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Thread: S3 - 300TDi conversion - Glow Plug question

  1. 11th June 2025, 04:16 PM #1
    Balmain Boy
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    S3 - 300TDi conversion - Glow Plug question

    Hi All,

    Thanks for reading - my S3 project is in the last mile but it is feeling like a marathon .

    Manage to get nearly everything electrical working - thankful that there is not much (still an issue with the LED and flasher unit so had to order a new one) but I am stuck on the Glow Plugs.

    The wiring harness was completed by Bruce from Bygone Spares and he matched the harness with the S3 diesel. I have attached a copy of the wiring diagram for this part. The other two photos show the connection where the resistor should go and the attached glow plugs.

    This is all new to me so I have no idea. Do I need a resistor? Or can the glow plugs take the 12 volts being wired in series? If not any suggestions on the type of resistor I need to put across the join?

    I have used the charge light on the dash for the glow plug light. I know they will come on when I turn on the ignition (currently wired into position 4 on the ignition) but what turns them off? I did find this chat on another forum but couldn't quite get the answer I was looking for.

    Thanks in advance for your help,
    Chris

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  2. 11th June 2025, 04:24 PM #2
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    Tins
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    Pretty sure you'll be fine. 300 TDi glowplugs are 12 volt. No resistor AFAIK. They do have a timer relay.
    ​JayTee

    Nullus Anxietus

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    2000 D2 TD5 Auto: Tins
    1994 D1 300TDi Manual: Dave
    1980 SIII Petrol Tray: Doris
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    Nanocom, D2 TD5 only.​
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  3. 11th June 2025, 04:38 PM #3
    Balmain Boy
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    Thanks Jay that is great to know.

    I did buy this relay but trying to work out if I needed it (sounds like I do) but now I need wire it in with the current harness.

    Anyone adept at interpreting this wiring and how to hook it into my S3?

    Thanks,
    Chris
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  4. 12th June 2025, 07:32 AM #4
    Fattima
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    As Tins mentioned you need a timer relay, I don't think that is one you have pictured. You can get aftermarket ones, do a quick Goggle. It might pay to get an auto electrician to wire it up if you are not certain how to do it.
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  5. 12th June 2025, 07:59 AM #5
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    Tins
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    I'm not recommending this particular relay, but here is an example. Thing is, the relay needs to cut the power to the plugs after a sufficient interval or the plugs will burn out. I believe it also needs to be turned off by the cranking of the engine, but maybe someone else can confirm that. Thing is, in our climate the plugs aren't really necessary, but I know why you want them to work. You would possibly need to wire in an indicator light as well. When I return from the dentist I can probably find a wiring diagram to help. It's simple enough if you have some skills.
    ​JayTee

    Nullus Anxietus

    Cancer is gender blind.

    2000 D2 TD5 Auto: Tins
    1994 D1 300TDi Manual: Dave
    1980 SIII Petrol Tray: Doris
    OKApotamus #74
    Nanocom, D2 TD5 only.​
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  6. 12th June 2025, 11:18 AM #6
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    Tins
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    Relay part number: PRC691

     This may be a trifle confusing but it's all I can find right now.

    Screenshot 2025-06-12 at 12.16.49 pm.jpg
    ​JayTee

    Nullus Anxietus

    Cancer is gender blind.

    2000 D2 TD5 Auto: Tins
    1994 D1 300TDi Manual: Dave
    1980 SIII Petrol Tray: Doris
    OKApotamus #74
    Nanocom, D2 TD5 only.​
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  7. 12th June 2025, 12:51 PM #7
    Balmain Boy
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    Thank you both for the help - it is getting me in the right direction. Surprised how much a relay costs $145. Might consider just setting up a manual switch and turning it off after a few seconds.

    I found this picture from a UK forum post - which will help with the wiring except for the confusing number 4 (white to any white in vehicle harness) - I can see that this is 85 and on the diagram from Jay that comes from satellite fuse box, do you foresee any issues with ignoring this plug?

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  8. 12th June 2025, 01:02 PM #8
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    roverrescue
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    300tdi
    In good condition
    In all but all Aussie conditions

    Glow plugs are optional

    May cough and splutter a bit but should start fine without em !!!


    In case you were inclined to let them be!
    '95 130 dual cab fender (gone to a better universe)
    '10 130 dual cab fender (getting to know it's neurons)
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  9. 12th June 2025, 01:08 PM #9
    Balmain Boy
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    Quote Originally Posted by roverrescue View Post
    300tdi
    In good condition
    In all but all Aussie conditions

    Glow plugs are optional

    May cough and splutter a bit but should start fine without em !!!


    In case you were inclined to let them be!
    might be the best option - thanks,
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  10. 12th June 2025, 02:19 PM #10
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    123rover50
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    In the series one I put a 200TDI in , I left the Glow Plugs disconnected. No trouble starting even in this weather. Minus 2 this morning at 7 AM, west of Gympie.

    Keith
    1963 ex Woomera Ambulance sold
    1950 Tickford Station Wagon sold
    1954 Royal Review sold
    Perentie 6x6 Forward Control Camper sold
    1957 SWB Hard top with 200TDI,.
    . 1965 2A Forward Control Camper.
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