
There is continuity from the fuse to the end of the loom in the rear 1/4 of the tub,
I have dismantled the switch and cleaned the contacts and this now works (didn't before)
the relay seems to work in that it will trigger power to light a test light when wired direct to the battery.
the diode passes 0.718 something or others (could be cabbages for all I know) when tested with a multimeter but as far as I can remember ( been tying too many things and I am now confused) there is no continuity to earth from the outlet (?) side of the diode, the wire goes to a voltage sensitive relay, is this fecked?
Edited to say the dash warning light wont work either but the bulb is good.
what else do I test ???
I think I checked every move with engine running, rather than just ign on.
All (most) replies welcome thanks.

I take it you have no voltage at the connection on the rear window?

have you tried to bi-pass the diode and voltage sensing switch to earth with a cable the relay should then energise the fault then lies with either the diode or voltage sensing switch if it doesn't energise assuming you have got voltage at the switch side of relay white green
then the relay coil is passed it sell by date F**ked

nobber said:

I take it you have no voltage at the connection on the rear window?

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That is correct, no power coming through the white feed to window element, though there is continuity in the loom from the r/h fuse connection, a good fuse had also been used.

mystery said:

View attachment 191599 have you tried to bi-pass the diode and voltage sensing switch to earth with a cable the relay should then energise the fault then lies with either the diode or voltage sensing switch if it doesn't energise assuming you have got voltage at the switch side of relay white green
then the relay coil is passed it sell by date F**ked

Click to expand...

I think the relay is good, though I tried a couple anyway. To test the relay I clipped a wire to the main power out with a 21w bulb in the line which I then put to earth, clipped a feed from the battery to the main power feed , then earthed the relay and energised it with another lead to the battery live. Relay clicked, bulb lit up, I guess the relay is OK. I'm sorry I can't quote relay connector numbers without going and having a look, but I think you can see what I've done.
So to test the diode/voltage sensing switch I can just take the connector off the diode (from the live side) and run a test wire to earth and see what happens?
By now you should know I'm as thick as two short planks when it comes to electrics, so what is the voltage sensing switch for anyway? could I safely by-pass it?

Thanks to both of you for your help by the way.

if you check the voltage on the relay on the white green wire should be 12 volt to ground ,then check the voltage on the black pink wire connection if you have 12 volt to ground then the fault is either the diode or the v sense relay ,if you get 12 volt on the black slate wire on the v sense relay connection to ground the diode is ok and the v sense relay is not switching ,if you remove black slate wire from v sense relay connection and connect a wire(preferably containing a fuse ) from the black slate to earth your relay should energise .
and the problem is the v sense relay NOTE the engine should be running for the v sense relay to operate correctly it purpose to stop the rear screen being left in circuit when starting and to only be usable with the engine running

@mystery Thanks for your help.
I shall try your tests in the next few days - bit busy at the mo - if the screen system does energise by bypassing the V sense relay would I be safe to just ignore the V relay and rely on me not using the screen heat unless engine running?
Just happened to scoot across to google and have a look at the prices for the V S R, Christ on a bike, they must be made of gold, I'd rather stop and do some heavy breathing to heat the screen.

yes it switches the earth to ground when the voltage from the alternator rises above 12.8 volt or higher basically to say the engines running on the later models with front heated screens this is done by the oil pressure switch
great price hike Chinese relays at EU prices that is if they work,and bit like Uni-part of old source parts from elsewhere ,place in Uni-part box instant profit sound familiar but now easier
not many marking on the blue box version

mystery said:

yes it switches the earth to ground when the voltage from the alternator rises above 12.8 volt or higher basically to say the engines running on the later models with front heated screens this is done by the oil pressure switch
great price hike Chinese relays at EU prices that is if they work,and bit like Uni-part of old source parts from elsewhere ,place in Uni-part box instant profit sound familiar but now easier
not many marking on the blue box version

Click to expand...

Just to be sure then, if I am careful to only have it running when engine is running then I can by-pass from the diode to earth and ignore the possibly duff VSR ?
If I put a fuse in the line from the diode would it be in order to use the same rating of fuse as that specced in the fuse box?

THE FUSE IS TO PROTECT YOU WHILST YOU CHECKING /TESTING

Oh, OK thanks.

An update,I put a fused link on the diode in place of the lead going to the VSR and connected to a good earth, turned on engine, switched the screen heater on and result, 14.3 volts across the screen terminals , so I guess thats going to work.
Still no light on dash panel. Had a look at bulb and connections, all seemed good, put back together and working guess just bad connections.
So everything working now. Just have to be careful not to switch it on until the engine is running.
Great thanks to everyone for the help given, really means a lot.

you do not need the fuse or the diode just connect the relay to good ground earth

mystery said:

you do not need the fuse or the diode just connect the relay to good ground earth

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Sorry, should have said, after testing I made up a proper earth tag and took it direct from the diode without the fuse and to a good earthing point.