top of page
MECHANICAL
Unsuccessful engine start?
Let's get started...
In this article, I would like to give an insight into those possible reasons when a vehicle doesn't want to start. I think so, this is actually a fairly common occurrence, and the reasons for these unpleasant issues are often quite simple, but it is also true that these can even be extremely complicated at the same time.
The fact that these are fairly common problems can also be well illustrated by saying that even I - as an average hobby mechanic - could easily give examples of varying degrees of difficulty in this pretty frightening subject - to make this topic even more interesting - only from the events of the last 2-3 weeks.
In the recent period, the situation was that I faced completely different challenges from the low starting current through the starter motor up to the main fuses and relays, and I almost forgot to mention the OEM anti-theft module along with the immobilizer and PCM module.
Moreover, if I had to recall older challenging cases, there was already broken starter solenoid or burnt starter motor, but I could even say the replacement of the contactors of the brush holder bridge as well, and last but not least comes the evergreen topic - which I've already written two articles on - is the maintenance and regeneration of lead-acid batteries, and the painful truth is that I didn’t even mention half of the possible reasons for this huge topic.
I think so, it is quite certain if there is an unexpected and more serious engine starting difficulty with our much-loved and seemingly very well known vehicle which is in addition in everyday use, then such an unpleasant surprise can quickly frighten even the most experienced drivers as well. Not to mention that for the average vehicle owners, the car - which is almost a family member - seems like an over-complicated aircraft technology - see illustration below - in these challenging situations, especially when the technical rescue service lists tons of possible reasons just above the opened hood, on the side of the highway.
Before I go deeply into the technical kinds of stuff, let me briefly mention the story that actually inspired this short topic. So the most beautiful true pearl of this month is that I'm dealing with a vehicle - unfortunately mostly as remote assistance, because I only got this car for 15 minutes for a short OBD diagnostic - that doesn't want to start at all for weeks, and on top of that, the main fault came out during driving right in the middle of the highway in abroad.
So, in short, the car simply stopped like a nail while driving and could not be started ever since.
I would also like to mention - pointing out the difficulty of the situation - that this vehicle has been inspected at a foreign car repair shop for a week and a half, but unfortunately, nothing worthwhile was found there.
By the way, this car also spent a few days at another official vehicle repair shop without results, and now the vehicle is being analyzed at a third service station - in the official car dealership - where it was originally purchased as a brand new one.
So in summary slowly for more than a month, the mechanics can not come up with a solution.
It can be seen from all this that in some cases the situation can even be extremely complicated.
Maybe the icing on the cake could be that the car is quite young, with a real mileage of about 100,000 kilometers, and it's a simple 1.8 intake petrol engine, so the story should not be too complicated in this respect.
I won’t even mention the exact brand because I don’t want to draw too far-reaching conclusions about this vehicle type, but I will show the DTC list that makes it pretty clear, that the situation is extremely serious, and the PCM control module itself needs a thorough examination as well, and besides these I hope the engine control unit itself has not given up the fight permanently.
Anyway, the car is under warranty, so for this reason - unfortunately - I can't inspect it, but in this case, the official car dealership performs this interesting and challenging task.
First of all, let me mention it too that before the beginning of a troubleshooting process, it is always a good idea to describe the entire history very accurately and in detail - as well as the individual experience - of the vehicle failure, to the mechanic. Definitely yes, this in itself often matters a lot in terms of repair results and time.
In this story, it simply looked like that the engine shook once without any sign after 400 km of continuous driving. In parallel, the yellow check engine MIL lamp lit on the instrument panel. After about 40-50 km, the engine began to shake and weaken, and finally, it stopped simply in the middle of nowhere.
More than 1 hour later, the technical rescue specialist was still able to start the engine with a cold start spray, but since then the situation has gotten worse and worse. Currently, the engine not starting and not cranking, moreover the PCM module doesn't want to respond either, which is no wonder since the control unit internal failures are also included in the DTC list.
I don't think it's a coincidence that the exact root cause can not be found because we are facing not a traditional electrical or mechanical fault. It seems more like some kind of serious SW issue, but anyway, this incredible process is not understandable to me either, but since I am familiar with automotive software code testing, the main task of the vehicle service would be - in my opinion - a deeper SW analysis in such cases, because when the PCM related internal integrity failures were stored in the memory, then the so-called freeze-frame data should also be stored
about the critical state where the software was stuck, and with the help of this useful data they could most certainly get more information on exactly what was going on in the engine control unit when these failures were stored.
So, neither the starter rotates nor the spark signal on the spark plugs, and even the starter relay doesn't receive the required supply voltage, and the door cannot be locked with the factory key because only the turn signals are flashing, in spite of the fact that initially, the battery was in excellent condition at full charge, but now the situation is that overnight the battery runs out of its reserves too quickly and simply discharges in the vehicle, but it can receive external charge perfectly and has excellent starting capabilities with high cold starting current.
I don’t accidentally pick these out because I started the investigation - in remote assistance mode - on this track as well. However, this is an especially difficult situation, and I don't like it when I personally don't have the opportunity to physically inspect the vehicle. I can only rely mostly on the phone conversations, where the exact details have to be pinched one by one.
​​​So it is a known fact that this vehicle had already conflicts in connection with the immobilizer system, moreover I still see - among many other failures - immobilizer-related DTCs as well. However, I would like to emphasize that the DTCs alone - with reference to the immobilizer system - can not clearly indicate that there is a specific problem with the immobilizer module, especially if there are several other much more serious error codes in the DTC list.
Perhaps that is not out of the question either - even if only partially - that this vehicle is locked by the built-in separate anti-theft module and immobilizer and kept in so-called self-protection mode. Anyway, this could in itself even explain - if the DTC list does not contain more complex failures - why the starter chain does not reach the main relays and fuses either, and this self-defense mode may even lead to the intended self-discharge process of the battery. In this case, it is advisable to carry out a thorough current leakage test in such a way that, 10 to 15 minutes after the central locking system has closed, the power consumption of the vehicle is not allowed to exceed - as the illustration shows - a few tens of milliamperes.
Is the engine not starting?
Let’s take a quick look - first without OBD diagnostics and measurements - at the components which may arise as slightly more complex possible causes of such and similar symptoms. So now let's not think about the running out of gasoline and similar obvious things.
My very first thoughts in connection with the above-mentioned story - during the first completely unexpected emergency phone conversation - revolved around the fuel system control unit and the fuel pump as well as the main relays and fuses in the ignition system.
Then, when I thought about it a little better, after at least 40-50 km after the first fault signal appeared, it somehow didn't fit so much into this type of - fuse and relay - electrical fault circles.
If we go even deeper into this subject, we could even consider a sensor that provides some kind of basic signal, such as a crankshaft position sensor, which is a simple magnetoresistive sensor and sends a very accurate position signal to the PCM control module for basic engine operation, which is stored in the PCM controller memory together with other important parameters such as injection duration or ignition data.
However, the question is how and where should I start the deeper investigation?
What should I do?
In connection with this story, I would list what steps I would have taken to try to find a solution.
In such a difficult case, when the engine does not want to start, and there are many DTCs from all directions, and somebody doesn't even know where to start the complicated examinations, then maybe the following few lines can help. Of course, always check the fuel level and the idle voltage level of the battery - suitable if it is between 11.6 and 12.6 V without charge and load - together with the cold start - see my articles on batteries for more details - current as well, but now I want to talk about a little more complex things.
Personally, I would do it in such a way that I would go along nicely from the beginning to the end on each component. First of all, of course, I would read the DTCs - in step zero - stored by the PCM controller via OBD diagnostics. If we are lucky, there is a good chance that this quick step will already identify the possible root cause of the problem, but of course, this is not always the case.
After this, I would check the main relays and fuses, which routine steps I will describe in detail in later sections to see exactly how. If we have an OBD diagnostic tool - of course something more serious, as these are already easily available retail devices - it's worth checking the main signals needed to start the engine since this step does not involve any serious disassembly yet, only the DLC connector needs to be connected to the OBD scanner. The OBD device must support the display of the live status of electrical signals, and with more experience and practice, we can even check where the chain of the engine starting process is stuck.
These useful parameters can usually be found around the following subgroups. Typically, first, go to the main group of the PCM engine control module, then to the live data subgroup, and finally select the electrical data. If we are pretty much in this place, then check for such and similar parameters and signals - after the ignition is switched on but before starting - which are shown below. Anyway, if the state of a given signal seems to be not correct, then I think so it can even be guessed intuitively too.
- Voltage signal => around 12.6 V - Ok?
- Ingnition relay feedback signal => status - Ok?
- Starter relay command => ON - Ok?
- Engine speed => 0 rpm - Ok?
- CMP sensor signal => 0 rpm - Ok?
- and so on and so forth.
Then, depending on the given situation, even one of my favorite routine disassembly steps could come on the to-do list. Namely, if it seems necessary - definitely recommended anyway - it can be checked with minimal disassembly even the starter solenoid and starter as well, since these components typically do not set any DTCs in the event of an electromechanical failure. I will cover - of course only to the best of my hobby repair knowledge - this interesting topic in detail a little below.
Of course, if the starter is already able to spin up but the internal combustion engine still does not want to start, then there can be many-many other reasons for this, such as either a faulty ignition - check the ignition wires and the ignition transformer, even the ignition distributor and the spark plugs - system or fuel - check the fuel pump, injectors, and the correct fuel pressure - system or even a defective crankshaft sensor or MAP sensor or even the EGR valve, but the root cause can be either a torn timing belt or insufficient compression if we want to take into account the very extreme cases as well.
I left one of the most complicated topics - after the engine controller unit - the immobilizer and anti-theft modules to the end because even the security system can cause unexpected surprises. I think everyone knows that the antenna of the small immobilizer unit - integrated next to the ignition switch - always requests on a given radio frequency a valid unlocking RFID code from the key to enable the start-up process. This unlock code is stored in a small copper coil, which is nicely read by the system at a given frequency, such as 315 or 433 MHz. Moreover, if a higher level OBD diagnostic tool would also be available that can also detect immobilizer-related status signals - approximately as shown in the list below - we could be even closer to the cause of the potential failure.
- Immobiliser system status => standby - OK?
- Immobiliser security code function => present - OK?
- Immobiliser security code programmed => yes - OK?
- Immobiliser security code accepted => yes - OK?
- Immobiliser security code lockout => inactive - OK?
- and so on and so forth.
​
Then if there is still no solution, I would take out the battery for half an hour and also turn on a consumer to drain the remaining stored energy from the control modules as well, because this complete disconnection - and after half an hour reconnection - step will perform an electrical reset to a certain degree, and restart the system - where possible - with new init values. It does not require too much effort or expertise either, although I don’t really like solutions like this.
Anyway, all of these proposals are - of course - very situation-dependent. Moreover, it is not possible to give completely general instructions that work on all situation-specific issues.
This is more like just my broadly summarized general scenario on the subject.
The starter motor
- Engine not turning over and there is no clicking noise at all around the relay box?
- No, even the gasoline pump works nicely to build up the necessary fuel pressure.
- Engine not starting but cranking?
- Yes, but slower than usual, however the battery is in medium condition.
​
As we have seen so far there can be plenty of reasons for engine starting difficulties, but now let’s narrow the circle down only to the starter motor.
Why can an engine crank but not start even though the battery seems to be at a sufficient level,
and even spark plugs produce all the necessary spark signals, and there must even be sufficient compression in the engine, but it seems like the engine not turning over fast enough?
I recently encountered such an ordinary case again, and it is a typical symptom, but it does not come from one moment to the next because it has gradual signs.
The sluggish spin of the starter motor can be heard very nicely, and one possible reason could be briefly that the starter motor does not rotate fast enough.
The solution is usually a very simple maintenance in this case, which requires a little cleaning and shaft lubrication - or in given case bronze brush holder bridge replacement - which quickly fixes this problem. Anyway, independently from all this, it is worth cleaning the area of ​​the commutators from metal dust and lubricating the shaft ends every 15 years so that the bronze brushes do not accidentally get grease, because it can easily insulate the metallic connection between the contact surfaces.
Keep in mind that even if these bronze brushes wear out or get stuck the starter will not start at all, even though the starter solenoid coil works with a well audible click noise.
In this unpleasant case, it is advisable to gently knock the back of the starter motor, as this strange but understandable step usually helps temporarily in such a situation, and this can also be applied to motorcycles and larger vehicles, however, do not use this rude solution on any of the latest starter motors.
I also encountered such a challenging issue, when the copper wire itself - coming out of the starter solenoid coil - was completely broken, but this was only visible when I had already disassembled the starter coil.
I repaired this on the spot there, because I soldered the broken threads precisely, and so it has been working properly for years. In this case the replacement is a much safer solution, since the copper wire was not accidentally broken at that point.
In the case of soldering - as diffusion-type bond - the resonance will be summed up in the same way as before, which thanks to the material rigidity due to the heating - the metal lattice bond is permanently rearranged around the soldering point - it is very likely that the newer copper wire rupture will occur much sooner.
Anyway, the failure of the freewheel mechanism of the starter motor is not a too rare case either, and of course the vehicle engine will not start even though the starter motor rotates.
Fuses and relays
Engine not turning over and there is no clicking noise at all around the relay box? Then, let’s move on to the topic of fuses and relays.
However, it should be clarified that instead of faulty fuses and relays, there may easily be other faults in the background, such as in some cases even the crankshaft sensor, or an unexpected software error related to the camshaft sensor output signal. In this case, the CMP sensor sends incorrect signals to the PCM controller immediately after the KL15 ignition line is turned on, that the engine is already running - although, in fact, the engine has not yet started - and the PCM controller no longer allows - for safety reasons - the restart process.
In this case, the sensor disconnection through OBD diagnostics - then reconnect - may help to solve this problem, but - as we have seen above - even the anti-theft module can easily cause such an unpleasant situation that the vehicle cannot be started at all.
So, let's start this relay topic right there to clarify a few basic notations that are used in the automotive industry - as certain electrical markings are even visible on the side of the relays -
and on top of that, we can easily come across them in other descriptions as well.
Let’s look at some of these more interesting ones in the next few lines.
- KL30: battery positive,
- KL31: battery negative,
- KLR: ignition switch accessory position => 1. power mode,
- KL15: ignition switch ON position => 2. power mode,
- KL50: ignition switch START position => 3. power mode,
- KL51: positive rectifier terminal on the generator,
- KL1: primary terminal of the ignition coil,
- KL4: ignition distributor high voltage terminal,
- and so on and so forth.
​
Anyway, the abbreviation KL comes from the German word klemme, which refers always to the electrical connection. As far as relays are concerned, these markings are usually the following, as shown in the list below.
- KL30: battery positive wire and protected, of course, by an appropriately sized fuse
=> basically this will feed the consumer through the terminal 87
- KL85: relay coil (winding end) negative contact - control circuit on the relay
=> suitable GND point on the vehicle chassis
- KL86: relay coil (winding start) positive contact - control circuit on the relay
=> + 12 V supply voltage coming from a low current switch
- KL87: Normally Open (NO) contact - output terminal to the consumer circuit on the relay
=> + 12 V to drive the consumer (like starter solenoid)
- KL87a: Normally Closed (NC) contact - first output terminal
=> output terminal to the consumer circuit on the relay
- KL87b: Normally Open (NO) contact - second output terminal
=> output terminal to the consumer circuit on the relay
I think so everyone knows that the main function of the relays is to switch a high current circuit - such as a starter motor with a current consumption of 120 Amps - using a lower current circuit. So, knowing all this, checking the relays is not a too complicated task either in case of engine starting difficulties. Terminal 85 of the relay is clamped to the negative pole of the battery, pin 86 to the positive pole, and in this case, it should click nicely and close the connection between terminals 30 and 87 for the high current consumer. This can also be checked with a multimeter to see if the desired 0 Ohm resistance is there between terminals 30 and 87.
What relays and fuses can be considered as a potential source of problems at all in the event of vehicle starting difficulties?
- Starter motor fuse and relay
- IGN main relays and fuses
- Fuel pump relay and fuses
- PCM control module fuses
Keep in mind that the integrity of the fuses should be checked using an open circuit test with a multimeter or a debug lamp to ensure that the tested fuse is not pulled out of its place to avoid undesired software and memory disorders.
Thank You for reading this article!
bottom of page