Figure 7: Step 3a. - Trace Each Control Circuit's Path ECM Applies Ground on Terminal 54 ONLY = Med Speed Cooling Fan The ground from terminal 54 is sent via the splice / buss bar (J127) the low speed fan relay. Voltage is dropped across 1 high wattage resistor within the cooling fan assembly resulting in the medium fan speed. the circuit in the event the service manual wanes in its effort to make me understand. I also like to box in subsections of the circuit to help make the page more manageable. You don't paint a house all at once. You paint it one wall at a time. The same is true with understanding complex schematics. switches inside it as the lower right hand corner of the schematic might indicate. There are most likely solid state driver modules inside of ICs (Integrated Circuits) containing up to 8 independent outputs. Figure 7 shows the active circuits when ECM terminal 54 applies ground to circuit 473. Two relays (fan Med and High) activate to get the highest fan speed setting. Figure 8 shows us that ECM terminal 41's output is grounded that both control relays activate resulting in the low speed fan setting. Finally we see in figure 9 that both control relays and all three of the speed relays all turn on resulting in the highest fan speed setting. Naturally, if at all possible, activate outputs with your scan tool. This can save you from the possibility of damaging circuits or components while performing live testing with a Power Probe or ammeter. But use caution when doing so to avoid damaging wiring or electronic components. Simplifying Schematics Step 2 Labeling Your P's & G's. (Powers & Grounds) Highlighting the B+, Ignition, and ground circuits in the schematic that are related to the system you are trying to diagnose is especially helpful with the monster 5 relay cooling fan circuit on this Chevy. (Figure 6) That gives you an idea of the labeling / location for the most common problems with electrical circuits - the "P's and G's." Simplifying Schematics Step 3 Trace Each Control Circuit's Path Regarding jumping or activating modules or relays in general; some modules use transistorized outputs that are current limited and reverse polarity protected from well-meaning techs who love to use jumper wires and Power Probes. Other modules get one sniff of reverse polarity or over current and the magic smoke gets released. Use EXTREME Next, if the OEM was so kind as to show you a little of what is going on inside of their "F.R.E.D.'s" head as in this Sonic's ECM (Figure 7) you can print out a page for each fan speed and then highlight the "P's and G's" that change when each of the ECM's two output drivers turn on a control ground. We know this Sonic's ECM doesn't have mechanical December 2016 5 MACS Service Reports