Backup Lights out ; rear fuse #32 blown

CajunWon

Well-Known Member
#1
Thanks to the below quote from 2013, was able to verify under hood fuses 27 gets power when key-On, and #4 gets power when transmission in Reverse.
Fuse #32 in rear fuse box (under seat) has blown and a new fuse blows within 5 seconds.
The b u lamps are 7watt LED and test good.
Trailer wiring and backup wiring look normal.
Could use guidance on how to troubleshoot for a short / crossed wiring.

Front fuse #27 gets power from the ignition switch RUN/START circuit. Then it goes through the Reverse switch on the tranny, and then through front fuse #4, and then to the light green wire (A) on the trailer connector. Does that work? That proves the reverse switch on the tranny operates.

Rear fuse #32 gets constant power from the battery in all ignition switch modes, so if there's no 12V on either side of that fuse, then either the fuse socket is erratic (twist the fuse blades slightly to improve the contact) or else your front 125A megafuse is dead, and I'm certain the plethora of other failures would make you mention those along with the backup lights being dead.

See how you can trace a bad section of wiring if only you know the voltages? :thumbsup::thumbsup:
 

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budwich

Well-Known Member
#2
basically, just disconnect at the "mirror system" (rear fuse box) as shown in your attachment. That eliminates the mirror system and rear tail light stuff. If the fuse doesn't blow, your problem is from the rear back. You can pull the plugs further back to see if the mirror system is an issue as opposed to the rear stuff. Note: if the fuse blows with that stuff disconnected, then the body module might have an issue as there is other stuff there beside what is shown.
 
#3
Have you tried regular bulbs? Maybe the LED are possibly bad or reversed causing the fuse to blow.
 
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CajunWon

CajunWon

Well-Known Member
#4
The LEDs have been in there >5 years; both stopped working at the same time (within the last 3 days); both light-up with 12v. I did put the oem bulbs in at first, no joy. I put in reverse and quickly tapped with a new #32 fuse and the reverse lights do work. So I just need to find the short.

The side mirrors still function without a #32 rear fuse. Tested the trailer plug, 2 pins are grounded (in addition to the ground pin), never tested before but doesn't sound right. The wires and conduit are in good condition, as far as I can follow.

Fuse #32 must only deal with Backup lamps and trailer harness through TBC#5 (Truck Body Controller 5). Everything else seems to work. Where is this controller?
 
#5
I think the mirror connection is only to signal the BCM to lower the passenger mirror automatically when reversing. I have that option on my Saab (which I turned off since it just find it annoying more than useful).
 
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CajunWon

CajunWon

Well-Known Member
#6
Tested the downstream side of Fuse #32 to ground. It's not a completely closed connection, ohm meter measures a factor of 5. With battery disconnected, the upstream side of fuse #32 is also grounded, ohm meter measures a factor of 10. The reverse lights and trailer wiring ohm meter measures a factor of 5.

I lifted the rear fuse box out of its cradle, there are 3 modules attached to bottom, don't see how this fuse box can be opened for further inspection.

Don't understand how this works: Underhood fuse#4 gets power from transmission in reverse position, but that doesn't turn on the reverse lights. Rear fuse#32 always has power. So why does fuse#32 blow only when in reverse, which also disables the reverse lights, yet fuse#4 does not blow?
And how can I locate this TBC#5?
 
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#7
Don't understand how this works: Underhood fuse#4 gets power from transmission in reverse position, but that doesn't turn on the reverse lights. Rear fuse#32 always has power. So why does fuse#32 blow only when in reverse, which also disables the reverse lights, yet fuse#4 does not blow?
And how can I locate this TBC#5?
I'm wondering if possibly there's an internal problem in the BCM? As best as I can tell the 'controller' IS fuse #32. Maybe try unplugging the harness exiting the BCM, and testing pin C3, since it's downstream of fuse #32 and see if it grounds out, or is open? Looking at the diagram, I think it should be open, but maybe it's grounding out somewhere, and that's causing your short?
 

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budwich

Well-Known Member
#8
^ "I'm wondering if possibly there's an internal problem in the BCM".... your testing / measuring has been done on a "non-operatring" bcm (ie. no command to the bcm to activate the backup lights). Hence all your measurements are good ... why? because as you know, fuse 32 doesn't blow when in this mode. You need to pull the bcm and look at the module. You will likely find your answer. Not sure how the bcm closes the power to the lights (ie. relay or FET power device) but it would appear that potentially has a problem... although I have a question on how you are measuring the wiring going to the backup lights .... where are you measuring that? and how? disconnecting at the fuse block?
 
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CajunWon

CajunWon

Well-Known Member
#9
re: measuring wiring to reverse lights. Remove the bulb from each side, measuring continuity from plug to downstream side of fuse#32 (fuse removed) with key off. Should have also tried with key-ON and in reverse.

What exactly the BCM? is this the entire fuse block, or are each of the 4 modules below the rear fuse block a BCM?
 
#10
What exactly the BCM? is this the entire fuse block, or are each of the 4 modules below the rear fuse block a BCM?
With the fuse block cover off, towards the front of the block there's a module with a ribbon cable going into it. That's the BCM.

Edit: The item to the left of the pic here...

 
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CajunWon

CajunWon

Well-Known Member
#11
Fixed! But back to oem darkness.
Replaced both with the original bulbs, and it works! Even with just 1 LED will pop the fuse. Odd this hasn't been an issue for 5 years. Perhaps the LEDs have worn-out some internal component which protects the fuse. But they light up fine with 12v

How can I use these 7w LEDs so I can see while backing?
 

coolasice

Well-Known Member
#12
Have you checked the amperage draw through them? It may be more then 7w (.6a). The limiting resistor may have shorted
 

budwich

Well-Known Member
#14
what do they measure for resistance with your "dime store AVM"... :smile: make sure you "bench mark" the reading first with the test leads touched together to see what they read first so that can be subtracted later on to get a "true reading".
 

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