Monday, October 22, 2007
Diagnostic Tech: '02 Olds Aurora
After spending some time trying to find a problem with the lights and having no luck, I decided to move on to the MIL complaint and come back to the lighting problem later. A lot of time can be spent checking a system but when the complaint can't be verified it can easily become time wasted. It's better to check for the cause of a problem while the problem is present.
I pulled a P0410 trouble code from the car's engine control module.
A P0410 will set when the engine computer has commanded the AIR pump on and the AIR valve solenoid open but does not see the O2 sensors go lean as they should when extra air is being pumped into the exhaust.
"An AIR pump?", you ask. Yes, an AIR pump. The modern version of the old belt driven smog pump is still alive if not well on many GM vehicles. The AIR pump no longer needs a belt. It is an electric motor and tucked away under the left front fender on the Aurora. The engine computer will provide a ground to the pump motor for testing or for normal system use. With a ground the motor should pump fresh air which is piped to a vacuum controlled valve at each exhaust manifold. With the vacuum control solenoid grounded by the ECM the vacuum will reach each valve and open them, allowing the fresh air to enter the exhaust and accelerate the catalyst operation. The fresh air entering the exhaust as it is commanded should lower the O2 sensor voltage to a lean condition. The lean condition should change the fuel trims. The engine computer does monitor the voltage on the command wire to the solenoid and will set a circuit electrical code if the voltage does not match the command. This means that when you see a code P0410 as a stand-alone code, you aren't likely to find an electrical problem.
One very common problem with the GM pumps is that the design of the air intake hose allows water to be drawn into the pump housing. This water will eventually cause the pump motor to be very hard to turn and even freeze up. GM Technical Service Bulletin 02-06-04-024G contains the related information. A replacement pump and an updated hose design to prevent the water contamination are available as a kit. The part number is 19515548. The original air intake hose filter is open to road splash from water while driving.
The new design is covered on the inner fender side and open to the engine compartment near the air filter housing. The install is fairly easy. The air filter housing and inner fender liner must be removed for access.
After finding the pump motor on this car to be water damaged and very difficult to turn I replaced it and the hose with the update kit.
While looking for the GM TSB and part number I had noticed something interesting. GM TSB 01-06-03-007A concerns headlamp and interior lights dimming in cold weather. The bulletin states that a revised alternator is available to fix complaints of headlamp dimming in cold weather and perhaps the blower speed slowing or changing pitch intermittently. The problem is due to intermittent low voltage. Engine vibration may cause the alternator to lose field continuity and the voltage regulator to reset. Now when the car owner dropped this Olds off he did say that the alternator had been replaced at another shop in an attempt to fix the light problem. I wasn't going to suggest yet another replacement. What really caught my eye is that the bulletin does go on to say that short term voltage drops with light flickers or fluctuation is a normal condition with high electrical demand AND that even after installing the revised alternator the car will exhibit short voltage dips and headlamp dimming DUE TO THE AIR PUMP CYCLING. Now just how much worse would this be if the air pump motor was water damaged and near frozen, causing much more current draw? It looked like the installation of the new air pump was going to help with the light dimming problem.
As important as it is to verify a complaint before starting a diagnostic job is verifying a repair after the work is performed. After clearing the trouble code a short test drive was in order. Everything appeared to work fine with no MIL on but a P0410 is a two trip code. In other words, if the air pump failed the monitor test during this test drive no code would be set and no MIL turned on. It would take a second ignition cycle and test drive to set a code. The air pump would need to fail the self test on two trips. To make things move along better and get the most of the test drive I usually take a scan tool along and record the ecm data.
A look at the scan tool data showed me that the air pump test had run and had FAILED during the drive. I would have liked to include the scan data capture in this article. You could clearly see the ecm activate the pump and solenoid three times and three times see no lean signal from the O2. It then gave up and logged a failure. Yes, I would have truly liked to include that but I did a software update on that scan tool earlier tonight not realizing it would clear the record memory.
Back in the shop, I used the command function of the scan tool to command the air pump on and verified the ecm could provide ground and the pump was fully functional. If the pump was working as commanded then the shut-off valves were the next place to check. Vacuum from the intake manifold is piped to the vacuum solenoid. When the ecm grounds the solenoid it opens, allowing manifold vacuum to reach the shut-off valves. The vacuum should then open the valves allowing the fresh air from the pump to reach the exhaust system. If the solenoid has an electrical problem the ecm should detect that and set a related solenoid circuit code. There was no circuit code so I used a simple vacuum gauge to verify 18" of vacuum to the solenoid. With the engine running and the vacuum gauge at one shut-off valve port I grounded the solenoid and got only 5" vacuum. I blocked the other shut-off valve port and tried it again. This time I had 18" of vacuum. The rear-most (right side) shut-off valve was bad and losing vacuum. With the vacuum loss, neither shut-off valve could open so no air from the pump was reaching the exhaust.
I actually do have scan data on the test drive done after replacing the shut-off valve. It was saved to the pc after being captured on a different scan tool so escaped the fate of the first data capture.
You can see the O2 sensor go lean when the pump is on verifying the system is operational.
I love my job!
Kenneth Hayes
aka Deranger
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