Keith Browning Posted March 21, 2008 Share Posted March 21, 2008 I started a topic about what settings you guys were using on your silly scopes when checking CKP and CMP sensors. The reason was I did not believe, or did not realize what I was looking at. I am diagnosing a 2004 SD 6.0L that has been towed in a few times for a cranks no start condition. I was not the original tech but I did assist on some diagnostics so I know the history. The tech was at his wits end and as the team leader I took the job over. I figured I would scope this thing and see whats going on. The problem was that this truck starts most of the time, except when it sits outside overnight. Code P2416. Passes all pinpoint tests, shows no FICM SYNC when it doesn't start and the CMP_F =YES at times. The tech discovered that a couple squirts in the intake and the truck would start and start every time after until it sits overnight. I had it in the shop setting it up on the scope for the next morning and I had some time to play with it. Here is a screen shot of the display of a capture WITH THE ENGINE RUNNING. I thought these wouldn't start or run without a CMP signal? This is why I did not believe what I saw. The original tech replaced the CMP and the PCM... the Hot Line diverted his attention to the CKP sensor with some silly explanation as to why. Long story short, I decided to recheck everything myself. This is what I found. The sensor on the left is the new sensor the tech installed less than a week ago. The sensor on the right is new, right out of the bag. Somebody here mentioned that they found the block rusted under the CMP on a truck so I looked closer at the block. No shit! Rust formed under the sensor and pushed it away from the block, distorting it causing the sensor to move away from the camshaft. With the air gap too great there was no signal generated. Tomorrow I will show you the rusty hole... Quote Link to comment Share on other sites More sharing options...
LARRY BRUDZYNSKI Posted March 21, 2008 Share Posted March 21, 2008 Oh nice... Some more shit to have to worry about...Like we need more... /forums/images/%%GRAEMLIN_URL%%/mad.gif Better question, WTF causes the rust???? Quote Link to comment Share on other sites More sharing options...
Jim Warman Posted March 21, 2008 Share Posted March 21, 2008 Keepppthfff.... I'm not sure we wanna see your "rusty hole".... /forums/images/%%GRAEMLIN_URL%%/smirk.gif Good catch all the same.... Quote Link to comment Share on other sites More sharing options...
LARRY BRUDZYNSKI Posted March 21, 2008 Share Posted March 21, 2008 Quote: Keepppthfff.... I'm not sure we wanna see your "rusty hole".... Lookie lookie Jim made a funny /forums/images/%%GRAEMLIN_URL%%/hahaha.gif Quote Link to comment Share on other sites More sharing options...
Keith Browning Posted March 21, 2008 Author Share Posted March 21, 2008 Oh nice... Some more shit to have to worry about...Like we need more... /forums/images/%%GRAEMLIN_URL%%/mad.gif Better question, WTF causes the rust???? Here in NJ we use salt on our roads in the winter which is bad enough. This truck is one that is used to spread the salt so everything underneath just rots away. Quote Link to comment Share on other sites More sharing options...
Mekanik Posted March 21, 2008 Share Posted March 21, 2008 Its kind of like an ABS front wheel speed sensor in a 95 explorer mounted on the hub. Same thing would happen. I don't think you need to post the pic of a rusty hole. We can all find that stuff on the web. Quote Link to comment Share on other sites More sharing options...
Bruce Amacker Posted March 21, 2008 Share Posted March 21, 2008 The tech discovered that a couple squirts in the intake and the truck would start and start every time after until it sits overnight. That sounds exactly like high air gap/low sensor output voltage would do. Give it a squirt, the RPM picks up as it starts, the AC style sensor (PM generator) puts out more ACV, and the PCM recognizes it. I had it in the shop setting it up on the scope for the next morning and I had some time to play with it. Here is a screen shot of the display of a capture WITH THE ENGINE RUNNING. Yes, by my captures it should have peaked positive about 14 CKP pulses before the CKP identifier and the bottom sweep should have been about 8 pulses before the CKP identifier. You missed about 2 whole divisions of CKP signal, but the spots where the CMP should have been are there. I thought these wouldn't start or run without a CMP signal? To my knowledge that's true, but I have never proved it out. Anthony pulled the CMP plug off an '08 DT and it started, and that is wired very similar to a 6.0. (Who's by-line says ,"How do you know that won't work? Have you tried it yourself?!) I've always thought if I was writing PCM software and the CMP dropped out I would tell the PCM to try firing injectors until it started. It's a 50/50 chance, so if it cranks long enough it will start. Some GM gassers are like that, and will revert to bank firing injectors and guessing at spark until it lights up. Some other GM's will not start with the CMP dead, but if you pull it off running it will continue to run. Remember in theory, the PCM only needs ONE CMP pulse to start it and drive a complete key cycle, however long that may be.The CMP is only there to tell the PCM whether the engine is on the compression or exhaust strokes. Got some spare time? Open the CMP while it's running and see if it stays running. A smart PCM should.... This is why I did not believe what I saw. The original tech replaced the CMP and the PCM... the Hot Line diverted his attention to the CKP sensor with some silly explanation as to why. Long story short, I decided to recheck everything myself. This is what I found. The sensor on the left is the new sensor the tech installed less than a week ago. The sensor on the right is new, right out of the bag. Somebody here mentioned that they found the block rusted under the CMP on a truck so I looked closer at the block. No shit! Rust formed under the sensor and pushed it away from the block, distorting it causing the sensor to move away from the camshaft. With the air gap too great there was no signal generated. This is a big pattern failure on GM pickup WSS causing false ABS activation at low speeds. Pull the sensor, clean the hole and it's fixed. It happens only in the rust belt. Tomorrow I will show you the rusty hole... Hush your mouth, boy! /forums/images/%%GRAEMLIN_URL%%/grin.gif PS I have to update a big error on my last scope post. That CKP wave wasn't 80v, it was 8 volts, I had the frigging 10x turned on on my scope. Duh. Quote Link to comment Share on other sites More sharing options...
Keith Browning Posted March 21, 2008 Author Share Posted March 21, 2008 Bruce, the missing divisions were on the original screen shot. I cropped the picture. We decided after looking at this to clean the scale from the block, hit it with some rust inhibitor and put in another sensor. Quote Link to comment Share on other sites More sharing options...
Bruce Amacker Posted March 21, 2008 Share Posted March 21, 2008 BTW, the scope teachers will always bitch at you for having a small waveform on a big screen. Get your settings set up where the waveform fills the screen. There's things that you can see when the wave fills the screen that you can't see when it's squeezed down in size. HTH, /forums/images/%%GRAEMLIN_URL%%/grin.gif Quote Link to comment Share on other sites More sharing options...
Jim Warman Posted March 21, 2008 Share Posted March 21, 2008 Good point on the amplitude, Bruce... I think that most that aren't at least a little familiar with the scope don't realize that the faster we move the magnetic lines of force across the coil in the sensor, the higher the peak voltages we will see. Quote Link to comment Share on other sites More sharing options...
Bruce Amacker Posted March 21, 2008 Share Posted March 21, 2008 Correct on an AC (PM generator) style of sensor, but not on a digital sensor. On an AC sensor, the faster the shaft spins, the higher the voltage output. On a digital sensor like a Hall Effect, there is no relationship between output voltage and shaft speed. To refresh memories there are two common styles of motion sensors used in automotive applications: AC Output, also called a PM Generator or Analog sensor. This is what we're looking at on the 6.0 CKP and CMP, and 90% of ABS WSS. The advantages of these sensors is that they are very reliable. One disadvantage is that the PCM cannot process an analog signal. This requires and extra component inside the PCM called an A/D Converter (Analog to Digital) to change the analog voltage into a digital signal that the PCM can understand. Another problem is that for an AC sensor to put out a signal, the shaft has to be spinning fast enough to generate a signal from breaking the magnetic field. Therefore, these sensors need "some" RPM to work. (Hall Effect and MR Sensors work at a creep speed, PM's do not) If you scope the CKP in (analog going into the PCM) and CKP out signal (digital signal going to the FICM) on top of each other, you'd understand this relationship, and the need for converting analog into digital. I drew an analog CKP signal on top of a real CKP out signal to show you what it would look like. Note the time base is expanded- 1ms per division. (Keith, what are your time and voltage bases in your waveforms? That's real important.) How do you identify it? Usually two wires with no VREF input. It may have some voltage on the line for CCM to verify circuit integrity. Fake (analog) CKP in overlaid a real (digital) CKP out This is a good example of what an A/D converter does inside the PCM. The second most popular motion sensor is a regular old Hall Effect, like the 7.3 CMP had. It is also called a Digital Sensor, and it's signal does not need to go through an A/D converter like an analog signal does, because it's already digital. Advantage- no A/D converter needed. Disadvantages- higher failure rate because there's electronics built in. Also, it usually costs more because of the electronics built in. Output is a square wave of any voltage depending on what the design is. How do you identify it? Usually three wires with a VREF input. Those two motion sensors probably comprise 90+% of CKP, CMP, and WSS on a car. The other sensor coming into popularity is a MR or MRE sensor, which stands for Magneto Resistive. These are getting more popular for WSS and some CKP sensors. They look like a two wire PM generator, but have a digital output like a Hall Effect. The signal is a unique square wave that is also directional, that means the waveform is different when the shaft spins forward or backward. Advantage- Extremely accurate, like measuring vehicle movement by the inch in a WSS. Regular PM generators need several MPH before they read vehicle movement. Output is a square wave, but commonly very small, like 1v in height. Application: Ford "Active" WSS sensors and the speed sensor on 4WD transfer cases. Focus rear WSS on drum brake cars. Also many WSS in high end Japanese and Euro cars. How do you identify it? Usually two wires, but one has a higher (12v?) voltage on it, because it needs a power supply. Also you can identify these by the price tag. ($$$) They may have a bunch of magnets built into the reluctor instead of a metal ring with tabs, like we're used to. (Focuses have the magnets built into the wheel bearing grease seal!) Why do you need this expensive, highly accurate sensor? Because with Traction Control, Stability Control and etc, AC sensors are too sloppy. You need a highly accurate sensor in a ATC system. On some VW vehicles, the WSS do not have any connectors between the WSS and the ABS module, which means the WSS has a 20' wire harness on it and you have to take half the car apart to change them. Why no connectors? Because of the low output voltage, they are afraid to introduce connectors and the associated problems with connectors. My soapbox just collapsed... /forums/images/%%GRAEMLIN_URL%%/grin.gif Quote Link to comment Share on other sites More sharing options...
Jim Warman Posted March 21, 2008 Share Posted March 21, 2008 Generally speaking (and statements this bold usually don't go unpunished), VRS sensor circuits will have the two wires twisted as a pair. This is to reduce the possibiliy of RFI inducing a spurious signal (much like a bad ignition coil can set the ABS light). Late model Ford (starting around 04 or 05) wheel speed sensors are no longer the VRS type, though they do have only two wires. I'm starting to grow a little too fond of your scope... While the scope in the IDS is much better than the cartoon we got with the WDS... I'd like the user interface to be a little easier to deal with. Quote Link to comment Share on other sites More sharing options...
Bruce Amacker Posted March 21, 2008 Share Posted March 21, 2008 Late model Ford (starting around 04 or 05) wheel speed sensors are no longer the VRS type, though they do have only two wires. Right- these are what Ford calls "Active" sensors, that the rest of the world calls MR sensors. I'm starting to grow a little too fond of your scope. The Pico IS the cat's ass. Bernie Thompson at Automotive Labs has an 8 channel PC based scope I tried, but I still like the Pico better. You wouldn't believe how fast, or how slow you can make it. As fast as 500ns per division (one half of a millionth of a second) or as slow as 50 seconds per division, with a multiplier that can go to many hours. That's great for looking at dropouts. if a dropout occurs, you can zoom in on a small portion of the screen to see which sensor fell out first. /forums/images/%%GRAEMLIN_URL%%/grin.gif Quote Link to comment Share on other sites More sharing options...
tecanic1 Posted March 23, 2008 Share Posted March 23, 2008 sure the block isn't magnetized....hahahahaha same exact thing on the one i had that the fsc was spewing magnetism about. sanded block and replace sensor just about three weeks ago and truck still starting...thank god. that one kicked my asss.. Quote Link to comment Share on other sites More sharing options...
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