Why Are Gauges Sweeping?
"Instrumentation problems. It's a long story." That was the symptom description submitted to our repair hotline. The long story involved a 2003 Chevrolet Impala with fuel and temperature gauges that would sweep back and forth when the ignition was first turned on when the vehicle was cold. The gauges would sweep a few times and then begin working normally. The technician had tried a number of tests but still had not found the root cause.
We started by looking at the trouble codes in all of the modules. There were plenty of codes, but none were related to our problem. We checked for scan tool communication with all modules on the vehicle, and all were communicating properly. Next, we looked at the fuel level and the engine coolant temperature in the powertrain control module (PCM) data while the condition was present.
The PCM sends Class 2 serial data messages through a data circuit to the instrument panel cluster (IPC), indicating the fuel level and engine coolant temperature. The values did not fluctuate at all as the gauges swept back and forth. We started to get suspicious of the IPC. I wanted to look at the coolant temperature and fuel level values in IPC data on the scan tool while the condition was present, but that data was not available on the scan tool. Because the problem only occurred when the vehicle was cold, the technician placed the IPC in the freezer overnight and left the vehicle in the shop so it would remain warm.
The next day, with a frozen IPC installed, the symptom was gone. This made it seem like the IPC was not the cause of the problem. Then we started to wonder if the PCM was sending fluctuating data to the IPC, despite the stable scan tool data. Though this seemed unlikely, none of the other modules in the vehicle should influence these gauges. We disconnected the PCM while the gauges were sweeping back and forth, and they stopped sweeping.
So it appeared the PCM was causing the gauges to sweep, but I wondered if something was affecting the data messages from the PCM to the IPC when the vehicle was cold. The technician had already replaced the body control module for an unrelated problem, so that was not likely to be the problem. He had also checked for add-on accessories that might be tapped into the Class 2 serial data circuits and found none.
We needed a quick way to rule out each module interfering with the PCM-to-IPC communication. Luckily, this vehicle has a splice pack connector that is a central connection point to tie all of the modules' Class 2 serial data circuits together. The splice pack (SP205), located under the left side of the dash, uses a shorting bar to connect all of the data wires together into one circuit. To make progress quickly, the technician removed half of the data wires from the splice pack, leaving the IPC and PCM circuits in place with the shorting bar.
The car was put outside to get cold. The next day, the gauges did not sweep on the cold start, so we knew that whatever was causing the problem had been disconnected from the splice pack. The technician reinstalled wires into the splice pack until the symptom returned. The wire for the sensing diagnostic module (SDM) was the one that brought the symptom back. With the wire back in the splice pack and the SDM disconnected, the gauges did not sweep when the car was cold. This meant there was nothing interfering on the wire itself. The SDM powers and ground were tested when cold and the symptom present; they were all good.
In the end, the SDM was causing the gauge-sweeping problem. The SDM had been interfering with the temperature and fuel level data that was being sent from the PCM to the IPC. The technician replaced the defective SDM, and the car was fixed.
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