Which Relays Are For What?

[Mouldy626]

Hi. I have a 2004 tdi that has an electrical gremlin somewhere. I have changed relay 109, crank sensor, found badly burnt cables in engine bay fuse box, cut back and fitted new crimps. But every other time I start it, it takes about 6 / 8 seconds of cranking to start, to which she instantly fires up. Sometimes when she fires up, after long cranking, she cuts out immediately, buts then starts ok. I've also fitted a new injector harness and no codes show up. I was just wandering what others relays I could try?

[Mouldy626]

Also when cranking over, the Led on the door stays on, not flashing.

[GreenSharanTdi]

First layer of relays, you'll find 1 with 53 on it, that's your fuel pump relay. I only know this as i'm having problems with 53 myself at the moment! Just an idea. Best of luck

[BrianH]

That sounds somewhat like its struggling to pickup diesel - either an air leak in the system (filter is likely location) or crap stuck in the pickup both being possibilities. i don't know if there is a tank pump on the diesel models, but if there is your best bet to see where fault might be is to remove the feed from the tank and put something on the end of the pipe to catch fuel, and get someone to turn the ignition on for you. You should have a flow of fuel coming out if there is a pump, if this happens the pump and pickup are probably ok.

 

Most diesels don't have a tank pump though.

 

Other possibility is a clogged fuel filter though I'd guess you've already looked at that?

[GreenSharanTdi]

That sounds somewhat like its struggling to pickup diesel - either an air leak in the system (filter is likely location) or crap stuck in the pickup both being possibilities. i don't know if there is a tank pump on the diesel models, but if there is your best bet to see where fault might be is to remove the feed from the tank and put something on the end of the pipe to catch fuel, and get someone to turn the ignition on for you. You should have a flow of fuel coming out if there is a pump, if this happens the pump and pickup are probably ok.

 

Most diesels don't have a tank pump though.

 

Other possibility is a clogged fuel filter though I'd guess you've already looked at that?

Yep 100% they have a lift pump in the tank, you get to it by removing the seats and rear carpet to get to the access panel, it's situated under the middle row of seats behind the driver

[GreenSharanTdi]

I've just had to replace a very weak and tired pump on mine so lift pump failure is possible

[Mouldy626]

Thanks Brian and Greensharan. I'll try relay 53 first as if left over night, it fires immediately in morning. It seems to be more when half warm or hot. But also could be tank pump ( which they defo do have .) as she's done 217000! I've got both 2 nd hand. I was just having a break to let some hair grow back, and then try tank pump. I didn't know about 53, so I'll do that tomorrow. Thanks chaps. Will get back.

[BrianH]

Thanks Brian and Greensharan. I'll try relay 53 first as if left over night, it fires immediately in morning. It seems to be more when half warm or hot. But also could be tank pump ( which they defo do have .) as she's done 217000! I've got both 2 nd hand. I was just having a break to let some hair grow back, and then try tank pump. I didn't know about 53, so I'll do that tomorrow. Thanks chaps. Will get back.

 

Testing for the fuel being pumped up should be easy to do then when you get chance to do so as pumped should give you a good flow if its working correctly

[GreenSharanTdi]

Thanks Brian and Greensharan. I'll try relay 53 first as if left over night, it fires immediately in morning. It seems to be more when half warm or hot. But also could be tank pump ( which they defo do have .) as she's done 217000! I've got both 2 nd hand. I was just having a break to let some hair grow back, and then try tank pump. I didn't know about 53, so I'll do that tomorrow. Thanks chaps. Will get back.

Just another possibility, reading through another thread someone had very similar problems and the actual cause was the conections on the board where relay 53 plugs into had opened up too much so the relay wasn't making a proper connection, the relay was still fine. Just an idea, it took them an auto electricion to diagnose this though as they were out of ideas!

[Mouldy626]

Hi Gents. Yesterday, I fitted a set of relays from another galaxy tdi, which I know work on other car, bearing the same numbers and checked all the connections on them. Switching on the ignition about a dozen times, relay 53 was clicking on and you could hear the tank pump buzzing and gushing. Started every time when cold. Took her for a drive last night, when warm, stopped in car park and she took about six secs to start. Apart from in tank pump, I've checked out every thing electrical. I'm now turning towards the tandem pump on side of engine as its mechanical and is sharing job of pumping or vacuuming air. It's not visually leaking, but at 217000 could be dropping on pressure internally. I think there is a test plug on it. In Haynes manual it says those pumps run at 3.5 bar at 1500 revs. Might connect a gauge up and see if it drops off when switched off. Catch you later!