Figure 8: Step 3b. - Trace Each Control Circuit's Path ECM Applies Ground on Terminal 41 ONLY = Low Speed Cooling Fan The ECM's applies ground at terminal 41 to activate control relay 2 which then activates the other control relay. Splice pack / buss bar J131 becomes grounded and turns on control relay (which does not have any effect on the circuit in this combination) and also supplies a ground to the low speed fan relay. Two high wattage resistors drop voltage to run the fan at low speed. December 2016 Dave Hobbs caution when attempting to activate a driver circuit by pack probing a module with power or ground. I personally like to disconnect the modules and back probe harnesses with a fused ammeter or Power Probe. This isn't completely fool proof, however. (Figure 11) Sometimes the connector you need to get at on a module can't easily be disconnected to get "F.R.E.D." out of harm's way. Sometimes you don't even realize you are working on a "module." (Figure 10) In the case of my brain freeze on the Sonic, I was trying to repair the higher priority complaint 1st (the no start / crank condition) while the problem was present. I wanted to see if I could activate the starter solenoid from the female spade terminal that the starter relay plugs into. Basically, bypass the relay. (Figure 10) I first looked at the schematic to determine which terminal on the fuse / relay center that was. Then I checked for voltage and resistance to ground to get a warm fuzzy feeling that I wasn't going to be probing 12-volts of power into a circuit that would not like 12-volts hitting it (sensors, grounds, etc.). There was no voltage present so I checked for resistance to ground. There wasn't any. What? Yeah, Figure 11: A partial disassembly revealed a multi layered printed circuit board that was very unforgiving when power was probed in the wrong terminal during diagnostics. New harness ends had to be spliced into the vehicle as well to restore the vehicle. 6 MACS Service Reports