1977 MGB

Anyone had to deal with this ??

The wires look correct bottom left side: 2 brown and 1 brown/white

2nd one up 2 brown/white

Thank you

I would suspect the glass fuse is has a loose contact in the fuse holder. I went through this same issue a couple of weeks ago while chasing intermittent electrical gremlins. I gave up on the original style fuse block and ended up putting in a fuse block with ATC/ATO fuses. Life is much better now.

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Who's version of right are we talking about? When you get 10 LBC owners in a room you'll get 12 different answers.

2nd from the bottom is normally your "accessory" fuse that powers all the ignition switched stuff. Typically, one side(unfused) is white and feeds the fuel pump and coil, while the other side is green and feeds the lights, heater blower, radio(if you have one) and the like. Basically it's anything that you can turn on and off, along with power to the gauges and the brake lights.

It sounds like yours might have had some "wiring modifications" however given the colors you describe.

Still, I'd proceed with assuming that it's still the accessory circuit.

I've never gone out and felt the fuse, but keep in mind that there are a LOT of different things running through it. I blew mine once with a short, and I think that the fuse I installed to replace it was a 15A/30A slow blow(basically it will sustain over 15A for a time but will blow immediately at 30A). If, for example, you have your headlights, wipers, and radio on it wouldn't surprise me if you're getting close to 15A on the fuse. I know that the brake lights draw around 3A by themselves(or at least that's what I measured the last time I replaced the switch). I'm feeling too lazy to look it up, but I think the headlights are 45W each or 90W total for 7.5A at 12V. John Twist is a big advocate of putting relays on the headlights, a job I keep putting off.

I guess my point is that it doesn't take a lot to get up to 15A, and the fuse is rated at that because it was what the circuit was designed for.

I'd probably go through all the green circuits and make sure there's not one with chafed or mouse-eaten insulation that could provide an intermittent or high-resistance short.

EDIT:

Sorry, I was wrong in some of the above and realized it after I went and checked the wiring diagram.

Specifically, the headlights are run off the unswitched bottom(purple) circuit, so they're out. Also the gauge lamps are run off the top fuse for a '77.

Still, a lot of stuff does come off this fuse, and I still stand by checking for a phantom short.



Edited 2 time(s). Last edit at 2017-05-18 01:01 PM by benhutcherson.

I started to write a troubleshooting method, but, there were too many assumptions to make and my description got too ling, so, instead let me ask a few questions.

1) To what are the two white/brown wire connected? I'm guessing the PO added an ignition relay to your car and that one white brown wire comes from the relay to the fuse connector, is crimped together with the second white/brown wire, and that wire goes to the coil + terminal. Is that correct?
2) Assuming the above is correct, what is the resistance of the ignition coil? If it is a 1.5 ohm coil and is powered directly from the ignition relay, the coil will get hot and the wires feeding it will also get hot. If it's a 3 ohm coil there shouldn't be a heat problem.
3) Is it just the left side of the third fuse that is getting hot or are both sides getting hot? Are the white/brown wires also feeling hot?

The reason for these questions is that heat is generated by current flowing through electrical resistance. There is resistivity in all wires based on their cross sectional area (wire gage)and there is contact resistance at all electrical junctions (e.g. crimp connections, spade cinnections, the connection of the fuse box spade terminals to the fuse clips, and, as mentioned in another reply, the connection between the fuse clips and the fuse. So, if something is getting too hot, the current draw is too high, the resistance is too high, or a combination of both.

An easy thing you can check; disconnect the green wires from the RH side of that fuse, turn the ignition on for a while and then feel if the fuse still gets hot, feel if the white/brown wires get hot, and feel if the ignition coil gets hot. If everything stays cool, then the problem is on the load side of the fuse. If so, turn off the lights, the heater fan motor, and the wipers and repeat the test. If the fuse gets hot, you probably have a short in a wire or component like one of the motors.



Edited 4 time(s). Last edit at 2017-05-18 12:19 PM by course2kid.

Remove the fuse
Disconnect the battery
Don't run it till you trace the short
Fuse shouldn't get hot
IF it does it should blow
Get out the multimeter and start testing
http://www.advanceautowire.com
Scroll down to your year

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"I'm a long way gone down this wild road I'm on
It's gonna take me where I'm bound
It's a long way around"



"These are the days that must happen to you"

RD2 Radar/ Electronic Warfare Technician
Vietnam 1969-1972

@ Ben,
To clear confusion to the OP perhaps another Edit is in order, or a strikethrough of the erroneous information?
I know you know this but I will repeat,
All purple, (P) wires are live but fused, and of course the headlamps are not fused.
Headlamps are fed by Brown wire, (N) that is un fused and always live, which is a good thing.

@ Gary F
I second Charles suggestion and would investigate the quality of the fuse block, and replace if suspect.

Have a read here,

http://www.mgexp.com/phorum/read.php?1,3041606

B

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Check your ego Amigo!

Heat is generated by resistance. Either the fuse is bad (which I have seen) or (more likely) one of the copper rivets in the fuse box has become oxidized. The only cure I've found for that is to replace the fusebox - fortunately they are available and not expensive.


Technically, these are not "Slo-Blo" fuses. American and British fuses are rated differently. An American 15 amp fuse is rated to carry 15 amps indefinitely. A British 35 amp fuse is rated to blow within a certain number of seconds when 35 amps are passing through it. A British 35 amp fuse is equivalent to something between 15 and 20 amps continuous.

A true "Slo-Blo" fuse (a trademark) has a little springlike winding on the end of the filament that allows the heat to dissipate and the fuse to run beyond its rating for a longer period of time. These type of fuses are too slow at tripping to protect our wiring.

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com



Edited 1 time(s). Last edit at 2017-05-18 12:42 PM by ingoldsb.

I would make sure someone didn't plug in a high amperage fuse ,changing the weak spot in the circuit to a wire behind the dash.

Good point, and done!

Found it.

I Removed the three green wires ... and it did not heat up !

When I attached the thicker of those three (green wires) it stated to heat up like a "micro waive" !

Now how / where / what should I look to find out where the short could be ??

And thank you very much for all your help

Gary
When you remove these green wire sequentially check to see which accessory has stopped working.
You will by process of elimination, whittle down the culprit this way, then you will know where/what to look for/at.

B

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Check your ego Amigo!

If there is a short (or a device drawing excessive current) it should blow the fuse (unless someone replaced the fuse with one of much larger value). Unplugging accessories will reduce the current and the heat will go away. However, unless the draw is higher than normal, that is probably not your root problem. Heat is caused by current flowing through a resistance - the question is, where is the resistance?

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

Reconnect the thinner green wire(s) that DID NOT make it heat up. Then see which loads are still working. Once you know the list of everything that works fine the temaining load(s) must be on the thick green wire that caused the heating problem. After that you can disconnect all the suspect load items, check each load item for low resistance to ground and check the loom for a short to ground. Since it's a thick green wire, it's most likely to be one of the higher power loads like the wiper motor, heater fan motor, or induction heater. That being said, it's obvious that the PO did some rewiring since there should only be two green wires connected to that fuse box terminal and both should be the same gage wire.

Corrosion + dirt = heat

A lot of MGB Wiring issues can be traced back to the FUSE BLOCK> There was a Post on this site last week about "STAKING" the connections with a punch. I do that and also BRIDGE SOLDER the connections on the back side. A LOT of intermitant electrical issues disapear after that. Have Fun