vehicle. That analog voltage then becomes a serial bus message shared with numerous modules on the vehicle and the command is carried out. By configuring the steering wheel control switches in series, multiple inputs from the switches can be sent over a single wire. Now the trick we have to do in the event the service manual doesn't give us anything more than resistance values for the switches by themselves is to run 'free style' diagnostics. Why not just measure each switch with an ohmmeter? If there are intermittent problems with the contacts breaking down or the resistance inside the BCM is having solder joint issues, the SWC switch resistance would be fine. So in order to load the circuit with a little realistic current to catch the intermittent, you need to do your own calculations and measuring. Once you determine the voltage supply from the BCM to the SWC (just back probe w/o any of the SWC switches pressed) write that down. Do this with the engine running or a clean charger / power supply connected so you don't have a dying battery messing up your testing. Key off, measure resistance to ground at the BCM's SWC input. In the case of the GMC van, it was 450 ohms. Now add 450 ohms to whatever SWC button's resistance is to get a total series resistance. ton, the Ohms Law app can show you that at 0.0066 mA X the value for JUST the SWC involved switches (1210 plus 301 = 1517 ohms). To calculate the voltage drop across VOL UP multiply the 0.0067 amps times 1517 ohms and the voltage drop across the SWCs will measure 10 volts. If you were to measure at the BCM you would see the other 3 volts dropped across the BCM's internal resistor. Always remember the sum of the voltage drops equals the source voltage and you'll never go wrong! Figure 2: If you don't like to do math - there's an app! Ohms Law calculators for both iPhone and Android! Simplifying Schematics Step 1 - Inventory Components / Divide Sections Sometimes it's just easier starting at the end of a circuit (the single cooling fan motor) instead of the beginning of the circuit. (Figure 5) Start by label Hate Math? There's an App! Using Ohms Law OR your handy smart phone app (Figure 2) you can then pop in total resistance for the circuit. Adding up the SWC switch resistor marked "Mute" (1210 ohms) plus "VOL UP" (301 ohms) and finally the 450 ohms you measured between ground and BCM pin 6 and the number is 1,961 ohms. Now pop in your measured B+ value (13 volts) to the calculator along with the total resistance for the whole circuit and you will get 6.6 mA of current calculated. Now, since you want to see if a switch or connection is breaking down under real working conditions, calculate the voltage drop that would be at each Figure 5: Step 1 - Inventory Components / Divide 'Speed' and 'Control' resistance when a button 1. Single cooling fan motor assembly w/ 2 resistors. is pressed. How do you do 2. 3 'speed' relays in underhood fuse block. (B.E.C.) that? Pop in what you know 3. 3 spike suppression diodes built into B.E.C. in the smart phone's Ohm's 4. 2 'control' relays in a separate dedicated relay block 5. ECM has 2 fan control related outputs Law app. To estimate a volt6. 2 related grounds and 3 fuses age drop at the VOL UP but- December 2016 3 MACS Service Reports