It is impossible to check the control unit using the devices available to the car owner. Only a workshop can do this. If it is assumed that the preheating system is faulty, the car owner can check it himself by performing the following steps.
1. Check control unit fuses 21 and 27.
2. Check the 80A fuse.
3. Open the cover of the central distribution block and make sure that the fuse link is not oxidized and has not burned out. Replace the defective link.
4. Check the voltage supply to the spark plugs if the fuse is normal.
5. Remove the power cable tip from one of the spark plugs (see illustration).
6. Connect one tester wire with a control diode lamp to the removed spark plug power wire, and the second tester wire to the ground (-).
7. Disconnect the coolant temperature sensor plug.
8. Turn the ignition key to the "Warm-up" position. The tester's LED should light up and stay that way for about 28 seconds. In this case, the power supply to the glow plugs and the warm-up duration control are normal. The control lamp on the instrument panel goes out after about 20 seconds. If the tester's LED does not light up, the fuse has blown or the relay is faulty. If the warm-up duration time parameters are violated, the cause is a malfunction of the control unit.
Open the cover of the central distribution block and check the relay. It is located in the block under No.12.
To check the glow plug itself, it must be removed.
9. Remove all glow plugs and place them on a fireproof pad. Before doing this, remove the spark plug tips and unscrew them with a 10 mm socket wrench (see illustration)
10. Connect the spark plugs directly to the battery using auxiliary wires with a cross-section of at least 2.5 mm. Observe the spark plugs to determine which one does not work or glows weakly.
11. Replace defective spark plugs. It is recommended to replace all spark plugs. Tightening torque of spark plugs is 15 Nm.
The following glow plugs are installed on AUDI 80 vehicles with a TDI engine:
- Beru 725 MJ;
- Rosch, N₂ article 0250 202 009;
- Audi, article no. 101 40101.
(The original article is located on the online resource AudiManual.ru)