[Dr_Solstice]

I agree. It appears to be isolated. There are no other electrical issues that I've come across. The car is salvage/rebuilt but very low miles The drivetran is in good shape. The total loss was due to a tree damaging the hood and top during COVID. There is no evidence of any electrical tinkering except maybe the rebuilder swapping in leather seats with the power raise I'm not sure if that was an option on base models.

DrS

 

[HHGadget]

Having had the tranny out twice now,
I can tell you the wiring harness is held in by vinyl covered metal tabs that are folded over the harness to keep it in place. It is almost entirely a “by feel” job as without a snake camera it is hard to figure out.
I had a camera and was able to scope out what i was doing first but still had to feel my way through it. It is tight Nd if you are on your back looking up at the tranny you will need safety glasses as all the tiny little dirat and road debris will shower down while you are feeling around.

 

[HHGadget]

Here is a photo of the manual trans out.
if you loosely at the gearbox side on the right you will see one sensor connector Nd the blCk vinyl coated prongs i mentioned above.

 

[Dr_Solstice]

So you guys got me thinking. There are two pink wires in the harness at C1 @ A10 and C12. I did not disassemble the fuse box and I wonder if there is continuity coming from fuse 46 to A10. It seems plausible that connection could be broken. I tested one of the pink wires at C1 but I can’t say for sure it was A10 that goes to the backup switch. I’m going to try to check that connection tomorrow before I go back to the shop to have a wire run. If that indeed is the issue I’ll get an add a fuse block to bypass the fusebox and jump the load side of fuse 46 to the proper pink wire. With any luck that will be the solution.

 

[JohnWR]

So you guys got me thinking. There are two pink wires in the harness at C1 @ A10 and C12. I did not disassemble the fuse box and I wonder if there is continuity coming from fuse 46 to A10. It seems plausible that connection could be broken. I tested one of the pink wires at C1 but I can’t say for sure it was A10 that goes to the backup switch. I’m going to try to check that connection tomorrow before I go back to the shop to have a wire run. If that indeed is the issue I’ll get an add a fuse block to bypass the fusebox and jump the load side of fuse 46 to the proper pink wire. With any luck that will be the solution.

The pink wire at C12 is Ignition Voltage 1 so is the same base circuit as A10, but does not route through Fuse 46.

Didn't you state in Post 17 that you have 12V at A10?

If you have 12V at A10, but do not have it at C4 when in reverse, the problem lies in the wires to the reverse switch, or in the revse switch itself.

You could try connecting a fused jumper from A10 to C4 with the car powered to see if the lights turn on. That would completely isolate the problem to the switch circuit.

 

[Dr_Solstice]

I was somewhat presumptive that I had chosen the A10 wire, which could be my mistake. Short of pulling the whole fuse box, I can't be sure which is A10 or C12. I plan to do two things:

1) Check continuity from A10 and C12 back to fuse 46. If neither provides continuity, then it's the switch or wires to/from the switch.
2) I've already verified there is continuity from C4 to the backup lamps, so it's definitely between C4 and fuse 46.

I watched a few YouTube videos about similar GM fuse boxes and it seems to be relatively common that there are wire trace failures. It's kind of scary that GM discontinued the fuse box for these cars, so if you have a problem you either have to repair it or source a used one. The repairs are straightforward, but I am concerned about inviting problems if I take my fuse box apart, without having a replacement at hand.

DrS

The pink wire at C12 is Ignition Voltage 1 so is the same base circuit as A10, but does not route through Fuse 46.

Didn't you state in Post 17 that you have 12V at A10?

If you have 12V at A10, but do not have it at C4 when in reverse, the problem lies in the wires to the reverse switch, or in the revse switch itself.

You could try connecting a fused jumper from A10 to C4 with the car powered to see if the lights turn on. That would completely isolate the problem to the switch circuit.

 

[JohnWR]

I was somewhat presumptive that I had chosen the A10 wire, which could be my mistake. Short of pulling the whole fuse box, I can't be sure which is A10 or C12. I plan to do two things:

1) Check continuity from A10 and C12 back to fuse 46. If neither provides continuity, then it's the switch or wires to/from the switch.
2) I've already verified there is continuity from C4 to the backup lamps, so it's definitely between C4 and fuse 46.

I watched a few YouTube videos about similar GM fuse boxes and it seems to be relatively common that there are wire trace failures. It's kind of scary that GM discontinued the fuse box for these cars, so if you have a problem you either have to repair it or source a used one. The repairs are straightforward, but I am concerned about inviting problems if I take my fuse box apart, without having a replacement at hand.

DrS

C12 is powered through Fuse 43 = ECM/TRANS

Fuses 43 and 46 are both powered through the Run/Crank relay, so you will have continuity from A10 to C12 if both fuses are intact and, of course, the circuit within the fuse block is not broken.

We have had a very few cases of bad circuits in the fuse block, but the effect has generally been wider-ranging than a single circuit failure.

 

[Dr_Solstice]

From testing today, it’s definitely a wire to the switch or the switch. There’s continuity at the fuse box. Back to the shop now for a bypass wire.

 

[JohnWR]

From testing today l, it’s definitely a wire to the switch or the switch. There’s continuity at the fuse box. Back to the shop now for a bypass wire.

You also need to verify that the switch is actually being actuated.

Another possibility that has just occured to me is that a pin may have pushed back in one of the connectors, either at the fuse block or at the switch.

 

[Dr_Solstice]

You also need to verify that the switch is actually being actuated.

Yes

Another possibility that has just occured to me is that a pin may have pushed back in one of the connectors, either at the fuse block or at the switch.

I don't think that's the case based on the continuity measurements at the fuse block. It could be the case at the switch.

DrS

 

[Dr_Solstice]

So, the suggestion that the switch might not be actuated in the transmission resonated with me. So, rather than pay for shop time to put the car back on a lift and take the closeout panel off again, I decided to solve the problem from a different direction. I bought a limit switch and mounted it at the shift boot and positioned it that it closed when placing in reverse gear. I tapped switched 12V at the in cabin fuse box and moved the 10V fuse from the engine fuse box to that location (opening that circuit). I then tied into the light green wire running along the passenger side of the car, and voila - backup lights.

Sorry for lack of photos, I'll take some the next time I have the shifter boot off.

DrS

 

[JohnWR]

So, the suggestion that the switch might not be actuated in the transmission resonated with me. So, rather than pay for shop time to put the car back on a lift and take the closeout panel off again, I decided to solve the problem from a different direction. I bought a limit switch and mounted it at the shift boot and positioned it that it closed when placing in reverse gear. I tapped switched 12V at the in cabin fuse box and moved the 10V fuse from the engine fuse box to that location (opening that circuit). I then tied into the light green wire running along the passenger side of the car, and voila - backup lights.

Sorry for lack of photos, I'll take some the next time I have the shifter boot off.

DrS

An inventive solution.

 

[Dr_Solstice]

Added bonus, there was no need to pass a wire through the firewall and the parts cost me about $25 total.

DrS