Rear Demister

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craigt

Rear Demister

Post by craigt »

Guys,

Now that the cold weather is here I've noticed how rubbish the heated rear screen is.

It works but take ages to clear even the smallest amount of mist. I've cleaned the terminals where they connect to the heating elements but it's made no difference.

What else should I be checking. :?:

Cheers
KC Ron Carter

Hmm,

Post by KC Ron Carter »

Here is a diagragm.

But wysiwyg.

Image

Later,
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sharkfan
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Post by sharkfan »

But wysiwyg.
Huh :?: :?
KC Ron Carter

OK

Post by KC Ron Carter »

Sorry for the riddle!

What you see is what you get.
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sharkfan
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Joined: Tue Sep 28, 2004 3:14 pm
Location: Guildford, U.K.

Post by sharkfan »

Thank you :D
craigt

Post by craigt »

Thanks for that Ron
horsetan

Post by horsetan »

But what happens when the individual elements of the demister start to fail?

Are we then looking at a complete replacement rear windscreen :?: :shock:
KC Ron Carter

Well,

Post by KC Ron Carter »

There is a copper impregnated paint for repairs.

You use tape to outline the needed area.

It looks alot like a finger nail polish bottle and brush.

It just provides connectivity for completing the circuit.

http://www.autoaccessconnect.com/perrearwinde.html

Later,
ScottAndrews

Some diagnistic tips

Post by ScottAndrews »

I might point out that there are many ways a demister can fail..

If yours demists fine, but only in some places, then you have broken elements, and the copper paint fix should work. However, this is pretty rare unless somethign has scratched the element (they are relatively thick). The other way they can fail is to just simply not be getting any power. THe final way is that they are getting power, but not enough. Remember this is a heating element, so it uses a fair amount of current to generate heat. If that current is not available, then it will heat less, and thus not work very quickly. This sounds to me like your problem.. so read on..

Following the diagram Ron provided, you might first check to see if the defroester is actually getting any power. To do this test the wires that attach to the elements (on each side of the rear window). Turn on the de-mister switch, and measure the voltage on each window connection. You should see 12 volts on one wire, and zero on the other. If you have zero on both wires, then you have a bad switch or a bad connection or wire somewhere between the battery and the element. If you have 12 volts on one side, and zero on the other, then you probably have some issue with the contacts onthe glass, or the element itself.

If, on the other hand the hot side is not 12 volts and/or the grounded side is not zero, then you have a bad contact somewhere along the way.

If the "grounded side" is not zero volts (but is lower than the other side), then you have a bad connection to ground. Trace that wire and figure out why.

If one side is zero, and the other is some intermediate voltage (like, say 7 volts), then you have a probelm on the current feed side of the circuit. The easiest way to test for this is to start at the demister switch. You should see 12 volts coming into and out of that switch when it is on, and 12 volts in and zero out when it is off. If this is not the case, then the problem is in the wiring upstream from the switch. In Ron's diagram this is Fuse 20.. so go check there. At some point between the battery and the element you will find some wire or component thaty is dropping the voltage.. and that is the culprit.

You have now inspired me. My de-mister has never worked.. so I am heading out to my car to test this...

S
craigt

Re: Some diagnistic tips

Post by craigt »

ScottAndrews wrote:I might point out that there are many ways a demister can fail..

If yours demists fine, but only in some places, then you have broken elements, and the copper paint fix should work. However, this is pretty rare unless somethign has scratched the element (they are relatively thick). The other way they can fail is to just simply not be getting any power. THe final way is that they are getting power, but not enough. Remember this is a heating element, so it uses a fair amount of current to generate heat. If that current is not available, then it will heat less, and thus not work very quickly. This sounds to me like your problem.. so read on..

Following the diagram Ron provided, you might first check to see if the defroester is actually getting any power. To do this test the wires that attach to the elements (on each side of the rear window). Turn on the de-mister switch, and measure the voltage on each window connection. You should see 12 volts on one wire, and zero on the other. If you have zero on both wires, then you have a bad switch or a bad connection or wire somewhere between the battery and the element. If you have 12 volts on one side, and zero on the other, then you probably have some issue with the contacts onthe glass, or the element itself.

If, on the other hand the hot side is not 12 volts and/or the grounded side is not zero, then you have a bad contact somewhere along the way.

If the "grounded side" is not zero volts (but is lower than the other side), then you have a bad connection to ground. Trace that wire and figure out why.

If one side is zero, and the other is some intermediate voltage (like, say 7 volts), then you have a probelm on the current feed side of the circuit. The easiest way to test for this is to start at the demister switch. You should see 12 volts coming into and out of that switch when it is on, and 12 volts in and zero out when it is off. If this is not the case, then the problem is in the wiring upstream from the switch. In Ron's diagram this is Fuse 20.. so go check there. At some point between the battery and the element you will find some wire or component thaty is dropping the voltage.. and that is the culprit.

You have now inspired me. My de-mister has never worked.. so I am heading out to my car to test this...

S
Great tips Scott, will check it out
bfloshark

Similar Issues

Post by bfloshark »

When I bought by 1980 633 in July, both the hazard and defogger (or demister for our British Brethren) switches were busted - would not lock when pushed in. I ordered new ones from BavAuto - the hazards work great, but I could not get the defogger to go.

I bought the ETM, and had the following issues: Green/Blue wire is hot from the car, but goes cold when I hit the switch. I cannot get the red/black wire to the defogger to get hot at all. There is also a pink ground that I am not sure where it goes on the switch.

One day I was fooling around with it again, and finally got the light on the switch to light, and the window to get hot. Don't ask how I did it. However, I have the same issue where it is not very hot at all. Will have to try and test again at a later date.
ScottAndrews

Post by ScottAndrews »

I'll check the ETM to try to understand your issue. Off the top of my head, I would say you have a shorted wire. That is why the Blu/Grn wire goes cold when you hit the switch (MAYBE). The failure of the hazard and def switches to lock is, unfortunately, very common. I have been through 2 hazard switches in 4 years, and I never even use the hazard switch! Just one day the latch seems to give up and you have permanent flashers..

S
ScottAndrews

Defroster Debugging (Long)

Post by ScottAndrews »

OK. First of all, the older cars have a simpler defroster switch. In the newer cars all that business with the diodes and the resistor are there to illuminate the lamp a little bit when the lights are on. So the newer swtich shown in Ron's diagram has a number of additional wires. Basically the lamp is powered through the 100 ohm resistor when the switch is OFF and the lights of the car are ON. When you press the switch the lamp is powered with no resistor, so it gets brighter. If the car lights are OFF, then the lamp gets no power. All of this has nothing to do with the function of the defroster/demister.

In the 1980 ETM, the switch is shown with three wires. The green and blue (Gn/Bu) comes from Fuse 13, which is powered by the "unloader relay" The unloader relay is basically the relay that controls power to all the things that only work when the ignition is on. For those of you who are electrically challenged, this relay is sort of a proxy for the ignition switch. Since the switch is rather small, they put only a small amount of current through it. When the switch is ON, it turns on the Unloader Relay which is a big switch that can control the current for a bunch of high current stuff in the car (the heater fans, cigarette lighter, etc)..

The black and red wire (Bk/Rd) goes to the hot side of the heating element. The other side of the heating element has a Bk wire to ground.

So, this circuit is super simple. Fuse 13 only protects the rear defroster and the sunroof (if you have one).

If you turn the switch on and the Gn/Bu wire goes from 12 volts to zero, or near zero, then one of two things is happening.

1) there is a short to ground somewhere between the switch and the heater (this would be the Bk/Rd wire)
2) there is high resistance between the battery and the switch

If Fuse 13 doesn't blow when the switch is turned on, but the voltage on the Gn/Bu wires goes way down (maybe not all the way to "zero"), then you have problm #2. If, on the other hand, the voltage goes to ZERO and stays there until you either jiggle or change fuse 13, then you have problem #1, and you need to find the short.

For #1 (Short): Use your ohmmeter, and start at the window. Unplug the Bk/Rd wire from the window and test the resistance of the window terminal to ground. If it is zero, then there is something weird about your window (highly unlikely)...If it isn't, then check the wire. If it is grounded, then go to the other end and unplug it from the switch. If it is still grounded, then you have a treasure hunt on your hands. Keep the Ohmmeter connected beween the wire and ground, and trace the path of the wire. Jiggle it and look for changes in the resistance (like it goes from zero to infinity). really check areas where it goes through a hole in the body and may have chaffed through the insulation. Check also for PO work that may have tapped into the wire for some other purpose (people do all sorts of stupid things over the years...like have the defroster on while installing a read CD changer or amp, and mistake the Bk/Rd wire for switched power..and then wonder why the CD only works when the defroster is on.. electronics is just not that hard, but it can make you look real stupid).

If you have problem #2. This is more interesting. Basically you have a resistance path that is dropping the voltage when the defroster is on. The defroster takes a fair bit of power. This is because it is having to heat the cold (sometimes frozen) window. In electronics, power is given by I^2*R (current squared times the resistance). This means to get heat (power) you want a low resistance and LOTS of current. However, this can work against you, since voltage drop is given by V=I*R. This applies in the heater as well as for the resistance at every connector, in every wire and in every switch and fuse. So, if the current is high and you have some small resistances in the circuit , they will drop the voltage. When the current goes down (ie when you open the switch), the voltage will come back up.

To find this, you need to measure the resistance between the switch and the unloader relay. Hook your ohmmeter between the green and violet wire on the unloader relay (this is on the distribution panel under the left side dash). If you have this problem, the resistance of this will be something like 1 to 10 times the resistance of the window heater. It SHOULD be very low (like less than 2-3 ohms, or even unmeasurable with a regular multimeter). This process is like finding the short, only in reverse. You first measure the resistance between the relay Gn/Vl wire and each side of Fuse 13. These should be the same. If not, then pull out the fuse. One side of the fuse socket should measure zero ohms to the relay and the other should be infinite (open circuit). If this is the case, try a different fuse. If changing the fuse eliminates the resistance, then you are done. If the resistance to the relay stays high, then remove the spade connector at the relay and examine it. If it is corroded, try cleaning INSIDE the connnector with a piece of folded sand paper and electrical contact cleaner. the resistance of the Gn/Vl wire between the fuse and the relay should be zero (or very very low). Similarly, the resistance of the Gn/Bu wire from the other side of the fuse and the def swtich should also be low. If it isn't, then you have to clean the contacts and/or trace the wire to find out why. If all of this checks out, then you should look at the Unloader relay itself. This is especially true if you find that the heater fan slows down when the defroster is on...

I suspect corroded contacts are the cause of defroster problems that involve poor defroser performance. Poor, vs inoperative.

A few months ago I was trying to debug my auxilliary fan. It would not come on. I found that the double fuse in the small fuse block on the left inner fender (near the battery) was blown.. melted.. I replaced the fuses, and tried it..ONE of the fuses blew instantly.. Weird..This is a double fuse. That is, both fuses are in parallel. When only one fuse blew, it became clear to me that the other was no seeing any current. It turned out that the fuse block itself was bad. The connecotr tabs on the block are riveted to the fuse clips and tot he plastic fuse block body. THis rivet creates the mechanical AND the electrical connection. After 24 years living in the engine compartment, the joint between these parts had corroded enough that they were no longer in electrical contact, even though they were touchingn each other. I cleaned the whole thing with cleaner, wire brushed it, and then soldered the joint, and voila..

Lesson: Electrical contacts can corrode and increase resistance to the poitn where things don't work, but when you measure continuity, things check out. So be suspect of every connection, and use deduction to systematically eliminate possibilities until you find the one that can't be eliminated.


S
Quendil

Post by Quendil »

This may be of interest and adhesive rear screen heater from

http://www.holden.co.uk/hvc-home.htm

Part# S080.109
ScottAndrews

A few more tips on the Demister

Post by ScottAndrews »

I went out to my car this AM and tested the demister circuit. I get solid 12 volts at the green and blue wire. I also have the "pink" wire which apparently is the "ground", although I have not tested it to see if perhaps this is for the newer style back lit switch.

The hot side of the windo element is the LEFT side. The righthand side is ground.

I tested 12 volts at the left side of the window, and 12 volts along the metal edge of the window. So the electrical feed is working just fine. The switch lights up when it is popped out (on).

HOWEVER, I get no defrosting action. I tested the element itself. It seems to have about 67K ohms of resistance, which I suspect is way high. From what I can surmise, the element is probably mostly corroded away, and this resistance is the oxidation on the lines. I can measure the voltage on any given line by running the tester probe along it. It goes rapidly from 12 volts to zero in the first foot or so. This indicates to me that the lines are probably broken, and the breaks have corroded over. If a line is working properly it should measure 6 volts right in the center. I suppose one could remove the glass and clean the demister element and then inspect it with a loupe to find the breaks, but this is probably a LOT of work.

Does anyone know what the resistance of a properly functioning rear window element is?

S
ScottAndrews

Post by ScottAndrews »

Here's a US link for defroster repair and replacement kits.


http://www.frostfighter.com/ff_pdt_2600.htm

S
bfloshark

Great Response

Post by bfloshark »

Thanks Scott for that detailed response. From looking at the RF archives, this question seems to come up a lot this time of year. I think we have the makings for technical archive article.

One question for you relative to the light in the button. You mention that it gets brighter when it is in use. Was that for the "simple" switch in our E12 based cars, or like the one in Ron's example? In mine, it doesn't light at all until I release the button (in use, I guess)

Dale
ScottAndrews

Post by ScottAndrews »

I suggest I write this all up and circulate it to those who have been following this thread. Gather new ideas and experience, and then we'll send it to Brian.

The older (how old? probably the pre 84 E12 based cars) have a lamp in the center of the green defroster button. This lamp ONLY lights when the button is released (popped out). On my 80 Euro there are three wires. The two business wires are the Green/BLue (current source) and the Black and Red (to the heating element). The other wire is not given a color in the ETM, but in my car it is pink and black. It goes to ground..Not sure why it isn't brown...

In the newer cars there are 5 wires. Basically the business part of the switch is the same (Gn/Bu and Bk/Rd). Instead of the pink/black wire there is a Brown/White and a Brown/Black wire, and there is an added Grey/Red wire. The Gy/Rd wire goes the the interior lighting (the wire that lights up all the dash backlighting). The Brown/WHite one goes to the interior lighting dimmer via a 100 ohm resistor. The other goes to the same dimmer but with no resistor. When the switch is OFF, the backlighting current flows through the bulb and the 100 ohm resistor to the dimmer. So when the switch is off and the lights are on, the dimmer will control the dimly lit bulb along with all of the other back lighting. WHen the switch is on, the bulb is fed by current from the Gn/Bu wire, and it goes through BOTH the 100 ohm resistor and a closed switch to the dimmer. The effect of shunting the 100 ohm resistor using a switch basically increases the voltage on the lamp, so it glows more brightly.


Wakatta?

So, desu ne..

S
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