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MG MGB Technical - Fuse blows

The 35A fuse (controls cooling fans, temp gauge, turn signals, and I think brake lights, keeps blowing. I'm not sure where to start to find an answer for this. I can drive about 15 minutes before it gives out. What suggestions do you guys have. Thanks in advance.

I know the turn signal switch was bad and it was replaced. Also, replaced the alternator, which was bad and the ignition switch. This was all done by a shop.

They are having a hard time finding a answer as to why it is blowing. They said somethng like 40 amps is going thru the circuit, but can't out why. Any help would be appreciated.
FJE Frank

My 1978 roadster had the same problem.found to be conection on brake switch behind pedal box.have a look?

What Ian says is a common cause of the green circuit fuse (2nd one up in the 4-way fusebox) blowing. Another common cause is the manifold heater wire detached and floating arround. However that fuse doesn't normally control the cooling fans as they have their own in-line fuse somewhere else, to a separate green circuit.

If they can measure a steady 40 amps going through the circuit then it should be easy enough to disconnect the circuits one by one at the various 4-way bullet connectors and find out which one stops it. You don't give the year, but some have three green wires at the fusebox, in two separate spades, so it is a relatively easy matter to find out which of this has the problem, then work from there. As an alternative to keep replacing the fuse connect a high wattage 12v bulb like an old headlamp bulb in place of the fuse. This will safely limit current, and glow quite brightly when the short is present and dimly or not at all when it isn't. This method does work best with a full short which blows the fuse instantly, but may also be helpful with this partial short.
Paul Hunt 2

Your post is similar to an MG Experience post with the same problem.,633082
The fans are not supposed to run through any of the fuses in the fuse box. If the fans are running through the fuse box that may be your only problem, correct that problem first. If the problem stills exists follow the trouble shooting suggestions mentioned by Paul.

For a wiring diagram go to:
On the left click on stock schematics, then MGB and find your wiring diagram in the PDF. You didn't say whay year car you have but I assume it's a 77 or later since you mentioned cooling fans. In the wiring diagram a white/brown wire from the ignition relay is shown on the input side fuse box. From the terminals on the fuse box a white/brown wire goes through a seperate line fuse to the cooling fans.

Clifton Gordon

FJE Frank, Washington, USA -

It's always helpful if you include the model year. Since you have mentioned electric fans, I am assuming you have a '77-'80. If a '77 without an ignition relay, the fans are connected to the unfused side of the fourth fuse. If a '78 to '80, the fans are connected to the unfused side of the third fuse. Wiring diagrams are available at click on stock schematics. If your car is a '77 and has an ignition relay, use the diagram for '78 and later. Note there is a typo on the wiring diagram. The wire marked NW from the starter relay should be marked WN.

Thanks for all the comments. My MG is a 1976 and I copied what was written on another post as it was very similar to my problem. I haven't owned a MG for over 20 years and had purchased this one about 1 1/2 months ago with none of the issues I spoke of. It has been at the shop for about a month now and now I find myself micro-managing the shop. One interesting thing that they showed me was that when the ignition is on, you can pull the handbrake and the car would start. I guess the handbrake light switch is causing this. I had them disconnect the light switch. This solved the problem with the handbrake. Again, I appreciate all your inputs and would welcome more. Thanks.
FJE Frank

You mentioned cooling fans, the 76 originally had a mechanical cooling fan. Do you have electric cooling fans? If you do, where are they connected?

Clifton Gordon

Clifton, I have a mechanically cooling fan.
FJE Frank

Thanks, Forget the comments about fan wiring and follow Paul Hunt's suggestions for trouble shooting.

Clifton Gordon

Diode failure will cause the starter to engage when the switch on the brake handle is closed while the car is on. Remove the diode and insulate the terminals on the wires to prevent shorts. When working, the diode allows the dash brake lamp to illuminate while the ignition switch is in the start position, a test feature for the bulb.

"a test feature for the bulb"

But totally superfluous if you use the handbrake!
Paul Hunt 2


I agree with you about the test feature for the dash brake bulb and for the emmissions system, that's why I always say to remove the diodes for the dash brake light and the emmissions system.

This what happens when the diodes fail.

Service counter lamp diode:

Service counter reaches a specified mileage point and closes illuminating the lamp. If the diode fails, feedback from the green/pink wire energizes the white/red wire causing the starter to engage. The green/pink wire gets power from the green circuit which gets power from the white wire circuit.

Brake warning lamp diode:

The brake warning lamp is illuminated when the handbrake handle is pulled up closing the circuit between the green wire and the green/orange wire. If the diode fails, then feedback from the green/orange wire energizes the white/red wire causing the starter to engage.

Removing the diodes and insulating the unconnected terminals will have no effect on the electrical system with the following exception, the bulb test feature when the ignition switch is in the start position will be removed.

This thread was discussed between 15/09/2007 and 21/09/2007

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