GM's ONLY 

The 1976 Cadillac Seville was a milestone for General Motors, as it introduced GM's first fuel injected vehicle. What seemed to be a complicated system back then, only laid the foundation for what was to come. I'll be doing some case studies from time to time to show what actually is involved in fixing vehicles in the 21st century.

When I first started in this business, I specialized in brake and front end work. While there is nothing wrong with this type of work,(other than being dirty and very labor intensive) I found that it became boring. Keep in mind this was before the advent of  ABS (Anti-Lock Brakes), electric power steering pumps, and other electronic controls that have taken the place of mechanical/hydraulic systems. At the suggestion of one mechanic at a dealership I worked at, I looked into getting into electrical and driveability repairs. Here it is 25 years later and the one thing I can say about this type of work is that it is any but boring. This especially true today with the rate that technology has been advancing. Working on one car manufacturer’s line of cars means that you become familiar with some of the same faults, but there are enough twists to keep the job interesting.

Case in point was a 2000 Oldsmobile Intrigue with a 3.5L, VIN H. The customers complaint was that the MIL (Malfunction Indicator Lamp) lamp was on. I hooked up a scan tool and found that the vehicle had a P0410 stored. The customer was pretty familiar with this code as it had been “repaired” at another shop a few other times. He was told that he could ignore the code after they had given up on it. The problem is the state of California would not ignore it as a smog test was due. Doing a search on GM’s website turned up a TSB(Technical Service Bulletin) for the exact same code...  #04-06-04-037: Check Engine Light On,

What the TSB describes, is by far has been the most common repair for this system that I have run into. Do I pull the gun and replace the pump at this point? Not so quick, there are some tests that need to be done first. First thing I did was turn on the secondary air pump using the Tech 2. The pump came on. If you take a look at the snapshot that was taken when turning on the pump, the HO2S Sensor 1 initially goes lean as I would have expected, but then goes rich about 2 ½ seconds later, not what I would expect, but… From GM’s flow chart:

Circuit Description

The AIR pump is used on this vehicle to lower tail pipe emissions on start-up. The powertrain control module (PCM) grounds the AIR pump relay control circuit, which energizes the AIR pump. The PCM also grounds the AIR solenoid valve control circuit, which energizes the AIR solenoid valve. The AIR solenoid valve opens allowing manifold vacuum to open the AIR control valves. The PCM enables both control circuits when AIR system operation is desired. When the AIR system is active, the AIR pump forces fresh air into the exhaust stream in order to accelerate catalyst operation. The AIR control valves replace the conventional check valves. When the AIR system is inactive, the AIR control valves prevent air flow in either direction. DTC P0412 applies to the AIR solenoid control circuit. DTC P0418 applies to the AIR pump relay control circuit. DTC P0410 sets if an air flow problem is detected.The PCM will run two tests using the heated oxygen sensor (HO2S) voltage to diagnose the AIR system. Both tests have two parts.

Alright, the PCM appears to have failed this system for Test 1, because the HO2S did not stay below 300 mV for 25 seconds.

Following the trouble shooting/flow chart at step 1 the answer is yes, so I’m directed to step 2. The answer for that is no, which brings me to step 3.

Step 3 and step 4 should actually be reversed, as it makes no sense to look for the fuse first while the scan tool is already hooked up to the car and it is much easier to turn on the air pump before looking for a blown fuse. Minor point, but when this is wrapped up you’ll see what I mean. I just skipped to step 4, which is a yes and from there I’m sent to step 10.

Step 10 is to verify there is air that being pumped. Besides step’s 3 and 4, here’s where we start running into trouble using the trouble shooting charts;

Step 10: Disconnect the AIR hose and pipe from the AIR pump. Turn the AIR pump ON with the scan tool.

Is air flow present at the AIR pump outlet?

This is a subjective test. I know from previous experience what is close to normal. If you never had worked on this system before, would you? I can say based on experience, the pump is working normally. I’m directed to step 11, which is, Is air flow present at both AIR hose and pipe outlets?

Step 12: With the engine running, check for engine vacuum at the AIR solenoid valve.

Is vacuum present at the valve?

Here again, based on the wording, this is a subjective test. How much vacuum is the engineer looking for? Unlike (from  step 10, Is air flow present at the AIR pump outlet?) Vacuum can be measured and a minimum specification should have been put into the wording as in, “Is there a minimum of 15” mm Hg vacuum present at the valve?”

I had 17” mm Hg so I’ll assume that is good.

Step 13: Reconnect the vacuum supply line to the solenoid valve.

Disconnect the vacuum lines at the AIR control valves

Command the AIR system ON using the scan tool.

Is vacuum present at both vacuum lines?

 Here’s what I have. Do we go to step 14 or to step 28? Using this flow chart requires some thinking. I’ve probably bored you to tears with this ‘case study’ so I’ll wrap this up. There was 5” mm Hg at both AIR control valves when the pump was turned on. So vacuum has dropped 12” Hg from the control solenoid to the control valves. The left side control valve would not hold vacuum at all when using a vacuum pump. The right side bled off quickly. I ended up replacing both valves. I explained to the customer that replacing the left side would probably turn off the MIL , but there was a problem with the right side. These valves are not inexpensive. The left retails for $267.08 and the right for $283.48. This does not include the labor for testing, or for replacement. Here’s a snapshot after replacing the valves and the car being fixed.

There are a few points I would like to make with this post. 

Even though TSB’s and past experience are helpful, they will not fix every vehicle and actually might not even apply. Be careful here. Another area to be aware of is DTC trouble shooting charts can be misleading and not accurate. There’s a saying that trouble shooting charts can get you into trouble. So that would leave us with how are we supposed to fix vehicles. I’ll leave it up to you to answer that. Hopefully your answer will be that you’ll bring the car to GM's ONLY  to get it fixed correctly, the first time.

Past Lives...