PicoScope 7 Automotive
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|Nick Hibberd | Hibtech Auto-Electrical Diagnostics
The vehicle was a Fiat Ducato 2004 2.8 JTD with a repetitive complaint of poor starting. The engine would crank at good speed but took a while before it eventually fired, and once started the vehicle drove fine. The customer was adamant that this problem only reared its head after a cam belt failure and major top-end repair work. The vehicle got passed round his local garage network with different verdicts ranging from timing misaligned to new sensors fitted all round. The vehicle arrived at one of my trade clients, who understandably didn’t want to take on someone else’s leftovers, and the fault was passed over one more time for my opinion.
Checking the system with the scan tool, I found one DTC logged relating to ENGINE RPM/CAM SYNCHRONISATION. This sheds light on the attached history and the new CMP and CKP sensors already fitted. The DTC would reappear each and every time the vehicle was cranked and, once the vehicle was eventually started, could be erased until the next start-up. Monitoring live serial data, I saw that the synchronisation PID mirrored the symptoms, taking a long time before it registered a “YES”; incidentally, the same time as the engine fired. This correlated nicely and gave a good indication of where to focus the test plan. Until the ECM is happy with the cam/crank indexing, it cannot identify the cylinders and subsequently deliver fuelling.
The scope was hooked up to the crank sensor and cam sensor outputs. This vehicle application uses a Variable Reluctance CKP (sine wave), and a Hall-effect CMP (square wave). Fig.1 was captured during a typical poor starting event and shows an obvious problem with the CMP signal. There is no signal at the beginning of the crank command with a progressive signal structure emerging as cranking continued. Towards the end of the trace we see the signal stabilise enough for the ECM to recognise a good indexing and allow a successful start.
From these initial findings the CMP sensor circuit needs to be explored in more detail.
Fig. 2 shows another poor start event with the same delayed and erratic CMP signal. Here we are monitoring the sensor’s power supply and ground which remain good throughout the fault. Although the signal line is erratic, there doesn’t appear to be any voltage leak outside the normal operating window: the two states 0 V and 5 V remain consistent as opposed to drifting to other potentials. Also the transitions between the states are clean switching, giving an excellent DC pulse, just not the correct pulse duration and not in sync with the crank. For the moment I’m not thinking that this fault is cabling or connection related.
Our problem is that the CMP sensor is unable to generate a good pulse at the start of the crank command. Why?
On this engine the CMP is hard to miss, sitting proud on the rocker top next to the cam gear. The sensor itself is bolted onto a bracket and can only be mounted one way. But the sensor mounting bracket has quite a lot of lateral movement available towards/away from the cam gear. On a positive note, this can be an advantage to fine-adjust any discrepancies, but also leaves the assembly open to agitation or becoming disturbed.
A common problem associated with CKP and CMP sensors is proximity between a rotating reference mark and the sensor face: the gap can be too large for the reference mark to interrupt the sensor’s magnetic field. Often this gap cannot be seen and although the external surface mount of the sensor appears a flush fit, this is far from conclusive that the signal output will be good. The ONLY reliable test is monitoring the signal through an oscilloscope and deciding from there if the signal is suitable or not. Fortunately we have a ringside seat for examining this CMP gap, which can confidently be ruled out. However, there is something else to consider.
We have to remember this sensor uses the Hall effect principle. An internal current-carrying IC is placed in a small magnetic field, and as our reference mark moves into the sensor’s field this changes the strength of that field and deflects the IC current path creating a new millivolt potential across the IC – it’s this millivolt potential which is the Hall voltage. From here the voltage is evaluated internally and emitted as a DC pulse switching between ON and OFF states.
One of the advantages of the Hall effect is that it is not speed-dependent, so it will recognise a given state at any speed, even at stationary. This operational characteristic is key. If we overlay this information to our CMP sensor and its mounting position we see a problem. Almost a third of the sensor’s face is obstructed by the cam gear during periods where the reference mark is absent. This OFF state is likely enough to fall into the magnetic reach of the sensor and incorrectly report that a cam reference is present.
The mounting bracket was removed and spacers placed underneath to elevate the assembly away from the cam gear, making sure the cam trigger would comfortably pass across the sensor’s face. This should now give the sensor a good magnetic distinction between the two states and hopefully improve starting performance.
Fig.3 is a typical capture taken after the bracket adjustment and shows an immediate response from the CMP together with a quick start-up; overall much better. The capture also reports a return of the CMP’s 5 V bias line when the ignition is switched on. Back in Fig.2 with the ignition on, our CMP’s output remained in the ON state (0 V), which we can now attribute to the cam gear hanging in the sensor’s magnetic field.
This was an unusual problem. The vast majority of CMP sensors are either fixed into the cylinder head or fixed adjacent to a diesel pump, where little user thought need be applied. It was clear that someone in the vehicle’s history had understood the importance of having a speed/position sensor close to a reference gear. However, as this fault-find demonstrates, it is not the only operational factor to consider.
Not related to this particular fault, but did you notice the recorded CKP signal levels? 32 VAC during cranking and 65 VAC at idle: surprising components are VR speed sensors. These values were a little aggressive and it was worth spending a bit of time tweaking this voltage down to a more manageable level. As a benchmark I use 15 to 20 VAC at idle speed, which usually creates a good induced voltage at cranking, and keeps a safe voltage at high rpms.
Tip: Don’t verify that the sensor is working by licking the end wires!
August 28 2021
great post, had issue not starting first time since had van 3 years , few washers as per your instructions starts 1st time every time great thanks so much for taking the time to put this on the net
May 18 2021
I recently purchased a Burstner 591 Motorhome with the 2.8 JTD engine and was a little disappointed that it would take more than one attempt to start, whether immediately from cold or after a long or even a short run,which was very frustrating. I then came across this really concise and clearly explained article which detailed one of the causal issues for the problem I was experiencing and decided to give the recommendation of elevating the camshaft position sensor slightly to achieve a more managed signal from the sensor. I could not believe the difference that 4 plain washers could make - the engine now fires immediately and starts without any hesitation or prolonged use of the starter motor, which would ultimately shorten its life-cycle. I cannot thank the author of this article enough - no more embarrassing turning the engine over endlessly, either on site or after refuelling, waiting for it to fire into life, much to the amusement of the other people present at the time (probably more down to my embarrassment than reality). Definitely worth a try for those experiencing the same symptoms and for the price of 4 washers and a few minutes of labour, extremely good value.
December 19 2018
Thanks for brilliant analysis on this problem. I was instructed by workshop staff, that the distance should be exactly 1 mm to prevent from DTC. This did not work. The effect was that the motor startet only at the second trial to start. Inspired by your analyses I increased the distance to 3 mm and it worked immediately. As second step I will measure the signals in different distances to make sure to find a distance that provides tolerance in both directions for save operation over temperature and other boundary conditions.
Many thanks. Ronald
October 31 2015
A massive thank you, I bought my 2006 bessecarr with what I was informed was a lazy immobalizer, when I came across your info, I thought I would give this a try and hey presto I raised the sensor using a £2 coin, only as I was at the store age place and was the only thing I had to hand. My motorhome started first time the first time she has done this in the six months I have had it. The motor home has never started correctly and normally cranked the engine 3 or4 times before she fired. Thanks to your information and a £2 coin best fires first time. Saved me a fortune not going to fiat.
April 17 2014
Great article Nick and one which I have been able to forward to a customer with a similar issue. Not only are these great reads and education for all but really do assist technicians in need. Thank you.
June 04 2010
Simple! just goes to show how easy it is to find and rectify these kind of issues using the scope and a litle bit of training. I hope you didn’t pass your secret on to the other garage !! and charged accordinly for your investment in equipment and knowledge base. Excellent article.
May 30 2010
Very useful information
May 29 2010
I can’t think of any cars that I see here in the USA that have an adjustable sensor, but I did have to use a .005 shim on a chevy Suburban CKP sensor this week. It was setting a p0300 code at 70 mph, plugs and wires had been replaced to try to solve the problem. A TSB said to pull the CKP and check for wirness marks on the sensor and sure enough it had been contacting the relucter.
May 28 2010
Great article. Excellent detective and diagnostic skills in relating the ‘scope readings to the CMP behaviour and its positioning.
May 14 2010
brilliant very informative
May 11 2010