CartFerrari 360 with Intermittent Non-Start
By Nick Hibberd
Hibtech Auto-Electrical Diagnostics
Starter Operation Details
The starter motor needs to develop a high torque to make the engine fire, and to do this it needs a lot of current. This simple illustration shows the path of the starter current. Once the exciter cable has energised the solenoid, current flows from the battery, across the now-closed solenoid contacts, and leaves the solenoid through an insulated fly-lead into the motor’s yoke.
Normally with this type of assembly, two positive brushes are set 180° apart (either horizontally or vertically) to pass current directly to the motor windings; then, to complete the circuit, two negative brushes are set 180° apart in the same plane. The unit was a warranty item, so unfortunately it was not possible to dismantle it to examine the construction method.
How the solenoid manages to get the heavy contacts to close, and keep them closed, is a clever arrangement. It uses two separate coils; a pull-in coil and a hold-in coil, both of which are placed around a sliding plunger in such a way that their magnetic fields push on the plunger. The sliding plunger tries to centralise itself within the magnetic field, thus closing the contacts at one end of the plunger. Electrically speaking, the solenoid has two main objectives: to engage the plunger, and to keep it engaged as long as required. It’s quite possible to do both jobs using just one coil, so why have two?
In any solenoid, the magnetic field needed to start the plunger moving is stronger than that needed to keep it engaged. If we had one coil for both jobs, the amount of current needed to generate sufficient magnetism to move the plunger would be more than is necessary to merely keep it in place. This would be a waste of energy, heat, and vital cranking amps that the battery is better spending on the motor itself. An improved solution would be to have a powerful electromagnet to get the plunger moving, and then use a smaller electromagnet once the plunger is in place (two coils of equal ampere-turns would also work).
The exciter cable feeds both coils with battery voltage at the same time. To energise both circuits, the hold-in coil is grounded through the solenoid casing whilst the pull-in coil receives a ground path through the motor’s windings. The combined force of both coils moves the plunger in place; as the solenoid contacts close, our pull-in coil has done its job and can now be disconnected. It can be seen that once the contacts close, battery voltage is supplied not only to the motor but also now to the ground side of the pull-in coil. Since the pull-in coil is already being supplied with battery voltage from the starter relay, the potential difference across the coil drops to almost 0 V, which cancels any current flow and subsequently any electromagnetism the coil once had.
The hold-in coil’s operation on the other hand is much simpler. Having a permanent connection to ground, from the moment a crank command is given, the coil remains fully energised until the crank command is released. This way the coil is always ready to hold the plunger whenever the pull-in coil is disconnected. That’s how it works.
From our test point where the measurements were taken, and the cranking current trace, we can safely say that current flowing into the starter assembly was intermittent, and for the moment that is all that can be deduced. So we now have to build a case using this sole fact arguing for and against possible causes.
A poor main power supply?
A current reading of any kind in a circuit without voltage present is impossible. From this we can deduce quite a bit. Voltage would have to be in good supply to keep the current level of the motor at 250 A plus. Also remember that the engine’s RPM during cranking seemed fine. It could be perhaps this main cable was making and breaking a contact, thus causing our current signal drop-outs, but this explanation didn’t seem to fit
A check of the voltage level at the power distribution block showed a good battery voltage present before, during and after the current signal lapses. This eliminated everything before the distribution block towards the battery. Admittedly, because our test location was in the middle of the power supply line, this did leave the power cable travelling from the distribution block to the starter motor. A cable connection problem onto the solenoid wasn’t considered likely because of the sheer nature of the current lapses. The drop-outs were quite sudden and cleanly defined, and this together with the lapses appearing very quickly didn’t make a strong case for the theory. A decisive factor against a power cable connection problem came later.
Neither was there any obvious sign of connection resistance. The scope never shows an insufficient current level, only small current peaks that haven’t had a chance to reach a normal operating level before being cancelled again. This signature is very different from a high resistance causing low current flow.
A low battery supply was immediately ruled out because this would affect the circuit’s overall current flow; not as in this case where our fault allows current flow to disappear, and then reappear.
A poor ground path?
This is very similar to the above case. A current can flow only from a high potential to a low potential. Both potentials would have to be present to see the recorded normal current level in between the lapses. It’s not a normal characteristic for a poor cable connection in either supply or ground path to cause a repetitive signature. In this instance, a load of 250 A plus on any circuit would itself find a connection problem and usually bring the circuit down permanently.
Not mentioned yet is that this vehicle model has a battery isolator installed between the battery’s negative post and the chassis. This is definitely a likely cause if the ground path was ever suspected as a problem. But the trace shows something that could completely rule this out. The exciter line potential was taken with channel A referenced to a nearby bolt screwed directly into the chassis framework. If this channel is still measuring a good potential difference then the scope’s ground connection, and therefore the isolator, must be good during our fault.
A solenoid problem?
This one couldn’t be ruled out as it’s the solenoid that delivers current to the motor, and if this isn’t done efficiently then problems will arise. One possible explanation for the current level drops is if the solenoid releases control over its plunger, thus opening the contacts. This might include the crank command being poor or indicate a problem with the hold-in coil, but we need to consider something. If the hold-in coil was failing causing the plunger to retract, this would in turn throw the pinion drive out of mesh with the flywheel; a noticeable “clunk” would be heard. As our fault is quite repetitive, the “clunk” would turn into a nasty “chatter”. Neither condition was evident at any point in the investigation. Clues were all pointing to the drive gear being held in place during the fault and it was backed up with a healthy voltage level present along the exciter line before, during, and after the fault.
We know the voltage was healthy because if the exciter circuit was going open and maybe even causing the fault, we would expect to see a higher voltage level than that shown; but we do not, indicating that an electrical load is still present on the exciter line during the fault. Any doubts here would have been further investigated with a low-level current probe.
A problem with the pull-in coil itself could be eliminated, simply because the starter motor always responded to the crank command given.
A commutation problem?
Commutation is the point where the motor brushes glide across the many commutator segments as it rotates. The commutator bars come in pairs, each pair connected to one insulated winding which is wound onto the armature. This basic illustration shows segments 1 connected to a power source. This creates a magnetic field in No.1 winding which works against the field poles (not illustrated); and this consequently rotates the armature which brings in the next segment pair and the process starts again. A typical armature houses about 30 segments, feeding 15 armature windings. These pairs are usually orientated on the shaft to match the polarity of the brush fixings (actually it’s more correct to say the polarity of the brushes match the armature windings).
A commutation problem isn’t ruled out. For instance, a broken brush spring will cause insufficient contact pressure on the commutator, thus developing a possible loss of brush contact. No brush contact means we will lose current supply. As a general rule, most starter units house two positive brushes and two ground brushes, so the fault would have to affect both brushes on either potential.
Motor armature problem?
An open circuit in one of the windings would cause a current signal dropout, but it would also leave a very distinct pattern in the captured current trace, certainly more regular than ours. A short between windings was ruled out, as this would show dramatic positive current peaks, and again this would be very regular.
Even though the capture isn’t yet telling what the fault is, it is starting to tell what it’s not. There was something else in the capture that needed explaining before arriving at any conclusion.
The voltage level along the exciter line is seen to drop whenever motor current demand is high. This comes as no surprise; the motor draws 250 A and will inevitably bring down battery voltage slightly. Our crank command comes indirectly from the same power source, so it makes sense that this will be affected to a similar degree; in this case down to a healthy 10.5 V. What is interesting are the peaks seen on the exciter line immediately after the motor releases electrical load (during our fault). The hashed line in the exciter line shows the voltage level present during a motor current lapse and can be taken as near battery voltage; it’s marked as being around 12.45 V. The hashed line in the current trace represents 0 A.
Both measurements show activity past their normal working values for a crank condition. We see the exciter line exhibit brief voltage peaks higher than battery voltage the exact moment we lose current draw; these are most definitely linked. Also, the current trace shows similar characteristics where current is no longer flowing from battery to engine bay components (generator and starter), very briefly flowing from the engine bay towards the battery.
We know what these peaks represent, but how are they linked to the fault?
Out of all the theories put forward only one stood from the rest which includes the fault and these “peaks”.
At the beginning of the investigation it was stated that the motor behaves like a generator that opposes the current flow supplying the motor, and as motor rotational speed picks up so does the EMF generated. It seems that it’s this EMF that is contributing to the “peaks” during the fault.
First of all remember this EMF is between chassis ground and battery positive, and it’s the same voltage reference that the scope is measuring on the exciter line.
Secondly, we know that the probe is seeing no current flow during the fault, which ultimately means a break somewhere in the motor circuit, and if there’s a circuit break within the motor yoke, any EMF generated is no longer referenced to chassis ground so the scope will struggle to pick this up. The fact that the scope is picking this up means the open circuit isn’t within the motor structure. We can go further and say that because the current probe isn’t registering the same degree of EMF burst below the 0 A line (current returning to battery), very little of the generated EMF is coming through the main supply line. The peaks seen on the exciter line were coming from somewhere else.
Applying these facts to the illustration can lead to only one realistic explanation — the solenoid contacts.
If the contacts are bad (making and breaking) this will immediately drop the overall main line current flow. Because the motor is intact, it continues to generate (for a very short time) reverse EMF proportional to speed. The EMF is allowed to escape through the pull-in coil and enter the vehicle’s electrical system.
All of what has been discussed sounds very dramatic, to the point where it’s hard to conceive that initially during testing no noticeable fault seemed to be present. But in reality the fault never lasted much more than 6 ms (0.006 s), and although it would be difficult to put into figures, the percentage of time spent with current flowing far outweighed the time spent with no current. So when you put this in perspective with the problem, without a scope this fault would have been near impossible to spot.
This case study might seem too involved for a humble starter motor complaint, but from the time it took to capture the first trace to eventually condemning the unit, the problem took little over 10 minutes and all done with only one scope hook-up location. What has taken up much of this study’s time is the need to explain the wealth of information hidden within the recorded waveforms.
The following waveforms were taken with a new starter motor installed.
Channel A measures starter solenoid current, and channel B is starter motor current
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