Car:
2002 PT Cruiser
Problem:
1. Overheats, blown head gasket.
2. Electrical issues relating to proper cooling, voltage regulation, and transmission problem.
3. Engine vibration
Test Drive:
No test drive, Dead on arrival.
Diagnosis:
This car came to me with a blown head gasket. And later overheated again after the head gasket was replaced, with a new thermostat and new water pump. This was an incredibly difficult diagnosis, because it isn't typical for a car to overheat after replacing major cooling components where it would lead to a major electrical problem within the car itself within the same malfunction. The root cause was electrical, where the initial diagnosis was a problem with the cooling system.
Fix:
Car came to me dead on arrival. Removed the engine due to limited work space in the engine bay. New head gasket installed. New water pump, new thermostat. Radiator didn't appear to be clogged as the coolant wasn't dirty or yucky coming out of the drain tube. If there had been residue buildup, I would suspect clogged plumbing. But there was none. With the head gasket replaced and passenger sidewall engine mount replaced, the engine was re-installed. Test drive indicated the cooling problem was resolved. Drive to the dealership to return the car and the car is overheating, boiling over. After much research and troubleshooting a few days later, I find a single post out of 100 that indicate the problem might be with the electrical system failing to turn on the cooling fans. But why?
Further diagnosis with the car....
Headlight dimming. Car idling funny when shifting into gear from P. Interior lights flickering. All of this indicates it may have a weak battery. New battery installed, and same symptoms still persist. Install a used alternator. Same problems still persist.
Further troubleshooting electrically.... Voltage meter on the battery reads proper output voltage from the alternator. and battery voltage. What gets me is that the headlights dim when you turn them on, and the engine idle dies down when you go to high beam. Turn on the AC and it idles even more roughly. Put it into gear and it almost dies.
Root cause analysis:
After all of this work, the problem is related to the electrical system. While the original head gasket did need to be replaced, the root cause of the problem is voltage regulation. After thorough troubleshooting, as this was a doozy to fix.... the root issue was with the car's main computer. Chrysler decided to put the voltage regulator in the main computer box. What was happening was that as the car idled, the voltage regulator was able to keep up with demand, however if you loaded the electrical system down by turning on the high beams, the AC unit full blast, and shift the transmission into gear, the computer has to decide which system to keep alive, and will shut down all unnecessary functions. The computer was resetting itself when the voltage/current demand was not enough to keep the main computer functioning, which caused the rough idle. When it resets, the idle air control system had to hunt for it's proper setting because, at the moment of reset, it did not keep in memory where the idle air control valve position. With lights flickering, and when the car's electrical load increased due to being in gear, the AC unit on, the system would reset. It could not regulate voltage properly, dropping voltage below the 5VDC threshold required to run the system computer. Research and discernment to the root cause was that the internal voltage regulator in the main computer was failing.
THE FIX:
After the head gasket was replaced and new thermostat and water pump installed, and the engine installed into place, and after further diagnosis with the electrical system......
The main computer in the car needed to be replaced. This Chrysler car main computer box is filled with a gelatinous substance, and the metal box is the heat sink, which is bolted to the rear firewall.
I ordered a used vehicle ecm from a company out of Florida via website. They needed the VIN code, and the mileage. Given that, and free shipping, the first computer they sent me was a refurbished unit that failed to even start the car. So I won't recommend them, or give their name here. The replacement computer arrived after another week of waiting for free ground shipping. I tell ya, customer service used to mean something, but not to these folks in Florida at this company. With the second car computer in hand, programmed with the right VIN code and mileage.... the car started up. The cooling fans came on properly, and the lights no longer flickered. The vehicle, even under full load with the AC running, car in gear, high beams on, and cooling fans running, the car held up and didn't die.
Post work assessment:
Everything on the car was working as it should have been. The cooling fans kicked on at the proper temperatures. The electrical system was no longer getting bogged down, and the idle was running smoothly. I had to make a minor adjustment to the engine tilt due to the new engine mount to eliminate engine vibration.
Like I said, this was a doozy of a problem car. While everything pointed initially to a cooling problem with this car, the end result turned out to be a problem with the main computer. Who would've thought? If I didn't have my 15 years of experience as an electronics technician, I would never have guessed the problem was electrical if my only experience was mechanical expertise. These newer cars require more in depth knowledge at times, to understand the input and output of system electronics. Some sensors are no longer just a high/low, on/off configuration. They're starting to pulse width modulate signals, meaning that where you thought a single wire was just carrying 12 volts to a sensor, might be multiplexed to carry other signals as well. That complicates things for some mechanics. I never would have guessed this overheating problem with a blown head gasket would lead to a root cause of the main computer voltage regulator.
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