Diagnostics aren't that easy at the top

Even the folks at the top encounter problems every now and then. Leaders at TST this month share their experiences when diagnostics saved them in the bays.

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BMW ABS Confusion
By Pierre Respaut, TST Vice President

This is a tale of an ABS controller failure and how we can be confident in diagnosing a module. Just keep in mind that our first intuition might be right, but we might not believe it and follow the wrong diagnostic track.

I closed my shop almost two years ago, but I am still in contact with some of the “A” customers. An “A” customer emailed me with a plea for help.

The car in question is a 1999 BMW 528i with 150,000 miles on the odometer. It had been in the shop for three weeks with multiple codes and symptoms and they couldn’t figure it out. They were sure, however, that it was not the ABS control module. I agreed to look at the car.

I scanned the vehicle and I realized what was going on. The technicians were getting lost in all the codes — there were more than two dozen of them. I figured the best way to start was to clear them all and see who came back first. He’d be my most likely culprit.

I cleared the codes and the first code to return was the ABS code 21 for a faulty control unit immediately followed by the MID code and the two rear fog lamp codes. The rear fog lamp and I-Bus telephone codes did not concern me, because they are typically false codes in all of the 528i series. As for the other codes, I’m sure that all would have returned if I had driven the car as the other shop had done. Maybe I would have been more confused as well.

Instead I decided to test the first code that appeared, the ABS control unit code. If all signals were good, then I would condemn the control unit. I tested all the control unit powers, grounds and signals Key On Engine Off (KOEO) and Key On Engine Running (KOER) with both a Digital Multimeter (DMM) and a scope. Voltages were within 0.1V (100mV) of system voltage and all grounds were within 15mV of 0.0V.

Because all the signals were correct, I condemned the control unit. As simple as that! I replaced the control unit which corrected all the codes in the other control units.

The moral of this story? When you get caught in a diagnostic trap, stop, take a break and start from the beginning. Ask for help or brainstorm with other techs. Most of all, don’t be too sure that it isn’t the component that the DTCs (Diagnostic Trouble Codes) and logic dictates it is. Always take an intelligent diagnostic approach, start from the basics and work your way up.

Repairing Rotary Engine Driveability Problems
By G “Jerry” Truglia

My son was asked by a customer whether or not his shop diagnosed and repaired Mazda RX-8s. The customer was frustrated that the dealer was not able to diagnose the problem he was having. He took in the job and his technician at the time, Kevin Quinlan, was entrusted with troubleshooting the vehicle. The Mazda had 93,000 miles showing on the odometer and ran really rough. I had a feeling this was going to be a tough one, and I took an active interest in their diagnosis.

The first thing they did was look at the maintenance schedule and saw that the vehicle mileage was approaching a scheduled tune-up. After pulling out a spark plug and seeing it was original, they figured that would be a good place to start.

They came to me and told me what they were planning on doing. I’ve only worked on RX-7s back in the day and hadn’t dealt with an RX-8. I told them just to remember to squirt some oil in before putting in the spark plugs. Knowing that this might be something I can use in one of my classes, I also asked them to take some compression waveforms. They were removing the plugs anyway, so this would not pose any additional work for them.

With the spark plugs out and new spark plugs on their way from the parts store, they used the shop’s labscope to capture engine compression waveforms, using a pressure transducer in place of the mechanical compression gauge. A pressure transducer and a labscope allow us to see compression as an image. This is often more useful for our diagnostics than a fluttering needle on a compression gauge. They did not understand rotary engines too well, so anything that would give them more information was helpful. This is what they found on the rear chamber (see Figure 2) and the front chamber (see Figure 3).

They looked up the specifications and found that the engine should have been generating at least 96 psi up to 120 psi. The rear chamber was low by at least 10 psi, and they had no idea what the rapid decline in compression in the rear chamber signified when compared to the front.

I thought this was interesting, but I had to get back to work. I told them to give me an update that night at class on how everything went. That evening they told me that the new spark plugs and motor oil did not help. I removed some cobwebs from my mind and remembered that I’d had success fixing RX-7s by decarbing them. I’d fog the intake manifolds with fuel system cleaner. I gave them this new advice and Kevin put it to work the next morning.

Believe it or not, the decarb worked. The car now ran great and we could see the improvement in the compression waveforms (see Figures 4 and 5).

The waveforms now matched up in shape, proving that the engine decarb cleared up the irregular compression issue that was responsible for the driveability complaint the customer told us about. However, rear chamber compression was still a little low. A few weeks later the vehicle came back with the same problem. Decarbing the engine worked again, but I knew this was not a permanent fix for this particular vehicle. At this point, I emailed a contact I had in Mazda engineering. He told me that RX-8s that year were subject to a selective recall.

So, even though this vehicle was ultimately fixed for good by a new engine, the principle we demonstrated was that excessive carbon build up can be an issue in rotary engines. Barring some major problem, such as a motor that was defective since the day it was built, decarbing a rotary engine can markedly improve drivability. This might not fix all your rotary poblems, but its another bullet for your gun belt.

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