By Dave Martin
In the automotive industry, "PI" stands for preliminary information and refers to a type of technical service bulletin used by General Motors. Some vehicles are almost impossible to figure out without this information.
When I was young, if I heard the word "PI" I would think of a pie, filled with blueberries or cherries. When I was in school, if I heard someone say "PI" I thought of pi, the ratio of the circumference of a circle to its diameter. In the automotive industry, "PI" stands for preliminary information and refers to a type of technical service bulletin used by General Motors. Some vehicles are almost impossible to figure out without this information.
Take the case of a 2003 Chevrolet Impala with cooling fans that never turned on. Not rocket science, right? This system uses two cooling fans. Both fans turn on in a series circuit at 223 degrees Fahrenheit or when the A/C high side pressure exceeds 190 psi.
Since the ground side for one fan feeds the power side of the other fan, each fan uses 6 volts of the 12-volt circuit. It's a great design because both fans run on a quiet and efficient low speed. If the system needs additional air flow through the condenser and radiator, three relays energize to switch the fans to a typical parallel fan circuit. Then each fan gets 12 volts and its own ground, so both fans run at high speed. This occurs when the engine temperature reaches 230 degrees Fahrenheit or the A/C high side pressure exceeds 240 psi. Engine temperature is monitored by the power train control module (PCM), and the PCM is in direct control of the relays. Great system when it works.
This car came in with the instrument cluster indicating an overheating engine. After connecting a scanner, the technician verified the engine was in fact climbing to more than 250 degrees Fahrenheit, well over the temperature that both fans should have engaged. There were no codes and other engine sensor data appeared normal. After checking both fan fuses and wiring, the PCM was disconnected. Each fan circuit could be turned on with a jumper wire at the PCM so he did what any skilled diagnostician would have done - he replaced the PCM.
Now things got really confusing. The new PCM reacted exactly the same as the old one - no fans - it didn't make any sense. After reviewing the scanner data one more time, it was discovered that the reason the fans were not on was because the PCM didn't request them to turn on. Here was a computer that saw an overheated engine, but did not turn on the fans to cool it back to operating temperature. So the replacement relays were tried ... same results.
The technician went back to the manual and read all the available information on the fan circuit. There was nothing that explained why this PCM refused to operate the fans. He reviewed his list of TSBs and there was nothing that related to this problem. At this point he called Identifix and explained the whole story. After asking him several questions, I knew he had tested the whole system correctly.
When this type of situation occurs, I have learned to step back and start over. Sometimes something has been missed or an error has been made. The tech had access to another brand of scan tool and we used that to confirm the readings. I went over the Identifix database and a complete list of technical service bulletins (TSBs). Our TSB list contains GM's preliminary information bulletins. These PI bulletins usually pertain to a particular geographic area or a small segment of market, and usually come out shortly after a problem has occurred. The PI may become an actual TSB, or just remain in a PI status.
As it turns out, the solution to this problem was specifically presented in a PI (No. PIC3045B). The problem had nothing to do with the cooling fans, wiring, relays, PCM, or programming. Instead, it resided in the body control module (BCM). One of the functions in the BCM is a "limp home mode" for the electrical system. If the BCM detects that the charging system is not working correctly, it will shed electrical loads to give the driver more time to arrive at his destination. In this case, the BCM was falsely detecting low battery voltage, and then gave instructions to the PCM over the class two data line to eliminate cooling fan operation to reduce the draw on battery. Using a scanner, the BCM data showed the problem ... the load management status was shown as "active." After verifying the battery and charging system were normal and the powers and grounds at the BCM were OK, the BCM was replaced, and the cooling fans worked normally.
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