Spitfire Mk4/1500 wiring issues

[Spitfire74]

Hi all

So I noticed that one of the green wires coming from the top fuse in my Spitfire had a melted cover and bare exposed wires (NOT GOOD!). Having traced it I've found that it connects to a double bullet connector and then that goes to the heater motor (which I've realised is off a 1500). Photos as below. I have also discovered an unconnected Brown/White wire which I am slightly confused as to the purpose of and whether it ought to be connected!?

 

 

So, I have a couple of questions;

 

1. I'm assuming the damage to the melted green cable is due to overheating!? But not sure why it would have happened, all the other wires in the harness look fine, just this one that is melted and frayed. Looking at the wiring diagram (for a RHD 1500) I think it is correct that it goes to the fuse box.

2. Does anyone have an idea what that White/Brown cable is for? I think from looking at the Mk4 and 1500 wiring diagram it looks like it should be the Oil Pressure warning light? Forgive me if this is a stupid question but is that not the green light on the speedo that says OIL?

 

Thanks in advance.

 

Nick

 

 

 

[RobPearce]

Given that the green wire is (or certainly should be) connected to the "downstream" side of the fuse, it's quite impressive that it managed to get melted like that! The last time I saw a wire in that condition it had been connected straight across the battery.

So, the first thing to check is that the other end of that wire is on the correct end of the fuse, and that the fuse is a correct 17A (35A blow) fuse. If you have a multimeter with a high current range (most don't go above a couple of amps, unfortunately) you could put it in circuit in place of that wire and quickly check how much is being drawn. In the absence of such a meter, a headlamp bulb would work. The heater motor should make it glow; if it achieves full brightness there's something wrong.

If the current looks sensible and there's no sign of any fault, you can probably just replace that wire. It's not impossible that it fell out of the bullet connector at some point and landed on part of the bodywork, then the DPO of the time figured he'd get away with just reconnecting it.

The connection of heater motor with a green wire to the fuse matches the early 1500 wiring diagram, not the MkIV, which would tie up with your 1500 heater. Based on that diagram, my guess is that the "brown and white" wire may be a faded white/pink. It doesn't look right for an oil light wire. The white/pink would be accessory feed for a radio, shown as number 5 and incorrectly labelled "wiper switch" in my copy of the Haynes manual (29 is also thus labelled and makes sense to be so). Because the factory didn't fit a radio, that wire was left dangling as you have.

You can check for that by connecting a voltmeter (or small bulb) between the wire and earth. If it goes live when the key is in the accessory position, that's what it is.

[Spitfire74]

Hi Rob

Thanks again for replying and being so helpful!!

When you say that it should be connected to the downstream end of the fuse, how can I tell which end is which? I will see if I can either put it in circuit or put  a bulb in.

Yes that's what I thought I was reading on the 1500 wiring diagram but wanted to make sure I was reading it properly! Thanks for confirming.

Ahh white and faded brown would make sense, there is a radio in the car but totally disconnected. I have been wandering about maybe getting it reconnected so good to know, will check it like you suggest! As an aside to do with that do you know if there was a standard placement of speakers for Spits fitted with radios? I know  I'm unlikely to hear anything whilst driving and actually the engine and pipes are a good enough radio by themselves but seeing as it has one it'd be nice to make it work haha!

Thanks again for all your help!

Nick

 

[RobPearce]

If you can see the fuse box wiring, that fuse should have a bunch of white wires one end and a bunch of green the other. If you take the fuse out (or fit a dead one - there's a quirk with some fuse boxes that means the fuse cap forms the connection between wires of the same colour) then the white wires should still go live with ignition on, but the green ones should not. White is directly fed from the switch, "upstream" of the fuse. Green supplies circuits that are protected, "downstream" of the fuse.

I expect there was at least one "standard" placement of speakers, although I think they would have been dealer fit so not necessarily consistent. I couldn't say what it was, though. On Heralds it would be a single, oval speaker beneath the radio console, but there's no room there on a Spitfire.

[Spitfire74]

Hi Rob

 

Thanks again.

 

Have tested the fuse connections and they were the correct way round. I've reconnected a new wire in place of the old one so hopefully all good now! The fuse was also a 25A fuse so I've replaced that with 35A fuse too.

 

Have checked the radio cable and you are correct, with the keys at the accessories position it goes live.

 

Thanks again

 

Nick

[Spitfire74]

So having put the dash back together just now I started the car to make sure everything was still working and noticed that the tachometer and the indicators were no longer working. The only thing that I could think that I had changed between them working yesterday morning and now not working was the frayed wire and the 25A fuse out for a 35A fuse

So I just put the 25A fuse back in and they are all working again. I know, Rob, you said that a 35A fuse was correct so do you think that this is/will be an issue? 

Thanks again, Nick

[RobPearce]

If it works with a 25A fuse then it will work with a 35A. Something else is up.

First, though, a note. I actually said a 17A (35A blow) fuse. You need to be careful of fuse ratings as they're not always specified the same way. The correct fuse should be rated for 17A continuous current and to blow "instantly" (for some value of instant) above 35A. If you subject it to 25A for an hour then it's supposed to blow some time before that.

Now, since your 25A fuse works and the 35A doesn't, there are two likely explanations. First, the 35A fuse you found was a dud (maybe an old one that was changed out because it had blown). Second, it wasn't fitted properly and wasn't making good contact with all the terminals. Either way, there's not much point fiddling; the 25A fuse that was in there before will work, it just won't protect the circuit from some weird corner-case conditions that the correct 17A one would.