'The Other 3800'Posted 7/18/2003
By Jeff Bach
In my last article, I wrote about a 3.8 liter engine that was hard to start in subfreezing temperatures. I also had another problem car, the same week, with the 3.8 powertrain. This was the other one and it was still extremely cold when we became stranded.
This one would start fine and run 25 or 30 miles, then quit. This car had been having this problem for quite some time and, like on the other 3.8, several parts had been replaced in an attempt to resolve the problem.
The owner said she could get it to quit by driving to her friend's house. She said it would not necessarily happen when going other places, but every time she went to a certain girlfriend's house, it would quit.
I decided to follow her to her friend's house since it wasn't that far away. Needless to say, it didn't stall and the girlfriend (recognizing her friend's car in the driveway) came out to investigate. The owner was still adamant that she could make the car quit, so we decided to drive around a bit in the subdivision. A potential breakdown in freezing cold weather must have sounded more entertaining than what her girlfriend had been doing, so she climbed aboard too.
As we test drove the car through the subdivision, I began thinking the stop-and-go driving may be the reason the car would reach the required quitting condition. There were, it seemed, half a dozen stops and starts per block ... and speed bumps. Just as I started to look into the mirror to see if I looked as green as I felt, the car started jerking rather violently. Then it stalled. We waited the 20 minutes she said it needed to cool back down, but it didn't restart.
I got out and pulled a plug wire to check for spark while I had her crank it. The spark was popping nicely but it seemed a little random, like Morse code. If it were any other car, I would say it sounded like a timing chain issue because of the way it popped back through the intake as if it were out of time.
The heat in the car was rapidly dissipating now, as the temperature had dipped below zero. Rather than gamble that the car would restart, I elected myself to walk back to my car. I had on my silk long johns under my uniform and a ski band around my ears. I decided I would be warm enough to walk (the half-mile?) without the army jacket, so I left it for them to share. I figured that by the time I got back to their car it would be sitting there idling.
It was such a nice day for a walk I decided to take the scenic route. After detouring a few extra blocks, I finally saw the street on which I had parked. By now, I was ready for some heat. I got in my car and drove back to the girls. Their car was still not starting.
After dropping them off, I headed back to the shop to round up some test equipment. Later I returned with my scope, some leads, a "pokaholer" and a current probe and connected one lead to the circuit that I thought would be the most telling: the 3X crank sensor signal.
It's amazing how clearly you can think when the still-chilled "logical" side of your brain says to the "curious, playful" side: "We're not playin' around out here pokin' and scopin' pretty signals in 'goose-egg' temperature; get the picture and let's go."
I'm really liking this light blue wire with the white stripe on it. I think it's neat because you get to notice many things from one signal. The first time I hooked a scope into one of these 3X circuits, I saw all that noise and just knew I had found my problem. The only trouble was, I couldn't get the signal "fixed." It took me a little while scoping to learn to make sense of the "noise" in this circuit that is common and found on many 3.8 systems. I thought everything about this signal indicated a problem. In Figure 1 you can see a 3X signal from a well-running 3.8 engine. The signal looks noisy but in actuality it is a good, clean 3X.
I call this the "castle signal" for obvious reasons. The castle effect seen in the 3X signal in Figure 1 has a few underlying implications that can be helpful in diagnosing problems. During cranking, the module in this system determines which coil to fire by looking at the 18X count during the 3X pulse. Since the 3X windows step up progressively, each one has a unique signature.
Notice in Figure 2 that 18X (outer ring) is showing one blade through the 3X windows middle slot, which accounts for two changes of state (on, off) in the Hall-effect signal. This is for the 3 and 6 coil. Now look at Figure 3and see that the narrow slot has only one change of state (1 and 4 coil) while the widest slot actually has three (off, on, off) for the 2 and 5 coil.
When either the slightest bit of noise shows up on the 3X or the 18X pulse, things can get confusing. Look at the 3X signal in Figure 4 from the subject car just before the engine stalled. The widest slot has an extra pulse, which causes the module to miscount the 18X pulses, causing it to fire the wrong coil.
When stacking the primary current pattern over the 3X signal, you can see how badly the module is misfiring the coils based on just a small amount of noise in the 3X signal (see Figure 5).
When this system is working right, during cranking you can see the coil primary circuit start charging during the first 18X signal after the 3X signature signal. Three 18X pulses later, the coil fires. The 18X signal shares the same power and ground supply and, due to loading of the circuit, can be seen riding on top of the 3X signal. This accounts for some of the "noise" seen in the 3X signal. The primary current can also be seen in both the 3X and the 18X signals with a little imagination, due to its loading of the circuit. This accounts for the remainder of the noise. In Figure 5, if you only had the 3X signal to look at, you could still see that the primary current is out of sync with the 3X pulses.
With one lead connected to the 3X and the current probe connected to the primary circuit, by looking at the 3X signal windows during cranking it becomes clear that the module has no idea which coil to fire when. Even though you could hold a spark tester to it and get a good spark, what chance do you have of knowing that the crankshaft sensor is working properly without seeing the 3X signal? The image in Figure 6 shows the primary loading the 3X in about the right spot, but looking at the window slots you know the coils are firing out of time. This car came in with a new crank sensor on the list of parts that had been tried. Looking at the crankshaft sensor signal, though, I knew the 3X was bad and when replacing the sensor I could see what had happened. Someone had bent one of the slots and it was slightly touching the sensor. Possibly, when the thing got up to temperature it was flexing slightly and rubbing the crankshaft sensor.
The way it was bent it looked as though someone had tried to get the sensor out without pulling the balancer off. It took some doing to satisfactorily straighten the pulley blades. Figure 7 shows a shot of the new crank sensor 3X signal with the primary current. By setting the scope to capture the first 3X pulses, you can see that before the first 3X signature, there are no primary current pulses, leaving the top of the castle flat and level. This can help identify a miss in one of these engines when it's due to a bad module or ignition coil.
When you see a "known good" 3.8 3X signal from a close-up, such as shown in Figures 1 and 7, it now becomes possible to see a second and third signal without actually hooking leads to them. I think that is way cool!
When you finish repairing a hole, make sure you use anti-seize on the new studs or bolts before you install them, so you (or the next tech) won't have to go through this the next time. On exhaust parts, use brass or stainless steel nuts and lock washers, so they don't corrode in place.
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