Getting the Rear Defroster working

[TomoHawk]

I want to go after the rear window defroster, which doesn't seem to be working; I turned it on one evening when it was foggeed up a little, and it didn't defog that much.

I don't think there are any gaps in the gridlines, so Would it be better to optimize the voltage at the defroster terminals, or should I measure the current capacity of the circuit, to see it it's drawing XX amps, or what?

thx

[Daniel]

Of course the easy terminal cleaning and volt check at connection would be great if that did it. I doubt it . With the window on, go side to side with a volt meter. Each line is small bare wire. Think like this, One bare wire with a volt meter on it will read no volts because the wire is the path of least resistance. Now cut the wire between the test leads. Now you have a voltage reading and have found your bad trace. Repairing is another can of worms :tapemouth . Hope this helps.

[EScanlon]

Daniel;

I may be wrong, but it seems that your logic has missed a step. Unless I have it all completely wrong.

The hatch glass' that I've seen, have a grid made up of 2 sets of resistance wires that are joined at the bottom edge of the glass and two spade terminals at the top of the glass. The resistance wires (let's say 12 total) need to ALL be cut on fully half of the grid, and all on one side, in order for there to be a lack of a circuit through the glass, and therefore a measurable voltage present.

I am deriving this from what I found when I did a continuity check to try to find a broken resistance element. Since only a couple of the wires did not work, I was only looking to repair those two elements. Unfortunately, no matter where I tested for continuity, since the circuit was NOT completely broken, I always got a tone and could not determine where the break was.

You're probably more of an electrician than I am, so I would sincerely appreciate knowing how to use this voltage method to determine the break in the elements.

Enrique

[Daniel]

Test searching for VOLTAGE not continuity.Please read my single wire example once again. The number of wires makes no difference If still confused I will be glad to explain further. I should have added that the probes will need to be slowly brought together . Whenever you have volts,you have a gap/break in the path between the probes. no volts= one solid path. O.k. one more..The volts across a switch that is on/closed =none The volts across a switch that is off/open= voltage of power source

[TomoHawk]

Having contiguous gridlines is essentialy understood by all. I think my question would be what else to look for, in terms of current draw, voltage drop, etc., to know if the thing is operating at the level it's supposed to be. I think there will probably be some degredation in the performance or the efficiency of the grid in total, but there should probably be some minimum specs to shoot for.

thx

[EScanlon]

I guess I'm dumb. I'll need further explanation.

In the pic I'm attaching, you're saying that there will exist a voltage across the two points of the "wire" denoted with RED. But won't you be reading the circuit across the blue line?

Enrique

WIRING.bmp

[Sean240Z]

My rear defogger was not working for quite a while. I finally found a blown inline fuse behind the radio on the passenger side. Replaced the fuse (and the inline fuse holder) and rear defog works great.

My $0.02.

Cheers,

[Daniel]

Enrique , Thats correct. The area between your red lines would show 12 volts because there is a break between the two test probes(red lines).This reading would be across the entire length of that blue line with the break and you have to move your probes until the voltage disappears. That would mean the break is no longer between the probes, therefore the path of least resistance is the silver trace, not the meter. That would mean that area is good. Ok one more example. You have 3 light bulbs sockets that have a + and - test point on them. They also have a pigtail of wire that enable us to connect to the + and - of the battery. We install all three socket + to the pos. and all three - to the neg. We install bulbs in all three sockets. The bulbs glow. At our socket test points we will show less than 12(about6volts or less) voltage , because the lamp is the path of least resistance and we are paralell in the circuit. Now remove ONE lamp. I would now have two glowing lamps that show small voltage at their test point, but the socket without the lamp will read 12v because the meter is now the path of least and is a series circuit. Between the battery pos and the socket + you will show no voltage ,Same with socket - to battery neg. BUT, once we have an open between our probes,that makes our meter the path we will show 12 voltage. Let me know if thats not clear and I'll try again.

[Daniel]

New example.. left side of grid is vertical (+ ) right side is vertical( -)... (+)probe on left vertical. Put( -) probe in the center of top horizonal trace. if its good you are now parallel in the circuit and therefore show reduced (about 6)volts. Row two has a break 4 inches right of the left vertical(+). With the probes on left and center as we did on row 1 we will read 12v as we are now a series connection. By sliding the (-) probe toward the left once we lose 12 we have found the break as is will be at the (-)probe tip. If the break was between the center of row two and the right (-) horizonal we would read 0 volts. By sliding the( -) probe RIGHT this time ,once we read 12 we know we just passed over the break and are again a series circuit. I think thats a better explaination.

[Bambikiller240]

Now I am confused. The wiring diagram from Enrique seems to show the meter (red lines) testing across the BLACK lines. Yes? No? If YES, how does testing the BLACK line that relate to the BLUE line at all, since the BLUE line doesn't appear to be connected to anything? (meter or BLACK lines)

[EScanlon]

Now I am confused. The wiring diagram from Enrique seems to show the meter (red lines) testing across the BLACK lines. Yes? No? If YES, how does testing the BLACK line that relate to the BLUE line at all, since the BLUE line doesn't appear to be connected to anything? (meter or BLACK lines)

The red lines denote where the meter would be put to. The blue line denotes how in a CONTINUITY check, you would get continuity between the two points the meter is hooked up to. I was trying to understand how you would have a voltage at that point if it still had a "wire" between the two leads.

I'm just going to have to go out and check this out with my meter.

Enrique

[Daniel]

Your example I should have used, as it appears to be an early vertical strip window unit. My 73 has horizonal. That would put you power + and - at the top and bottom instead of left and right as in my example. Your photo shows what would be read as 12v. Sure you have continuty all over that window, thats why you search for voltage as it only cares about the top to bottom voltage thru each individual trace. All of those traces make a flat parallel circuit. They are just small wires, your volt meter just acts as a jumper cable across the broken area of that individual wire and therefore reads 12v.