Hi all new to this forum. Have had my mk4 tdi for a while now. Has the usual tdi stuck/sticking turbo veins. Car mechanic by trade. The mechanical side of things would be no issue for me. Its more the wiring side if I was to do this conversion. Would the original tdi throttle pedal and wiring work with a 20v? Would all the wiring loom connecters to the car/to the engine be the same? I know I will/ more than likely have to run wires for the fuel pump in the tank. The ecu will need unlocking for the immobilizer. Is there major wiring differences between petrol and diesel? Some people would say just sell my tdi and buy a 20v but I have had this car for a while and would rather keep mine. More the amount of £££ and the amount of time I have in it. Any help would be appreciated.
https://www.clubgti.com/forums/inde...install-faq-now-with-added-dieselness.267196/ Have a look at that, see if cross reference petrol to diesel bits. But I'm sure that @rubjonny is the genius to answer this fully. Is the lift pump in the mk4 tdi in the tank and so possible there is wiring there? Can't remember if there is a fuel cooler under the car on them which wouldneed removing so new fuel line, I was converting a caddy to pd130 about 6 years ago so my memory is a little hazy regarding some bits (albeit the other way).
converting a diesel mk4 to petrol is a little difficult electrically, as you have to do a fair bit of re-jigging on the interior electrics to make it work. unless its an early one theres a fuel pump in the tank already so thats no biggy, but instead of running from fuel relay on the relay carrier like a petrol it has its own relay clipped separate (glowplug relay in carrier instead) the pedal needs changing as the diesel ones not compatible. as you can see from my thread a couple pins on the coloured plugs have different functions petrol to diesel as well. can be done if you have a real attachment to the car though petrol engine mk4 are way easier as its mostly plug and play, only difficulties is if you have a really early car with later engine or vice versa as you need to flip a couple pins on the coloured plugs, plus if converting manual to dbw or vice versa need to mess with pedals
Hi thanks for your replies. I've had a look at my car and it has only 2 wires so no lift pump. I've bobbed my head under and can't see a cooler in the fuel line either. My tdi is dbw no cable. So this is do-able? Id probably be better using a 20vt that is dbw? I could also go a local scrap yard and get a sender/pump from a petrol mk4 with the plug and a length of loom. Plus a petrol throttle pedal. Another issue I can think of is the evap as tdi isn't piped for it. But could get that mapped out with a non-immobilized ecu. Although wouldn't stop it from running either way as long as it doesn't create a vacuum leak. Ill a study of your post regarding the pin layout. Again thanks for your input guys.
ok so as well as what I said wiring tweaks on the inside you'll also need to run the fuel pump wiring, but all doable no problem. as long as you pick a dbw 20v that'll make life easier for the pedal side just get a petrol pedal to replace the diesel unit T6/4 -> ECU relay power on diesel, needs to be changed to permanent live T6/5 -> ECU relay power on diesel, needs to be changed to fuel relay or switched live (ecu relay is triggered by the ecu!) T6/6 isnt used so can ignore it T14a/5 & 6 -> ECU relay power on diesel, needs to be changed to fuel relay or switched live S/3 -> ECU relay on diesel, needs to be ignition live With T6/4 make sure its a good chunky fused live, this powers the coilpacks as well as the ECU and you want a 15a fuse in case the coilpack wiring cracks and shorts out which is a common issue. theres main live '500' and '502' terminals bottom of relay carrier you can use no biggy there. For the fuel pump feeds theres a fuel relay terminal '504' on the bottom of the relay carrier as well, so can run a couple 10a fuses off that. main issue here is finding the T14a/5 & 6 wire in your harness to tap into it All of these are fed off the ECU relay so you can start there and chase its feeds hopefully the crimped joint will be accessible so you can split them out. but worst case you could run them all off the relay terminal with a slightly higher amp fuse inline. S/3 sends ignition live to the fuel relay socket, thats a plug round the back of the relay carrier. plenty ignition feeds around the dash you can tap into same as above its tapped to ecu relay power so hopefully you can find the crimped joint and split them all out. I have all the diagrams in my signature link anyway look for g4agralhahfasvecu.pdf and g4aumauqecu.pdf
So the brown t6 connector needs permanent/switched live going from inside the car going to the engine ecu loom? S/3 is in the relay carrier under dash on drivers side? t14a connector I found today on the inner wing next to the battery. Are the fuel pump supplies from the ecu come out of the t14a connector to the fuel pump?
so the coloured plugs are on the ecu loom which you unplug in the scuttle, T14a is the 14 pin plug in the engine bay near the battery. S is one of the plugs on the back of the relay carrier. you need to modify the car side of the T6 plug behind dash to give it the right feeds, and find where the t14a plug pins 5 and 6 are in the dash wiring to sort those out as well what you need to do to start with is locate the diesel ecu relay 109 and chase the wiring from it, this has most of the wiring you'll be messing with from it. there should be a crimped joint on the ecu relay output feed to all those feeds I mention, hopefully it'll be somewhere easily accessible under the dash. you can chop that joint and split off all the feeds I mention to re-direct where they need to go need a 409 fuel relay, or can use a 67, 80 or 167 that'll do just as well if you have a good crimp tool and extra patience you can re-route the ecu relay feeds to the fusebox where they would be from the factory, grab a section of loom from a donor mk4 which goes from the fuel pump stud '504' bottom of relay carrier to the fusebox to power up the relevant fuses. on the diesel '504' powers the glow plugs and runs off to a chunky 2 pin plug in the engine bay. otherwise you can grab a 409 relay and run all of your power feeds straight off the 504 terminal with inline fuses so the wires I mention above plus your fuel pump feed
having another look at the diagrams, what you could do is cut the pin off the back of the ecu relay. run that straight to the 504 terminal on the relay carrier, this will power up the engine loom fuses that feed t6/5, t14a5 and t14a/6 which means all that side sorted. you'll need to find S/3 and cut it from the diesel ecu relay power feed above, dont have to find the joint just find the pin and cut a decent length to work with. then find yourself an ignition live, an easy one could be the black/blue ignition live feed to the clutch and brake switches, its close by. sometimes you find unused ignition live wires crimped into one of those joints I mentioned earlier just flopping around loose in the loom then all you need to do is add your fuel pump power to the 504 terminal with inline fuse, or if you have the patience tap it into the back of the fusebox to use original fuse 28
i have took the trims off from around the steering column and can see relay 109. There is 2 small nuts holding the relay holder in. Is terminal 504 behind this? Is there a link for the mk4 fusebox and pin diagrams on this site? Or will 504 be marked on the pin? I have had scuttle off and I can see where all the small connectors go through the bulkhead on the o/s. Does sound doable but would want to make sure i can do this before I cut/join/buy anything. You say cut the pin of the ecu relay which is relay 109? to power up t6/5 t14a5/6?
the terminals are along the bottom of the relay carrier, for some reason I had it in my head they were labelled the same as the wiring diagrams but now I look they arent so thats my bad the one you want for the diesel relay output wire is '87F/diesel' so on the 109 relay you have 3 wires, a chunky red which is permanent live tapped into one of the permanent live stud '30' on the bottom of relay carrier, thin blue/yellow which is ecu trigger and last one is chunky blue which is the output from relay to all the fuses. the last one you cut off the relay, crimp a ring on and pop it to 87F/diesel terminal
Ok that seems easy to understand although do i need to do anything with terminal 504 or is it 87f? Just to clarify. That will sort the feeds for the 2 wires in t6 and t14a. I don't need to do anything with s/3 because that bypasses it? Can i just use a bog standard relay to power the pump in the tank wired to switched ignition or have a to use a genuine vag relay? Maybe repeating ourselves but i would very much like to clarify before I start chopping wires on my car lol. Will I still need to put inline fuses in anywhere or does the way you show above use all the original ones in the fuse box?
504 = 87F, so you need to put the 109 relay blue wire here, no need to add inline fuses as this feed already goes inside the fusebox to power the relevant fuses. if you dont want to open the fusebox to add the fuel pump live inside you can put the pump power to the 87f terminal as well but you'll need to run an inline fuse as the 87f terminal is not fused internally you do need to find S/3 wire and cut it with a good length of wire, then splice it into an ignition live in the dash harness somewhere. ignore what I sdaid about the clutch and brake switches, just remembered on the diesel those are both spliced to ecu relay power as well so once you have finished modifying the loom they'll be fuel relay power. look in the harness for black/blue wires, if you can spot any loose ends going nowhere those will do or taped up joints in the loom can go there. Just check they are ignition live, 99% of the time they will be but best to be sure you could wire in a separate fuel relay but that would be more effort
