Rear screen diagnostic check

[HITECH]

Hi, I got my rear screen replaced after it shattering on me but got the car back with the demister not working. Ive run a dagnostic with nothing showing up and also done a voltage check from the button all the way thru to the SAM rear screen 40A fuse and it shows a 12 volt feed at the fuse when the demist button is pressed. This would say the screen is dud ? I checked the elements on the screen and they were like 0.1, 0.2 volts. So must be a bad connection or just a dud element in general ? Anybody got some input on this? Looks like its going back to the body shop to get screen replaced again. btw its a late 2004 W03 C220 saloon.

 

[whitenemesis]

Presumably it worked before the replacement, so the body shop needs to sort it for you

 

[Ted]

If the elements are printed on the surface of the screen, this is how to test.
With screen turned on, using a testlight with one end connected to earth, firmly (not TOO firmly!) draw the testlight down the middle of the screen across each element in turn.
Testlamp lights dim - element is working
Testlamp lights bright - break between where you are testing and earth
Teslight stays off - break between where you are testing and supply.
You can then use the above to track (pun intended) down exactly where any breaks are.

 

[HITECH]

Waiting for them to get back to me. Already left it in there to be re-checked. Now awaiting call to go back in because unfortunately insurance gave the job to a company who subcontracted to another company to fit it. so 1 company supplied it to another company who fitted it. The company who fitted it have to contact the cmpany that supplied it.

Presumably it worked before the replacement, so the body shop needs to sort it for you

 

[HITECH]

Already done voltage tests on the element tracks. Very low readings. Certainly not enough to light up a bulb because if there was then it would demist in certain places. Its been an insurance replacement glass job so I just was double checking for my own peace of mind really. Handy to do for any future customers who have their demister stop working. Unfortunately my new back screen it appears the element is dead. After the feed 12volts does it have to go thru the relay before the element ?

If the elements are printed on the surface of the screen, this is how to test.
With screen turned on, using a testlight with one end connected to earth, firmly (not TOO firmly!) draw the testlight down the middle of the screen across each element in turn.
Testlamp lights dim - element is working
Testlamp lights bright - break between where you are testing and earth
Teslight stays off - break between where you are testing and supply.
You can then use the above to track (pun intended) down exactly where any breaks are.

 

[Ted]

I don’t know the specifics, but usually the switch would switch a large current via a relay.
In your case (purely a guess) this might be done electronically via a power transistor in the rear SAM with the signal coming via the canbus.

Edit: read your post more thoroughly and it looks like the current does come via the SAM.
As you have 12 volts at the SAM and bugger all at the elements, I’d be looking at the connection to the screen - especially if it’s been replaced. Either fell off during fitting, or not connected at all.
If the latter, suspect the other side as well.

 

[HITECH]

Yeah thats what im thinking. Just a dead element. Rather annoying in this freezing weather right now Was just wondering if the 12volts at fuse (in the SAM) is supposed to activate thru relay before getting to the element. The circumstances of the screen smashing was. Got in the car, went to turn the key and the glass shattered. So had turned the key after it shattered. Element was switched off at the time. So doubt its damaged in any way anything else or the fuse woulda been 1st thing to go normally. These guys fitting the windscreens are supposed to sort all that stuff out arent they ?

I don’t know the specifics, but usually the switch would switch a large current via a relay.
In your case (purely a guess) this might be done electronically via a power transistor in the rear SAM with the signal coming via the canbus.

Edit: read your post more thoroughly and it looks like the current does come via the SAM.
As you have 12 volts at the SAM and bugger all at the elements, I’d be looking at the connection to the screen - especially if it’s been replaced. Either fell off during fitting, or not connected at all.
If the latter, suspect the other side as well.

 

[richard]

Fuse is before relay contacts. Relay contact connect directly to windscreen. - I.e there is no signal measuring so no error logging if rear window fails or is disconnected or fuse blows. Fuse is F49 40Amp.

If you are getting courageous, the cable coming out of the SAM is 4.0mm2 black cable - on connector 1H (single wire connector)

Cheers
Richard

 

[Ted]

And the other side will be to earth. Possibly brown.
As far as I can see there is NOTHING on a heated rear window that could possibly blow a fuse. The only thing would be a short to earth in the high current cable from sam to screen.

 

[richard]

BTW, you need to check there is permanent power to one of the relay's contact pins (the other going to the screen). The switched 12v you see is what turns on the relay.

You can test the screen wiring by measuring the resistance between the other relay contact pin and earth ! If its open circuit you know the problem is the screen or the screen not plugged in !

R

 

[HITECH]

When testing relays I usually just whip the covers off them and get somebody to press the button. if they are arcing its always obvious to see and a wee clean up is usually the answer. I might try that simple check. Just recently sorted an octavia with an intermittent fuel pump relay. Sometimes it would work sometimes it wouldnt which led other mechanics up the garden path. When I opened it up to look the burn marks inside it were obvious to see what the problem was. Any idea which relay it is. looks like there are a few in there. All fuses have got 12 vlts going to them. Apart from the demister one but 12 volts are present once the button is pressed. Thanks for the info Richard. i will try suss out which connector the 12 volts comes into the relay at . it should show easily enough with the relay out if i get somebody to press the button and probe each relay socket for 12vlts. Then i will have to put in a wee wire from the relay output side wont I to see if it switches the 12 volts over with the relay in situ?

BTW, you need to check there is permanent power to one of the relay's contact pins (the other going to the screen). The switched 12v you see is what turns on the relay.

You can test the screen wiring by measuring the resistance between the other relay contact pin and earth ! If its open circuit you know the problem is the screen or the screen not plugged in !

R

 

[richard]

Relay C

Richard

 

[Ted]

When testing relays I usually just whip the covers off them and get somebody to press the button. if they are arcing its always obvious to see and a wee clean up is usually the answer.

Not always a good idea!
Back in the day, we had people doing this on voltage regulators.
This would cause the current to flow through the pull in coil and passthrough to the dynamo, turning it into an electric motor which was of couse stalled and would burn out the dynamo, regulator and associated wiring.
The more current that flowed, the more the contacts were held together.
Nice little earner that was.
Obviously, no dynamos today, but not knowing exactly how a circuit works, it isn’t the way I would test a relay.

 

[HITECH]

Them SAMs and their power distribution on the older c classes are quite simple compared to the newer ones with sealed regulators and chips. Not a lot to have to know about them thankfully. One of the reasons Ive kept it going lol. Its getting colder here BBBBRRRrrrrrrrrrrrrrrrrrrrrrrrrrrrr.....

When testing relays I usually just whip the covers off them and get somebody to press the button. if they are arcing its always obvious to see and a wee clean up is usually the answer.

Not always a good idea!
Back in the day, we had people doing this on voltage regulators.
This would cause the current to flow through the pull in coil and passthrough to the dynamo, turning it into an electric motor which was of couse stalled and would burn out the dynamo, regulator and associated wiring.
The more current that flowed, the more the contacts were held together.
Nice little earner that was.
Obviously, no dynamos today, but not knowing exactly how a circuit works, it isn’t the way I would test a relay.

 

[Ted]

The old Rover P5 didn’t have a hrw.
It had a fan blower in the rear parcel shelf

 

[HITECH]

Hahahahahahahahahah the thought DID CROSS MY MIND lol Any idea where one of them old P5s are ? I might just need the parcel shelf blower at this rate if I dont get my own swapped again ASAP lol

The old Rover P5 didn’t have a hrw.
It had a fan blower in the rear parcel shelf

 

[HITECH]

Screen getting replaced next week. Wonder if I will get a chance to get a meter on one getting removed out of the car.

 

[HITECH]

Now we are almost a year later. The screen did get replaced and still doesnt work. NIGHTMARE. Been such a busy year and ive had so many diffirent cars to drive i didnt get round to bothering with it. until now again when winter has come and it needs to work for me. I just prefer the comfort of driving this car than any of the rest. Treated it to new shocks and springs and arb bushes etc etc so winter handling is perfect. I have another rear sam there i got a hold of for this model. Is this a straight swap? Its all its gotta be. Everything checks out. light on demister switch comes on as normal and u hear the relay in the rear sam. but that does not mean that its feeding the proper current thru to the rear screen. I CAN just swap this rear sam out cant I? One thing i didnt think to add about this problem was I swapped out the boot lid brake light with a cheapo chinese version and on occasion it would not work and come up on the dash as centre brake light malfunction. so 2 questions.
1. Can I swap out the rear sam. Plug n play?
2. Would the rear centre brake light have any effect on the demister circuit 15R ?
I will get back onto this on Sunday.... hopefully somebody can shine some light on those questions.........

 

[richard]

If you can hear the relay the electronics in Sam is good. Change relay and fuse and check the power output at SAM. Then check at window. And check ground at window. Could do all of above backwards too ...

 

[HITECH]

If you can hear the relay the electronics in Sam is good. Change relay and fuse and check the power output at SAM. Then check at window. And check ground at window. Could do all of above backwards too ...

Ive instances where relays have clicked but when the required current is desired its not enough in which case its farting against the wind as they say. more resitance to the under current power being pushed along. them sams are plug and play are they ?