20vt conversion wiring help please

Discussion in '1.8 & 1.8T' started by Ianm, Mar 20, 2011.

  1. Ianm Forum Member

    Mar 19, 2011
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    I have an issue with the wiring harness on my mk2 20vt, so for completeness I will walk through what ive done, car is a 90 spec gti with the hazard switch on the steering column

    Engine going in is an early AJH, similar to an AGU block from the early golfs but with the airbox on the left of the engine instead of the right.

    Ecu has been professionally de-imobilised

    So when I removed my old loom I cut out the following wires at the fusebox, leaving enough to strip and solder to the new loom

    All except the last one are on the 3 white connectors on the back of the fuse box, with a bank of red connectors on one side of them and green on the other. I numbered them 1-3 with stickers to help my wiring, number one is closest to the Red connectors and 3 is closest to the green

    (im hoping to those of you who know the late model fuse boxes this is making sense)

    so, I cut.....

    Connector 1

    Pin 1 Green/black Oil temp sender
    Pin 3 Yellow/Red Coolant temp sensor
    Pin 10 Yellow High oil pressure switch
    Pin 11 Blue/Black Low oil pressure switch

    Connector 2 (the middle of the three connectors)

    Pin 3 Yellow/Red Fuel pump relay
    Pin 5 Brown/White Ground cylinder head
    Pin 8 Red/White Oxygen sensor power
    Pin 10 ECU power

    Connector 3

    Pin 4 Thick Brown Block earth
    Pin 5 Digifant ECU Pin 1 thermotime switch

    And finally I had to cut out the thick wire to the white connector that is in a line of 4 in the bottom left corner of the fuse box which is for the fuel pump after run control unit (its red)

    Now I got my engine with the loom for the coil packs/knock sensors etc still attached, I just had two connectors, one for the ECU and one which joined the other bigger part of the loom. That other part also connected to the ECU (bigger connector)

    I stripped out the stuff I knew I didnt want like the wires to the old dash clocks and for the door open alarm on the donor car, SAI and the usual valves etc.

    and I was left with the following (all came from the bigger of the 2 connectors on the ECU, the engine part I have not modified in any way)

    Pin 1 of the ECU which is a thick black wire and I assume is the live feed.
    Pin 2 Thick brown wire
    Pin 22 Thin brown
    Pin 3 Red/Yellow Engine control unit fuse (3 amp)
    Pin 4 Yellow/Blue Fuel pump relay pin 1
    Pin 1 Black to +ive connection

    So I did the following bits of wiring.

    Pin 4 of connector 3 to the block (I used the forge SAI blank as a seemingly good place to get a clean ground)

    Pin 2 of ECU thick brown to earth point on chassis leg just behind battery

    Pin 3 of connector 2 to pin 4 on the ECU

    Pin 5 of connector 2 to cylinder head (again I cleaned up a bolt and grounded it where I thought best.

    Pin 10 of connector 2 to pin 3 of the ECU

    Pin 1 of the ECU to the wire I cut on its own for the fuel pump after run thingy.

    Pin 10 of connector 1 to the yellow wire that used to feed the old clocks for high oil pressure

    Pin 11 of connector one to the blue/black wire that used to feed the old clocks for low pressure

    Pin 3 of connector 1 to the yellow/red wire that used to feed the old clocks for coolant temp

    Pin 1 of connector 1 to the Yellow wire that used to feed the old clock for oil temp

    I did not wire pin 8 of connector 2 to anything, as the oxygen sensor on the 1.8T draws its power from the ECU via the plug which is all still there.

    I did not wire pin 5 of connector 3 to anything because as far as I know the 1.8T does not have a thermotime switch.

    Now, ive had the car turning over no problems and got oil pressure, but when I plugged my coilpacks back in to see if she would fire it just turned over and over.

    I removed the fuel feed line to see if it had pressure and there is a small amount of fuel there but no real pressure.

    Any wiring help would be gratefully received as this really is my weak point and some of the above was guess work.

    Thanks guys

  2. rubjonny

    rubjonny Administrator Staff Member Admin

    Oct 31, 2003
    Likes Received:
    are you keeping the mk2 clocks? if so you need to keep the yellow, blue/black and green/black sensor wires for the oil pressure and mfa, the mk4 type sensors aren't compatible plus there is only 1 oil pressure switch anyway. you may need a T piece to get all 3 MK2 sensors in though

    this may help:

    and this:

    and i assume you have found the fusebox FAQ? there is also an FAQ on how to read VW wiring diagrams which will be useful.
  3. Ianm Forum Member

    Mar 19, 2011
    Likes Received:
    Eh? whadya mean they arent compatible? now im properly confused, my oil pressure sensors run to the ECU, and then from there to the old wires in the fusebox

    If I run the old wires straight to the fusebox wont the ECU kick off?

    Starting to think ive bitten off more than I can chew here lol

    I think ive solved the start problem tho, the ECU switched live was not on the circuit that stays live under cranking, it soon will be tho. Then hopefully it will run.
  4. rubjonny

    rubjonny Administrator Staff Member Admin

    Oct 31, 2003
    Likes Received:
    the oil pressure, level and temp sensors go nowhere near the ecu, they run direct to the clocks. see the wiring diagram above ;)

    what you do is run the old wires to mk2 sensors so the dash works as it should. if you wired your oil sensors to the ecu no wonder its having problems :lol:
  5. Ianm Forum Member

    Mar 19, 2011
    Likes Received:
    I never wired them anywhere, they were part of the engine side of the narness, which goes to the ECU, it only has one other 6 pin connector that joins to the other half of the loom, never really looked as it was already on the engine, i assume then those wires run through that plug and intot he car?

    haha, I must sound like a proper mong.
  6. rubjonny

    rubjonny Administrator Staff Member Admin

    Oct 31, 2003
    Likes Received:
    they are part of the engine loom aye, but they dont go to the ecu :)

    looking at this diagram there is an 8 pin plug, where the engine loom sensors connect to the fusebox side of the loom:

    T8J - 8 pin connector on engine left:
    1 - blu/blk - oil pressure switch (0.3bar)
    2 - yel - oil pressure switch (1.8bar)
    3 - grn/blk - oil temp sensor for mfa
    4 - pur - dash coolant temp sensor
    6 - wht/blu - speedo sensor pin 2 (speed sensor output to dash)
    5 - brn/wht - earth for dash
    7 - blk - speedo sensor pin 1 (ign live feed)

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