How to fix the rear defroster?
10-01-2003, 10:42 AM
#1
Burning Brakes
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How to fix the rear defroster?
My rear defroster doesn't work. I already know the heater wires glue to the hatch glass are corroded, and they have too high resistance.
Can you buy somewhere a metal pattern that can be glued on top of the original one? Or maybe a roll of metal (heater) tape with adhesive so that it can be layered on top of the old one?
Thanks, Stan
Can you buy somewhere a metal pattern that can be glued on top of the original one? Or maybe a roll of metal (heater) tape with adhesive so that it can be layered on top of the old one?
Thanks, Stan
10-01-2003, 04:48 PM
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More to this than you may think.
The connectors to the grid are soldered on not glued.
The power for the grid comes from the drivers side wire to the strut (gas spring). The strut conducts the current to the wire at the hatch end.
The grid is a resistor and heats up as the current flows through it to the other strut.
Here's the secret; the passenger side strut is a conductor like the other side BUT the pillar end has a contact in the socket and grounds out to the ball stud on the car.
That silly little wire is NOT the ground for the grid. It is the ground for the cargo area light. The PS strut has either a mechanical contact or a mercury contact switch on it's end just for the cargo light. So you have to buy a right and a left strut when you replace them.
There is sufficent wire in the pillar to repair any prolems with the grid power wire at the DS pillar. Use the original connectors even if you have to solder the wire to it.
The connectors to the grid are soldered on not glued.
The power for the grid comes from the drivers side wire to the strut (gas spring). The strut conducts the current to the wire at the hatch end.
The grid is a resistor and heats up as the current flows through it to the other strut.
Here's the secret; the passenger side strut is a conductor like the other side BUT the pillar end has a contact in the socket and grounds out to the ball stud on the car.
That silly little wire is NOT the ground for the grid. It is the ground for the cargo area light. The PS strut has either a mechanical contact or a mercury contact switch on it's end just for the cargo light. So you have to buy a right and a left strut when you replace them.
There is sufficent wire in the pillar to repair any prolems with the grid power wire at the DS pillar. Use the original connectors even if you have to solder the wire to it.
10-01-2003, 05:11 PM
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Burning Brakes
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Thanks, but I was missunderstood. It is the heater grid (resistor) that is defective, and needs to be replaced. The resistance is too high to allow any meaningful amount of heat being generated under 12V. I need a resistive heater tape, or the actual heater grid.
10-01-2003, 06:01 PM
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Re: Re: How to fix the rear defroster?
Originally posted by CR_944
SoCal, thanks for that info, I've been wondering how this worked for a long time.!!!
SoCal, thanks for that info, I've been wondering how this worked for a long time.!!!
"Everything is simple -- once you understand it."
10-01-2003, 06:03 PM
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Originally posted by Stan944
Thanks, but I was missunderstood. It is the heater grid (resistor) that is defective, and needs to be replaced. The resistance is too high to allow any meaningful amount of heat being generated under 12V. I need a resistive heater tape, or the actual heater grid.
Thanks, but I was missunderstood. It is the heater grid (resistor) that is defective, and needs to be replaced. The resistance is too high to allow any meaningful amount of heat being generated under 12V. I need a resistive heater tape, or the actual heater grid.
Still I'm wondering if the connectors to the grid are the problems.
Did you use an ohm meter to check each of the small grid "wires"?
I have only seen two or three grid wires "broken" on 944's.
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10-02-2003, 11:06 AM
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Burning Brakes
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Thanks for the replies.
I need to replace the whole grid. The grid wires are uniformly corroded (my car is 20 years old!).
There is 12V on the grid (and it doesn't produce any measurable heat), which means it is the grid at fault.
It was a while since I checked the resistance - don't remeber the value now.
Anyway, power = U^2 / R, here (12V)^2 / R.
If the resistance R is too high, there is little power generated.
The paint would be a great suggesten. Unfortunately, it's impractical to paint the whole grid.
I need to replace the whole grid. The grid wires are uniformly corroded (my car is 20 years old!).
There is 12V on the grid (and it doesn't produce any measurable heat), which means it is the grid at fault.
It was a while since I checked the resistance - don't remeber the value now.
Anyway, power = U^2 / R, here (12V)^2 / R.
If the resistance R is too high, there is little power generated.
The paint would be a great suggesten. Unfortunately, it's impractical to paint the whole grid.
10-02-2003, 01:07 PM
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Must have had the hatch "tinted" with the plastic. Removing the plastic with the wrong tools can damage the grid lines.
Still I don't think we are communicating on what can and does cause the heating grid NOT to work.
You have to check each of the grid wires or traces with an continuity meter. Ohm meters have this setting. If there is continuity for most of the traces then the grid should heat up when 12 volts is directed through it.
If you find a grid trace that is open, no continuity, then you have to find the spot where the grid trace is broken. In my 83 I found three breaks when I bought the car in 85. I've had none since.
Still I think we need to back up to square one. There should be 12 volts at the electrical connector coming out of the drivers side (DS) pillar. There should be 12 volts at the electrical jumper from the end of the DS strut to the grid connector. From the grid connector there should be 12 volts on the buss bar before the grid traces.
If any of these test points show 0 volts then you have to fix this.
On the other side, passenger side (PS), You test the continuity of the grid connector to ground. If there is no continuity, an open circuit, then the jumper between the PS grid connector and the strut or even the pillar end of the strut where it contacts the ball stud is a concern.
NOTE: The struts are rights and lefts, DS and PS. The reason is the DS strut is an isolated conductor and the PS strut grounds at the ball stud on the pillar.
The grid connectors for the jumper from the struts are soldered on. These can be resolderd with a very hot soldering gun. Don't give it more heat than necessary and hold the connector in place with a small screwdriver till the solder cools.
Still I don't think we are communicating on what can and does cause the heating grid NOT to work.
You have to check each of the grid wires or traces with an continuity meter. Ohm meters have this setting. If there is continuity for most of the traces then the grid should heat up when 12 volts is directed through it.
If you find a grid trace that is open, no continuity, then you have to find the spot where the grid trace is broken. In my 83 I found three breaks when I bought the car in 85. I've had none since.
Still I think we need to back up to square one. There should be 12 volts at the electrical connector coming out of the drivers side (DS) pillar. There should be 12 volts at the electrical jumper from the end of the DS strut to the grid connector. From the grid connector there should be 12 volts on the buss bar before the grid traces.
If any of these test points show 0 volts then you have to fix this.
On the other side, passenger side (PS), You test the continuity of the grid connector to ground. If there is no continuity, an open circuit, then the jumper between the PS grid connector and the strut or even the pillar end of the strut where it contacts the ball stud is a concern.
NOTE: The struts are rights and lefts, DS and PS. The reason is the DS strut is an isolated conductor and the PS strut grounds at the ball stud on the pillar.
The grid connectors for the jumper from the struts are soldered on. These can be resolderd with a very hot soldering gun. Don't give it more heat than necessary and hold the connector in place with a small screwdriver till the solder cools.
10-02-2003, 01:55 PM
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I actually had a bad wire coming out of the DS pillar and one of the grid connectors came off while I was test the darn thing. Now it works better that the front defrosters! The next fix-it on the list.
10-02-2003, 02:11 PM
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Originally posted by Manning
Do you actually ever have to use a defroster in Costa Mesa?
Do you actually ever have to use a defroster in Costa Mesa?
10-04-2003, 06:17 PM
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If you measured 12V on the actual resistor grid, then you must be right, but I have never seen uniform corrosion! Try the old kettle test: put a boiling kettle behind the rear seat, wait until the window is all fogged up, then turn off the kettle and turn on the defrost power. Do you really see lines of defrost slowly and evenly expanding away from all the wires?
Another option, of course, is to mount a small fan behind the rear seat and hook it up to the defroster wires. That was all I ever had for my first 30 years of driving! It works.
Another option, of course, is to mount a small fan behind the rear seat and hook it up to the defroster wires. That was all I ever had for my first 30 years of driving! It works.
