Car Mechanics (UK)
Glow plug issues
I have a 2001 VW Bora 1.9 diesel. In the winter, on turning on the ignition switch, the glow plug light on the dash only shows for about three seconds and the engine starts after about 20-30 seconds with a cloud of white smoke from the exhaust and a strong smell of fuel. In the summer, the light shows for about one second. Once the engine has been running for a while it will start first time.
All four glow plugs read 0.8-1 ohm. There is no voltage to them on switching on the ignition. The glow plug relay (Position 4a, 97 on the wiring diagram) clicks on operation of the ignition. I took it out and tested it with a voltage supply by connecting 12V to the terminals 86 and 85 for the solenoid and the voltage meter got continuity on terminals 30 and 87.
With the relay out, there is 12V on terminal 30, and when switching on the ignition there’s 11V between terminal 86 from the voltage supply relay and 85, which I presume goes to the ECU. When the relay is in place, I do not get 12V on the two heavy cables under relay 103 which supply the four glow plugs. I disconnected these two cables, but there was still no 12V on their terminal; as the relay works fine, I would have expected to get 12V here. The two cables to the glow plug harness are OK for continuity.
When I disconnect the coolant sensor, which has four terminals, and turn on the ignition, I now have 12V at the glow plugs. The glow plug lights for about 12 seconds and it starts up straight away. I kept the meter at the terminal for the glow plugs under relay 103 and, after five minutes, it still had 12V. I thought this would cycle off after a period, once the engine had started. I am going to replace the coolant sensor with a genuine VW part, but I would like your opinion on this issue before I buy a new sensor. I read on the internet about a guy with similar conditions on his VW Golf. When he replaced the coolant sensor, it reverted back to no voltage at the glow plugs. I don’t know if he used a VW sensor.
I am using the Haynes manual for information. Attached is the wiring diagram. On the wiring diagram it shows a ‘low heat output relay (glow pin heater only)’ and ‘coolant heater element’. Would these affect my problem?
The low heat output relay operates the three cooling system heater plugs, but this system is also operated by the ECU from data retrieved from the coolant temperature sensor, so this may be relevant to your problem. The temperature sensor uses two wires to control the temperature gauge and two to inform the ECU of the engine temperature. With the temp sensor disconnected, the ECU will go into default cold mode and this is why the glow plug operation is working correctly.
The main relay feed (number 30 on your diagram) will be constant as it is the relay that controls the switching of the glow plugs; it is the terminals 85, 86 that will alter according to the ECU control. Terminal 87 will only be live when the relay switches to active the glow plugs.
If you check your wiring diagram, you will see terminals 1 and 2 are for the gauge and terminals 3 and 4 are for the ECU control. Using an ohmmeter between terminals 3 and 4, you should check the resistance of the sensor with a cold engine and as the engine heats up. The resistance should lower as the engine heats – you should see somewhere around 4000Ω when cold and 200Ω when hot. If the resistance does not vary greatly then the sensor should be changed.