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Gt6 Mk1 repeatedly blowing fuse - it's back


Colin Lindsay
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Hi guys; just before I dismantle the entire front of the GT6 - yet again - I'm open to thoughts tips and experiences of blowing fuses.

My Mk1 is repeatedly blowing the top fuse. This one covers indicators, reversing lamp, brake lights, heater circuit, windscreen wiper, fuel gauge and temperature gauge.

On the way to MOT the top fuse blew after only three miles leaving me with no temp or fuel gauges  - that's the first thing you see as an indicator of the fault; I replaced it and it performed perfectly all the way to the test and the eleven miles back. The car can sit and idle all day in the garage or drive and nothing happens, no matter what you do. Three miles down the road on Sunday, it blew again; I replaced it and got another two miles, then it blew again, then again after about 200 yards. 

Nothing on that circuit is on, or was being used at the time - it was a straight stretch of road, no braking, no indicating, no heater nor wipers. I can't make the fuse blow under controlled conditions by turning on any of the circuits on that fuse, or wiggling the loom or fusebox about.

I'm therefore suspecting a live circuit that is shorting out; and possibly the overdrive which being a J-type takes power from the reversing lights - overdrive was not working at all but nor was the fuse blowing if I flicked the switch. It must be something happening as the car drives. 

Any handy tips for narrowing down the problem? I got my first MOT in two years and don't want the car off road and in bits yet again.

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Exposed wire (damaged insulation or a loose terminal) somewhere - possibly the overdrive sub-harness - randomly touching ground when rattling about as you drive.

I've also had the live-side of the reversing light switch make contact with the gearbox top cover, but that was rather more definite than your problem.

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23 minutes ago, NonMember said:

Exposed wire (damaged insulation or a loose terminal) somewhere - possibly the overdrive sub-harness - randomly touching ground when rattling about as you drive.

I've also had the live-side of the reversing light switch make contact with the gearbox top cover, but that was rather more definite than your problem.

Just been out for an hour and tried everything to make it blow, to no avail; then it blew when I wasn't watching - no idea why.

I've turned on wipers, heater, indicator, everything else on the circuit at the same time, but no good. THEN I put it into third with overdrive on, and it blew first time. Thinking that's the problem I got the camera set up ready to try again, and this time it blew as soon as the engine was started, with overdrive NOT on, nor in gear. 

Ever watch one of these blow? Scary...

 

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Colin I had an issue with my vitesse and blowing fuses / burning out flasher relays, initially it only happened when the car was loaded up, and I suspect it was a trapped or chaffed wire, never found the damage but I cured it be rewiring the near side indicator circuit.
 

Just watched the clip you posted - I’m glad I wired in a modern fuse box at the same time as that looks risky.

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3 hours ago, dougbgt6 said:

Is your OD a switch on the gear knob? This has a common fault, the wiring inside the gear stick frays, flaps about and shorts.

Doug

It's not, Doug - the gearbox was Dolomite 1850 with J-type and the wiring was extended - by me - to the column stalk. I've ripped the entire loom out, some of it is getting quite elderly but there was one small section of black - I'm assuming earth - cable that had chafed through:

D194D160-A8AB-4040-A7E5-FC4647876314_1_105_c.jpg.38b268543e7af4147d8721ef59c602dd.jpg

I don't know if that's the culprit but in any case it needs replaced. I'm debating making a loom myself based on the existing cables, but has anyone bought a pre-assembled version recently? None of the colours of the looms that are for sale match anything on my car, which are all black, green and red. The currently available looms are all various shades of yellow, so I'm not sure they're suitable.

EFBE02E1-7930-458E-9D02-E17B7603F4CC_1_105_c.jpg.60f1fed7adf979eeebd9086c02a38a0e.jpg j-type-gear-box-wiring-loom_1448463615_1920x.jpg.537239739dbdb2b8a8bdfa6cefe4217c.jpg

 

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That assumes it's actually an earth! If it came out from anywhere in the gearbox loom - and given that we're discussing a J-type wired from the reverse lights as per Spitfire 1500 - then there aren't any earth wires down there.

The yellow colours on the overdrive sub-harness were original factory colour for the column switch, which drove a relay on the D-type fitment. On my Spitfire I decided to retain a relay even though I have a J-type, which leaves me with a more Mk3 style loom, although it's a bit strange in the detail.

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1 hour ago, Pete Lewis said:

but an earth wont blow a fuse ?????

pete

Technically there is an earth but it's the really short wire from the solenoid to the bolt, only a few inches long. This chafed one, although also black, ran from the column stalk to the other terminal on the solenoid so while I guessed earth because of the colour it's the other way round and carries the power back from the switch, which gets to the switch along the red wire - the red wire, in turn, splices into the white/green circuit from the overdrive inhibitor switch.

68F3B0DA-E206-4D12-B515-4C54A8FDF2D3_1_105_c.jpg.6f241d99503968669de7b51c28d3aac5.jpg

I wonder if the system is live when the gearbox cam activates the inhibitor switch, and the column stalk simply completes the circuit? This makes me think that's what blew the fuses - getting to third gear so that the overdrive cam came into play; it's been intermittently chafing at speed and once in the proper gear for overdrive the current blew the fuse. But this begs the question of how it blew at idle in the garage, with the gears in neutral? In any case I'll rewire the entire circuit, rebush the gearlever mechanism and clean up a few other things which are now accessible. I'm also debating replacing the clutch, which has been there for twenty years now, but I'll need to identify the required plate.

Looks like I'm on my own for the loom, but I should have more than enough of the colours required - I might even have the right-angled male spade connectors. 8 amp cable ok? I have lots of it plus a lot more 17 amp but think that's too heavy.

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1 hour ago, Colin Lindsay said:

 But this begs the question of how it blew at idle in the garage, with the gears in neutral?

Easy. The inhibitor switch on the small gearbox (unlike the big saloon one) doesn't detect whether you're in gear, it only detects whether the lever is in the 3rd/4th plane. It can still be in neutral but leaning to the right and the switch closes.

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Worth replacing all the wiring around the gearbox area as they will be very brittle & easily broken and internal cable oxidized. I think I've replaced 90% of the cables in the engine bay near the engine for that reason, replacement colours well Yes, but I have marked the handbooks wiring diagram accordingly, also with any additional equipment wiring.

Non-members explanation re isolation switch interesting & worth investigating.

 

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Latest update: I've stripped down the entire reversing light and overdrive loom and replaced all of the wiring, bullet connectors and spade connectors, not necessarily in the correct colour but all new wiring plus I've removed a lot of the connections and just strung long cables to the column switch, thereby eliminating weak points at joints. Reconnected the battery and the fuse blew the minute it was put into third gear. The only parts I have not replaced are the inhibitor switch and the solenoid itself. The reversing lights work as usual, but it's only selecting third gear that blows the fuse.

DSCF9396.jpg.035156dce0c01f1fc8d5fbf533ad336f.jpg

So: two questions: I'm going to replace the switch in a minute or two but the switch that is fitted is the original kind, you can see it in the photo above - the replacement I bought some years ago at some show or other is a more modern version so has anyone tried these newer switches, are they likely to be better, or should I wait to go for the same as is currently fitted?

DSCF9400.jpg.8b14250b96886e4a41bc08437ba61d7c.jpg

Secondly: if the switch is not at fault what are the chances of it being the solenoid? I don't understand why it blows immediately no matter if the overdrive switch is on or off. Four hours I've just spent lying in the footwells only to find it made no difference at all, so need a bit of moral support and suggestions before going for the next four. Any ideas?

 

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Colin - Can't offer you any thoughts on this one other than a short to earth around the o/drive solenoid/cut out. However, for what its worth, you have all of us routing for you and hoping that you get it sorted... soon. Tea/coffee or some other beveridge from over the sea, may help lighten your darkness. Good luck

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22 minutes ago, Badwolf said:

Colin - Can't offer you any thoughts on this one other than a short to earth around the o/drive solenoid/cut out. However, for what its worth, you have all of us routing for you and hoping that you get it sorted... soon. Tea/coffee or some other beveridge from over the sea, may help lighten your darkness. Good luck

Thank you, that's very appreciated! If I didn't have grey hairs before, I'm getting them now.

I've replaced the O/D switch on the gearbox - it made no difference at all except it now blows even if you wiggle the gear lever in neutral, and don't even get as far as 3rd or 4th.

I also disconnected the solenoid - left the earth connection on but disconnected the power input - and it STILL blew. I would suspect the column wiring but I replaced the column stalk switch already. 

What sort of power does the solenoid draw?

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1 hour ago, trigolf said:

Colin,

IIRC the reversing light switch and the overdrive cut out switch use the same type of switch. Can you try swapping the switches and see if the reversing light fuse then blows instead ?

Gav

They do; I've just replaced them both to no avail. If I disconnect the entire circuit, reversing lights and all, the fuse won't blow; if I disconnect the overdrive from it by removing the bullet connectors at the switch, it doesn't blow either, but the reversing circuit works and I have reversing lights. I just cannot work out where the short or drain or surge is that blows the fuses - that's absolutely everything replaced bar the solenoid itself.

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Colin, can you trace and list all the wiring of the overdrive for us, please? Something like:

   - Green wire from fuse to inhibitor RH contact
   - Red wire from inhibitor LH contact to ...

and so on. If you want to draw it, that would work too 😁

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Colin when I installed my overdrive in the Vitesse I fitted the official column switch and shroud, I was very concerned when having to fit the extra wires into the column wire cover when tightening the column mounting to the dash the wires obviously were heavily compressed in my mind too much and I've always been concerned of insulation damage, but as it worked and the wiring is static I've not dare undo it all and check! Followed Dads rule if it ain't broke don't fix it!

Its worth checking the O/D column switch wiring to see if its not or is earthing, there should be some bullet connectors where the wires come out of the column under the dash. 

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10 hours ago, NonMember said:

Colin, can you trace and list all the wiring of the overdrive for us, please? Something like:

   - Green wire from fuse to inhibitor RH contact
   - Red wire from inhibitor LH contact to ...

and so on. If you want to draw it, that would work too 😁

I'll try, but I replaced the wiring in different colours from original but thankfully took lots of photos. 

Originally - before rewiring - there are two wires from the loom, green / red and green; the green goes to the overdrive inhibitor switch where it splits and the other length goes to the reversing light switch. From the O/D inhibitor there is a short length of white ' green which then joints to a red bullet connector, and a long red wire which doubles back to the switch (it would have been the gearlever but I extended it to the stalk). The other wire from the reversing light switch runs back to the loom connection on the bulkhead.

From the switch a long black cable runs to the solenoid, then from that to earth. It has run like this, untouched, for many years. For some reason the red cable was routed to the solenoid in a large loop but not connected to anything other than the switch, so I've shortened that.

I replaced the wiring like for like, with the old loom spread out and the new wires parallel until all of the connections were the same, and if anything has been changed round it's the connections at the reversing switch, but this won't matter - it's just an in / out from either direction and I replaced the switch after testing the replacement. I have replaced the short section of white / green with one long single red cable and the dark green circuit is now light green. The two black cables at the bellhousing are the loom / power in - insulated - and the switch connections with the plastic sheath over them.

Before and after shots below:

DSCF9375.jpg.e4f400570c568b02df8b54d8495afde1.jpg

DSCF9396.jpg.ba2d6228bc750f9b4591fa7ac728e269.jpg

This morning I'm going to try with a replacement switch - just any two connection switch from the spares pile - to bypass the column stalk. Everything works until O/D is activated, even the reversing lights, and if the O/D circuit is disconnected then nothing blows, but the slightest touch of the gearlever towards 3rd blows the fuse immediately.

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