[Mendonmafia]

Light comes on for a few minutes. I hear a small click. I get 12v across the grid at the two wires that connect to the grid. Measured resistance at 12.5 ohm across the grid. I swapped the breaker with the power window breaker (my car has manual windows so not sure why the breaker was there but it was) and still not working. Any ideas? I did a lot of searching but couldn't find a solution. Thanks in advanced!

 

[Z09B4U]

I do not drive the Caprice in winter so I have not really paid any attention to it.

The FSM says the first time you tap the switch you get 10 minutes. After that 5 minutes.

The FSM does not give the amp draw for the defroster but several people have reported it "wore out" the 30 amp breaker.

Using just math:

Measured resistance at 12.5 ohm across the grid.

volts/ohms=amps 12.5V/12.5Ohms=1Amp
VoltsXAmps=Watts 12.5VX1Amp=12.5Watts

I do not think GM would put a 30 amp breaker on a 1 Amp circuit and I doubt 12.5 Watts will heat a back window.
I expect the defroster to draw 15-25 Amps which would be 0.8-0.5 Ohms.
Most meters are not accurate under 1 Ohm.

When you set your meter to Ohms touch the probes together. If the meter does not show zero subtract that number from what you measure on the grid.

From the math I expect the grid should be under 1 Ohm

Another way to check would be to measure the current draw. Not many people have a meter with a 30 Amp range.

For testing only:
If you replace the breaker with a 30 Amp fuse (and it does not blow) the math says the two contacts on the top of the fuse will have [email protected] Amps or [email protected] Amps. 30-50miliVolts requires a good Volt meter.

 

[Mendonmafia]

I do not drive the Caprice in winter so I have not really paid any attention to it.

The FSM says the first time you tap the switch you get 10 minutes. After that 5 minutes.

The FSM does not give the amp draw for the defroster but several people have reported it "wore out" the 30 amp breaker.

Using just math:

volts/ohms=amps 12.5V/12.5Ohms=1Amp
VoltsXAmps=Watts 12.5VX1Amp=12.5Watts

I do not think GM would put a 30 amp breaker on a 1 Amp circuit and I doubt 12.5 Watts will heat a back window.
I expect the defroster to draw 15-25 Amps which would be 0.8-0.5 Ohms.
Most meters are not accurate under 1 Ohm.

When you set your meter to Ohms touch the probes together. If the meter does not show zero subtract that number from what you measure on the grid.

From the math I expect the grid should be under 1 Ohm

Another way to check would be to measure the current draw. Not many people have a meter with a 30 Amp range.

For testing only:
If you replace the breaker with a 30 Amp fuse (and it does not blow) the math says the two contacts on the top of the fuse will have [email protected] Amps or [email protected] Amps. 30-50miliVolts requires a good Volt meter.

I live in Florida so winter here rarely gets below 50 F for the lows but every morning my rear window is covered in dew and it persists after I'm parked at work.

My meter does say 0 ohms when the probes are shorted. I agree with your math so it must mean that most of my grid is open but I cannot see any breaks. Guess I'll need to spend some time checking them closely and attempt to repair them. However I would expect the grid lines that are still good to defrost locally but as far and I can tell none of them do anything. I tried to measure the current draw and it was at most .25 amps but my meter wasn't really measuring anything if 8nhad to guess. When zeroing it out it fluctuates almost that much. I don't have a 30 amp fuse so I didn't test the voltage drop across the fuse but I will probably just test it with a fuse I do have tomorrow. Thanks for the quick reply!

 

[Fred Kiehl]

Check your ground.

 

[Z09B4U]

so it must mean that most of my grid is open but I cannot see any breaks.

Not necessarily.
From a quick read on the forum and personal experience on other cars it could be a connection issue between the electrical connection between the wiring connector and the defroster connector.
More scary are reports of resoldering the connector to the vertical (left/right) grid.

What you might try is to measure the resistance from the outside edge of the pasenger vertical to the drivers vertical.
Again I have not done this on the Caprice but on other older cars the verticals were metal foil with a paint like cover.
Really sharp probes worked on older cars.

 

[Z09B4U]

Found a picture.
The defroster connector (marked with square red) may need to be cleaned at the harness connector.
You may not be making a good contact with the harness plug or the meter probes.
The red dashes _ _ are where the connector solders to the window foil (I think)
What you might try is to measure the resistance from the outside edge of the passenger vertical to the drivers vertical. (green dot area)

 

[Mendonmafia]

I will check resistance direct to the foil tonight. Hopefully I can get to it without to much damage to the coating. I do have sharp probes. I did measure the resistance to the connector tabs that come off the foil and not the wires that connect to them, so maybe the solder joints are bad? Nothing looked bad or felt loose. I also cleaned up the tabs and connectors when I was in there with a bit of Emory.

Not sure the ground would affect the resistance across the grid but I'll check it out again. If I recall the ground is in the trunk on the passenger side? The resistance from the passenger side connector to ground (vehicle body) was about 0 ohms iirc. I checked this about a year ago. It hasn't worked since I bought the car over 4 years ago. Also not working in my 79 z28 but that's another project for another day.

 

[Z09B4U]

Just to provide information on the defroster circuit.
Diagram:

http://www.goldsswagon.com/diagrams/impala_defogger.pdf

The dash button connects to a timer on the back of the HVAC controls.

The Orange wire comes from the breaker. If the defroster only runs for less than five minutes (should run 10 the first time and five after that) the breaker may be damaged. the best check would be to test for power at the timer. If the breaker trips the orange wire will lose power.

In the real world it will be winter and you will not want to pull the dash. So several threads here have suggested substituting a 30 Amp fuse for the breaker. If this results in close to factory times you can replace the breaker. At this time I would suggest a new breaker and several large companies make replacements.

If the timer lights the button for the correct times but the defroster does not heat:
Check the resistance of the defroster with the driver's side window connector unplugged. As I stated above in theory it should be 0.5-0.8 Ohms but most Ohm meters were not designed to read well below 1 Ohm.

If the window resistance checks out or you want a double check of the circuit you can first check for voltage between the two supply wires. FYI damage to wires, connectors, and relay contacts can in rare situations show voltage but will not carry power. In this case you might measure voltage with the defroster disconnected but no voltage when connected.

If you do not find power at the window you can check for power at fuse 44 (heated mirrors). I am not sure if all cars are wired for this. If you do not have heated mirrors there still might be contacts for the fuse. If you find power here there may be a break in the wire between the splice and the rear window.
If there was no power at the mirror fuse the next place you will have to check the back of the HVAC.

You can inspect C200 for damage and check that the purple wire has power from the timer. If you find power here there is a problem between here and the rear window.

 

[HockeyJohn1]

I do not drive the Caprice in winter so I have not really paid any attention to it.

The FSM says the first time you tap the switch you get 10 minutes. After that 5 minutes.

The FSM does not give the amp draw for the defroster but several people have reported it "wore out" the 30 amp breaker.

Using just math:

volts/ohms=amps 12.5V/12.5Ohms=1Amp
VoltsXAmps=Watts 12.5VX1Amp=12.5Watts

I do not think GM would put a 30 amp breaker on a 1 Amp circuit and I doubt 12.5 Watts will heat a back window.
I expect the defroster to draw 15-25 Amps which would be 0.8-0.5 Ohms.
Most meters are not accurate under 1 Ohm.

When you set your meter to Ohms touch the probes together. If the meter does not show zero subtract that number from what you measure on the grid.

From the math I expect the grid should be under 1 Ohm

Another way to check would be to measure the current draw. Not many people have a meter with a 30 Amp range.

For testing only:
If you replace the breaker with a 30 Amp fuse (and it does not blow) the math says the two contacts on the top of the fuse will have [email protected] Amps or [email protected] Amps. 30-50miliVolts requires a good Volt meter.

Light comes on for a few minutes. I hear a small click. I get 12v across the grid at the two wires that connect to the grid. Measured resistance at 12.5 ohm across the grid. I swapped the breaker with the power window breaker (my car has manual windows so not sure why the breaker was there but it was) and still not working. Any ideas? I did a lot of searching but couldn't find a solution. Thanks in advanced!

Light comes on for a few minutes. I hear a small click. I get 12v across the grid at the two wires that connect to the grid. Measured resistance at 12.5 ohm across the grid. I swapped the breaker with the power window breaker (my car has manual windows so not sure why the breaker was there but it was) and still not working. Any ideas? I did a lot of searching but couldn't find a solution. Thanks in advanced!