Midget fuse blowing at speed
Discussion
Hi, hoping for so ideas on this one.
The 35a glass fuse from which the rev counter etc. runs on my MG Midget 1500 keeps blowing when the speed approaches 70mph.
Seem ok at lower speeds - so assuming that for some reason the alternator is generating too much output as the speed increases, or something like that.
Car has an electronic ignition conversion and coil and cables etc. all look pretty new.
Any ideas????
Thanks.
The 35a glass fuse from which the rev counter etc. runs on my MG Midget 1500 keeps blowing when the speed approaches 70mph.
Seem ok at lower speeds - so assuming that for some reason the alternator is generating too much output as the speed increases, or something like that.
Car has an electronic ignition conversion and coil and cables etc. all look pretty new.
Any ideas????
Thanks.
If it is the alternator (regulator) generating to much voltage it'll also cook you're battery.
Quite easy to check, voltmeter on the battery terminals at no engine should be around 12.8v, start engine and you should see around 14v rev the engine. If it goes higher than 14.8v, time for a new alternator or regulator.
Quite easy to check, voltmeter on the battery terminals at no engine should be around 12.8v, start engine and you should see around 14v rev the engine. If it goes higher than 14.8v, time for a new alternator or regulator.
try asking on the Midget and Sprite Technical forum of the MG BBS as there are a couple of electrical experts that post there
also have a look in your Driver's Handbook to see what comes off each fuse but do also allow for latter wiring or additions of electrical items by previous owners as some people can do the strangest of things with electrics
also have a look in your Driver's Handbook to see what comes off each fuse but do also allow for latter wiring or additions of electrical items by previous owners as some people can do the strangest of things with electrics
Somethings shorting, maybe the ignition switch. I've not come across a coil shorting like this but it's a cheap enough thing to replace.
If the fault occurred after fitting the electronic ignition, I would be checking the wiring carefully.
The coil maybe a red herring plenty of other equipment supplied by that fuse.
Nigel's advice above is sound.
If the fault occurred after fitting the electronic ignition, I would be checking the wiring carefully.
The coil maybe a red herring plenty of other equipment supplied by that fuse.
Nigel's advice above is sound.
Edited by PositronicRay on Sunday 22 February 15:26
I've jumped over from the MGBBS to add my bit 
Clarification question(s)?
1, Is the fuse that blows the one feeding the "green" circuits i.e the one that has power only when the ignition is on?
2, Are you saying the car stops when the fuse blows and won't start again until the fuse is replaced?
3, Which electronic ignition unit is fitted?
Before I get the answers I'll give you my first guess just for fun
The electronic ignition has been wired in to the wrong side of the fused ignition on circuit and is drawing too much current at higher speeds (if it's getting worse it could be failing slowly or you are using more of the other items on the circuit at this time of year)
Clarification question(s)?
1, Is the fuse that blows the one feeding the "green" circuits i.e the one that has power only when the ignition is on?
2, Are you saying the car stops when the fuse blows and won't start again until the fuse is replaced?
3, Which electronic ignition unit is fitted?
Before I get the answers I'll give you my first guess just for fun
The electronic ignition has been wired in to the wrong side of the fused ignition on circuit and is drawing too much current at higher speeds (if it's getting worse it could be failing slowly or you are using more of the other items on the circuit at this time of year)
Edited by Geordie MGmike on Sunday 22 February 20:59
Edited by Geordie MGmike on Sunday 22 February 21:00
PositronicRay said:
The coil maybe a red herring plenty of other equipment supplied by that fuse.
It is. The coil isn't fed by the fuse it's actually fed by an unfused 12v from the ignition switch (via a ballast wire on the 1500) which also supplies the live side of the fuse for the "green" circuit. Edited by PositronicRay on Sunday 22 February 15:26
Geordie MGmike said:
I've jumped over from the MGBBS to add my bit
Clarification question(s)?
1, Is the fuse that blows the one feeding the "green" circuits i.e the one that has power only when the ignition is on?
2, Are you saying the car stops when the fuse blows and won't start again until the fuse is replaced?
3, Which electronic ignition unit is fitted?
Before I get the answers I'll give you my first guess just for fun
The electronic ignition has been wired in to the wrong side of the fused ignition on circuit and is drawing too much current at higher speeds (if it's getting worse it could be failing slowly or you are using more of the other items on the circuit at this time of year)
Firstly, thanks for jumping on the thread - much appreciated. Clarification question(s)?
1, Is the fuse that blows the one feeding the "green" circuits i.e the one that has power only when the ignition is on?
2, Are you saying the car stops when the fuse blows and won't start again until the fuse is replaced?
3, Which electronic ignition unit is fitted?
Before I get the answers I'll give you my first guess just for fun
The electronic ignition has been wired in to the wrong side of the fused ignition on circuit and is drawing too much current at higher speeds (if it's getting worse it could be failing slowly or you are using more of the other items on the circuit at this time of year)
I will have another look and see what wires are connected to the fuse. To try and be as defined as possible, the car drives along perfectly well, then as the speed builds - like I say, it used to be 70 but last time was 50 (I might have had the wipers on at the time, not 100% sure) the fuse will suddenly blow, the rev counter drops to zero instantly, but the ignition lights remain on, the car dies and coasts to a stop. Then the fuse needs replacing before it will start again - starter motor turns, ignition lights on but no firing and no rev counter movement until the fuse is replaced.
I bought the car in its current 'state' so I am unsure as to which ignition is fitted. The car and wiring, other than the electronic ignition, look very 'original' and everything looks very new under the bonnet. I will have a nose around and see if I can work out the brand of the ignition system (or check paperwork to see if it is listed there).
MrTickle said:
Firstly, thanks for jumping on the thread - much appreciated.
No prob's, I'm usually lingering in here MrTickle said:
I will have another look and see what wires are connected to the fuse.
Probably more important to know where the cables for the ignition unit are connected to.MrTickle said:
To try and be as defined as possible, the car drives along perfectly well, then as the speed builds - like I say, it used to be 70 but last time was 50 (I might have had the wipers on at the time, not 100% sure) the fuse will suddenly blow, the rev counter drops to zero instantly, but the ignition lights remain on, the car dies and coasts to a stop. Then the fuse needs replacing before it will start again - starter motor turns, ignition lights on but no firing and no rev counter movement until the fuse is replaced.
All this fits with my first guess theory so far. The ignition warning light is not fuse dependant and the rev counter, wipers, heater blower etc are on the same fuse.MrTickle said:
I bought the car in its current 'state' so I am unsure as to which ignition is fitted. The car and wiring, other than the electronic ignition, look very 'original' and everything looks very new under the bonnet. I will have a nose around and see if I can work out the brand of the ignition system (or check paperwork to see if it is listed there).
It's good to know what unit is fitted if only to ensure it's connected correctly. However, it's not essential and just knowing how many cables it has and where they are connected will help understand what's going on. Most ignition units have three or four wires, 12v +ve, ground, coil -ve and possibly one for the tacho. In your case it appears the 12v supply has been taken from the wrong side of the fuse.OK - I gave had chance to take a look in daylight and have taken a picture to assist. I think you may well be right Mike.
Here is the fuse box (the blue one is the one blowing)
Most wiring seems to be coming from the original loom with the exception of the black one with the different spade connector. That is a new wire and is connected to a red one coming out of the distributor (presumably the electronic ignition module).
Should this be connected on the other side / somewhere else?
Here is the fuse box (the blue one is the one blowing)
Most wiring seems to be coming from the original loom with the exception of the black one with the different spade connector. That is a new wire and is connected to a red one coming out of the distributor (presumably the electronic ignition module).
Should this be connected on the other side / somewhere else?
MrTickle said:
Should this be connected on the other side / somewhere else?
Yep connect to the white on the other side of the fuse. A word of caution here.... the ignition circuit should not draw enough current to blow a 35A fuse on it's own (more like 3 to 4A max) but will/does have peak surge currents that may. However, when combined with your comment about it getting worse over time, I'm concerned the electronics unit may be failing and the fuse blowing is a symptom of a short circuit when "hot". Therefore, If you get ANY miss fire or the engine still dies with the unit connected as above, switch off as soon as possible and replace the ignition unit.
Best of...
Geordie MGmike said:
Yep connect to the white on the other side of the fuse.
A word of caution here.... the ignition circuit should not draw enough current to blow a 35A fuse on it's own (more like 3 to 4A max) but will/does have peak surge currents that may. However, when combined with your comment about it getting worse over time, I'm concerned the electronics unit may be failing and the fuse blowing is a symptom of a short circuit when "hot". Therefore, If you get ANY miss fire or the engine still dies with the unit connected as above, switch off as soon as possible and replace the ignition unit.
Best of...
Brilliant, I will keep an eye on it. Thanks again for your help!A word of caution here.... the ignition circuit should not draw enough current to blow a 35A fuse on it's own (more like 3 to 4A max) but will/does have peak surge currents that may. However, when combined with your comment about it getting worse over time, I'm concerned the electronics unit may be failing and the fuse blowing is a symptom of a short circuit when "hot". Therefore, If you get ANY miss fire or the engine still dies with the unit connected as above, switch off as soon as possible and replace the ignition unit.
Best of...
if you're also able to take a photo(s) of the igniter unit(s) and with the dissy cap on and another photo with the dissy cap off this might (or might not) help with explaining the overload
I've seen that many igniter heads can cause problems when their delicate wires are trapped or chaffed either inside or out side the dissy
ETA: having that black wire in red instead would have made more sense as it's connected to a red wire but as long as you record it as being what it is it doesn't really matter that much
I've seen that many igniter heads can cause problems when their delicate wires are trapped or chaffed either inside or out side the dissy
ETA: having that black wire in red instead would have made more sense as it's connected to a red wire but as long as you record it as being what it is it doesn't really matter that much
Edited by nta16 on Wednesday 25th February 12:03
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