moosemobile

Thanks 157db, now I need a little education. Since there is not a fuse for the trunk light in the SJB, how would replacing it, or it being toast affect the circuit? How do I determine that the SJB is what needs to be replaced? Could it be that the BEC is toast instead, and how can I know which one is bad?

157dB

If you look at the diagram linked to above (click the highlighted text)
you will see the power (a little line drawn on the image) comes from the BEC
and THRU the SJB via the low power LOGIC BOARD.
Thats whats a toast.
The BEC is just that, a BUSSED ELECTRICAL CENTER.
It distributes fused power. It can be corrodded, but not 'fried'.
The SJB is more like an electrical component with items that
switch on and off items. Thats the LOGIC part of it.

moosemobile

Thanks 157db for all the help and the crash course in mustang fuse boxes 101. Armed with your tutorial, I went out to make my last stand against the forces of electrical evilness and this is what happened:
After checking the Bec to confirm voltage at fuse 59, I put the the voltmeter on the plug prongs for the trunk light again and it still showed just .2 volts. Then I pressed the trunk switch and the voltage did not change. I know the switch works, so with it not having any effect on the plug my guess was that the Sjb was indeed toast. I started buttoning things up and called my son over to show him what I'd found. Then I showed him the rapid pushing of the switch and how the test light would flicker on. Then, on a whim, I plugged it all together, put the bulb in (no light) and rapidly pressed the switch. The light came on!!!, but would go out shortly after. I took off the plastic lens, connected jumper wires from the plug to the bulb, and it works perfectly, and it measures 11.8 volts. Apparently there is a slight break somewhere in the metal in the clear plastic lens/lamp holder that can not be seen.

Couple of things I find interesting:
1. When I disconnected one side of the bulb and then reattached it, the light would not come back on until I pushed the switch (once). So, if anyone replaces their trunk bulb, you'll have to push the switch before you can tell if the new bulb is working or not.
2. The Sbj apparently can tell that the bulb is not connected, and therefore does not send the 12 volts to the plug until it is, meaning the circuit can not be tested without a load on it.
3. The trunk release handle does indeed glow in the dark.

As for the defroster, I found that the contacts were badly corroded and loose. After some sanding and crimping, the test light lights up all the time. I will have to wait until morning to put it to the test, but the test light lit up on all points of the contact pad on the passenger (hot) side when tested to ground and also when tested to the drivers side contact pad, so I have high hopes!

Now, if I can just stop that water leak........

Thanks again to 157db and to all of you that offered advice. It was all helpful, invaluable and appreciated.

moosemobile

Crap. The window didn't clear.

Stoenr

Ok guys, picked up some LED's from Amazon, as Ive had good luck with the 194 style bulbs, I figured I would try them on the Festoon style that goes in the trunk bulb.
The LED only lights up for a second then goes out. Cycling the trunk switch results in the same, a one second light up.
Stock bulb works normal.
I am thinking the SJB is not seeing enough voltage go through and killing power with the LED?

These are the ones I picked up

CJ 06

Was having the same problem with my trunk light inoperative and followed this post and bam, got mine to work also. First I replaced the plastic lens but at the same time I inserted an LED. For the past week I've been chasing through the wires and this post. Today on a whim I tried the regular incandescent bulb, put it in, pressed the trunk switch once and waalaa, it's working. I'm guessing that there's not enough voltage drop using an LED which is kind of strange seeing I have LEDs in the map lights, trunk break light and license plate. But to the people from years ago that placed this post, thank you kindly

barry711

LED’S have such a low current draw that your switch contacts can oxidize add quit working-long term

Joshmittie

I had a working light stopped no reason checked bulb split broken tried meter and bulb no luck but didn't leave in so probably fixed because of you and don't need to loose hours tracing wires