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MG Midget and Sprite Technical - How can I trace the short?

I know it should be easy, but I am failing to find the short on my 1275 Healey Frogeye.
There are two fuses in the standard midget-type box, and the lower one blows even when the ignition is off. The components on this side, as far as I can tell, are the Kenlowe fan, the horn and the cigarette lighter. With the fuse blown, obviously none of these work. Everything on the ignition fuse side (top fuse) is fine and the car is running perfectly all the time I don't need fan or horn.
Right. I disconnected the fan connectors, put a new fuse in and it didn't blow. The cigarette lighter now works. The horn doesn't. Thinking it might be a failed fan, I connected it direct to battery and it whirred into life.
The car is fitted with air horns so I have disconnected the terminals (one single mauve and black and a double mauve wire). I don't fully understand multimeters but I set mine on circuit test and common probe, tried it on the horn terminals and it swung across in the same way as when the probes are touched. With the ignition off, I then tried the probes on the wires themselves - and nothing. Right, I thought, the horn circuit must be the problem. As I knew the fan itself was working I left the horn disconnected, reconnected the fan and flicked the interior Kenlowe switch. And the fuse blew.
Suggestions as to what to do next, or obvious things I have missed, will be appreciated.
C Whiting

The reason it blows when the ignition is off, is because it has a permanent feed, not reliant on the ignition.

When did it first start blowing fuses? Had you done anything just prior? If all the devices, horn, fan, lighter etc, work independently, then look for a trapped live feed wire shorted to earth. Just keep on as you are, eliminating each circuit and wire as you go and you'll find it.

If you aren't far away, drive over to me. I'll find the fault for you.
Lawrence Slater

If the fuse blows when you flick the fan switch it suggests the fuse is not rated high enough to cope with the fan's startup current surge.

Have you tried a higher value fuse? Sorry for stating the obvious but that is the first thing to try.
JB Anderson

Thanks for both suggestions - Lawrence, I'm near Faversham - where are you? JBA - stating the obvious might be well worthwhile here. I used one of the spare fuses that was in the Lu8cas box without thinking it could be under-rated. I think it was a 10 so quite possibly too light. Tried to buy a selection of glass fuses this morning without success, but will try in Canterbury.
C Whiting

It might be a bit risky (I Used To Do It)but if you used a bulb in place of the fuse, then disconnected the different circuit sections one at a time you would know where to begin isolating the probable short when the lamp went off.

The ease "NOT" of locating glass fuses these days was one of the reasons I fitted two separate fuse boxes to my car

One "Ignition Live" fed for always live parts. The other "Permanantly Live", then each piece of the circuits covered got its own individual blade type fuse

Seen here being connected


Bill1

The electric fan and air horns are both post-factory fits. What are you basing the fuse rating on because if there was one there originally for other components, it's bound to need uprating. Just need to watch the rating of the live feed wire to the fuse.
Can you see a rating plate on the fan and horns? If you take the wattage figure (if you can see one) and divide by 12, that will give the current consumption but it will take a little more on start up. Ideaaly you need to add the current for all three together to get the fuse rating, but that assumes you use all three together! It might also be a pretty high number!
Graeme Williams

"If the fuse blows when you flick the fan switch it suggests the fuse is not rated high enough to cope with the fan's startup current surge."

That could be the case, or it could be that a short has occured in the wiring from the switch to the fan. Putting a large fuse in as a test is not a sensible way to find a fault. If a short exists, you draw more current than the wiring was intended to take. So before upping the fuse value, do as graeme suggested and calculate the size you need.

I'm in T-Wells btw.
Lawrence Slater

Ahhh... im going with its a 10amp fuse your using to power 3 hungry circuits

Id try a 20 amp and see what that does

Prop

Prop and the Blackhole Midget

Crispin: what size is the fuse at the moment?
Graeme Williams

Unfortunately I can't find the fuse that blew (but that had been in the car for at least three years without popping). The two spares from the Lucas box aren't marked but the wire is thinner than the original, which was more like a strip than a bit of wire. One has popped and one is now in the system unpopped but with the fan disconnected. I have ordered some 17-35s which seem to be the usual rating and can buy 20s locally today, I hope. Still have to find where the short is though...
C Whiting

I still dont think its a short...the fuse is to light

You said it was 10 amp

If the ciruits on the fuse work individually but blow as a group... the fuse is to light

Prop
Prop and the Blackhole Midget

Doesn't somebody on here have the manual with original fuse spec for a frog?
Lawrence Slater

My BMC AN5 Shop Manual, in the specifications pages, lists both fuses as 35 amp. In the Electrical section, the fuse box illustration shows the switched fuse as 35 amp, but the unswitched fuse at 50 amp.

One of my later BMC manuals, which covers the early 1275s, shows both fuses as 35 amps.

No mention is made as to 'continuous' or 'intermittent' ratings. As I recall from the early '60s, the commonly available fuses were 50 amp and were used in both positions. Later, fuses were available in other amperages and were labeled with 'continuous' and 'intermittent' ratings - e.g. "35/50".

Bill Gavin

the car a 1275 Healey Frogeye

Crispin,
I'm surprised it's only got the two fuses but if that's how it is then that's how it is

the original glasses fuses were thin wire with paper backing one side, 17 amp continuous 35 amp blow, the modern glass fuses are a slightly different size (length and width?) and are a metal strip across rather than wire

I wonder if fan at least was added after manufacture, having those three items on one fuse seems unusual to me but I could be wrong

I don't know how much electric 'power' the air horns draw but the fan motor would draw a bit and so would the cigarette lighter so I'd have though the fan would have separate supply with an in-line fuse of it's own
Nigel Atkins

The fan is aftermarket add on, he said its a kenlow...a very popular aftermarket fan

So I think nigal is correct, a differant circuit on its own in line fuse would be the starting point after a bigger amp fuse is tried

Prop
Prop and the Blackhole Midget

Thanks to Lawrence, the problem is now sorted. He's less than an hour away so I drove over this morning and he had a good look. As mentioned before, I didn't take a note of the rating of the fuse which had been in for years but blew last week (one of the two spares wasn't rated and the other was a 10), but I drove over with a 17/35 fitted, which meant I could run the fan but not the air horn, which was still not functioning.
To cut a very long story short, he reckons it was probably a combination of factors. There was a distinct possibility that the wires to the control box of the Kenlowe fan (which were fitted standard to Healey Frogeyes, or Isle of Wights as they are also known)might have shorted against the copper temperature line which runs to the head of the radiator. One of the spade terminals to the air horn compressor was a bit loose too. As for the fuse, he calculated that the fan and the horn and the lighter socket would be ok with a 10, although that got a bit hot, so will be using a 15 or 20.
He might wish to explain further as his understanding of the multimeter readings is light years ahead of mine, but whatever, thanks very much Lawrence - much appreciated. And thanks to everyone who made suggestions. I'm off to email Meadcam Spridgets the official IoW dealer importer to see if they hold IOW wiring diagrams.
C Whiting

just a side note the fuse is there to protect the wiring so make sure the cables can take the current of the fuse fitted
mark 1500 nearly on the road

Fortunately (for me) Lawrence is fully aware of all the loadings, amps, volts and wiring thickness and worked it all out with a multimeter and calculator - if he says 15 or 20 is fine for the car's wiring, I believe him! I have a 15 in at the moment and everything is working fine.
No further use for the heavy-duty jobs I managed to track down yesterday, but £2.25 is a small price to pay for sorted Spridget.
C Whiting

Yup a combination of two seperate, but related problems.

The fused circuit in question is permanently live. It feeds the horn, fan, and ciggy lighter. Most probably, the fuse which could have been the original, gave up the ghost. But as Crispin said, there was a live wire feeding the horn, that was trapped tightly between the copper line from the thermostat control for the kenlow, and the sharp edge of a jubilee clip. If it hadn't actually shorted, it would soon. There may still be an intermittent short elsewhere. If there is, it'll show up by blowing another fuse.

Anyway. The air horn couldn't work, 1), because the fuse had blown, 2), because the +ve connection on the compressor was loose, and 3), the horn push wasn't making an effective earth to feed to the other side of the compressor.

The Kenlow is plate rated at 150W/12.5A. But actually it only pulls just over 10A on start, and runs at 7.5A. -- according to my multi-meter. A 10A fuse in place doesn't blow with the sat-nav on, horn blasting, and kenlow running. But it does get hot. So I reckon a 20A maximum size fuse should be used, but see if a 15A will do it, for the very reason I gave earlier, and Mark 1500 just confirmed.

Very nice frog that Crispin. :).
Lawrence Slater

Glad the suggestion was of use.
JB Anderson

It wasn't. Bunging in a larger fuse as a fault finding method, when there's the possibly of an intermittent short, isn't the most sensible approach , even if some people think that an obvious thing to try. In fact, I'd describe it as not useful. :).
Lawrence Slater

Crispin,
I'd ask the dealer if they also have an Owners Manual for the car

I did wonder if the Healey Frogs had an electric cooling fan but couldn't remember hearing one on the only IoW Frog I know of and IIRC the cars did vary slightly

as already said putting a larger fuse in when there are problems could (almost?) literally be adding fuel to the potential fire

I'm still surprised the car came with a two instead of four fuse box

a post about the original fuses to follow
Nigel Atkins

Larger fuse was a part of the solution. But could only be a safe solution by calculating the total loads. Especially in the circumstances where there was both a doubt about the capacity of the original fuse and a lack of manufacturer's specification. Measurement and calculation would be the only safe way of working it out. And not that hard to do anyway!

But it was a good guess! ;-)
Guy W

I'm sure if there was an owners manual it would have come with the car - there is a large folder of everything including the letters placing the order and agreeing spec - and the final bill for £14,200. There is small green IOW owners book, but this only has details like factory commission number, engine number paint code and suchlike. I have emailed Paul Mead about a wiring diagram. I suspect that as you say, there was considerable variation in cars coming out of the factory and that there might not be a handbook - unless anyone knows differently, of course.
I'll go off and look through my collection of period magazines covering the cars and see if there is any mention (or picture) of fuse boxes, just out of interest. The two fuse system might have been an attempt to keep it more MK 1 sprite-like, although that doesn't make huge sense as so many other things were modernised and improved, which was the main purpose of the project.
C Whiting

I can't remember the details of IoW cars and if/what parts were from donor(s) but that would lead to considerable variations so probably no Handbooks then

bit of info on glass fuses

photo below -
(sorry about the not so good photo and my lack of measuring callipers)

the top and bottom fuses are the modern equivalent 35 amp – they are 30mm x 6mm (approx.)

the middle fuses are two versions of the original type of 17 amp continuous/35 amp blow – these are 28.5mm x 6mm (approx.)


Nigel Atkins

You mean theres math to figure stuff like that out

In the words of presidential canadate Mr. Sweater vest..."how elitists" ....whats next, dinosaurs didnt roam the earth 5000 years ago, and charles darwin knew what he was talking about.

Science and math are straight from the pit of hell....the bible says so, (somewhere ive heard)

Go tea party

Prop
Prop and the Blackhole Midget

This thread was discussed between 04/10/2013 and 07/10/2013

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