Checking the injector coil with a multimeter and SIT-12

For many people to find that measurements made with different instruments can give results that differ by tens of percent is very strange. Is this a result of the abysmal accuracy of the instrument? This can also be the case, but the fundamental problem is the method of measurement. In the following text, we will look at checking modern injectors from the electrical side.

Many available injectors, despite having different designations and numbers, can be divided into several groups with similar parameters. This remark applies not only to electrical parameters but also to the control parameters of such injectors (e.g., the voltage of the so-called boost, holding current). Based on such information, it is possible to determine several ranges of parameters that occur in piezoelectric and electromagnetic injectors. However, before ranges of values can be presented, a bit of news about measurements needs to be presented and systematized.

NOTE! When checking the injectors, remember all the safety rules associated with fuel systems in the car (containing diesel or gasoline under significant pressure). Also, be cautious due to the high voltages of control or test signals fed to the injectors.

ELECTROMAGNETIC INJECTORS

This type of injector is actuated by an electromagnet. By regulating the current flowing through the injection coil, the controller activates the injector mechanism. This leads to the injection of fuel, which is burned in the cylinder. In the case of gasoline injectors in indirect injection engines, the solenoid directly opens the valve, and the fuel enters the intake manifold. In common rail injection systems, the operation of the injectors is more complicated. Also, the electronic circuitry in the controller is different. In this case, the signal from the controller fed to the injectors is usually generated using two levels of voltage: a higher so-called “voltage”. “boost” for shorter opening times and lower for sustained opening.

MEASUREMENTS WITH A MULTIMETER

The multimeter, as the name suggests, is used to measure a number of electrical quantities.
Depending on the manufacturer and price, in addition to the standard measurements of voltage, current and resistance (resistivity), options are available to measure capacitance and inductance. In most instruments, the manufacturer tries to maintain some compromise between price and capabilities and ranges. This results in the versatility of a given device but does not allow precise measurements at the extremes of the ranges. An example of such a measurement is the determination of resistance. Most multimeters in ohmmeter mode make a measurement by supplying the measured component with a current of known value and reading the voltage that will occur on it. This allows using the well-known Ohm’s law R=U/I to calculate the resistance. Power supply and measurement are carried out by the same wires which results in the resistance of the wires being included in the total result. Inexpensive meters allow measurement in the lowest range (often 200, 400 or 600Ω) with a resolution of 0.1Ω. The better ones go down to 0.01Ω. Only lab-grade equipment or specifically dedicated devices (milli-ohm meters) can detect changes in the range of 0.001 or 0.0001Ω or less. This is due not only to the price but also to the complication of the measurement itself. In a meter that is supposed to measure resistance with a resolution of 0.01Ω or better, the effect of the resistance of the probes themselves should already be taken into account. This resistance is 0.2 – 0.3Ω for cheaper meters. The better ones have it in the range of less than 0.1Ω. In addition, as mentioned earlier, the resistance of copper wires varies with temperature. Therefore, meters often have a function that allows them to take the initial resistance of the probes as the “0” point thereby allowing more accurate measurements. Bypassing these problems as well as further increasing the measurement resolution is possible by using a different technique called “the measurement resolution”. four-terminal measurement called Kelvin measurement. In a nutshell, it involves supplying current for measurement with two wires and measuring the voltage drop across the component under test with a second pair of wires. Such a measurement gives repeatable results despite changes in temperature, cable length, etc. This method of measurement is precisely what is used in the SIT-12, which makes it possible to obtain a result that distinguishes such a small resistance as 0.001Ω (that is, 1mΩ).

CHECKING THE COIL OF THE SIT-12 INJECTOR

Such an example measurement where the way the value is measured strongly affects the result is inductance. Many methods of testing this parameter require applying a signal of a certain frequency to the measured element. There are two automatically selected frequencies for the SIT-12 range:

1. up to 60µH it is 100kHz
2. From 60 to 800µH it is 50kHz

This means that the results measured by the SIT-12 should relate to the results of other devices measuring at just such frequencies.

Some conclusions can be drawn from the ranges of measured inductances. The low inductance suggests an injector coil with a low number of turns. And this in turn already gives information that the coil will be small in size. Low inductance, on the other hand, communicates the high currents needed for operation, especially in the opening phase where speed is important. The low resistance of the coil itself also comes from this. An example of such an injector is, for example, the popular DELPHI EMBR00002D production model. The set measured SIT-12 has the following parameters:

Second similar type EMBR00203D:

Remember that copper has a positive temperature coefficient of 0.0039. For example, this means that a 70ºC change in coil temperature (that is, a typical value between 20ºC (the conventional ambient temperature) and 90ºC, the engine operating temperature) will increase the resistance by 27%. Which, for the above injector, means an increase to about 0.255Ω. Such a change against 0.2Ω at ambient temperature shows how important it is to make measurements under similar conditions. Very often, reference values are given just for temperatures of 20 – 25 ºC.

The next group of popular injectors is the type labeled “CRI 1” and the example representing it is the BOSCH model 445110141:

Another type is “CRI 2-18” and the BOSCH model 445110369:

Another type with lower injector coil resistance is the “CRI 2-16 M2” and BOSCH model 445110418:

The injector that has a coil with higher resistance is DENSO and model 095000-613:

The above examples involved used but operational electromagnetic injectors. General conclusions can be drawn from these data:

1. The resistance of common rail injectors is in the vast majority of cases in the ranges of 0.2 to 0.5Ω (you can find DENSO injectors , which are characterized by a large coil resistance, of the order of 1Ω up to 6Ω). Gasoline injectors are characterized by higher resistance.
2. The inductance for low-resistance coils (0.2Ω) is a single µH (3-4).
3. Insulation measured at a test voltage of 250V is beyond the measurement range of the SIT-12 (on the 500V range, this will also be the case in most cases).
4. Measurements of the sets show a very high similarity of results among themselves.

In general, testing all injectors from a given engine allows you to better identify the inadequacies of a particular injector based on a comparison of each parameter in relation to the others.

A separate topic is the temperature of the injector under test. These components, as they are known, operate over wide temperature ranges – from starting up in the cold to driving fast on the highway during a hot day. This causes some insufficiencies to show up on a hot, running engine. This is especially evident in the measurement of insulation. Winding elements change their position as a result of temperature changes. Thousands of such cycles can cause insulation abrasion and operating problems (especially symptoms associated with error code P0200). Therefore, it is good practice to test the injectors after heating and compare parameters. Of course, controlling the temperature of the removed injector (while it is being heated) is cumbersome, but you can, as previously mentioned, use the change in resistance. An increase by a factor of 1.27 will give us an estimate that the winding temperature has increased by about 70ºC.

Summarizing the damage to electromagnetic injectors, two groups of symptoms can be distinguished:

1. Interruption of the coil circuit – resistance out of range;
2. Short-circuit within the coil – reducing resistance and inductance;
3. Problems with the insulation between the injector body and one of the leads.

Piezo injectors

The triggering element of a piezo injector is a piezo stack (sometimes incorrectly referred to as a “piezo coil”). The application of voltage leads to a change in the shape of the wafer and their many layers allow to achieve greater dimensional changes. The stroke of such an actuator is small, but it is able to occur very quickly, allowing the fuel to be accurately dosed by the injector. Piezoelectric injectors first appeared in common rail systems, and are now also used for gasoline injectors in direct injection technology.

How to detect a faulty injector with a meter?

The piezoelectric stack exhibits the characteristics of a capacitor but not the typical one used in electronics or electrical equipment. In addition, it has some resistance seen in measurements as being included in parallel with the capacitance. This fact means that capacitance measurement, depending on the method, can level this additional resistance more or less. One of the best methods is to measure the charging time of a capacitor from a constant current source. Such a measurement allows for a good elimination of the influence of additional resistance as well as for making quick measurements. Such a better method is used in SIT-12.

It should be borne in mind that cheaper multimeters may measure on a different basis. In this case, a capacitor with good parameters (that is, close to ideal, without additional resistance and good insulation) can be measured correctly. However, the piezoelectric stack no longer necessarily. Measurements can be severely adulterated. Moreover, in the case of a complex component, which is precisely the stack, each multimeter can give a different result. This will be a normal phenomenon, after all, in these devices the capacitance measurement is calibrated to good quality capacitors and not complex components. As an example, let’s measure the capacitance of a polyester capacitor type MKS4 from WIMA with a rated capacitance of 3.3µF, a tolerance of +/-10% and an operating voltage of 250VDC with a resistor attached in parallel (simulating the internal resistance of the stack).

As can be seen from the measurements, even in a slightly better class multimeter, the effect of additional resistance on the capacitance measurement result can be 4% (3.29 µF vs. 3.43 µF).

Measurements using the SIT-12

The second type of injectors are those that use a piezo element to open them. In this case, the electrical test is for resistance, capacitance and insulation. Example measurement results are as follows for BOSCH type “CRI 3-18” and model 445116059:

These data are for two defective and two good injectors. As you can see from the resistance test, the first injector has a significantly underestimated value. Capacity has also increased. Very significantly, the insulation has deteriorated and the value obtained is very low – 11MΩ. The second injector has a good resistance, but the capacity is starting to deviate from the third and fourth injectors. A measurable insulation condition value of 288MΩ also appears. This suggests beginning problems with this injector.

Example of two working BOSCH injectors type “CRI 3-16” and model 445115064:

Set of BOSCH injectors type “CRI 3-18” and model 445116019:

The above data show that, despite the lack of measurement with the SIT-12 device of the stack resistance at high voltage, it is possible to pick out the damage. Working injectors have a capacitance of 2.2-2.4µF, the defective one has 1.91µF. In addition, the resistance measured by the SIT-12 in the case of a defective injection has a resistance lower (164kΩ ) than the others (174 – 186kΩ). These results show once again that, having a set of injectors, it is possible to pick out a defective unit without knowing the correct ranges.

Summarizing the results of piezoelectric injector measurements, the essential information can be distinguished:

1. Isolation is a very important parameter due to the high voltage control of piezo stacks through the motor controller. Measured values on the 250V range, less than 300MΩ means, depending on the result, either damage or deterioration of performance and risk of sudden failure.

   NOTE: All insulation measurements are sensitive to moisture. Therefore, do not measure injectors that have not reached ambient temperature. Water vapor that condenses on the body combined with dirt can significantly underestimate the result of the insulation test. In particular, this effect can be seen in measurements at 500V when even a slight chuckle at the body and then performing a test will show a drop in insulation to hundreds of MΩ. Of course, after evaporation, this effect will disappear.

2. The stack resistance for the vast majority of types should be in the range of 170 – 210 kΩ. DENSO injectors are distinguished by a value on the order of 1000 kΩ (1MΩ).
3. Capacitance ranges from 2 – 2.5µF for injectors made by BOSCH. For Siemens/VDO, it is 3.3 – 3.5µF.
4. When the piezo element is damaged, the insulation between the stack and the injector housing deteriorates. The second is a decrease in resistance and a change in capacitance (increase or decrease – depending on the design and type).

By: Product Engineer Bartlomiej Nowojowski