Rear Defroster / Fuse Box ... How boned am I? - JeepForum.com
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post #1 of 47 Old 12-05-2016, 09:47 PM Thread Starter
regrebierf
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Rear Defroster / Fuse Box ... How boned am I?

So, it is getting to be that time of year where things get a bit hazy in the morning. This is my first winter with the Jeep, and I have never really needed to use the rear defroster until now. To give you some info, 1998 Sahara with Hard Top from factory. My build sheet (attained online from VIN) says that it came with the soft and hard top from the factory, but I have never seen the soft top, as I am the third owner. Unclear which it left the factory with.

Anywho, I get in the car this morning and go to turn it on and the switch did not stick, nor did the indicator light come on. It's a little cold outside and I am a student with no garage, so I didn't exactly have the time or facilities to take the dash apart. So, I decide to quickly look at the fuse and notice that there is not fuse in the #16 slot, where my rear defroster fuse should be. Upon further investigation, I find that there are no retainers either *refer to pictures*.

This brings up a few questions...
  1. Can I fix the fuse slot?
  2. If so, how can I fix the fuse slot?
  3. If not, do I need to get a new fuse block / suck it up and not have rear defrosters?
  4. Would the fuse not being there cause my switch to return (not work) and not light up?
  5. How boned am I?

Any help would be much appreciated...

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post #2 of 47 Old 12-06-2016, 02:35 AM
Jonny Jeep
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Fuse 16 is for the rear fog lamps and headlamp levelers fitted to export models. Your rear demist switch is supplied by fuse 17 (10A) HBL SW means Heated Back Light Switch.

The switch is non latching. It activates a timer in the cluster which turns on the demist relay in the under hood PDC (Power Distribution Center). There should also be a 40A fuse in slot 15 in the PDC. Without that fuse and relay the switch will not illuminate and the demister will not work.
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post #3 of 47 Old 12-06-2016, 06:54 AM Thread Starter
regrebierf
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You just blew my mind, and brought me much relief there, Johnny Jeep. Thanks so much! I'll investigate today after classes and let everyone know what the deal is. Why would the not put the rear defrosted as "REAR DEFR" or something?!
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post #4 of 47 Old 12-06-2016, 07:44 AM
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Quote:
Originally Posted by regrebierf View Post
You just blew my mind, and brought me much relief there, Johnny Jeep. Thanks so much! I'll investigate today after classes and let everyone know what the deal is. Why would the not put the rear defrosted as "REAR DEFR" or something?!
For the same reason that what we call a drive shaft is actually a propeller shaft. Just because we call something by a name doesn't mean that is actually what it is. If you compare the front defrost system to the rear, you'll note that one uses warm air and the other uses a heated element to melt ice and remove condensation thus the Heated Back Light nomenclature. I suspect it is easier for folks that used to be in construction to understand since all doors with glass in them are referred to by the number of lights they have. Pretty similar to the use of the word "bulb" when referring to what are actually lamps and we really mess that one up when we talk about head lights.

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post #5 of 47 Old 12-06-2016, 09:56 AM
Jerry Bransford
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Quote:
Originally Posted by mrblaine View Post
...I suspect it is easier for folks that used to be in construction to understand since all doors with glass in them are referred to by the number of lights they have.
All the other informational was educational, this one has me scratching my head though.

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post #6 of 47 Old 12-06-2016, 04:23 PM Thread Starter
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Update: I checked under the hood and the fuse and relay appear to be okay. The fuse in the defroster spot was different from the other fuses (all the others were the the same, so I assume stock...), as it had a little S (like the smaller fuses) instead of a plate. Still rated at 40 amps and not blown, so I presume it is okay.Not sure if this means there have been issues with the defroster before I owned it, though, as it's difference is an indication it has been replaced. Who knows?! I pulled two fuses to see if the heat marks on the prongs is what the issue is, and they both had it.

Next course of action was to look at the lift gate, itself. I am no electrician, but all of the wires are intact and none appear to be frayed or cut. The only suspect thing I found was that one of the "clamps" (for lack of a better phrase) was a bit bent. Sooooo, just a bad switch? How do I test this to see if the switch works? I have a multimeter...
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post #7 of 47 Old 12-06-2016, 06:18 PM
Jerry Bransford
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Do a continuity test across the rear window heater wires with your multimeter, make sure there is good continuity. Sometimes the painted on heater or its connections are damaged and lose continuity. You can buy a repair kit for the rear heater element is damaged.

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post #8 of 47 Old 12-06-2016, 09:11 PM Thread Starter
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Jerry... As I mentioned before, I am no electrician. So, just to clear things up, I would flip the switch in the front, then go to the back and see if there is current running? Where exactly do I put the leads?
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post #9 of 47 Old 12-06-2016, 09:31 PM
Jerry Bransford
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Set your multimeter to its RX1 setting, that is how you measure resistance. With the power off, connect the meter leads to the defogger's electrical connectors on the glass and see if there is a connection (continuity) between them. There must be continuity between them for everything to work.

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post #10 of 47 Old 12-06-2016, 10:13 PM
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What jerry said & if good check for power at the element.

As I read the FSM, the switch indicator needs fuse 17 to light the indicator light but if everything else is in order power can still be going to the element even if the indicator is not on (bad or missing fuse 17 or bad indicator bulb). You should check for 12v power to the grey wire at the back glass using the DC voltage scale with the red lead on the grey wire terminal & the black one well grounded. Since it is a momentary on/off switch you need to check it after one click & if nothing click it one more time & recheck. If not working it is possible that the timer in the cluster is bad & not sending a grounding trigger to the relay.
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post #11 of 47 Old 12-06-2016, 11:29 PM Thread Starter
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Jerry, thanks for the clarification!

Rubi, I am an idiot. Should have looked at the FSM before coming here. Learned a lot about the function and testing of the whole circuit. Thanks. For future reference of anyone who looks at this post, here is the little section I am talking about.
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post #12 of 47 Old 12-07-2016, 02:49 PM Thread Starter
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Jerry and Rubi: With the help of you guys and the FSM, I went through some continuity and voltage tests. The first thing I did was test continuity between the electrical connectors on the window. The DMM read about 0.7 Ohms, which seems a bit low to me, but there was a reading.

So, next thing I did (per the FSM - pg 8N-2) was check the voltage to the same terminals after putting the ignition in the 'on' position and hitting the defogger switch... and it read no voltage. The FSM then says to check the fuses in the PDC, which I already did and confirmed they are all good. Moving forward, it says to turn the key into the on position and hit the switch. If the indicator light does come on, go to the Defogger Relay diagnosis section.

On to the Defogger Relay Test we go! I pull the relay and check all the terminals and cavities for corrosion. No issues. Then, I check the continuity of the relay terminals:

  • Continuity between 87A & 30 --- 0.08 Ohms
  • No continuity between 87 & 30 --- check
  • Resistance of 75 +/- 10 Ohms between 85 & 86 --- 69.35 Ohms

It tells you to connect a battery to terminals 85 & 86, but I don't have any alligator clips or wire or 5 hands to really do that well, so I skipped it. On to the Relay Circuit Test. This is where one question comes to mind... I am not exactly sure how to test if the terminal cavity is "hot". Basically, what I did was turn on the Jeep and put my negative lead to the negative battery terminal and the positive lead on the terminal cavity. I did this for both 30 and 85. Please correct me if I am wrong and I will re-do the test.

  • Terminal cavity 30 is "hot" --- yes, read 14V with my method
  • Battery voltage at 85 when ignition on --- yes

On pg. 8N-4 of the FSM, it says "The relay normally open terminal (87) is connected to the common feed terminal (30) in the energized position. This terminal supplies battery voltage to the rear glass heating grid and the defogger switch indicator lamp. There should be continuity between the cavity for relay terminal 87 and the rear window defogger relay output circuit cavities of the rear glass heating grid and defogger switch connectors at all times. If OK, go to Step 4. If not OK, repair the open circuit(s) as required."

So, where the heck am I supposed to put my DMM leads? What terminal cavity goes to the "output circuit"? This is the one I presume I will be connecting to cavity 87 and checking for continuity...

I tested for continuity between cavities 30 & 87 and there was NO CONTINUITY. Not sure what this means, or if it is even the right thing to do, but this is where I hit the wall of knowledge. Please help me make up for my lack of knowledge.
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post #13 of 47 Old 12-07-2016, 02:53 PM
Jerry Bransford
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I'm on my way out so I can't write much but that .7 Ohms for the heating element is about right. It gets hot because it is very nearly a dead short across 12 volts.

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post #14 of 47 Old 12-07-2016, 03:06 PM Thread Starter
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Oh... duh! I am forgetting some basic principles here. Thanks for the quick reply. I am eager to find out what the heck is going on here.
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post #15 of 47 Old 12-07-2016, 09:30 PM
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There would only be continuity between 30 & 87 when the relay is closed. So your test verified that, no continuity when no power, ground to 85 & 86 & when power & ground are supplied to 85 & 86 it should close & get continuity (connecting 30 to 87). Then they are saying to be sure that power (from 87) is getting to the back. 87 is the output terminal.

This all depends on the relay actually getting power & ground signals to 85 & 86. You got power to 85 (meaning it is hot) but the real issue is whether you are getting a ground signal when you hit the switch. That signal is sent momentarily through the switch to the cluster timer & that grounds the relay coil, 86 in this case.

You actually jumped ahead to the relay test, which appears to test good, when the FSM has you testing the switch first. Then the relay, then the cluster. I would actually have you test the cluster after the switch, then the relay but not that important. My experience says if you are not getting power to the back glass/indicator light & both fuses are good the cluster timer is the most suspect. If the timer is bad there are external timers you can use to bypass the cluster timer.
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