Seemingly Unrelated Problem Causes 2002 Alero's No-Start
A technician called recently with the following problem: The vehicle - a 2002 Oldsmobile Alero GL with a 3.4 liter engine - would not start in the morning. There was no injector pulse; yet there was good spark. It appeared to be a theft system problem, except that when fuel was added, the car would start and run, and would continue to restart until the engine sat for eight hours or more.
A quick check for codes revealed none in the body control module (BCM) and only P1546, for the air conditioning (A/C) compressor clutch circuit, stored in the powertrain control module (PCM). There were no theft codes stored in either the BCM or PCM.
A review of the PCM data did not find anything out of place. There was spark and the scan tool did show an rpm signal while the engine was cranking. This was also confirmed by monitoring the "3x" signal from the ignition module on the purple wire with white tracer. This signal should be a 5-volt direct current square wave with engine cranking. Checking the "24x" crankshaft position sensor signal confirmed a good square wave on the light blue wire with white tracer while the engine was cranked.
Before replacing what appeared to be a defective PCM, we decided to review theft system scan data:
1) The auto learn timer status, lockout timer status, passlock data, passlock data voltage, passlock power and security lamp status all showed the correct values.
2) The PCM theft data, "PCM in vehicle theft deterrent (VTD) Fail Enable," displayed the correct value of "No."
3) VTD Auto Learn Timer displayed the correct value of "Inactive."
4) VTD "Fuel Disable Until Ignition Off" displayed the correct value of "No."
However, while watching the VTD Fuel Disable value with the engine cranking, the scanner would show "Active" instead of "Inactive." Surely, this must be some sort of PCM problem because it displayed a wrong value; however, there were no theft codes stored in the PCM or BCM. Remember, the only code that was stored in any module was the P1546 for an A/C compressor clutch relay circuit problem in the PCM. It was interesting that the customer was not complaining of any A/C problems, only the starting problem, which had been going on for nearly a month before he brought it into the shop.
So, what is code P1546? The service manual points out that it is an improper voltage level on the A/C clutch relay circuit. Basically, a wrong voltage is coming from the compressor clutch relay to the PCM. A quick glance at the compressor relay wiring diagram shows that this relay is powered by the A/C BFC (fuse 37) in the underhood fuse block. Is there a chance it supplies power to the PCM or BCM? Yes; it does supply "key on" and "start" voltage to the BCM.
Luckily, this vehicle hadn't been started yet today. After replacing the fuse, it started without adding any additional fuel. So, why did the fuse blow? Checking the compressor clutch circuit, it was soon evident that the compressor clutch had shorted. Each time there was a request for the compressor to come on, the A/C BFC fuse would blow. With this fuse blown, the BCM was not completely powered up, thus under certain conditions it would not give all the proper information to the PCM. Because the PCM failed to see the proper information, it would not allow injector pulse but would not set any theft codes.
On today's cars, you never can tell what might be the cause of a customer complaint, even when a problem doesn't seem to be related.
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