Table of contents: Main relay and fuel pump relay…🠇 Main relay 🠇
Main relay and fuel pump relay (single double contact relay)
The Bosch Motronic electrical system is controlled by a single 15-pin dual contact relay (see illustrations 11.41 and 11.42). The supply of direct voltage to terminals No. 8, No. 11, No. 14 and No. 15 of the relay is carried out from the positive pole of the battery.
11.41 15-pin double contact relay
11.42 15-pin relay terminals
When the ignition is on, the electronic control unit connects terminal No. 7 of the relay to ground, from which power is supplied to the primary winding of the relay.
When power is supplied to the relay coil, terminal #15 is connected to terminals #4, #5, #6 and #13. This allows voltage to be supplied to terminal #37 of the ECU and to the injectors, idle air control valve and other actuators.
When the ignition is on, the ECU grounds relay terminal #10 at ECU pin #3. This supplies power to the secondary winding of the relay, which closes the second contact of the relay and supplies voltage from terminal #11 to terminal #9, thus providing voltage to the fuel pump circuit. After approximately one second, the electronic control unit opens the circuit and the pump stops. Short-term operation of the fuel pump increases the pressure in the fuel lines and ensures a quick start.
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The second circuit will remain open until the engine starts or the crankshaft is cranked. Once the ECU receives a signal from the crankshaft angle sensor, the ECU will re-energize the secondary winding and the fuel pump, ignition system, and fuel injection system will operate until the engine stops.
Main relay
| Terminal | State | Relay | Volts |
| 8, 11, 14, 15 | The ignition is off | Connected/disconnected | nbv |
| 3 | The ignition is on | Connected/disconnected | nbv |
| 7 | The ignition is on | Connected | 1.25 (max) |
| 10 | The ignition is on | Connected | nbv |
| 10 | When turning the crankshaft/with the engine running | 1.25 (max) | |
| 4, 5, 6,13 | The ignition is on | nbv | |
| 1, 9 | When turning the crankshaft/with the engine running | nbv |
| Terminal | Source/Destination |
| 1 | Relay output voltage to ignition coils: t2, throttle body heater: t2, oxygen sensor: t2 |
| 2 | Not used |
| 3 | Main relay power supply to relay: t5 |
| 4 | Relay output voltage to the electronic control unit: t37 |
| 5 | Relay output voltage to VASC: t2, relay: t3 |
| 6 | Relay output voltage to idle speed control valve: t2, carbon filter solenoid valve: t2 |
| 7 | Control relay, ECM: t36 |
| 8 | Power supply from battery to relay: t30 |
| 9 | Relay output voltage to fuel pump 42 |
| 10 | Control relay, ECM: t3 |
| 11 | Power supply from battery to relay: t30 |
| 12 | Not used |
| 13 | Relay output voltage to injectors: t2 |
| 14 | Power supply from battery to relay: t30 |
| 15 | Power supply from battery to relay: t30 |
See illustrations 11.2, 11.3, 11.4, 11.5, 11.6