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Count Draguar Benge Collins
County Durham, Barnard Castle, UK   GBR
A 35-amp fuse keeps going in my TR7 drophead. It's appears to be used for the cigar lighter, horn, interior lights and clock as all of these stop working when the fuse blows. I am unsure what else may be fed by this fuse. The interior lights are in the door panels in the drophead and I thought I had solved the problem when I fixed one of those lights because the wires had been left training in the door, however the fuse has blown again.

Is this a common issue with a likely quick fix or do I need to study the wiring diagram and trace all of the connections.

Cheers

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kansanbrit Avatar
kansanbrit Phil H
Bonner Springs, KS, USA   USA
1952 MG D-Type Midget
1964 Morris Mini-Minor
1980 Triumph TR8
1980 Triumph TR8 "TR8 Holley"    & more
I would try disconnecting the cigar lighter first and see if that cures it. Does it blow immediately every time?

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Darth V8R Avatar
Darth V8R Vance Navarrette
Beaverton, OR, USA   USA
1980 Triumph TR7 Drophead "The Great Pumpkin"
1980 Triumph TR8 "Wedgie"
In reply to # 1601543 by Count Draguar A 35-amp fuse keeps going in my TR7 drophead. It's appears to be used for the cigar lighter, horn, interior lights and clock as all of these stop working when the fuse blows. I am unsure what else may be fed by this fuse. The interior lights are in the door panels in the drophead and I thought I had solved the problem when I fixed one of those lights because the wires had been left training in the door, however the fuse has blown again.

Is this a common issue with a likely quick fix or do I need to study the wiring diagram and trace all of the connections.

Cheers

When you have an intermittent problem like this, it becomes very difficult to troubleshoot.

You didn't mention the model year or whether it had A/C, as all of these affect the wiring diagram.

I would start by pulling the fuse, and getting an ammeter across the fuse clips, and measure the current draw as you toggle various things off and on, open and close the doors, activate the cigarette lighter, etc. To find a meter capable of handling up to thirty five amps is difficult, but they are out there. An alternative is to insert a 10 watt low value resistor in place of the fuse, say 0.2 ohms, and place a voltmeter across the resistor. If you find an accessory that causes the voltage across the resistor to spike up near one volt, you have found the culprit. I would expect a 0.2 ohm resistor to get quite hot as it could be tasked with dissipating up to 17 watts when a short occurs.

In addition to the items you mention, the seat belt warning buzzer also draws power on this circuit. It is an always hot lead from the ignition switch that supplies the circuit, so even the ignition switch itself is suspect. Make sure the switch is secure in its mounting where it attaches to the lock barrel in the steering column (Is that where the switch is in the UK? In the north American market the key also locks the steering column - dunno how it was done across the pond).

Like I said, you will need to be systematic and determine which leg of the circuit is the offending one. I cannot think of another way to do it, but perhaps someone else can.

Vance



1980 Platinum Metallic TR8, frame off restoration, complete.
1980 Vermilion TR7 Sprint replica, in progress.

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Count Draguar Benge Collins
County Durham, Barnard Castle, UK   GBR
In reply to # 1601588 by kansanbrit I would try disconnecting the cigar lighter first and see if that cures it. Does it blow immediately every time?

I removed cleaned and replaced the cigar lighter and it still blows, but I'll try disconnecting it just in case. It doesn't blow immediately and everything works when it isn't blown. Unfortunately I can't see anything in common with the times it has gone.

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Count Draguar Benge Collins
County Durham, Barnard Castle, UK   GBR
In reply to # 1601617 by Darth V8R
In reply to # 1601543 by Count Draguar A 35-amp fuse keeps going in my TR7 drophead. It's appears to be used for the cigar lighter, horn, interior lights and clock as all of these stop working when the fuse blows. I am unsure what else may be fed by this fuse. The interior lights are in the door panels in the drophead and I thought I had solved the problem when I fixed one of those lights because the wires had been left training in the door, however the fuse has blown again.

Is this a common issue with a likely quick fix or do I need to study the wiring diagram and trace all of the connections.

Cheers

When you have an intermittent problem like this, it becomes very difficult to troubleshoot.

You didn't mention the model year or whether it had A/C, as all of these affect the wiring diagram.

I would start by pulling the fuse, and getting an ammeter across the fuse clips, and measure the current draw as you toggle various things off and on, open and close the doors, activate the cigarette lighter, etc. To find a meter capable of handling up to thirty five amps is difficult, but they are out there. An alternative is to insert a 10 watt low value resistor in place of the fuse, say 0.2 ohms, and place a voltmeter across the resistor. If you find an accessory that causes the voltage across the resistor to spike up near one volt, you have found the culprit. I would expect a 0.2 ohm resistor to get quite hot as it could be tasked with dissipating up to 17 watts when a short occurs.

In addition to the items you mention, the seat belt warning buzzer also draws power on this circuit. It is an always hot lead from the ignition switch that supplies the circuit, so even the ignition switch itself is suspect. Make sure the switch is secure in its mounting where it attaches to the lock barrel in the steering column (Is that where the switch is in the UK? In the north American market the key also locks the steering column - dunno how it was done across the pond).

Like I said, you will need to be systematic and determine which leg of the circuit is the offending one. I cannot think of another way to do it, but perhaps someone else can.

Vance

The car is a 1980 5-speed drophead with the original 2 litre engine, no Air Con.

I don't have an ammeter that will handle more than 20A and it has managed to blow a 50A fuse before. The battery does not seem to discharge while the fuse is in (it has worked for a few days before), so I don't think the draw can be much but I'll test this. If I'm careful I should be able to test the current draw of each individual component. I wasn't aware there was a buzzer for the seatbelt as there is a light on the dash but sound. Perhaps this is at fault, where is it located? The ignition switch is very secure, but will check that after.

Thanks your knowledge is just what I was looking for.

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tapkaJohnD Avatar
tapkaJohnD John Davies
Lancaster, Lancashire, UK   GBR
If it blows a 35A fuse then it's not some trickle short, like a corroded earth, but a dead short, full contact of the 12V positive with an Earth.
There must either be some loose terminal that touches the bodywork, or damaged insulation, ditto.

As this will happen as the car moves, it's impossible to trace with a multimeter. It's no good trying to fix one then the other at random. You need a plan.
Suggest you disconnect all the likely suspects (as above), drive the car to confirm that the fuse stays intact.
(I'm surprised that those items need a 35A fuse. A cigarette lighter normally draws about 10A tops. There may be others on that fuse)

Reconnect the items, one at a time, and test drive with each.
When the fuse blows again, at least you know which wire to follow, to find where the short is occuring. TRace it's full length, but concentrate on where it goes through bulk heads.

Good luck!
John

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tapkaJohnD Avatar
tapkaJohnD John Davies
Lancaster, Lancashire, UK   GBR
AND,
I find this thread in this forum: https://www.triumphexp.com/forum/tr7-and-tr8-forum.3/early-tr7-fuse-box.1597458/

You'll note the fuse list included:

1) 15A red green - feed to aux lights

2) 2 x 35A white - vent fans, wipers etc

3) 3 x 50A brown - air conditioning / cigar lighter / heated rear window.


So should the cigar lighter have a 50A fuse - you need to do your own research!

JOhn

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Count Draguar Benge Collins
County Durham, Barnard Castle, UK   GBR
In reply to # 1601809 by tapkaJohnD AND,
I find this thread in this forum: https://www.triumphexp.com/forum/tr7-and-tr8-forum.3/early-tr7-fuse-box.1597458/

You'll note the fuse list included:

1) 15A red green - feed to aux lights

2) 2 x 35A white - vent fans, wipers etc

3) 3 x 50A brown - air conditioning / cigar lighter / heated rear window.


So should the cigar lighter have a 50A fuse - you need to do your own research!

JOhn

Hi JOhn,

It seems the early ones did indeed use 50A but by the time mine was made that circuit was 35A, not sure why but the fuse board cover confirms it is supposed to be 35A. I did try a 50A fuse because I didn't have a 35A but after a few days it blew too. I found a circuit diagram which I have been looking over which seems about right.

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Count Draguar Benge Collins
County Durham, Barnard Castle, UK   GBR
In reply to # 1601781 by tapkaJohnD If it blows a 35A fuse then it's not some trickle short, like a corroded earth, but a dead short, full contact of the 12V positive with an Earth.
There must either be some loose terminal that touches the bodywork, or damaged insulation, ditto.

As this will happen as the car moves, it's impossible to trace with a multimeter. It's no good trying to fix one then the other at random. You need a plan.
Suggest you disconnect all the likely suspects (as above), drive the car to confirm that the fuse stays intact.
(I'm surprised that those items need a 35A fuse. A cigarette lighter normally draws about 10A tops. There may be others on that fuse)

Reconnect the items, one at a time, and test drive with each.
When the fuse blows again, at least you know which wire to follow, to find where the short is occuring. TRace it's full length, but concentrate on where it goes through bulk heads.

Good luck!
John

I will do exactly this, nothing is essential that's fed by it and most are easy to get to. I assume if the fuse still blows with everything disconnected then I am going to have to trace the cabling around the car which sounds like fun. How easy is it to remove the dash in it's entirety in this car?

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kansanbrit Avatar
kansanbrit Phil H
Bonner Springs, KS, USA   USA
1952 MG D-Type Midget
1964 Morris Mini-Minor
1980 Triumph TR8
1980 Triumph TR8 "TR8 Holley"    & more
In reply to # 1601588 by kansanbrit I would try disconnecting the cigar lighter first and see if that cures it.

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Darth V8R Avatar
Darth V8R Vance Navarrette
Beaverton, OR, USA   USA
1980 Triumph TR7 Drophead "The Great Pumpkin"
1980 Triumph TR8 "Wedgie"
In reply to # 1601781 by tapkaJohnD If it blows a 35A fuse then it's not some trickle short, like a corroded earth, but a dead short, full contact of the 12V positive with an Earth.
There must either be some loose terminal that touches the bodywork, or damaged insulation, ditto.

As this will happen as the car moves, it's impossible to trace with a multimeter. It's no good trying to fix one then the other at random. You need a plan.
Suggest you disconnect all the likely suspects (as above), drive the car to confirm that the fuse stays intact.
(I'm surprised that those items need a 35A fuse. A cigarette lighter normally draws about 10A tops. There may be others on that fuse)

Reconnect the items, one at a time, and test drive with each.
When the fuse blows again, at least you know which wire to follow, to find where the short is occuring. TRace it's full length, but concentrate on where it goes through bulk heads.

Good luck!
John

John:

Don't forget that the 35A fuse rating is a bit <Ahem> different than you might expect as it is British. Forgive me if I am stating the obvious. Don't mean to be snooty, just want to cover this in case you were unaware.

US fuses are rated by Underwriters Laboratories out of California. A 35 amp fuse is rated to carry 35 amps without ever blowing and is physically sized in imperial units (0.25 x 1.25 inches).
English fuses are rated using DIN (German Industrial Standard) so a 35 amp fuse is guaranteed to blow at 35 amps within 1 second and is 6 x 30mm.

To translate the English/DIN fuse rating of 35 amps to a US fuse rating, multiply by 0.5. So that 35 amp English fuse will only carry 18 amperes indefinitely, which makes sense if the lighter draws 10 amperes.

Inserting a British fuse in place of a US fuse is benign, as the fuse will open at a much lower current than the identically rated US fuse. What us yanks do, however, is insert a US fuse of 35 amperes in place of a British 35 ampere fuse, and then wonder why the wiring goes up in smoke. confused smiley This assumes of course that the US fuse actually fits, as it is a wee bit longer than the British fuse, and may require some persuasion to fit. In my TR8, US fuses physically will not fit unless you use a hammer. I know because I tried. I gave up and ordered the proper English fuses. But I am sure someone made a US fuse fit, and then cursed the Triumph and Lucas engineers when all hell broke loose. In my TR6 however, a US fuse fits just fine, as long as you derate it accordingly to avoid offending the prince of darkness. Hah!

To prevent us hapless yanks from setting ourselves on fire, I noticed that UK fuses often have 35 amps stamped on them, but there is also a piece of paper inside the fuse that gives the continuous rating a'la underwriters labs. Of course that assumes the paper label actually gets read, and we know what happens when that word "assume" gets used. <poof!>

Vance



1980 Platinum Metallic TR8, frame off restoration, complete.
1980 Vermilion TR7 Sprint replica, in progress.

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Count Draguar Benge Collins
County Durham, Barnard Castle, UK   GBR
In reply to # 1601876 by Darth V8R

John:

Don't forget that the 35A fuse rating is a bit <Ahem> different than you might expect as it is British. Forgive me if I am stating the obvious. Don't mean to be snooty, just want to cover this in case you were unaware.

US fuses are rated by Underwriters Laboratories out of California. A 35 amp fuse is rated to carry 35 amps without ever blowing and is physically sized in imperial units (0.25 x 1.25 inches).
English fuses are rated using DIN (German Industrial Standard) so a 35 amp fuse is guaranteed to blow at 35 amps within 1 second and is 6 x 30mm.

To translate the English/DIN fuse rating of 35 amps to a US fuse rating, multiply by 0.5. So that 35 amp English fuse will only carry 18 amperes indefinitely, which makes sense if the lighter draws 10 amperes.

Inserting a British fuse in place of a US fuse is benign, as the fuse will open at a much lower current than the identically rated US fuse. What us yanks do, however, is insert a US fuse of 35 amperes in place of a British 35 ampere fuse, and then wonder why the wiring goes up in smoke. confused smiley This assumes of course that the US fuse actually fits, as it is a wee bit longer than the British fuse, and may require some persuasion to fit. In my TR8, US fuses physically will not fit unless you use a hammer. I know because I tried. I gave up and ordered the proper English fuses. But I am sure someone made a US fuse fit, and then cursed the Triumph and Lucas engineers when all hell broke loose. In my TR6 however, a US fuse fits just fine, as long as you derate it accordingly to avoid offending the prince of darkness. Hah!

To prevent us hapless yanks from setting ourselves on fire, I noticed that UK fuses often have 35 amps stamped on them, but there is also a piece of paper inside the fuse that gives the continuous rating a'la underwriters labs. Of course that assumes the paper label actually gets read, and we know what happens when that word "assume" gets used. <poof!>

Vance

That's useful advice and explains why one of the 35A fuses I found to fit was very slightly too big. I managed to get it in the slot with a lot of persuasion but I ordered some british (German?) fuses to replace it with because I didn't want to permanently deform the slot. By the time the new fuses arrived it had already blown though, so it seems it's getting over 35A.

This reminds me that the fuse box wasn't held very well behind the glove box. I think that the back of the glove box is supposed to be screwed onto the bulk head through the fuse box to secure everything in place? On mine the holes don't line up so the back of the glove box is loose and the fuse box is held onto the bulkhead loosely with one screw. Is this common due to bad workmanship and if so is it easy to fix? I suppose this could be what causes the fuse to blow too, but I'd imagine othe rfuses would blow too if there was something metallic behind that it was coming into contact with.

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HowardB Avatar
HowardB Howard Brissenden
Potton, Bedfordshire, UK   GBR
On the UK 81 DHC the only 35A fuse is the feed to the heater fan. There is a 50A to the following circuits:-

Boot ltg
Headlight dip/flash
Clock
HORN

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HowardB Avatar
HowardB Howard Brissenden
Potton, Bedfordshire, UK   GBR
IGNORE FIRST MESSAGE

On the UK 81 DHC the only 35A fuse is the feed to the heater fan. There is a 50A fuse with PURPLE wires to the following circuits:-

Boot ltg
Headlight dip/flash
Clock
Horn
Fog lightd

Cigarette lighter
Door lights
Hazard lights

Make sure you are using a correct lucas fuse as a 35A lucas fuse is equivalent to a standard 17A Fuse. Lucas quote the current to blow it instead of the continuous current which is the normal way of rating fuses!

If you PM me I will email the latest version of my simplified drawings which should assist your fault finding

Cheers

Howard

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Count Draguar Benge Collins
County Durham, Barnard Castle, UK   GBR
In reply to # 1602092 by HowardB IGNORE FIRST MESSAGE

On the UK 81 DHC the only 35A fuse is the feed to the heater fan. There is a 50A fuse with PURPLE wires to the following circuits:-

Boot ltg
Headlight dip/flash
Clock
Horn
Fog lightd

Cigarette lighter
Door lights
Hazard lights

Make sure you are using a correct lucas fuse as a 35A lucas fuse is equivalent to a standard 17A Fuse. Lucas quote the current to blow it instead of the continuous current which is the normal way of rating fuses!

If you PM me I will email the latest version of my simplified drawings which should assist your fault finding

Cheers

Howard

The TR7 is getting the starter motor reconditioned at the moment so can't try anything until next week, however the back of the fuse box cover definitely states 35A for this circuit which includes the Cigar Lighter, Horn, Interior lights, Seat Belt Alarm buzzer and clock. I have attached a wiring diagram I found which I believe is right. My car is 1980. I wasn't aware that Lucas fuses were different, I just measured the size of the fuse and ordered ones the right size. They match exactly however the fuses which are already in the car though it is possible someone replaced all the fuses with standard fuses at one point. The fuse does seem to blow whilst none of the components on the circuit are being used so there is some sort of short I need to find so will start with the cigarette lighter and assuming that doesn't help I'll unplug everything on the circuit.


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