# Rear defogger = not working



## 94GA16DE (Apr 29, 2005)

For a while now my rear defogger has been intermittenly working, mostly when I don't need it to. I got in the habbit of pressing the button whenever I got in the car to see if I could get lucky and it would work. It stopped working altogether the other day, and as it happens, living in upstate NY, its getting a bit cold here and I would like it to work. I initially thought it was the relay, but considering the switch was fairly cheap as well, I ordered and installed both today with no luck. It was then that I decided to look further into the wiring and came across the term "Timing Control Unit" upon further inspection I found it refered to as a SMJ or Super Multiple Junction. I searched the forums and found it to be described as the white box behind the fuse block. Is this the correct box? Although initially thought to be unrelated, my interior light dimmer switch does not work either, after taking the switch apart, it appeared that the connections were fine. I'm wondering if anyone else has had problems with this unit or knows if this will solve my problems, I have traced the wires for the rear defroster from the rear to the front of the car and have found no breaks, the connections on the back of the windows is fine w/ factory connections. Just trying to get my rear defroster back before winter hits. Thanks in advance


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## jakehale (Aug 8, 2004)

*Well*



94GA16DE said:


> Thanks in advance



Well the interior dimmer is (from what I heard) a very common break down part. very easy to replace.
As for the defrost. Have you checked the reistance in your wire grid? the Tabs that the wires connect to could be loose or there could be a break in the grid itself. Very easy to check. Just get a cheapo volmeter and set it for ohms and check for 0. if your getting anything but zero then you have a bad connection back there..

good luck and keep us posted with what you find. Helps others in the furture when they do searches and come across old posts..

:cheers:


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## 94GA16DE (Apr 29, 2005)

I did check the embedded wires in the window, everything checks out, none of the wires have any breaks, and I checked the connections at the window to the wire at each end and they are good connections, no corrosion. The fact that it did work before on and off tells me that its not a connection problem with wiring, but more of a relay related issue. Unless wires are really old, subject to weathering, crimped in a door, or otherwise, its really not a wear issue, I mean that its not a moving part. I mistakenly thought that there was just the one relay that controlled the rear defrost. The secondary timer that automatically shuts off the rear defrost is within the Timing Control Unit, which is basically a timed secondary relay. You push the switch which tells the first "loud" brown square relay next to the fuse block to turn on, it switches on the timer relay which in turn makes the second relay in the big brown relay to turn on which finally turns on the rear defroster. That is my take on the wiring diagrams which I have been over and over and over the last few days trying to straighten it out. I'll keep everyone posted with my results, and as for the dimmer switch I have an alternate that I haven't toyed with yet, it was a five finger discounted item at the local pick n pull.


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## 94GA16DE (Apr 29, 2005)

well i went to the pick n pull yard today and I got myself a couple of Timing Control Units, for those of you who don't know what it looks like/where it is, it is attached to the driver side under dashboard metal bracing. You remove four screws at each corner of the plastic under dash and pull it off. Then remove the under dash metal bracing. Depending on what year your car is there will either be 8 or 6 screws holding it in. When you remove the metal brace, there is a black box about 2" x 4" in size attached to the back of the metal bracing, on it will say "Timing Control Unit" and then 28550 64Y00 and made by UTA, other models I found in the yard had similar boxes however they had a different number of pins on the connector and the boxes were white in color.

The Timing Control Unit is what fixed the problem, the rear defrost now works and I am on to my next issue which is determining why the dimmer switch fails to do anything. So if you are having an issue with your rear defrost intermittenly working, try changing your TCU, I have access to 2 others that I took off of other cars and I still have the extra one that I bought if anyone is in need of one. Just for reference it takes about 2 minutes to remove from a car in the yard.


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## blackmaxima88 (Mar 20, 2005)

that makes sense too, cos the time control unit automatically shuts the defogger off after like 15 minutes.

as for your dimmer, I'd just replace the control knob. mine cuts out intermittently when I have the lights on at night, and suddenly everything goes dark. the potentiometer inside the dimmers wear down over time, and the "shavings" from the variable resistor get on the electrical contacts inside and screw stuff up. ever have an old radio that made a loud crackling noise whenever you adjusted the volume knob? I'm sure everyone's experienced this. it's the same thing, those knobs just wear down. you can probably get one from a junkyard, e-bay, maybe even at an auto parts store (we sold them for old fords). or if nothing else, a nissan dealer. that's a really easy fix, since there aren't any computers or control units for the instrument panel lights. a new dimmer will fix your problem. I lay money on it.


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