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MG MGB Technical - Electrical Problem

I know I've seen this problem before but cant find it in the archives. My tach, gauges, signal lights, and brake lights will suddenly stop working. The engine still runs and everything else appears OK but these items all quit. They may be off for a minute, sometimes longer, sometimes shorter, then they all kick back in. I know these are all fed by the same circuit. I've checked he connections I could get to without dismantling but havent found the source yet. The frustrating part is I cant recreate the problem while the car's stopped. Today I drove for about 10 miles on the highway with all these items off, when I got to where I could pull over to check they came back on. I was driving in my neighborhood when they quit another time and immediately pulled over and again they came back on.
william fox

Sounds like a bad ground. If your alternator is charging, the car will keep running. Check the battery cable to ground. Also check connections at the fuse box and clean all terminals. Good luck!

Ira
Ira Spector

William. I do not remember what year your car is. On my 79, all of these circuits are powered by the number three fuse, which is powered by the ignition relay. The ignition relay puts power into the third fuse and the ignition coil. Coming out of the third fuse are your green wire circuits which control the brake lights, gauges and hazzard warning lights/turn signals. Since you are loosing all of these, the power input is the place to start. Les
Les Bengtson

Les: Its a 73 GT. Powered by the #3(third from top) fuse. Diagram showsthe power is a branch off the fuel pump and ignition circuitry but those continue to work. Its a pain to track as I cant get it malfunction while its stopped. I patched in a test light to the power side of the fuse to see if its loosing power to the circuit or if the problem is after the fuse. Of course all works as it should now. I'll leave it connected for a few days and see.
william fox

I had a similar problem in my 80B. Except mine was a hard failure. Cleaning the fuse clips and dielectric grease fixed my problem. I know this is a safety thing but the next time it fails and you pull over (if possible) leave the parking brake off. I wonder if it (brake light) may be supplying a more solid ground for the failed systems to come back online again. My failure started when I applied the parking brake. At least this might aid in troubleshooting why it is an intermittent problem. Good luck and please post what fixes it.

Aloha,
Jasson
Jasson Moore

I know it s not a supply problem. The tach stopped briefly a few times on the way to work and home last night. My test light stayed on and bright the entire time. Now to check the other side of the fuse...
william fox

On my 79, there are a bunch of ground (black)
wires that are screwed down onto the right
hand side of the engine compartment. Seems this
connection could easily be poor causing bad grounds
and a lot of weird behavior.
Ronald

I would tend to believe it is the ground dropping out as Ira and Roland have mentioned. Try this to determine whether it is ground or supply.

Use an aligator clip with wire to the back post of one of the gauges, positive side, and hook a light bulb to it and hook the ground wire from the bulb to another ground and see if it stays lite when the others drop out. If it does, then your ground is dropping out. If it drops out at the same time, then you have a supply problem between your fuse box and the gauge connection.

Could be either one but I'd suspect the ground.
Steven O. Vandal

the exact same thing happened to me. check, polish and tighten the fuse holders - the vibration from the engine combined with the 20 plus years of wear can break the electrical contacts.

rn
RN Lipow

Amen to what RN Lipow said. That was what I left out of my earlier thread: "Replacing the bad fuses that checked good with an ohmmeter" should have been before, "Cleaning the fuse clips..."

Jasson
Jasson Moore

If you look at the backside of the typical MG fuse holder, you will generally find at least some, if not a fair amount of corrosion has taken place to the electro/mechanical connections made by the rivets that hold the fuse clips and wiring spade runners in place.

If you choose to soldier on with the original fuse block, clean it up both physically with a small wire wheel in a dremel mototool if you have one, and chemically with some good corrosion cleaner such as you can find at an Electrical/Electronics store.

Once every thing is cleaned up back there, you can then take some rosin core or multicore solder and a soldering iron and heat up all those riveted joints and flow solder into them. This will not only make a decent electrical connection, but a better mechanical one as well.

The same can be done to a brand new fuse block if you choose to go that route. I then "pot" (fill in) the backside of the fuse block with silicone seal or you can use "liquid electrical tape", a brush on goo that will seal up that backside from the elements and all the other bad influences in the engine compartment.

This just another way of dealing with the inevitable, and certainly will improve the performance of an OE block still in use. FWIW.
Bob Muenchausen

This thread was discussed between 10/05/2002 and 14/05/2002

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