How to test your camshaft position sensor

[hear]

This sort of thing comes up often but I've never really seen any sort of diagnostic process documented. We like to blame sensors for all sorts of things, and if you're like my wheeling crew, you keep a spare set on hand and then just replace them willy nilly whenever your poopbox isn't running right. So I figured it would be helpful if we actually had a process to test as many aspects of these sensors as possible. We can't test everything because we don't know everything. Like we might validate that the camshaft position sensor (aka CMP) is producing a periodic pulse, and we can even measure the duration of the pulse, but do we know how long a pulse the computer is expecting? Stuff like that. So let's get to testing what we can actually test and not worry about the stuff we can't.

The hardest part is knowing what the pinout is for your year, and then being able to use your expert spatial reasoning to figure out which pin on the sensor maps to which cavity in the plug. It's also fun to see the variation in FSM graphics over the years.

1997, 5V supply in cavity 1, sensor signal in cavity 3

1998-2004, 5V supply in cavity 3, sensor signal in cavity 1.

2005-2006 changes back to the 1997 configuration with the 5V supply in cavity 1 and the sensor signal in cavity 3. Why these things needed to change, I'll never know.

Bench Test
Many of these sensors operate on a +5V reference supply supplied by a specific pin on the PCM. If you have the sensor out of the jeep, you can test it with a phone charger & a sacrificial USB cable (or a USB to red/black alligator cable which you can get off amazon for a few bucks): https://www.amazon.com/dp/B09PQFCJJ3?_encoding=UTF8&psc=1&ref_=cm_sw_r_cp_ud_dp_QBYMFRBS9K0YD698TKBC

• Connect the 5V supply (+) to the 5V supply pin on the sensor
• Connect the 5V supply (-) to the sensor ground pin
• Connect the voltmeter (+) to the 5V supply (helps if you use alligator clips to extend the sensor pins out in the open.
• Connect the voltmeter (-) to the sensor signal pin

Your voltmeter should read +5V. If you pass a piece of metal like a screwdriver between the magnet and the actual sensor in the housing you should see the voltmeter drop to 0V.

+5V Supply Test
Verify you get 5V at the connector by turning the key on (engine off) and sticking a probe in the 5V supply cavity on the connector, and then grounding the voltmeter at the negative battery post (or other good ground).

CMP Signal Continuity Test
The CMP signal goes from the sensor to PCM pin A18 (except for 2005 & 2006, which are on PCM C34). Check for 0Ω or continuity between the sensor signal cavity and PCM A18.

CMP Ground Continuity Test
The CMP ground comes from PCM pin A4 (except for 2005 & 2006, which are on PCM C27). Check for 0Ω or continuity between the sensor ground cavity and PCM A4.


CMP Running Signal Test
This is the fun test because I got to use a tool I hadn't used since college: an oscilloscope. I got this Arduino based oscilloscope off Amazon for $33. It came with the power supply & a BNC connector with red/black alligator clips on it. The instructions were lacking, but I was able to calibrate it eventually.

• Oscilloscope red to back probe on sensor signal wire. This back probe was a challenge to get in, and you should probably do a continuity test on the corresponding cavity to ensure you actually have the probe in place.
• Oscilloscope black to chassis ground
• Start the jeep.

You should see a nice clean square wave with a ~5V peak & 0V trough. And if you give it some gas, you should see the pulse width shrink. It is left as an exercise to the reader to determine the engine speed in RPM based on the pulse width. Happy testing!!

 

[hear]

Ok fine. I'll do it.

I'm going to estimate that the pulse width is ~55msec, based on the 20msec/div scale shown. I'm also going to guess that there is a single "trigger" in there, meaning every full revolution of the distributor shaft will trigger the sensor once, aka duty cycle of 50%. This is just a guess, I haven't looked at it. We'll attempt a proof by contradiction. So our angular velocity is 180*/0.055sec, or 3273*/sec. Multiplying by 60 gives us rotated degrees per minutes, and since there are 360* in 1 revolution, dividing that by 360 gives us ~545RPM, which is in the ballpark. So I think I'm right that there is just one trigger inside there.

I don't know what this TJ idles at off the top of my head, my guess would be in the 650RPM range, so it wouldn't take but a small error in that pulse width (either through my eyeball measurement or calibration) to make the math work out.

 

[Mike_H]

Doesn’t the crankshaft rotate 2x for the cam? I don’t see where you accounted for that. Or maybe the speed is corrected in the distributor shaft?

 

[hear]

Doesn’t the crankshaft rotate 2x for the cam? I don’t see where you accounted for that. Or maybe the speed is corrected in the distributor shaft?

Maybe you’re right, but also as I describe it, it wouldn’t be able to give any useful info about the cam position. So my math is wrong somewhere.

I got excited about doing math and using my new toy, I may need to open it up to better understand HOW it all works. But this was primarily about how to TEST the sensor circuit, so nothing changes there.

 

[Mike_H]

Maybe you’re right, but also as I describe it, it wouldn’t be able to give any useful info about the cam position. So my math is wrong somewhere.

I got excited about doing math and using my new toy, I may need to open it up to better understand HOW it all works. But this was primarily about how to TEST the sensor circuit, so nothing changes there.

I think the video you did was great. Very clearly presented. I do know that one revolution of the cam shaft operates both the intake and exhaust, and in a 4 stroke engine that is two revolutions of the crank (intake, compression, power, exhaust). So, the cam is running 1/2 speed of the crank. I just don't know if the cam gear then speeds the dizzy shaft back up to make it 1:1.

I also think the math is more complicated than you've laid out...you're looking at time in degrees and the width of the "post" that the sensor would pick up needs to be accounted for (a wider tooth will have a longer signal). By rule of thumb, you're probably pretty close, but there are some questions that need to be answered to trust the rule of thumb.

 

[hear]

I also think the math is more complicated than you've laid out...you're looking at time in degrees and the width of the "post" that the sensor would pick up needs to be accounted for (a wider tooth will have a longer signal). By rule of thumb, you're probably pretty close, but there are some questions that need to be answered to trust the rule of thumb.

I made an assumption about the length of the "post" in my initial (faulty calculation). I guess I could work backward from the known RPM value and figure out how it works inside.

 

[hear]

Forgot I had my 97 distributor laying around. The CMP works the same regardless of distributor or sans-dizzy, and what do we have? The “post” is 180*, just like my initial assumption.

I’m guessing that the start/stop positions of the trigger is aligned with cyl 1 and spans the compression to exhaust exactly.

 

[sab]

There's a good thread on another forum about diagnosing problems with cam sensors using a 'scope (not sure if it's okay to post a link to another forum here, so I won't). @Chris, is it okay to post a link here? You really need a two channel scope to diagnose issues because it's the relationship between the crank position and cam position that is screwed up by the crappy aftermarket sensors. In that thread, it was learned that aftermarket cam position sensors have a wider pulse width, and the increased width can push either the leading or trailing edge of the waveform past the crank pulse, screwing up the number of crank pulses occurring under the cam pulse. The participants in that thread were successful in rotating the OPDA to get the pulses closer, and they used a DRBIII to trigger the re-learn/sync, if I remember correctly.

 

[hear]

Post the link. If you need permission to post a link then I don’t understand why we’re all here.

But just to reiterate, the point of my post was not definitively test once and for all every single aspect of the sensor & interdependent systems. I even specifically mentioned the pulse width as something we don’t necessarily have a quick lookup for the correct value.

 

[Chris]

There's a good thread on another forum about diagnosing problems with cam sensors using a 'scope (not sure if it's okay to post a link to another forum here, so I won't). @Chris, is it okay to post a link here? You really need a two channel scope to diagnose issues because it's the relationship between the crank position and cam position that is screwed up by the crappy aftermarket sensors. In that thread, it was learned that aftermarket cam position sensors have a wider pulse width, and the increased width can push either the leading or trailing edge of the waveform past the crank pulse, screwing up the number of crank pulses occurring under the cam pulse. The participants in that thread were successful in rotating the OPDA to get the pulses closer, and they used a DRBIII to trigger the re-learn/sync, if I remember correctly.

Go ahead and post it up!

 

[sab]

Post the link. If you need permission to post a link then I don’t understand why we’re all here.

Go ahead and post it up!

I didn't post it because this is @Chris 's forum, and the link is to a competitor's forum. Some forum owners don't like links to competitors. So, without further ado:

https://www.jeepforum.com/threads/l...ensor-waveform-for-2005-wrangler-4-0.4434961/

But just to reiterate, the point of my post was not definitively test once and for all every single aspect of the sensor & interdependent systems. I even specifically mentioned the pulse width as something we don’t necessarily have a quick lookup for the correct value.

No need to reiterate. I was simply adding to the thread, not correcting you in any way. I appreciate the efforts you went to in creating this thread, as I have some interest in better understanding why aftermarket cam sensors don't work and how to fix that (but since I don't currently have issues, and I have a long list of to-do items for my build, I haven't dug in, yet.) I also have some engineering experience with solving cam/crank sensor timing issues almost thirty years ago on a fuel-injected V-twin race motor. Hall effect sensors can be a real PITA. The output is affected by many factors, and it can be challenging fixing problems with them. A two-channel oscilloscope is an important tool to the diagnostician, and I just wanted to point that out.

 

[hear]

That's a great discussion. I did have lots of questions in my head about the cam sensor itself, you can even peer into my brain up above as I'm doing the math to estimate the RPM off the dwell time on the scope with respect to how the PCM can determine cam position from that huge trigger signal. And then I saw the inside of the OPDA on the "05 won't start" thread currently on the front page and it had the 1/2/3 notch pattern which I couldn't reconcile with what I have in my 97 & 02, but makes more sense from a "how does this even work?" standpoint. Knowing that the flywheel/flexplate & cam sensor changed waveform on the 05-06 may help someone someday.

 

[hear]

There's another nugget on page 3 of that thread, where they mention if your PCM happens to lose the 5V reference signal you can wire in a 5V phone charger to supply that reference signal to drive the sensors. That's pretty freaking clever.

Also, that multi-channel scope is only about $150....still more than nearly anyone needs to invest here, but good to know it exists if you ever need to troubleshoot the cam/crank sync.