1988 XJ 4.0 AX15 swapped (harness conversion was done correctly they have worked for 2 years).

My reverse lights suddenly stopped working, no work was done just a bulb check and voila.It is weird if both bulbs are removed I have 12.1v left and 11.9 right, as soon as I put a bulb in I lose all voltage. All other tail light functions work as should, as well as front, with no interference on other bulbs with function changes. I have done the following as of now, any suggestions appreciated.
- Verified bulbs and tried new
- Verified both sockets, good connections (volt meter +to center - to side brings aforementioned voltages)
- Changed fuse #4 20amp (fuse looked fine as well as connectors in Fuse block but I've been down that road)
- Verified ground behind rear plastic (already had upgraded, but still checked)
- Verified reverse switch harness and splice I had made on the AX conversion at the transmission
- Checked cleaned and greased both tail light harness connections
Thoughts that I haven't tried
- Could it be a bad socket (I have verified them both have good connections and the lock slides for the bulb are clean for a ground)?
- Could it be the switch i.e. it has volts but when it pulls amps it craps the bed?

 

You have a bad, high resistance connection or switch somewhere. Having voltage doesn't mean you have current. Think of a pinched hose, you have pressure until you squeeze the nozzle. You need to start measuring voltage along the path with the bulbs plugged it to see where you're dropping the volts.

 

#4 fuse. Is that the one labeled "trans"?

 

It is labeled Turn B/U on my fuse block, and labeled as such on the FSM.

 

Check the "trans" fuse.

 

Fixed... thanks for the tip thankfully it was a quick one on this balmy 23 degree morning. I'm sure glad I didn't have to start chasing the whole loom.