PCB Tech - Car oil level sensor gear failure caused by PCB

The oil level sensor of a certain vehicle model is a reed switch structure, and the sensor gear failure is manifested as the fuel gauge pointer does not respond below 3/4 gear. After investigation and experimental analysis, the cause of the failure is that the PCB board installation angle needs to be adjusted when the sensor is produced. However, at this time, part of the protective resin has been filled between the PCB board and the outer plastic tube. The resin prevents the PCB board from twisting, and external force is applied to the reed switch. It caused damage to the reed switch in the 3/4 position. After a certain period of use, the reed could not be disconnected. The maximum output resistance of the sensor could only reach the resistance value of the 3/4 position, so the oil gauge pointer was in the 3/4 position. No response below the position. To avoid such problems, the outside of the PCB is no longer filled with resin, and both sides of the PCB are protected by sealant and rubber seats.

1. Failure phenomenon of oil level sensor

A certain type of fuel level sensor has a malfunction on the entire vehicle. The specific manifestation is that the fuel gauge pointer does not respond below 3/4 gear, that is, the fuel level decreases and the pointer does not move. The pointer can swing normally above the 3/4 gear of the fuel gauge., The same failure occurred on 3 cars, and the sensors were of the same batch.

Sensor circuit schematic

The circuit and principle of the oil level sensor is shown in Figure 1. The reed switch is a kind of magnetic switch, which consists of two magnetic reeds wrapped in a glass tube. The oil float is equipped with a permanent magnet. When the reed switch is attracted by the permanent magnet, the resistance below the reed switch is short-circuited. Floating with the oil level, so the oil level has a certain corresponding relationship with the sensor resistance. The maximum design resistance of the sensor is 107.8 Ω, which corresponds to the neutral oil stop of the oil gauge. The maximum output resistance of the faulty part measured is 29 Ω, which corresponds to the 3rd and 4th gear of the oil gauge. There may be two reasons for this failure: one is that the circuit board itself is short-circuited, the other is that the reed switch fails and cannot be disconnected, both of which can short-circuit the resistance below 3/4 block, so that the maximum output resistance is only 29 Ω.

2, oil level sensor troubleshooting

2.1. PCB board short circuit troubleshooting

The sensor structure is shown in Figure 2. From left to right, there are casing, plastic tube, epoxy resin, and PCB board. The PCB board is put into the plastic tube and then into the casing. The plastic tube is filled with sealant first, and then epoxy resin is filled after fixing the PCB board. After the resin is cured, the PCB board is protected and sealed. Epoxy resin is fluid during filling and hard after curing, which also makes it difficult to dismantle faulty parts completely.

Check whether there is a short circuit on the PCB board. The wiring surface of the PCB board needs to be exposed. The PCB board after being soaked in epoxy resin solvent is shown in Figure 3. The solder joints are smooth and firm, and there is no continuous soldering phenomenon, which can eliminate the PCB board short circuit problem. . Because the solvent has a corrosive effect on the glass of the reed switch, the reed switch after soaking has been damaged.

2.2. Reed switch failure troubleshooting

The reed contact of the reed switch is sealed in the glass tube, and the interior is filled with inert gas, and the contact contains inert precious metal rhodium, which can reduce the loss of arc discharge on the contact surface. After the glass tube of the reed switch is damaged, the internal inert gas leaks, and the life of the reed contact will be shortened.

The pins of the reed switch need to be bent 90° from a straight line before welding. After the pins are bent, they are inserted into the PCB holes for welding. The state of the pins before and after the bending is shown in Figure 4. Bending pins generally use special tooling. The tooling has grooves and magnetism, which can prevent the reed switch from moving when the pin is bent, and avoid damaging the reed switch when the corner is bent. Even if the reed switch is damaged, after welding It will also be found during testing. During the inspection on the production site, a batch of reed pipes were randomly bent. After testing, the performance of the reed pipes reached the standard.

The reed switch cannot be disconnected because the reeds are stuck together. In order to verify the adhesion phenomenon of the reed switch, the following experiment was carried out: the reed switch is connected in series with the oil pump motor as a magnetic switch. The power supply voltage is 12 V. The permanent magnet is used to attract the reed switch. Sparks flashed between the tube reeds. After a period of time, the permanent magnet was removed, and the motor was still running. The reed tube could not be disconnected and failed. The heat generated by the sparks made the reeds stick together. However, this phenomenon will not occur in the whole vehicle. The current in the reed switch reaches 0.9 A during the test, which has exceeded the normal working current of the reed switch by 0.5 A. This test is only to verify the failure mode of the reed switch adhesion.

When reviewing the production records of the sensor, it was found that the installation angle of the PCB board in the casing was adjusted for the batch of faulty parts during production. This is to keep the reed switch away from the pump core and reduce the interference of the magnetic field on the reed switch. When adjusting the angle of the PCB board, the lower part has been filled with resin, and the resin is gradually solidified. At this time, the PCB board will be distorted by external force, which may cause damage to the reed switch. When disassembling another faulty part, it was found that the PCB board was obviously distorted

In order to verify the speculation, a normal PCB board was twisted to a certain angle, and then the PCB board was connected to the oil meter and put into the electromagnetic coil. The electromagnetic coil closed the reed switch at a frequency of 10 times/min. After 8 hours, the sensor The maximum output resistance is 29 Ω, which is consistent with the phenomenon of malfunctioning parts of the vehicle.

Put the failed reed switch into a 300x magnifier to observe, you can see that there is no gap between the reed contacts, and the normal reed contacts have visible gaps, as shown in Figure 6. Normal reed switches can observe the pull-in action of the reed after the magnetic field approaches, while the failed reed switches have no response.

The oil level sensor fails after the PCB board is twisted and used for a period of time because the gap of the reed is reduced. When the magnetic field of the oil float makes the reed tube open and close, it is easy to form an arc discharge, and the heat of the discharge spark makes the reed touch Point adhesion, after 3/4 of the retaining reeds are bonded, the following gears will fail.

3, sensor failure rectification measures

The function of filling resin around the oil level sensor PCB board is to improve shock resistance and sealing, but when the sensor fails, it is not conducive to complete disassembly, resulting in difficult failure analysis, and the resin needs a long curing time, and the production efficiency is not high. Since it is necessary to manually install the PCB board into the sleeve, it is easy to adjust the angle due to

Filling resin causes damage to the reed switch. In order to avoid this problem, the sensor no longer fills the plastic tube with epoxy resin. Instead, a small amount of sealant is filled at both ends. The sealant does not cover the circuit part, and the two ends use rubber seats to fix the PCB board.

4. Summary

The failure of 3/4 block of the oil level sensor is because to adjust the installation angle of the PCB board during the production of the sensor, the external force twists the PCB board to cause damage to the reed tube. After a period of use, the arc discharge between the reeds makes the contacts of the reeds stick and cannot be broken. Open, the maximum resistance of the sensor stops at 29Ω, and the minimum oil level of the oil gauge can only display up to 3/4