Quick Tip Sunday
This particular vehicle had the transmission going into limp mode.
It can be easy when hooking up your lab scope to go after the gravy. Looking at the diagram, pin 18 (green arrow) is the control for the transmission relay. Pins 19 and 28 (red arrows) are the output of the transmission control relay.
Simple job, right? Check for power, ground and control at the relay. If all is good and it doesn't have an output, replace the relay. If still no good, throw another one at it. If still no good, it must be those cheap Indochina relays the parts store gets. Go get a dealer relay. If that doesn't work, what then?
Hold on now!!
You (generally) have 4 channels at your disposal. USE THEM!!!! Hook up to power, control output AND ground. If any one of them are faulty, the transmission won't get power.
As you can see, what appears to be a perfectly fine ground, isn't. As the relay tries to energize, the ground is not able to carry the load. The ground jumps up to 10v due to a bad ground down stream, not allowing the relay to power the module.
The same technique goes for sensors as well. A problem in the power or the ground can affect the signal going out. The point of all this is to use your equipment to its fullest capability.
I want to thank Pedro Sanchez from Maven Automotive Consultants for providing the capture.
Thank you for reading
Yes, that's a valid answer. Another good test is monitoring all the wiring and components in their native environment. In any diagnosis you want to disturb as little as possible for fear of "fixing" the issue without knowing it. The next step for me after catching this issue would be, of course loading the ground.