300tdi wiring help, engine cranks when battery connected

[Sharp]

I've had my dash and steering wheel/column etc. off to do my master cylinder and some welding and I'm having some electrical issues. I have no idea what's gone wrong but I've reconnected the wiring and when I connect the battery negative now the starter begins cranking. This was without the key in at first, then put the key in and turned whilst it was cranking. It didn't start but the battery is weak and it sounded like it was starting to struggle.

Does anyone have any ideas what I might have done? I have checked photos of the ignition switch before I started and after and the wires are in the same place. I haven't put the dash back in yet as I have some welding to do on the passenger side and wanted to bleed the clutch and check all was well there first. That means I don't have things like the radio in or the switches to the bottom right of the steering wheel with the hazard warning lights etc. connected. I also don't have the steering wheel on. I had assumed none of that would make a difference? It's a fairly standard unmodified 1998 300tdi.

Here's a list of what I disconnected (apologies for the non-technical descriptions!)

Ignition switch before:

Ignition Switch after:

It was connected to the steering wheel when I connected the battery up.

Little black plug that comes off coiled wire around ignition:

Black cable connected to steering column hanger bolt:

Three plugs from controls on steering column:

Plugs for hazard lights, rear heated screen and fog lights (have not connected these up yet)

The only other wires are to the radio. I took the green box (imobilisor?) and two relays off the bulkhead but didn't disconnect them:

That was all I removed. I've been around and checked for any spare cables I might have missed and pressed all the connector blocks together in case anything had a loose connection. Can't see any damaged wires.

Any help greatly appreciated as I'm pretty stumped.

[simonpelly]

Looks like it's going to be one of those processes of elimination, buzzing it out with a multi-meter, etc...

Clearly you've checked for trapped/chaffed wires, etc... 🙂 Wires damaged as they pass through the bulkhead? Any changes to the connections under the bonnet?

Out of interest, does it occur if the ignition switch is not connected at all? Eliminate the switch being the problem?

 

 

[92a]

The red and white is the wire that starts the engine,I would  pull this off and see if it cranks when you reconnect the battery, (this terminal should only have 12volts when you are turning the key to start the engine)

[Sharp]

Thanks for your replies. I'm totally out of my depth with anything electrical so I'm pulling my hair out a little bit (what's left of it anyway).

Under the bonnet I think the only wires I disconnected were the brake fluid sensor, the bonnet alarm switch and the A pillar door switch (both of the switches were broken and I'm waiting on new ones coming)

I've had a pretty good look at the wires to check their integrity and worked my way back over the wires from the ignition but I'll have another look over again in the morning. I don't really know which wires could be the culprit, my head spins a little when I read on here about alarm spiders and solenoids and imobilisors and what not but it looks like I'm going to have to try and educate myself on how it works.

The welding was on the panel the pedal boxes are mounted to so single skin and there were no hidden wires to have melted. I could quite easily have damaged or pulled something getting the steering column back in but I can't see any damage.

Thanks for the suggestion with the red and white wire. I'll try that tomorrow morning and see if it still tries to turn over.

Thanks again for your help

[monkie]

Yes, that's my first thought too. My advice is remove all the wires from the ignition switch except for the main feed. Then see what terminals are live with the key in which position. The white/red wire should be connected to a terminal that is live only with the key held in the start position. It is this white/red wire that activates the starter relay to power the solenoid up on the starter motor. The only thing is that my 110 is relatively simple and has no immobiliser to add further complications... 

 

EDIT: looking at your ignition switch wiring (ignoring the red and the black wires as they look like additions and I don't know what for)... The brown wire is the main feed from the battery via the positive bus on the starter solenoid. The white wire must be live with the key in position 1, 2 and 3. The white/orange must be live with the key in position 1 only and the white/red wire must be live with the key in position 3 only. 

If that doesn't help. Remove that red wire from the switch and see what happens as it looks like a permanent unfused live taken off the main feed from your photo. 

[Peaklander]

If your wiring is un-molested, then the feed to the starter solenoid isn't directly from the ignition switch or starter relay.

The red/white wire goes to the starter relay coil but the other connection to the coil is taken from the alarm unit (the green box that you mentioned). So that box allows the relay to operate if the alarm conditions correct.

The switched contact in that relay is then taken to the immobiliser 'spider' which is in the battery box. This is the 'final check' which provides the starter motor output and the feed to the fuel solenoid. The white cable on the ignition switch (ignition ON) is taken to that black box.

So the only way that the starter can spin is if the cable from the battery box immobiliser 'spider' becomes live. That cable and the one that goes to the fuel pump solenoid are not colour coded, they are black. Have you done any work near the cables that come out of that box?

Edit: Perhaps check if you have that spider as it is often removed or bypassed.

Alternatively you might be better tracing back from the starter solenoid to see where that cable runs.

 

Edited March 24, 2021 by Peaklander

[missingsid]

Maybe I am confused by the flash on the second photo but.. the red wire looks older, thinner and with shorter insulation in the fist pic to the second! Are you sure you have not picked up a different wire?

[Sharp]

Missingsid, I think you might be onto something. There are two red wires:

It's difficult to tell but I think you might be right and that it was the thinner of the two that was originally connected (and I had put the thick one back on). The thin wire runs to the radio connections, the thick wire goes through the firewall and into the engine bay and connects behind the block:

I have a photo of it loose from when I was taking things apart and I have a feeling it might not have been connected to anything, or that I had somehow unfastened it when removing the dash and then taken a photo of it to note that I wasn't sure what it was. The cable tied wires are the wires from the ignition (the repair section visible in the background is from the previous owner!)

With the thick wire attached I disconnected the red and white wire and it still tried to turn over when I connected the battery. When I swap over to the thin red wire and connect the battery up it doesn't try to start and the ignition switch starts working as expected, i.e. one turn and the heater blower and dash lights come on. When I turn it all the way nothing happens, no clicks or anything.

So I think it looks like it was the thin wire connected originally, which leaves the question was the thick red wire just loose before from someones previous work or has it become unplugged from something and is that the cause for the non starting?

 

[Bowie69]

Where does the wire from that connector run to?

[Sharp]

Ok I've had a wee go with the test lamp and tested each connection in the different positions of the key. I had the test lamp connected to the negative terminal (with the battery connected up) and then touched each terminal on the back of the ignition.

Off but with key in

brown - live

white - no reading

white/orange - no reading

white/red - no reading

Position 1

brown - live

white - no reading

white/orange - live

white/red - no reading

Position 2 (click from engine bay when turned into this position)

brown - live

white - live

white/orange - live

white/red - no reading

Position 3 - when I turn the key to this position nothing happens at all, fan switches off and there are no noises.

brown - live

white - live

white/orange - no reading

white/red - live

 

I did the above without either of the red wires connected as well and that didn't make any difference to the results. I checked the unidentified thick red wire in each position also and the test lamp didn't come on. I can't find out what it is online, it looks quite new and there is only about 8 inches or wire coming out of the bulkhead, i.e. it doesn' stretch any further than the back of the dash panel. In the engine bay it dissapears into a thick bundle of wires in tubing at the back of the block, one tube goes back along the transmission tunnel, one goes around the nearside of the engine and the other wraps around the offside of the engine with wires coming out to the fuel pump and then continuing around to the alternator. I'm guessing it goes to the starter motor and the liver brown wire is what was sending power to the starter via that wire.

[Peaklander]

Sorry what I originally wrote in this post is rubbish so I have deleted it. I have just re-read your last post and understand what you are saying. The thick red appears to be connected to the starter solenoid but for it to work it would need to replace or piggy-back onto the crank terminal (red/white wire).

Edited March 24, 2021 by Peaklander

[Eightpot]

Just quickly scan reading this and not fully reading replies, I would suggest that someone has had an ignition problem in the past and wired directly to the starter motor with that red wire - i.e. a bodge.

Ignition switch takes permanent live from brown wire. Red/white takes power from this when key in correct position. The Red/white runs to the starter relay under the fuse panel & triggers the relay. The thicker red/white supplies power to relay on top of starter motor.  The alarm module provides the switched earth path to the starter solenoid under fuse panel - this can cause problems and may be the cause of original fault - no earth, relay doesn't work.  Using a multimeter check to see if the relevant pin on the relay is earthed - if not you can cut the wire, put a ring terminal on and attach it via one of the fuse panel screws to bulkhead (or get a replacement alarm module if you want to keep immobiliser)

Edited March 24, 2021 by Eightpot

[Sharp]

Peaklander, I saw what you originally wrote and it seems to have worked! I removed the red/white wire from the ignition and replaced it with the thick red one and boom, it started up! So I guess someone has bipassed the imobiliser and when I was taking it apart I have just messed up with labelling the wires.

Thank you so much for everyone's help, I will have to try and tidy things up at some point but I'm just glad to hear it fire up for now and I can get on with finishing the repairs.

[Sharp]

Thanks for that Eightpot, I think I will have to reread that a few times but it's starting to make a little more sense. So the red wire is probably someone's fix when there were issues with the red/white circuit of some sort.

[Eightpot]

1 minute ago, Sharp said:

Thanks for that Eightpot, I think I will have to reread that a few times but it's starting to make a little more sense. So the red wire is probably someone's fix when there were issues with the red/white circuit of some sort.

Exactly, and with a 300tdi it's usually a problem with the alarm module not providing the earth path on that yellow relay.  It's not ideal running a wire direct to the starter as it's a fire risk if not going through a relay, so worth reinstating the standard wiring. 

[Sharp]

Thanks for your help, the label in the fuse box cover has the left relay labelled as start but looking at the wires on the back it looks like the right hand one:

It has the white and red tracer, a thick brown and a thick brown and red cable. The thinner black one has an orange tracer and I can see that coloured wire coming out of the green imobiliser box so I think that looks to be the wire you're talking about.

To test it do i reinstate the ignition wiring and then check the resistance by poking one end of the probe in the back of the relay and the other against a ground point? Or do I turn the key and check for voltage between the two?

Either way it sounds like if I just wire it to the fuse panel screw and it starts working then that confirms it was the issue. Rained off just now so will have another play tomorrow. Really pleased to get it started up and thanks again for everyone's help.

 

 

[Peaklander]

On my 1996 300Tdi as you look at the three relays from the front, the starter relay is the one on the right. The challenge in re-instating the correct wiring is that you need to be sure that the alarm (the 10AS green box) is un-arming correctly on the key fob. This needs the doors and bonnet to be closed and registering that back at the 10AS. If you can set and reset it then that's a start.

Next though is the black box in the front of the battery box. It is this unit that is actually the immobiliser. As I said up the thread somewhere, coil of the starter relay is energised by the key at crank position, through the white/red wire and  the 10AS alarm unit through the black/orange wire. The switched contact in that yellow starter relay passes +12V from the brown down the brown/red wire to the immobiliser in the battery box (if it is still fitted). This "black box" then provides the actual feed to the starter motor solenoid and the fuel solenoid. It is this unit that seems to fail (dry solder joints) and people replace it with a plug that has been wired to do the same thing (I think people call it the spider replacement).

So you have to get the yellow starter relay to click on the turn of the key and then make sure that the outputs from the immobiliser "black box" are being turned on. What makes it harder is that those wires are all black.

[Sharp]

Thanks for such a comprehensive reply, I think it's making sense to me now. I have the black immobiliser box, although it is under the central cubby.

I think long term I'll get rid of both and fit something aftermarket. I already have my own security on the vehicle and I imagine anyone trying to steal it wouldn't be held up less by the stock system than by something unfamiliar.

Would terminating the red wire at a new relay on the bulkhead and running a beefier wire from that to the starter be an acceptable solution to the fire risk? It looks like they have only run the red wire to the connector on the back of the block which looks like the original wiring after that.

Once I've finished the welding repairs it'll be a good time to try and neaten up the wiring and get the bulkhead sealed and insulated properly. I've just got some basecoat and laquer from buzzweld and I'm looking forward to getting it properly sorted

[Peaklander]

Simpler would be to fit an in-line fuse just behind the ignition switch but don't forget that the cable is only live for the time it takes to crank and start. Run it in a bit of conduit and it will be safe from chaffing.

The solenoid on the FIP will possibly also have been 'bodged', to be energised with the key in the IGN position (white wire has +12V). Do you know where that cable runs? 

[Sharp]

Held up with work and the weather for a couple of days. Much simpler solution, thanks. What fuse would be appropriate?

The white cable dissapears into the rest of the wiring loom and I can't really see where it goes without separating it. I've got it partially reassembled just now so I can get to work next week but will hopefully have a chance to finish the rest of the welding and tidy it up in a week or so. I take it that if original then should the white wire end up at the black box under the cubby?

[Peaklander]

In the wiring diagram you can see that the white/red from ignition switch to coil of starter relay (R102) isn't fused except back at the 60A Link2 in the engine bay main feed. I presume that this is sufficient because, as I said earlier, this cable is only live for as long as you hold the key at the crank position.

On the contact side of the relay, again the +12V is fed from Link2 and the brown/red cable that goes to the black box / spider isn't fused. Again, it's only live for a second or two. Personally I think a further fuse is unnecessary.

You can also see that the connections in and out of the black box / spider, labelled 'ECU-Engine Immobilisation (D139)', are all made with black cables. This does make tracing back difficult and the only way to do it is with a meter, buzzing the cables with all power off.

It would be interesting to know how your fuel solenoid is wired-up. In the diagram it's labelled as 'Solenoid-Fuel-Cut-Off (N101) as this will have been bypassed too if the alarm / immobiliser isn't working.

 

[Sharp]

Well that diagram is making a little more sense to me than it used to so thanks again! I'm not sure when I'm going to have a chance to look at it again and I've had issues putting the dash back* so this might end up sitting on the back burner for a while depending on work. When you say buzz the cables do you mean checking the resistance of each wire? So clip one lead to the wire coming out of the solenoid and touch each wire on the immobilser until I get a reading greater than infinity?

*steering wheel seems to be binding slightly on the plastic box behind it which resulted in the permanent live wire on the ignition coming loose at a pretty inconvenient junction yesterday. Managed to tidy things up a bit more and tightened the spade connectors on the ignition so I don't think it will happen again but I am still getting a squeak when I turn the wheel.

[Peaklander]

Yes 'buzzing' is checking for continuity. Checking from one end of a cable to the other is going to read almost 0 Ohms and a meter selected to give an audible indication when 'continuity' is present saves glancing at the display. If you don't find the correct other end then the meter will read very high (M ohm).