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Sensors Get 'Cranky' on '96 STSPosted 7/23/2007
By Jeff Bach
"What's it doing?" I ask, to get a feel for whether or not it's a quick fix.
"It won't start," the guy says.
"Does it crank?" I ask.
"Oh yeah, it spins over fine but it just won't fire."
"What kind of Cadillac is it?" I ask, hoping he says something besides "Catera."
"It's a '96 STS."
"Great," I'm thinking. These are right up my alley. I have plenty of experience and spare parts for them. I even have one that I drive. Should be a piece of cake.
"What do you think it will cost to diagnose?" he asks.
A no fire on a Northstar, I'm thinking, shouldn't take more than a couple of hours to figure out, but I usually find myself underestimating my time - so, to err on the side of caution, I tell him my price.
"When can you look at it?"
"If you get it here tomorrow, I'll work it in sometime during the day."
Later that afternoon, I see a "rollback" coming up the street with a '96 STS with that it's-been-sitting-in-the-yard-a-while-look, and I start to get that sinking feeling that I took on another project.
"Where do you want it?" asks the driver. I have him put it in front of the bay door where I have another Northstar spread out on the floor undergoing a head gasket and lower block seal job, thinking, "I'll get this one running before I get the engine job done." As I take a closer look at the car, I can see that it's had some recent rearranging of the harnesses and hoses that makes me realize that this didn't just quit while running down the road.
"I bought the car from a friend of mine," he says. "Haven't even heard it run, yet. The guy blew the engine and bought another from a junkyard and put it in and then he couldn't get it running. We put in a new module, coil pack, computer, cam sensor and both crank sensors and it still won't fire. I hope you have better luck with it than I've had."
"Great," I'm thinking. An amateur engine installation on one of these is a prescription for a headache. I look at the car, and there are grounds hidden on nearly every corner and one invariably twists off at the stud in a place that's not what I would call "easily accessible." "I don't really have time for a long, drawn-out diagnostic dilemma," I'm thinking. I get started, grabbing the feed wire to the coil with the current probe. She's a flat line. Next, I check for ignition power and ground. Both test good. Knowing now that I have power and grounds but no primary current, I decide to jump right to the crank sensor signals. The wires on each end of the plug to the module are the crankshaft position (CKP) signals. I put a piercing probe in the positive circuits of both A and B sensors and connect my scope. The results are shown in Figure 1.
I can see now that I have a problem with the all-important polarity on the crank sensor in question. This engine uses two crank sensors mounted about 17 degrees apart from each other to determine crank position. To confirm my suspicion and make sure of the correct polarity, I consult my waveform database and pull up a known good set of sensor signals shown in Figure 3.
The pattern in Figure 3 clearly shows the correct polarity for the sensors. This lets me know that the wiring to the crank sensors has been repaired at some point. Access to the wiring on the sensor end is limited at best and requires having the car on a lift. Since the lifts are all tied up for now, I decide to reverse the polarity at the module end. The signals now look to be the correct polarity but the engine still doesn't start and has no primary current. After studying the pattern I now have - compared with the known good one in Figure 3 - I realize that the wrong sensor is in the lead position. I begin to wonder if the "fool proof" crank sensor plugs have somehow become swapped also. I raise the car with a floor jack and roll underneath to observe. I discover a suspicious-looking lump in the crank sensor harness. I also notice that the top sensor connector that is normally a gray color with an extra plastic leg protruding down from it makes it nearly impossible to plug on the bottom sensor, which has been replaced with what looks like a generic coolant sensor connector. The lump, as it turns out, is a set of butt connectors. The modified harness connectors now make it possible to mix the two sensor circuits and thoroughly confuse the powertrain control module (PCM) and ICM. I switch the wires on the two sensors and have another look at the scope pattern. The car now fires right up, which is the first time this engine has run since the transplant.
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