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15A IG fuse blowing

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09 Nov 2022 13:15 - 09 Nov 2022 13:16 #245846 by Wailin
15A IG fuse blowing was created by Wailin
Hi there, I was driving on the motorway recently when the engine cut out and would not restart. After getting my 02 Jimny 1.3 towed to a mechanic they struggled to find the cause but eventually discovered the 15A ignition (IG) fuse was blown. After replacing it it started fine. But it has since blown again and if I replace the fuse and try start, it blows everytime on cranking. I put in a 30A fuse just to get it started, which it did, then I pulled the fuse with engine running and it cut out. Should this happen? I thought the IG fuse should not blow once the engine has been started, only during starting?
Last edit: 09 Nov 2022 13:16 by Wailin.

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09 Nov 2022 13:39 #245850 by DrRobin
Replied by DrRobin on topic 15A IG fuse blowing
Pulling the ignition fuse will kill the engine straight away, just like switching the ignition switch off.

I would be a bit careful using a 30A fuse, the wiring won't be rated. Probably better to find out what is drawing all of the current as there could be a few things hanging off the ignition circuit.

2011 Blue Jimny SZ3, Hankook Dynapro/Maxxis Worm-Drive, BB Classics BB6 Sports Seats, Radius Arm protectors, Android Head, DAB, 360 Camera, TPMS, K&N, Bosch Aerotwin, boot light, parcel shelf, boot box, dash box, Osram Nightbreakers, CTEK Battery Sense, Front & Rear Recovery points, CB Radio.

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09 Nov 2022 18:47 #245855 by fordem
Replied by fordem on topic 15A IG fuse blowing
Start by finding a competent mechanic - if they struggled to find a blown IG fuse, they're going to struggle even more to find out what is blowing the fuse.

The IG fuse powers the ignition system and numerous other circuits, among other things it is the power source for the check engine light, charge light, low oil pressure light and several others on the instrument cluster - if you turn the key and you don't see those lights come on, you know you have an electrical issue, and you start looking at the power source.

Finding the blown fuse should have been a five minute task, finding why the fuse blew or continues to blow is likely to take much, much longer - you need to get hold of the wiring diagram for the vehicle, find every circuit that is supplied from that fuse and eliminate them as a possible problem, one, by one.

I suggest a close look at the wiring to the ignition coils, make sure it is not chafed or nicked and possibly shorting to the cam cover, the same goes for the wiring to the alternator.

Intermittent problems are an absolute pain to track down, you never know if you've found the problem - good luck.

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12 Nov 2022 12:38 #245913 by Delux
Replied by Delux on topic 15A IG fuse blowing
DONT JUST REPLACE THE FUSE WITH A BIGGER ONE!
Unless you have a fetish for firemen! If a circuit is supposed to have a 15a fuse it means that that circuit cant handle/shouldnt be running at anything over 15a. By fitting a bigger fuse you remove the safety device and allow much more current into the faulty part. In effect the wiring becomes the fuse. When the fault happens the wiring will burn out instead of the fuse. In a way this would fix the problem but not in the way you want it to!

If you want to fix the problem yourself you need to understand the issue is the power the fusebox is sending to something is contacting the earth before it gets there.

Step 1 is understanding what you are looking at. Bit of googling and you can work out everything connected thru that fuse. If it helps, get a pic of the wiring diagram, find the fuse that keeps blowing then, with a highlighter colour that circuit in.

Next, carry out a visual inspection, you know what that circuit does so follow the wiring back. Is there any corrosion? Is the insulation damaged? Have you recently fitted anything electrical?

If you dont find anything, next step is disconnecting all the things that use that circuit. This could include relays, coil packs etc. as Fordem pointed out , as its the IGN fuse this could include loads of things. Thats another reason you need the wiring diagram! When you are done, turn the ignition on. If the fuse still blows you have a wire shorting out. If you have the wiring diagram that will help trace it back.
If the fuse doesnt blow you need to go back over and reconnect everything one by one. Obviously, whatever you connect that blows the fuse is the fault.

You can save some time/money/swearing by replacing the fuse with a bulb connected across the fuse terminals, or a test light. If the light is on the fault is there, when its off the fault isnt.

If you dont feel confident enough, when to take it to a garage/auto electrician be sure to give them as much info as possible. The easier you make their job the faster they will find it! Does it blow as soon as you put the ignition on? Do you have to start it to get it to blow? Does it only blow when its raining/turning left/when you put the radio on on etc.

But please, never replace a blown fuse with a bigger one!

Busiest Mechanic in Scotland!
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12 Nov 2022 21:03 #245918 by fordem
Replied by fordem on topic 15A IG fuse blowing

If you want to fix the problem yourself you need to understand the issue is the power the fusebox is sending to something is contacting the earth before it gets there.
 



Not necessarily - a fuse can blow for other reasons than a short circuit to ground, and even if it's a short circuit to ground, that short is not always "before it gets there", it can be internal to whatever is powered by the fuse.

Example #1 - a vehicle might be fitted with two reverse lamps, that's 2x21w bulbs for 42W, some bright spark decides he needs more reverse light off road, and hangs two 55W fog lamps off the back bumper and wires them to the reverse lights, you now have 152W of lights, powered from a fuse meant to handle 1/3 of that, it's going to blow, and with nothing "contacting the ground"

Example #2 - the bright spark in example #1 learns his lesson and swaps out the 55W fog lights for a pair of 18W LED bars (the little ones with 6 LEDs each) and removes the bulbs from the factory reverse lights - he's now feeding a 36W load rather than a 42W load.  Now I don't know if you've ever opened up one of those LED bars, but if you haven't, there's a fair bit of electronics inside, essentially a PWM (pulse width modulated) current limiter.  Now, it's not uncommon for the *ahem* less expensive, asian manufactured LED bars to to not live up to their advertised IP (intrusion protection) ratings, and when this happens, the current limiter can fail - it's not a short to ground, but it does allow excessive current flow through the LED string, and it's a toss up between which fails first, the fuse or the LED string - point is, the fault has occurred internal to the device, and not before it got there.

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13 Nov 2022 00:45 #245919 by Delux
Replied by Delux on topic 15A IG fuse blowing
Hi Fordem,
Of course, there are exceptions and examples where a fuse can blow without shorting out but my reply was focussed more on helping the OP.

I was trying to highlight what would be involved in safely and practically diagnosing the fault.

There is a time and place for discussing Ohms law, electron theory and the quality of Chinese LEDs but when someone has a Jimny that doesnt work and has tried something dangerous to try and 'fix' it I thought that should be the focus of my response.

Wailin, let us know how you get on!

Busiest Mechanic in Scotland!
My pics (inc. pics of my latest Jimny project)...
www.flickr.com/photos/159422664@N08/albums
My ebay stuff for sale...
www.ebay.co.uk/usr/deeelux

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