Car Mechanics (UK)

Glow plug is­sues

- Denis Jor­dan Iceland · Belarus · Austria · Belgium

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.

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