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Vauxhall zafir cam position sensor fault help

posted Jun 23, 2016 19:23:09 by WakasAhmed
Ok so basically i have a vauxhall zafira 1.6 petrol 2001 with a camshaft position sensor fault i have changed both the cam and crankshaft sensor to no avail. When i tested the vehicle originally it would run and had a good cam signal but when it warmed up i lost my cam signal and car cut out so i changed the sensor for a new one along with the cranshaft sensor aswell, but its still the same when it warms up i lose cam signal.
Is there anything im missing?
Or is the new sensor i brought no good?

By the way the sensor is a hall effect and it has a good 5volt supply and a good ground. I even tested the signal wire for a short to ground but it was fine i really would appreciate aome guidance incase ive missed anything
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16 replies
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Dylan said Jun 23, 2016 20:43:36
By the way the sensor is a hall effect and it has a good 5volt supply and a good ground. I even tested the signal wire for a short to ground but it was fine


Ok you did some good work then. So there's no short to ground on the signal wire. Can you tell what you read on the signal wire. You used DVOM if I'm correct. In case you have scope you can always post it, just saying. The new cam sensor, OEM? Aftermarket? Not saying it is bad, just some feedback.
Belgium, Europe
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WakasAhmed said Jun 23, 2016 21:01:28
Thanks for your reply
I used the snap on verus scope to do all my checks and the sensor is new genuine sensor which could potentially be faulty

The signal wire is reading 0 volts all the time and increase by a few hundred milivolts on cranking i disconnected the sensor and connected a testlight from battery positive to the signal wire and it did not light which shows there no short on the signal.

I have also been told that the pcm is a common fault on these engines with the exact same symtoms as i have but that still doesnt explain why i dont have a cam signal with a good 5 volt feed and a good ground unless the sensor is faulty

I even in desperation i even cut the signal wire from the connector side of the sensor to see if it would give me a signal by taking the ecu out of the equation but still no signal which is leading me to believe i must have faulty sensor and potential pcm unless im missing something
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Dylan said Jun 23, 2016 21:38:26
The signal wire is reading 0 volts all the time and increase by a few hundred milivolts on cranking


Oh so not only when it warms up. How much is your ground voltage? Less then 100mV.
Am I wrong when reading your post that there's a difference between the old and new sensor?

Do you have VIN?
Belgium, Europe
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WakasAhmed said Jun 23, 2016 21:53:39
Yes my ground is less than 100 mv and it did start with the new sensor and was producing a good signal then once it warmed up it cut out just like the old sensor and i lost my signal aswell. i was unable to start it after that, i will have to try again tommorow it may start up after sitting all night but i just cant seem to understand why i bave intermittent signal when i have a good 5 volt feed and a good ground to the sensor
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cheryl hartkorn said Jun 24, 2016 00:54:48
you need to monitor the 5 volt ref and signal along with the ground at the same time on a scope to see if your losing the 5 volts ref when it warms up or if your just losing the signal. also see if the ground shows anything other than 100mv.and go from there
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WakasAhmed said Jun 24, 2016 01:16:02
Thank you i will try that next time and hopefully get to the bottom of this
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Tyler said Jun 24, 2016 01:31:12
Just curious, but what exactly is the CMP sensor reading? Cut outs in the rear of the cam, a tone wheel, the cam sprocket? Sorry, not familiar with this engine in the states...

Lots of great suggestions so far. You could also try removing the sensor from the engine (if it isn't too hard), leave it plugged into the harness, and wave something ferrous in front of of the sensor. Scope the sensor and watch for signal.
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WakasAhmed said Jun 24, 2016 01:56:25
The sensor is bolted directly next to the cam sprocket which hs some indentation where the sensor reads off. I did actually remove the timing cover to see if the camshaft is turning and it was. That is a good suggestion to remove the sensor and wave it infront of a magnet but i would of thought that would be a permanant fault rather than intermittent but none the less i will try it and see what happens
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Dylan said Jun 24, 2016 17:00:07
Just curious, but what exactly is the CMP sensor reading?


Yep. I'd like to see or read that as well.

Also a nice suggestion from Tyler to do that test.
Belgium, Europe
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Dylan said Jun 24, 2016 17:04:38
Sorry, not familiar with this engine in the states...


I know the feeling Tyler. Not familiar with 80% or more on all these US cars/engines, lol
Belgium, Europe
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WakasAhmed said Jun 24, 2016 18:54:52
I think i figured out where i was goin wrong so when i was checking the signal wire for a waveform i didnt think if this is a pull up or a pull down design. So when i was reading a flat line 0 volts on the signal it was due to not having a 5 volt reference on the signal wire to pulse to ground and produce a signal. This 5 volt ofcourse is supplied by pcm, so i went ahead today and replaced the pcm which fixed my problem. Thanks everyone for your suggestions
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Dylan said Jun 24, 2016 19:29:01
So like Cheryl mentioned you were losing the 5v reference. Nice job Ahmed. Thank you for giving us the feedback.
Belgium, Europe
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Tyler said Jun 25, 2016 18:59:58
Nice going, sir! Never easy to call a bad PCM.

The sensor is bolted directly next to the cam sprocket which hs some indentation where the sensor reads off. I did actually remove the timing cover to see if the camshaft is turning and it was. That is a good suggestion to remove the sensor and wave it infront of a magnet but i would of thought that would be a permanant fault rather than intermittent but none the less i will try it and see what happens


I misunderstood, thought that the symptom had changed from intermittent to hard after the sensor change. My mistake!

I'm really glad you brought this diagnosis to us, sir, as I think it makes for pretty darn interesting study material concerning pull-up/pull-down circuitry. It seems like a pull-down sensor that isn't getting supplied voltage on the signal wire will *appear* to be a pull-up sensor during initial testing.

I'm not criticizing you at all, just trying to think of ways to identify this condition, beyond knowing circuit design beforehand. Wondering if this sensor would light an LED test light connected to battery positive (backprobed into the signal wire)? That would suggest the sensor is working, and that it's a pull-down design. Also, if the crank sensor is also a hall effect, maybe see if it's pull-up/pull-down?
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WakasAhmed said Jun 25, 2016 21:05:46
Thabks Tyler. Not knowing circuit design can really throw you off as it proved to be the case in this diagnosis. I myself was completely thrown off when i changed the sensor and the fault continued because i was confident the cam sensor was faulty and it would fix the problem, but it was only when i was watching one of pauls case studies when he talked about pull up and pull down circuitry thats when i realised my mistake.

So a big thank you to paul danner for the video's he makes they are very informative and help poeple like me who are atill learning to become better qt diagnosing faults.
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Tyler said Jun 26, 2016 00:54:11
Agreed, big thanks to Paul for introducing this concept. NOTHING in any of my automotive classes talked about hall-effect circuit designs.

Thinking about it some more, I should have suggested doing the unplug test on the sensor when the engine was cold. This (I think) would have shown the presence of 5V on the CMP signal wire, which would have disappeared once the engine got warm. I'm learning!
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