Electrical/guage problem

[111]

All the guages and accessories in my MGB have worked just fine, with the occasional cold start RPM guage issue. However today all my guages went out with the exception of the oil pressure and speedometer. Furthermore my interior light isn't working, not are the turn signal, or the horn. However the headlights are working.

Any ideas?

 

[Deleted member 8987]

Probably washer motor, reverse lights, and brake failure light out too, among other things.
Fuse.
Either blown or corroded, or maybe the junction point in the green feed.

 

[Deleted member 8987]

Look for the green wire(s) coming off the fuseblock.
That's the fuse.
Use a test light and see if you have power in (should) and power out at the green wire.
Could be the ignition relay, but a test light will tell you, if you have no power into the fuse.

 

[111]

Assuming it is a fuse, what are the size/s?

 

[Deleted member 8987]

That information I do not have at hand.
Pull the fuse, and it should be printed on the metal part.
Now, early Lucas stuff is odd, in that the rating is not what we use today.
Let's say you have a Lucas 30 amp fuse (for the purpose of this example), a current replacemet US would be about a 15A.

I am not sure when or if the later (1980) cars had changed that to US spec.

 

[111]

My test light is fried, but I changed the fuse leading off the green wire. At first everything remained dead, but it slowly came back to life. However for lack of a better way to describe it, everything was acting slow. Also the blinker lights came on without actually blinking. Yet the windshield wipers worked normally, as did the brake lights.

So problem solved, right? Wrong.

Within a short period of time, everything went out again. The natural assumption would be that the fuse blew, but it looks intact. For that matter, the old fuses thin wire looks intact as well. Also on a side note, the old fuse was a Lucas fuse that had a tiny piece of paper inside the glass. It says 17amp continuous, but also says 35amp, so naturally I installed a new 35amp fuse, but am not sure if there is significance to the 17amp continuous one?

Any advice will be appreciated.

 

[David_DuBois]

The Mark I eyeball is the worst possible instrument for determining if a fuse is good or bad. A multimeter set to read ohms is needed to determine if a fuels is good (the "little wire" inside can be broken/detached up under the end cap where it can't be seen). From the description of the problem, it sounds like bad/corroded connections throughout the system. It is time to bite the bullet and get a new test light (if you are pressed for cash, get a 12 volt light bulb and solder some wires to it for a cheap test light), this is not something that you can troubleshoot without some kind of an indicator.

Fuses used in the MGs are a British style 17/35 amp (17amp holding forever, 35 amp blowing instantly and currents in between blowing in decreasing amounts of time at the current approaches 35 amps). American 3AG or AGC style fuse that is closest to these parameters is a standard 15 amp fuse.
Cheers,

 

[Deleted member 8987]

I could have sworn I told you earlier that Lucas is double rating of what we consider "normal".
30A Lucas = 15A US.
You put a double-rating in there, you are asking for a visit from the local fire department.

Did you clean the contacts for the fuse?
Did you clean where the wires connect, both side?
Get a VOM, and actually test the voltage under load (key on) both sides.
Now, if gauges don't work, they ground through the senders.
Rest of it has separate grounds, but if they are all out, the chances of every ground going at once, and all the sender units, while beyond belief, with Lucas, anything is possible.

 

[Deleted member 8987]

Well, if it starts and the headlights work, the main grounds are there, battery to engine and chassis.