We are going to do something a little different this time in the Fine Tuning department. We ran into a diagnostic case study you might be interested in. It involved a 2001 Chevrolet Impala with about 40,000 miles on it. The A/C fan and cruise control would intermittently stop working while the customer was driving. There seemed to be no rhyme or reason to the failure. The vehicle could be driven for days before the problem would occur.
As you know, intermittent problems like this can be extremely frustrating and time-consuming to diagnose. Here are a few tips that might help you speed the process. Listen to what the customer says, no matter how insignificant it sounds at the time. Gather as many details possible. Take the time to absorb and mentally process the information. Test drive the vehicle with the customer and try to duplicate the problem. This will give you an opportunity to explain that this diagnosis could be a time-consuming venture. Always check for TSBs.
These were the same steps we took. Darn, no TSBs were found. We then took a close look at the wiring diagram (a simplified version is shown here). Notice the A/C fan and cruise control are fed from the same ignition switch terminal. This was the first real clue. Could the ignition switch be at fault? Possible, but to be sure we still need to get the problem to occur. The ignition switch is in the dash on this vehicle, which makes access difficult. It’s not a five minute steering column ignition switch replacement.
Before a road test, let’s take a little closer look at the wiring diagram. Notice the A/C fan and cruise control are both fused. If both of these circuits are failing at the same time, they are probably losing current at the same time too. We decided to backprobe the fuse leading to the A/C fan. Notice the ignition switch is fed by a single wire. Could this wire be losing current, knocking out the circuit and the customer just doesn’t notice the other failures? At marker #4, you will see the PCM/BCM cluster fuse which is fed by a different terminal of the ignition switch. We also backprobed there.
We grabbed two DVOMs and went on a road test. After about twenty minutes of driving around, the fan went out and the cruise control quit. We connected the meters – one to each backprobe – and there it was. Backprobe #1 read 2.23 volts and #4 read 12.25 volts. Based on this information, we can be reasonably certain that the ignition switch is defective.
Think the problem through when access is limited. It isn’t always necessary to go directly to the suspected component if the circuit is fused. Here is my question to you: Let’s say we only had one DVOM to work with. Which terminals could we have connected to and achieved the same test results? What would the DVOM read when we experienced the problem? What is this test called? Use the markers to describe the meter connections in your answer.