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First time in years that the Bonnie has thrown a code. Middle of last week, I started getting an intermittent SES light. Comes on for a few second, then goes out. Car continues to run ok, but the gas milage took a hit.

Hooked a scanner up and only one code is stored, 41 Loss of signal on Cam Position sensor. Makes sense with the manual's description of the code. Car runs ok, but the gas mileage took a hit as the injectors pulses are no longer timed with the opening of the intake valves, so it has a one in six chance of delivering the fuel right to the cylinder instead of creating a cloud in the intake runner like a batch fire system.

So far, I've reseated the connectors on the engine harness. Removed the sensor and confirmed the magnet is present on the cam gear. That was fun, had to borrow a makeup mirror from my daughter to be able to look in the hole while turning the motor by hand. Also checked the wring from the ignition module to the cam and crank sensors. The plastic shielding on the harness disintegrated, so I replaced it and re-wrapped the harness. Didn't find any damage to the wiring itself. Cleaned the contacts, plugged it all back in and cleared the codes.

Drove the car around for a while, figured I had this problem solved... on the way back from dropping Carolyn off at the apartment of "she who must not be named", car threw the 41 code again.

Gotta work today, but when I get home, I'm going to swap the cam sensor and clear the code again. If that fails, I guess the next stop is the ignition module under the coils, then the ECM itself. At least that is the plan of attack. On a bright note, got a lot of little odds and ends taken care of, seafoamed the motor, cleaned the MAF, and plugged a nagging little leak from the coolant tank.

Thoughts on the code 41 anyone?

Headed down the right track there... Cam sensor, see what it gets you.. Just Curious, did we ever force you into the Delco II ICM swap?

Jrs3800 wrote:Headed down the right track there... Cam sensor, see what it gets you.. Just Curious, did we ever force you into the Delco II ICM swap?

Oh yeah, years ago the Magnavox coil pack/ICM ended up in the trashcan. I've got a spare ICM I can swap after the cam sensor.

Cam sensor is one of those magnetic kind right? I have seen that same kind of technology get sluggish on paper sensors in printers, sometimes a finger flick will dislodge them long enough to get back. Wonder if the sensor is just getting tired?

Very Possible... After all the sensor has to be 20 years old.. Its amazing on the old 3800's things were built to last..

Welp, my spare sensor was off a series II 3800 and slightly different in construction. Instant code 41 instead of an intermittent error when it was installed. Guess it's off to the parts store tomorrow during lunch for a new one...even if it isn't the actual problem I gotta start somewhere.

Still chasing this problem.

Tried following the diagnostic flowchart in the manual. The test of voltages at the cam sensor connector showed 10vdc on pins B-C, but only 6vdc on pins A-B. The mismatched voltages are supposed to indicate a problem with the cam sensor signal wire or the ignition module. I've checked the wiring once and swapped the ignition module. No change in those readings with the spare ignition module or a new cam sensor.

When the engine is running, cam signal going back to the ecm is running about 4.3vdc (varies as it should) but should be close to 10vdc. So, that's where I'm at as of now. I have to wait until the weekend to have another go at checking the wiring harness (maybe I missed something.) In the interim, I'm swapping the spare ECM into the car to see if there is something dragging the cam signal voltage down.

If your magnet has worn down over time and the air gap is too big, I think it would cause low voltage. But maybe I'm wrong in thinking how it works.

J Wikoff wrote:If your magnet has worn down over time and the air gap is too big, I think it would cause low voltage. But maybe I'm wrong in thinking how it works.

That would imply that the magnet is rubbing on something, which it shouldn't be. Wiring issue maybe? PCM putting in for retirement?

Well, changed the ECM with a spare. No change in the symptoms. With all three components swapped and the magnet physically present on the cam gear, that only leaves the wiring.

Time to dip into the spare parts bin again and see if I have a spare harness (or at least the section for the Cam/Crank/Ignition sub harness.) I guess y'all know what I will be doing on this coming Sat morning!

The problem is also heat related (I think). When I clear the codes at night, and the engine is sill hot from the drive home, the code comes back within a few seconds of running. When I start the car in the morning, no code until I get about a mile or so from the house, then it comes on.

It's gotta be in the wiring harness. I must have missed it on the last go-around.

Something isn't right someplace... And its interesting how it seems to be heat related... Have you tried reseating the main bulkhead connector?

If you don't have a Spare I am sure I could find a donor here...LMAO

Jrs3800 wrote:If you don't have a Spare I am sure I could find a donor here...LMAO

No, I haven't reseated that one yet. Probably should give that a go first on Sat before taking the harness apart.

Hey, aren't you supposed to be fixing that car instead of using it as a donor??

Still interested in the outcome of this one for sure. And wondering if I too have a sub harness somewhere for ICM to sensors.

Still not resolved. I've had the entire harness off from the firewall. Can't find the problem or clear the frigging error.

I did notice something different with the wiring on newer ignition modules that isn't present on my '90. Pin K in the newer harness has a ground wire on it. Pin K isn't used on my '90. I'm wondering if I'm missing a ground the module needs. How it worked for the past three years would be beyond me, but that's the only thing I can see different in the wiring. Two ignition modules tried, but have a really low cam signal output.

Ok.. tried it, that's a fail too.. no effect on the signal output.

I' m stumped...

Jrs3800 wrote:I' m stumped...

Me too... I'm getting ready to drop $120 on a new ignition module (just in case both the ones I have are an issue) along with a cam magnet, oil pan and timing cover gasket set. Definately ordering up the gaskets and magnet today, since they are special order and if I need them tomorrow or Wednesday, I gotta order them today. If I don't have to use them, well, so be it.

I may be driving this car 2000 miles to PA and back on a SES light. It's a hard failure now, nothing intermittant about it anymore. Turn the key, it starts up, and throws a 41 and stores it as a history code.

I'm going to start back at zero tonight by looking at the cam position magnet again and making sure there is a magnet in the holder. There was one there the last time I looked and touched my fingers on it. But at this point, I gotta start somewhere and looking at the magnet looks like the place to start. This time I'm going to pull the fender well and look at it directly instead of using a mirror. Perhaps the holder is there with no magnet or it backed up somehow through the hole in the timing gear. I'm just speculating at this point. I know it's not in the wiring. Multi-meter is showing 0.002 ohms on every wire end to end...that's a perfect score since touching the meter leads together is also 0.002ohm. Can't be wiring.

I figured I was on to something with this empty pin K on the ignition module, but I put a contact into K and tied it to engine ground with the rest of the harness grounds. Had zero effect on the signal output. Still lower than spec.

Did you check for continuity between any of the cam sensor wires and ground? Something may be shorted but not broken.

J Wikoff wrote:Did you check for continuity between any of the cam sensor wires and ground? Something may be shorted but not broken.

Yep, even went so far as to strip all the tubing off the harness, clean it, and re-wrap the entire length. There's no contact between the signal wire and ground (or any other circuit.) The sensor ground and ref voltages are in spec, and also power the crank sensors which are working fine.

The cam signal going into the ICM is not even used there. you could jumper pins F and J on the ICM harness before buying a new ICM.

I also read in my FSM that the magnet causes the signal output to go to zero. Have you tried puting the magnet in front of the sensor hole and replacing the sensor and then manually rotating the engine each way to see what the output from the sensor is? (pin J on ICM)

myfirstbonnie wrote:The cam signal going into the ICM is not even used there. you could jumper pins F and J on the ICM harness before buying a new ICM.

I know the circuit description in the manual says the signal gets passed through...but to me, why bother putting it through the ICM in the first place? I'm thinking that GM used the solid state drivers within the ICM as part of the "pass through"... like the module is providing the ground and signal reference, feeding it through a internal line driver, and spitting the output out the other pin back to ECM. That's speculation on my part.

GM didn't make it easy to get the ICM connector apart. I have yet to try isolating the sensor output from the ICM and getting a reading from it powed up. I've only done the tests in the manual flowchart which have the entire circuit connected (except the first, where the voltages on the A-B and B-C are supposed to be over 8 VDC and the same, which mine failed. I've got 10 vdc on pins B-C (which is the signal ref voltage and ground ins of the sensor) and 6 VDC on the A-B pins, which is the signal wire and ground.... which was supposed to indicate a wiring fault or a bad ICM.

myfirstbonnie wrote:I also read in my FSM that the magnet causes the signal output to go to zero. Have you tried puting the magnet in front of the sensor hole and replacing the sensor and then manually rotating the engine each way to see what the output from the sensor is? (pin J on ICM)

Worth a try. I've got an extra cam sensor harness, I figure I can put wires on the ends to simulate pins, plug it in to the cam sensor and use it to read the output of the sensor directly while rotating the engine. At that point, all the ICM is providing is the supply reference voltage and the ground...which I already know are good values.

If what I suspect about the ICM is true, I'm not going to see anything on the signal wire even though I have the sensor powered and I'm passing a magnet by it.... been a long time for me, but a hall effect sensor uses the magnetic field to mess with the current flow, not the voltage. I.e. impedance, not resistance So to get a reading, there has to be some current flowing through the signal wire. (Kinda like the pickup on an electric guitar...no amp hooked up = no signal.)

I'm willing to try just about anything at this point to narrow down which part is causing this. Through process of elimiation, I though of the ICM because it is the only one that I haven't tried a new part on (the spare I had was used and never tested..so it's not impossible that I had a bad spare.)

The more I talk this out, I'm thinking of starting tonight with divide and conquer approach. The weather-pak connector on the top center of the motor is where I can seperate the cam signal from the ecm. Take the cam signal pin out of the ICM side of the weather-pak...essentially eliminating the ECM and body wiring from being involved, leaving the ICM sub-harness, ICM, Cam and Crank sensors in the circuit. Since the motor is going to be hot when I get home, I can at least start there while the sucker cools off from the drive home.

That fails, then I'll jump to the other side of the harness and make up a test harness to plug inbetween the cam sensor and harness, so I can break out the signal before the ICM.