You are correct that the diode prevents a back feed through pin 86. Think of it this way. When the magnetic field collapses at the coil the voltage will instantaneously spike. You don't want that going back to anything like a controller with sensitive electronics.
You can wire up a relay...
Change out your fuel, oil filters and engine coolant on a regular basis and stay on top of your services. Watch the quality of fuel that you fill up with. Keep the engine bay clean and change out oil cooler lines and radiator hoses.
There was a way to make some money on the side when I was in the military. It required you taking a couple of glow plugs and solder them in series on a four foot lead. Call an infantryman over and say "hey watch this!" and demonstrate to him how you could heat up a cup of coffee with it and...
It would be nice if a PCM could do an auto-test on glow plugs. That is not the case because of era involved for GM 6.5. The computer could take a resistance or current reading on all eight GPs give you results.
Your transmission is telling you that the handwriting is on the wall. If you are lucky you can repair the electro-hydraulic portion and come out ahead. If you drop the oil pan and see a lot of "sparkly".. this will add onto your OD problems and issue.
What you are going through with your 4L80-E...
For the hydraulic side of things. The transmission is told what to do by the TCM and in your case this begins with the torque converter solenoid receiving an electrical signal of pulse width modulation. Think of it as a way to increase the length of throw to the TCC valve.
When the...
If your transmission is reaching operating temperature and drops out of overdrive it could be that your Torque converter clutch (hydraulic system) from the TCC solenoid valve (leaking) all the way through the actuator feed limit valve (side loading) ect.. When the fluid gets thinner you will see...
I don't know where the data is read from at the TCM whether it is the positive side or the negative side of the Transmission Fluid Temp sensor. I would first suspect that you have a short in the TFT circuit. Consider taking a resistance reading of the TFT, an Ohms reading through the TFT and...