Trucks that are difficult to start when cold

1997 year: Mazda – Titan model: P – WGLAT engine model: SL (24V specification)

Engine cold start difficult to start

At the time of starting the engine, the glow indicator lamp was lit normally, but it was troubled that the engine was difficult to start when cold. Also, even when starting, it was that many white smoke appeared.

Confirm work done at maintenance shop

  • Compression is normal for all cylinders at about 28 kg / cm 2
  • Air cleaner / element exchange
  • Exchange of fuel filter
  • The injection timing is normal

Based on the above results, it is suspected that the cause of the trouble that cold start engine is difficult to start is the preheating system. This vehicle has a glow indicator lamp, but there is no glow plug. Instead, there is a preheating system at startup, so I decided to check it.

Air Heater System

Since this vehicle was an air heater system, the voltage of the ① terminal and ② terminal of the air heater, which is easy to measure in cold time, was measured (lower right circuit diagram)

  • When the engine switch is turned on, the battery voltage takes 24V for both terminal ① and ② terminal. … There is a possibility that it is normal or current flows from the terminal ① to ② terminal.
  • After the glow indicator lamp turns off, no voltage is applied to both terminals ① and ②. Abnormality
  • Even if the engine / switch is set to ST, no voltage is applied to both terminal (1) and terminal (2). Abnormality

Based on the above measurement results, we will focus on checking relays, whether or not the interior of the air heater system operates normally.

When the air heater system is operating normally

Operating condition of the system at normal startup at a water temperature of 20 ° C or lower

Figure 1


Fig. 1 is a graph showing the relationship between the air heater temperature and the ON / OFF and energization time of the relay, and Fig. 2 is a circuit diagram.

  1. When the engine switch is turned on, the indicator timer (T1) and the brief heat timer (T1 ‘) are activated, T1 lights the indicator lamp, T1’ heats the air heater / relay 1 and 2 Is heated.
  2. When the indicator · timer finishes energizing, the indicator lamp turns off and the driver is notified of the start of starting. At this time, the 5-second timer is activated to continue energizing the air heater / relay 2.
  3. When the engine switch is set to ST, the air heater relay 2 is energized, and the air heater is heated.
  4. When the engine is started and the engine switch is turned back on, the after-heat timer (T 2) is activated and the energization to the air heater relay 2 is continued for the time T 2.
  5. After the time T 2, the air heater relay 2 is turned OFF and the energization to the air heater is stopped.

As inferred from the above documents, the indicator lamp is operating normally so that the relay 1 operates normally and heats the air heater, but if the cause of white smoke is caused by a start failure and the engine Even if it takes, it is because the compressed air is burning defect at low temperature, the air heater can not be fully heated. If the relay 1 is normal, it is considered that the air heater is not energized due to malfunction of the relay 2 because of relay arrangement of the circuit diagram. So we checked the air, heater and relay 2 individually.

Individual inspection of air, heater and relay

Single inspection
  1. Remove the negative terminal of the battery.
  2. Remove the air heater relay.
  3. Inspect conduction between the C-D terminal (green: white-black) of the relay
  4. When battery voltage is applied across the C – D terminals of the relay Confirm that there is continuity between A – B terminals (terminal of air heater body: see the above figure).
  5. If it is in trouble, replace with relay ASSY.

Since the inspection result of the relay was normal, checking the air heater control unit showed that there was no output voltage of the L terminal, so it turned out that this was the cause of not energizing the relay 2, so replace the unit Then it began to work normally.

The trouble of this time, from the symptom that white smoke comes out immediately after starting, the heating of the air heater is not enough, and the reason is that as a result of checking in detail by energization of the relay, the control energizing the relay · It was a bad unit. Troubles that can be understood as acquiring sufficient data and pursuing the cause from accurate inspection following the procedure are promising to prompt and accurate repair work flow Met.

Air / Heater Inspection

  1. Detach the wiring to the air heater.
  2. Using the circuit tester, measure the resistance value between each terminal. (Resistance values are shown in the above figure)
  3. Remove the air heater and visually check adhesion of soot and dust.
  4. If soot is attached, wash it and install it after thoroughly drying.

Inspection of air heater control unit

  1. Remove the control unit.
  2. Use the circuit tester to check each terminal voltage of the unit.

  • When the engine cooling water temperature is 20 degrees or more, disconnect the connector of the water temperature sensor, connect a resistance of about 600 ohms to the vehicle harness side, and use it as a pseudo signal at the time of measurement.
  • The time within the measurement condition is described with reference to the time when the engine coolant temperature is about 20 ° C.

Reference material

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