Poor performance from rear window defroster

lunchbox3

lunchbox3

Member
Joined
Nov 12, 2018
Messages
90
Location
Denver
I'm trying to get my rear defogger to work. As far as I can tell, the lines might just barely warmer than the glass (I'm not even sure if I was just imagining it) but not warm enough to actually work.

Here's the situation:

-The light on the rear defogger switch illuminates when I press it. (I think this rules out a problem before the relay?)

-With the defogger switch "on" and illuminated, the voltage measured between the two "terminals" on the rear window was about 9 volts, though one reading I got 10 volts.

-The fuse in the glovebox panel looks fine and passed a basic continuity test with the multimeter. I haven't tried replacing it.

-I tested the conductivity of the strips on the glass, they all seem fine.

The FSM says to test part of the instrument cluster and the relay in the engine compartment. I'll test the relay, but here's a thought for you all - could this be dirty connectors at the rear window causing the undervoltage and thus the shitty performance? Or is this 10 volt reading just a red herring because I'm incompetent and can't use a multimeter?

Suggestions for cleaning the connectors?

Thoughts? Where should I go from here?

Edit: it's a '97 TJ and as far as I can tell it's got the same hardtop it had from factory, and (obviously) is factory wired for it.
 
Last edited:
Use a voltmeter to test voltage at the plugs and check the conductivity of the wires going through the glass.
 
TJ_2000 said:
Use a voltmeter to test voltage at the plugs and check the conductivity of the wires going through the glass.
Click to expand...
Sorry TJ, I forgot to mention that I had done the conductivity test - I updated my original post to include that.

How should I check the voltage at the plugs - just stick the probes one in each plug, while the plugs are disconnected from the window? I was worried that would have too high amperage for the multimeter, but I barely understand how to test this stuff.
 
lunchbox3 said:
Sorry TJ, I forgot to mention that I had done the conductivity test - I updated my original post to include that.

How should I check the voltage at the plugs - just stick the probes one in each plug, while the plugs are disconnected from the window? I was worried that would have too high amperage for the multimeter, but I barely understand how to test this stuff.
Click to expand...

Where the wires lead into the plastic plug there is room for a test lead to fit. I just did all this the other day trying to fix my defrost as well.
 
Thanks for the heads up. I’m getting 9.5 volts at the plugs, battery reading 12.5 on the gauge. Does that indicate an issue with the relay? Maybe the resistance has been thrown out of wack?
 
This is interesting. When I tested the plugs disconnected from the window I got 12.3. When I plugged them back in and tested I got 9 something. Hmmm...
 
On one of my old vehicles my rear defroster didn't perform very well anymore. I found it was due to the little lines on the glass having been partially chipped away / eroded over time. A new window fixed it 100%.
 
TJ_2000 said:
This is interesting. When I tested the plugs disconnected from the window I got 12.3. When I plugged them back in and tested I got 9 something. Hmmm...
Click to expand...
Yeah same thing here. When I first tested I actually got 6.something. I used some steel wool on the window terminals and that’s when it jumped to nine.

It works now, though, even with the nine reading. I think it was an issue of cleaning and reseating the terminals. I’m gonna pick up some connector cleaner and clean the terminals and the spades, and maybe throw some dielectric grease on there too. But as of now, even with the 9.6v reading, they’re working fine.
 
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