Not quite clear to me what you were doing there. The non-coil contacts of the relay (the wide pins) are just a switch, there's no power source there, current goes in from the fuse on one of them, out on the other to the screen elements.

Hi Juan

 

Are you testing the original relay out of the car ?  Apply 12 volts across the coil connections (the two smaller pins).  You should hear the relay click as it is energised.  Then use an ohmmeter connected to the contact connections (the two larger pins).  This should read zero ohms when the relay is energised.  Remove the coil power and the resistance will go up to infinity (open circuit).

 

If you are seeing any voltage on the contact connections, it's probably due to a few megohms of electrical leakage.  This could be due to some damp still in the relay.  But it's an unusual test to be performing, not sure what you are trying to achieve there.

 

If you want to know how the new relay is working in circuit, you need to monitor the voltage on the contacts on its downstream side (output side).  Either at the relay or at the heated screen.  I appreciate that it's not easy to access, you might have to fix up a thin temporary test wire and bring it out to somewhere you can get to with your test meter.  If all is well there should be no more than a fraction of a volt difference between the input and output contact connections.  Or wait till a frosty morning !

Yes testing the old (suspect) relay, and was really just checking that "switch on" makes a circuit. 

 

If you like, with the relay "on" I used the output terminals as part of a circuit to test the battery voltage, and didn't get the full 12v (approx) as expected.

 

I'll check the resistance as suggested.

Just been out to the car (-3°C). Light dusting of frozen snow melted by heated windscreen alone in 5 mins. Success!

Edited 25 January, 20215 yr by juan27

Resistance across the output did not drop to zero when the coil was activated on the old relay.

 

I opened it up and the cable connecting the moveable armature to the terminal separated from the armature with almost zero force, looks like it was a  dodgy spot weld to me 

 

Probably manufacturing fault. I wonder if the heated windscreen has ever worked.

 

 

 

 

Edited 25 January, 20215 yr by juan27

Bosch used to make car relays in Portugal, but I think they were taken over by Tyco.  Must have been having a bad day when yours was made.

2 hours ago, juan27 said:

I opened it up and the cable connecting the moveable armature to the terminal separated from the armature with almost zero force, looks like it was a  dodgy spot weld to me 

Did that connect to connection numbered 3 or the number 5 one?

19 minutes ago, Wino said:

Did that connect to connection numbered 3 or the number 5 one?

 

3. Why do you ask?

Edited 25 January, 20215 yr by juan27

I was just wondering if the corrosion evident on no.5 was the result of moisture that had also tracked inwards and damaged the flexible wire. Seems not though. 

 

 

1 hour ago, Austin 7 said:

Bosch used to make car relays in Portugal, but I think they were taken over by Tyco.  Must have been having a bad day when yours was made.

 

Yes hopefully an isolated rather than design fault. Nobody seems to have had a similar issue on the forum so that gives me hope the fix should last..

Having seen your picture of the inside, I'd say the spot weld probably failed as a result of corrosion rather than a manufacturing fault.

 

If you get moisture bridging two metals carrying different voltages, the corrosion is greatly accelerated.  It acts like an electroplating bath, with metal being eaten away from one side and deposited on the other.

 

I deal with electrics for motorhome owners from time to time.  They often have issues like this due to their vehicles being laid up for months at a time and getting condensation which lingers.  Similar vans used every day with plenty of engine heat and ventilation are much less affected.

1 hour ago, Austin 7 said:

Having seen your picture of the inside, I'd say the spot weld probably failed as a result of corrosion rather than a manufacturing fault.

 

If it is corrosion it's very localised to the spot weld. To be honest it looks more of a rivet maybe as it's through a piece of chromed spring steel and forms the contact. On closer examination I reckon it's just pressed to mushroom it maybe. 

 

 

You could well be right about lack of daily use. The car had only done 10k miles when we bought it at 26 months old, but has been in more or less daily use since we got it, so the damage probably happened in the first couple of years.

 

I must say however that in any case I'm disappointed that it failed - are the camper vans you mention less than four years old?

 

Probably spent far too much time thinking about this relay, but thanks to everyone for their help

@juan27 Thanks for detailing this fault, hopefully it will help others in the future. 
 

Great that you’ve got it sorted. Bit of a head scratcher- It really looks like some kind of liquid corrosion, though they are high current and any high resistance joint will soon get very hot and damage will occur.

 

SWMBO has got the heated screen fitted, great this time of year. 

 

Thanks again for posting 

 

Can someone point me in the direction of where the heated screen connection points are please.

On 15/01/2022 at 02:35, vRSG60 said:

Can someone point me in the direction of where the heated screen connection points are please.

Top centre of windscreen. Only noticed when I had a new windscreen fitted and technician pointed it out to me.